From Donald Sutherland today...

Some quick early evening thoughts...

1. The pattern change continues to evolve as anticipated at this point in time.
2. A major winter storm will impact the Plains States into the western Great Lakes region tomorrow and Thursday. Parts of the region could pick up more than a foot of snow with isolated amounts exceeding 15". Rain could end with accumulating snow in Chicago.
3. There is a growing prospect that the above-noted storm will bring a moderate to significant snowfall to the Ottawa area.
4. In the longer-range, the AO is likely to remain negative, the PNA should become neutral (and could head positive in January) and the EPO could become neutral or even negative. Prospects for a significant Arctic outbreak will likely increase at the start of January.
5. Storminess appears likely to continue for at least the next 2-3 weeks.

Gearing up in Oklahoma and Texas...

Special Statement
Statement as of 10:06 AM CST on December 18, 2012

... A large winter storm may impact Oklahoma and western North
Texas sometime around Christmas night...

A rapidly evolving upper storm system will move into The Rockies
early next week near Christmas eve. The storm then appears it
will move across portions of Kansas... Oklahoma... and Texas late
Christmas night through Wednesday. Snowfall will accompany this
storm system. Although it is still too early to determine exact
snowfall amounts and the precise timing of the system... some
locations could experience white out or blizzard conditions.
Travel may become treacherous late Christmas night and the day
after Christmas... especially across portions of Oklahoma. Be
prepared to have your travel arrangements cancelled or changed
for the upcoming Holiday. Monitor the latest updates from the
National Weather Service as more accurate information becomes
available later this week.

From Mr. Sutherland last night:

"Last winter, week after week, most of the guidance and the ENSO-teleconnection analogs were painting warm outlooks. After a pause following November, the scenario is a cold one.

Below are the latest monthly forecasts for January 2013 from the CFSv2 and also what's showing up on the analogs for January 1-7, 2013 coupled with the month as a whole for those analogs. Notice how similar the CFSv2 and analog ideas for January are.

http://www,daculaweather.com/images/2012_12/January2013prelim.jpg

The ENSO-teleconnection analogs are based on ENSO Region 3.4 anomalies in the -0.35°C to +0.40°C range, AO in the -3 to -1 range, and the PNA in the -1 to 0 range for January 1-10, 1950-2012.

The analog years that came up were 1960, 1969, 1979, 1982, 1994, and 2004. March 1960, February 1969, February 1979, April 1982, and February 1994 had snowstorms. January 2004 had a major snowstorm in the Plains States and a massive lake effect snowstorm."

Basically what Don is saying is that we are going to be in for a very cold January. The swing in the pattern is occurring now and tomorrow is the first push in that direction. We can expect a stormy weather pattern to persist on into the new year.
This is going to be fun! :-)

Look for Friday to be VERY windy. No doubt that a Wind Advisory will be issued.

VERY STRONG WINDS AND COLD AIR FILTERING INTO THE REGION THURSDAY
NIGHT AND FRIDAY MORNING BEHIND THE COLD FRONT WILL MAKE FOR A VERY
BRISK DAY ON FRIDAY. STRONG SUSTAINED WINDS OVER 30 MILES PER HOUR
WITH GUSTS NEAR 40 MILES PER HOUR ARE POSSIBLE ACROSS ALL OF NORTH
AND CENTRAL GEORGIA ON FRIDAY. STRONGER WINDS WILL BE POSSIBLE
ACROSS THE MOUNTAINS OF NORTH GEORGIA DURING THE DAY ON FRIDAY.

From Atlanta NWS this morning:

BEGINNING LATE SUNDAY INTO EARLY MONDAY THE FORECAST CONFIDENCE
BEGINS TO GO DOWN DUE TO UNCERTAINTIES IN THE TIMING OF THE NEXT
TROUGH KICKING OUT OF THE WESTERN US. ONE THING THAT HAS REMAINED
FAIRLY CLEAR THE PAST FEW DAYS IS THAT THERE WILL BE A STRONG
SYSTEM MOVING INTO THE SOUTHERN PLAINS ON OR AROUND CHRISTMAS DAY.
THE GFS AND ECMWF OPERATIONAL RUNS HAVE YET TO COMPLETELY AGREE ON THE SHAPE AND DEPTH OF THE OVERALL PATTERN BUT HAVE GENERALLY AGREED THAT SOMETHING SIGNIFICANT WILL EVOLVE. THAT SAID...HAVE STARTED INTRODUCING CLOUDS LATE SUNDAY INTO MONDAY AS ISENTROPIC LIFT COMMENCES AND WE SEE SATURATION OCCURRING IN THE LOW LEVELS PARTICULARLY. THE ECMWF IS A LITTLE FASTER IN CRANKING OUT PRECIP WITH THIS PATTERN BUT IT ALSO INTRODUCES A SHORT WAVE TROF WITHIN THE GENERALLY SOUTHWEST FLOW ON MONDAY. IN BOTH CASES...GFS AND ECMWF...RAIN CHANCES GO UP MONDAY AND THUS HAVE INTRODUCED LOW RAIN CHANCES INTO THE FORECAST ON MONDAY. FOR NOW...HAVE KEPT THUNDER OUT OF THE WEATHER GRIDS DUE TO OVERALL LACK OF RUN TO RUN CONSISTENCY...BUT IF THE TROF IS AS DEEP AS SOME PREVIOUS RUNS... CANNOT RULE OUT ELEVATED STORMS LATE ON MONDAY.

CHRISTMAS DAY AT THIS POINT LOOKS CLOUDY AND COOL WITH QUITE A BIT OF OVERRUNNING RAINS AROUND...HOWEVER...HAVE KEPT POPS IN THE CHANCE CATEGORY FOR NOW DUE TO OVERALL LACK OF RUN TO RUN CONSISTENCY AND UNCERTAINTY IN THE TIMING OF THE TROF KICKING OUT.

I love reading these forecasts and appreciate all the work that goes into them. But, it would be nice if the weather man would translate some of this into regular language! I do understand a lot of it, but could it be broken down into something the regular person could understand? Are we talking a possible snow event after Christmas?

Terrie, thanks for reading! I'm sorry... sometimes I get too weather geeky!

What we looking at is a pattern change that will bring us much colder temperatures, as well as systems that skirt along the Gulf coast and across the southeast. As it looks right now, the month of January will be well below normal for temperatures and normal to above normal precipitation. That type of pattern translates to a potential winter weather period. I suspect January will be a fun period for tracking winter storms. I'm personally very optimistic.

However, the Christmas system is evolving as we speak... this from the HPC this morning...

NEW HPC OVERNIGHT MEDIUM RANGE GUIDANCE WAS PRIMARILY DERIVED FROM A 50-50 BLEND OF MORE COMPATIBLE GEFS/ECMWF ENSEMBLE MEANS. A COMPOSITE GEFS/ECMWF ENSEMBLE MEANS BLEND MAINTAINS DECENT HPC CONTINUITY AMID UNCERTAIN POTENT FLOW. THIS SOLUTION WOULD FOCUS ACTIVITY INTO THE CENTRAL US CHRISTMAS WHERE SRN PLAINS
CYCLOGENESIS/DEEPENING GULF INFLOW/DEVELOPING HEAVIER PRECIPITATION TUCKS UNDERNEATH/INTO A MID-HIGHER LATITUDE DOME OF COLDER HIGH PRESSURE.

THE DEEPENING WINTER LOW HAS AMPLE DYNAMIC POTENTIAL TO
DEEPEN TO ANOTHER THREATENING MAJOR WINTER SYSTEM FOR THE
E-CENTRAL US WED...GIVE OR TAKE A DAY.

And honestly, I don't know that we're out of the woods for next week either. It's possible that as the low strengthens and ejects to the northeast on Wednesday, that the strengthening will bring wrap around moisture and cold into our area, even if only briefly. All options are on the table right now.

Watch this loop of the jet stream depicting winds above 30,000 feet. These are the upper most rivers of wind that steer systems through the atmosphere. Watch how these winds really get cranking with winds greater than 150 mph. It will be important to watch how they influence our January weather.

http://www.daculaweather.com/4_gfs_06_200_nws_loop.php

Also notice as the frames progress, how the stream gets pushed further south. In the beginning of the loop you see both the subtropical jet stream as well as an arctic jet stream and they appear to phase or merge together. This merging creates an even stronger wind.

And as I expected...

URGENT - WEATHER MESSAGE
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
301 PM EST WED DEC 19 2012

...WIND ADVISORY IN EFFECT FROM 6 PM THURSDAY TO 1 AM EST SATURDAY
ACROSS PORTIONS OF FAR NORTH GEORGIA...AND FROM MIDNIGHT THURSDAY
NIGHT TO 7 PM EST FRIDAY FOR THE REMAINING AREAS OF NORTH AND CENTRAL
GEORGIA...

.STRONG WINDS ON THE BACK SIDE OF A DEVELOPING LOW PRESSURE SYSTEM
AND BEHIND A POTENT COLD FRONT ARE EXPECTED ACROSS MUCH OF NORTH
AND CENTRAL GEORGIA...WITH NORTHWEST WINDS OF 20-25 MPH AND GUSTS
OF 35-40 MPH POSSIBLE. SLIGHTLY HIGHER WINDS ARE EXPECTED IN THE
HIGHER ELEVATIONS OF NORTH GEORGIA.

...WIND ADVISORY IN EFFECT FROM MIDNIGHT THURSDAY NIGHT TO 7 PM
EST FRIDAY...

Also...
WITH THIS STRONG COLD ADVECTION WILL SEE A JUMP TO BELOW NORMAL
TEMPERATURES FRIDAY...LOWS IN THE UPPER 20'S AND 30'S AND HIGHS ONLY IN THE MID 40'S TO MID 50'S. THESE TEMPERATURES...COMBINED WITH THE HIGH WIND SPEEDS...WILL RESULT IN WIND CHILL VALUES IN THE TEENS TO LOW 20'S IN THE MORNING.

Very strong storms entering Georgia and continuing back though MS.

You can following the rain on these radars:
http://www.daculaweather.com/grlevel4/4_ffc_br248.php
http://www.daculaweather.com/current/misc/google-maps-radar/4_google_radar.php
http://www.daculaweather.com/4_ridge_georgia.php

And severe weather watches here:
http://www.daculaweather.com/national_alerts/4_severe_alert.php

There has already been more than 16 tornado warnings this morning across the southeast.

Some of you may not know this, but I have a specific page for cellphones.
http://www.daculaweather.com/cellphone.php

This firs batch of showers that is just now moving into the Atlanta area is moving from the southwest to northeast at 40 - 60 mph!

EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD
200 AM EST THU DEC 20 2012

VALID 12Z SUN DEC 23 2012 - 12Z THU DEC 27 2012

...MORE ACTIVE WINTER WEATHER COMING...
UPSTREAM...AMPLE MID-UPPER LEVEL TROUGH ENERGY DIGS FROM THE WEST COAST CHRISTMAS EVE TO THE S-CENTRAL ROCKIES/PLAINS BY CHRISTMAS WITH SURFACE LOW DEVELOPMENT AND PRECIPITATION UNDERNEATH AND OVER SOUTHWARD SURGED PLAINS HIGH PRESSURE. THIS SYSTEM REALLY IS EXPECTED TO DEEPEN MID-LATE WEEK OVER THE EAST CENTRAL AND THE EASTERN US INTO A MAJOR WINTER STORM WITH AN EXPANDING HEAVIER PRECIPITATION SHIELD. THERE IS AMPLE ROOM FOR COLD AIR TO SETTLE TO THE NORTH OF THE LOW AND DAMMED DOWN THROUGH THE MID-ATLANTIC IN ADVANCE OF THE SYSTEM.

In other words... a major winter storm is in the works for the Christmas period for many people in the US. Changes coming today for us is pretty much the beginning of what will be a cold a stormy period.

80% chance of a Tornado Watch for SW Georgia.

From my buddy Larry in Savannah, my notations in ().

"This Goofy (GFS) run is the best/most entertaining run of the season to date and one of the best in several years with one of the biggest and most widespread SE snowstorms being modeled for days 11-13 as well as it being about the coldest run for the SE US for days 7-16 for the season to date with nearly nonstop cold. Keep your sanity, folks, but also keep hope alive.

Besides the modeled snowstorm, getting cold late this month would be quite helpful climo-wise since it would bring Dec.'s warmth down just enough to help Jan. correlations to cold/wintry precip."

LOL! And you think it will be cold and windy here. Check out Grandfather Mtn in NC.

.THE FORECAST FOR GRANDFATHER MOUNTAIN NC AT 5946 FT...
...HIGH WIND WARNING IN EFFECT FROM 7 PM THIS EVENING TO NOON EST
SATURDAY...
...WINTER WEATHER ADVISORY IN EFFECT FROM MIDNIGHT TONIGHT TO
MIDNIGHT EST FRIDAY NIGHT...

.TODAY...CLOUDY. SNOW SHOWERS IN THE MORNING. RAIN SHOWERS THROUGH
THE DAY. IN THE CLOUDS AT TIMES DURING THE AFTERNOON. HIGHS IN THE
LOWER 40S. SOUTH WINDS 40 TO 45 MPH...INCREASING TO 50 TO 55 MPH IN
THE AFTERNOON. GUSTS UP TO 78 MPH.

.TONIGHT...CLOUDY. RAIN SHOWERS UNTIL MIDNIGHT. SNOW SHOWERS LIKELY
THROUGH THE NIGHT. PATCHY FOG AFTER MIDNIGHT. IN THE CLOUDS AT TIMES
DURING THE EVENING. LOWS IN THE MID TEENS. WEST WINDS 40 TO 45 MPH...
INCREASING TO 45 TO 50 MPH AFTER MIDNIGHT. GUSTS UP TO 74 MPH. WIND
CHILL VALUES IN THE SINGLE DIGITS BELOW ZERO AFTER MIDNIGHT.

.FRIDAY...CLOUDY. PATCHY FOG IN THE MORNING. SNOW SHOWERS LIKELY
THROUGH THE DAY. HIGHS IN THE UPPER TEENS. WEST WINDS 50 TO 55 MPH.
GUSTS UP TO 88 MPH. WIND CHILL VALUES IN THE SINGLE DIGITS BELOW
ZERO.

I'm going to continue to post Don Sutherland's comments about the medium/long range forecast. His forecast always appear to be spot on. This will be a multi-comment post he made around noon today.

"Large-Scale Pattern Change Imminent...

A blizzard is currently raging in parts of Iowa and Wisconsin. That storm represents yet another event in the ongoing evolution of the pattern that will culminate in a much colder pattern locking in over a large part of the United States and southern Canada. Already, parts of Wisconsin have picked up 10" or more of snow. Strawberry Point has received 12.2".

Meanwhile, this morning was the 6th consecutive day on which Fairbanks, Alaska registered a low of -40°F or colder. To date, the monthly mean temperature is a frigid -19.4°F (16.3°F below normal). However, there are hints on the ensembles and also the CFSv2 that Alaska could grow notably warmer in the extended forecast range. As that happens, Arctic air could press southward, leading to what will likely be a colder than normal January across much of the U.S. (except for perhaps the Southwest and Florida) and southern Canada, if the leading analogs are accurate. The CFSv2 has been signaling a cold January for some time now."

... continued.

... continued
"Such a break in the pattern featuring the warming in Alaska is not without precedent. For example, November 1977 saw Fairbanks come out with a mean temperature of -7.6°F (10.4°F below normal). November 2012 had very similar readings, with a mean temperature of -8.8°F (11.4°F below normal). The first 17 days of December 1977 were brutally cold in Fairbanks. The mean temperature was -32.8°F. 3 days had high temperatures of -40°F or colder. The lowest temperature during that period was -52° on December 13 and the highest temperature was -11°F on December 1. A rapid transition to milder condtions took place afterward. The December 20-31 period had a mean temperature of a toasty 10.8°F. 11/12 days had high temperatures above 10°F and 4/12 had highs in the lower 20s. 10/12 days had low temperatures above 0°F. The lowest temperature was -8°F on December 25 and the highest was 22° on December 26."

... continued

... continued

"January 1978 had a mean temperature of +0.1°F (12.0°F above normal). January1978 had temperature anomalies that were fairly similar to those forecast on the December 20, 2012 run of the CFSv2 for January 2013.

Through December 19, the mean temperature in Fairbanks has been -19.4°F. The lowest temperature has been -48°F on December 17.

The latest ensemble forecasts suggest that the EPO could go neutral or negative toward the end of December. The PNA could go neutral or positive (and support for a turn to positive has been growing). The AO will likely remain negative. As a result, the forecast teleconnections suggest that the focus of the cold could shift south and eastward from Alaska into the CONUS and southern Canada. As 2013 begins, the risk of an Arctic outbreak appears likely to increase. One has seen such an outbreak appear on a few of the recent runs of the GFS for the start of January (including today's 12z run). "

This is along way off but...my comments in ()

"Speaking of dreaming, the 12Z GFS (US model) puts down 0.87" of liquid equivalent in the form of pure snow at KATL (Atlanta... which could be 6-10" possibly) on 1/1-2! Once the snow starts early on 1/1, KATL falls to below 32 and pretty much doesn't get back above it through the end of the run (7 AM on 1/5)...96 straight subfreezing hours. Over a deep snow cover, it gets down to 12 F 1/3 AM! A second small snow falls early on 1/5 due to a 2nd/weaker Miller A! This is the winter wx dream of a lifetime for me. Who knows when this good of a dream will come along again?"

SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
754 PM EST THU DEC 20 2012

...WINDY AND COLD TONIGHT AND FRIDAY...

STRONG GUSTY WINDS HAVE MOVED INTO NORTH AND CENTRAL GEORGIA
BEHIND THE COLD FRONT. THESE HIGH WINDS WILL CONTINUE INTO
FRIDAY...AND ACROSS THE NORTHEAST MOUNTAINS INTO FRIDAY NIGHT.
SUSTAINED WINDS OF 20 TO 25 MPH WITH GUSTS OF 35 TO 40 MPH ARE
LIKELY. SOME GUSTS COULD REACH 40 TO 45 MPH IN THE HIGHER
ELEVATIONS OF THE NORTHEAST MOUNTAINS. THE COLDEST AIR OF THE
SEASON WILL BE USHERED INTO THE AREA AS WELL...WITH OVERNIGHT
MINIMUM TEMPERATURES TONIGHT RANGING FROM THE MID 20S IN THE
NORTHWEST PORTION OF THE STATE TO THE MID 30S IN THE SOUTHEAST.
THE COMBINATION OF COLD TEMPERATURES AND STRONG GUSTY WINDS WILL RESULT IN WIND CHILL VALUES IN THE TEENS TO MID 20S. LOWER VALUES CAN BE EXPECTED IN THE HIGHER ELEVATIONS. FRIDAY WILL ALSO BE A COLD AND BRISK DAY WITH MANY AREAS IN NORTH GEORGIA ONLY GETTING UP TO THE MID 40S FOR HIGHS...WHICH WILL FEEL MORE LIKE THE MID 30S DUE TO THE WIND CHILL.

ANYONE EXPECTING TO SPEND EXTENDED PERIODS OF TIME OUTSIDE SHOULD DRESS APPROPRIATELY.

Serious question - when did we start naming snow storms?
Apparently the storm that occurred over/through Iowa was called Draco?

http://www.weather.com/news/why-we-name-winter-storms-20121001

R, it's the most stupid thing I've ever heard of, as well as every meteorologist I've heard from. Strictly a marketing ploy by The Weather Channel. If you watch their programming, you'd understand why. TWC went from a great weather tool to just a tool. Now that they've started purchasing other great companies like Weather Underground, we're all expecting those sites to begin going downhill also.

Great area forecast discussion from Dallas TX last night.

YOUR REGULARLY SCHEDULED UPDATE AFD ENDS HERE. SINCE IT IS A QUIET
NIGHT...ITS WEATHER CLASSROOM AND LAB TIME FOR THOSE INTERESTED...

THE CHRISTMAS NIGHT FORECAST IS ON EVERYONE/S MIND. WE HAVE BEEN
WATCHING THIS SYSTEM IN THE EXTENDED MODELS FOR SEVERAL DAYS...AND CONTINUE TO RIGHTLY ASSESS THAT A POTENTIAL FOR A WINTER EVENT EXISTS. UNFORTUNATELY MODEL GUIDANCE DOES NOT HAVE THE SKILL IN THAT TIME RANGE TO ACCURATELY PREDICT THE INTENSITY AND LATITUDE THE UPPER LEVEL SYSTEM WILL TAKE BECAUSE OF THE UNCERTAINTY ON EXACTLY WHERE A COLD FRONT WILL BE ON TUESDAY. AS WE HAVE SAID IN THE LAST 2 AFDS...THIS COLD FRONT IS THE KEY TO THE FORECAST.

BUT TO UNDERSTAND WHY...IT IS HELPFUL TO DISCUSS BAROCLINICITY
AND ITS AFFECTS ON THE MOVEMENT AND STRENGTH OF WEATHER SYSTEMS.
ESSENTIALLY THESE STORM SYSTEMS ARE BLOBS OF UPPER LEVEL
ENERGY...OR VORTICITY ...OFTEN CALLED SHORTWAVES OR DISTURBANCES.
IN AN ATMOSPHERE WHERE THERE IS VERY LITTLE TEMPERATURE DIFFERENCE FROM LOCATION TO LOCATION...THE MOTION OF THESE DISTURBANCES IS BASICALLY DICTATED BY THE FLOW THEY ARE A PART OF. THEY BEHAVE MUCH LIKE A LEAF WOULD IF IT WERE FLOATING DOWN A RIVER.

continue...

continue...

HOWEVER WHEN HORIZONTAL TEMPERATURE GRADIENTS EXIST...SUCH AS
WHEN A COLD FRONT IS NEARBY...THIS SIMPLE RULE GOES OUT THE
window. THIS IS CALLED A BAROCLINIC ATMOSPHERE . A DISTURBANCE THAT
MOVES INTO A BAROCLINIC ZONE WILL INTENSIFY. THIS IS BECAUSE LOW
LEVEL TEMPERATURE ADVECTION BEGINS TO INTERFACE WITH THE UPPER
LEVEL SYSTEM. IN SHORT...THERMAL ADVECTION IN THE LOW LEVELS
CAUSE HEIGHT CHANGES IN THE UPPER LEVELS...WHICH IN TURN
STRENGTHEN THE UPPER LEVEL VORTICITY CENTER...WHICH IN TURN
STRENGTHENS THE LOW LEVEL THERMAL ADVECTION ...AND ETC. THE WHOLE THING BECOMES A FEEDBACK LOOP. AN UPPER DISTURBANCE UNDERGOING BAROCLINIC INTENSIFICATION WILL TRANSFORM INTO A CYCLONE AND WILL NO LONGER MOVE WITH THE MEAN FLOW ...BUT WILL CURVE TOWARD THE POLE. IN THE PROCESS OF TURNING TOWARD THE POLE THE CYCLONE OFTEN OCCLUDES...MEANING THEY LOSE THEIR ACCESS TO THE TEMPERATURE GRADIENTS AND THEY WEAKEN. WE HAVE ALREADY SEEN MANY MODEL RUNS OF A BIG WINTER CYCLONE DEVELOPING OVER THE SOUTHERN PLAINS AND CURVING OFF TO THE NORTH. IN THE LATEST MODELS WE NO LONGER SEE THAT SOLUTION AND SEE A WEAK TROUGH MOVING OVERHEAD.

continue...

...continued

THE REASON WHY THE MODELS HAVE BEEN ALTERNATING BETWEEN A POWERFUL WINTER CYCLONE IN THE PLAINS AND A WEAK DISTURBANCE HAS BEEN DUE TO THE LOCATION OF THE BAROCLINIC ZONE /OR COLD FRONT / WHEN THE DISTURBANCE REACHES THE SOUTHERN PLAINS. THE TREND IN MOST OF THE MODELS HAS BEEN TO MOVE THE FRONT /BAROCLINIC ZONE INTO SOUTH TEXAS. AS A RESULT THE UPPER LEVEL DISTURBANCE TRAVELS OVER TEXAS WITHOUT STRENGTHENING BECAUSE THE TEMPERATURE GRADIENTS ARE WEAK SO FAR BEHIND THE FRONT . THE SYSTEM THUS DOES NOT INTENSIFY UNTIL IT REACHES THE NORTHERN GULF COAST WHERE A BAROCLINIC ZONE EXISTS.

THIS SCENARIO...WHICH WE ARE CURRENTLY FORECASTING...IS ONE THAT
WOULD ONLY PRODUCE LIGHT AMOUNTS OF WINTER PRECIP. THIS IS BECAUSE
THE SYSTEM WILL BE WEAK AND HAS NO ACCESS TO THE THERMAL GRADIENTS
IT NEEDS TO INTENSIFY. AN INTENSIFYING LOW WOULD BE ABLE AUGMENT
THE WIND FIELDS AND THUS TAP INTO THE GULF FOR MOISTURE .

continued...

... continued

IN ORDER TO GET A SIGNIFICANT SNOW STORM HERE...WE WOULD NEED THE
FRONT TO BE FARTHER NORTH THAN CURRENTLY FORECAST...BUT NOT SO FAR NORTH THAT THE SYSTEM BEGINS ITS POLEWARD TURN AND STAYS NORTH OF THE REGION ALTOGETHER. WE NEED A GOLDILOCKS POSITION OF THIS FRONT . AGAIN...KNOWING THE PRECISE LOCATION OF A FRONT BEYOND 5 DAYS IS THE DIFFICULTY WITH THIS FORECAST. THANKFULLY THE LEAD SHORTWAVE RESPONSIBLE FOR THE LOCATION OF THIS FRONT WILL BE ONSHORE IN CALIFORNIA SATURDAY AND WILL BE SAMPLED BY THE BALLOON NETWORK (for more accurate data samples). IN ADDITION...WE WILL BE IN RANGE OF THE NAM BY TOMORROW FOR HAVING ANOTHER OPINION ON THIS FRONT'S STRENGTH AND LOCATION MONDAY NIGHT AND TUESDAY... EVEN IF WE DON'T GET AN EXPLICIT DEC 25TH FORECAST FROM THE NAM UNTIL SATURDAY.

Whew... lots of talk but I thought it was a very good primer on how some of the winter storm processes work, and why it can be so difficult to get it right.

There's AFD's are a wealth of information, if you have an interest in the weather, I highly recommend reading them to gain insight to what's going on with the weather.
The link to the Atlanta office AFD is here: http://www.daculaweather.com/4_afd_ffc.php

There are links to all of the southeast weather office forecast discussions on that page.

One page you can bookmark is the short term forecast discussion from the Hydrometeorological Prediction Center (HPC) http://www.daculaweather.com/4_hpc_short_range_disco.php

The HPC is responsible for the prediction of precipitation for the US. The page I listed above also has links to the other HPC discussions. There might be some weather geek talk in those but you should be able to get the jest of the discussion.

It is Ed! Still just 36º here. Gusty winds will make for some cold wind chills today.

Mt Washington NH. Notice the current wind speed...
http://www.daculaweather.com/4_mt_washington.php

Right now on Grandfather Mountain in NC:
Temperature: 14.4 °F (-9.8 °C)
Dew point: 12 °F (-11.1 °C) Relative humidity: 90%
Wind: West (256°) at 62 mph with gusts of 95 mph
Pressure: 815 mb (24.07 inHg) station pressure

http://www.nc-climate.ncsu.edu/cronos/?station=GRANDFATHR

Oh... by the way, today is the other official start to winter! The Winter Solstice occurred at 6:21 am this morning.

Welcome to Winter! :-)

Don Sutherland this morning... a two parter. Sounding better, and better!

"Quick thoughts...

On 12/15, some ensemble members had begun to hint that the PNA, which has now been negative for 46 consecutive days, could go positive near the end of December. Historical cases related to long-lived PNA- regimes also suggested that such guidance might have some merit. Today, there is developing consensus that the PNA is likely to go positive toward the end of the month or near the beginning of January.

The AO, which has again fallen below -3 is likely to remain negative through the extended period consistent with a number of long-lived blocking regimes, including those of winters 2009-10 and 2010-11.

With the blizzard departing the Great Lakes region after having brought more than 15" snow to Madison and Chicago its first measurable snowfall since March 4, 2012, along with a surge of warmth and moderate-to-heavy rain in the East, the final pieces of a pattern change to generally colder than normal weather across much of the U.S. and southern Canada are now falling into place.

... continued

... continued

"As the colder pattern unfolds, the active subtropical jet suggests that storminess could coincide with the cold. Hence, opportunities for at least some accumulation of snow appear likely.

Finally, once the PNA goes neutral and then probably positive, the risk of an Arctic outbreak could increase. The potential exists that sometime during the next 2-3 weeks, Albany could see its first subzero temperature since February 11, 2011 when the mercury fell to -5° and New York City could see its first single-digit reading since January 24, 2011 when the temperature fell to 6°."

This afternoon from the Atlanta forecast discussion:

THERE COULD BE ENOUGH WRAP AROUND MOISTURE WEDNESDAY
NIGHT...AND ENOUGH COLD AIR MOVING INTO NORTH GEORGIA WHERE SOME OF THE RAIN SHOWERS COULD BE MIXED WITH SOME SNOW AND HAVE ADDED THIS TO THE EXTREME NORTH FOR LATE WEDNESDAY NIGHT. TEMPS WILL BE TRICKY AND BASED ON THE TIMING OF SHORT WAVES. AT THIS POINT HAVE NO REASON TO GO AGAINST GUIDANCE.

http://www.daculaweather.com/4_afd_ffc.php

Off Topic, but the winds today are incredibly strong. I have a tree that fell down sometime during the day in my backyard. Fortunately no damage to anything though. If this potential winter storm develops as advertised it's going to get nice and cold, maybe a little wintry

No, not really off topic Charles! A tight pressure gradient between the departing low pressure area and the cold Canadian high pressure system, brought us those high winds today. But you think it was windy here? Look at the max wind gust on Mt. Washington today http://www.daculaweather.com/4_mt_washington.php

Huntsville AL NWS had this to say about the Christmas system:

AM A LITTLE CONCERNED ON WEDNESDAY ABOUT THE 12Z-18Z TIME PERIOD
WHERE WE COULD BE SEEING SUBSTANTIAL COOLING IN THE LOW-MID LEVELS OF THE ATMOSPHERE TO SEE EITHER A MIXTURE OF RAIN/SNOW OR A BRIEF FULL CHANGE OVER TO SNOW...BEFORE THE SYSTEM LIFTS FURTHER AWAY FROM THE FORECAST AREA BY 00Z THURSDAY. WILL CONTINUE WITH HIGHER PRECIP CHANCES FOR WEDNESDAY AND WILL BE ANXIOUS TO SEE FUTURE MODEL RUNS OVER THE NEXT COUPLE OF DAYS TO SEE SOME MORE CONSISTENCY BETWEEN THE GFS AND ECMWF BEFORE HEDGING ONE WAY OR THE OTHER RELATING TO THE PRECIP TYPE.

I love what a meteorologist just said...

"I am liking what I am seeing between euro and GFS....winter is coming. wow. winter....hard to believe."

Excerpt from the 7:05 pm AFD tonight from the Atlanta NWS:

AREA FORECAST DISCUSSION...UPDATED FOR AVIATION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
705 PM EST FRI DEC 21 2012

ALSO LOOKS TO STILL HAVE A POTENTIAL FOR SNOW MIXING INTO NORTH GA WEDNESDAY NIGHT FROM WRAP-AROUND MOISTURE ON BACK SIDE OF SYSTEM. HAVE LOWERED TEMPS A BIT WEDNESDAY AND WEDNESDAY NIGHT AS BLEND OF LATEST GUIDANCE AND INFLUENCE OF AFOREMENTIONED COLD FRONT. REST OF FORECAST LOOKS TO BE ON TRACK.

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
815 PM EST FRI DEC 21 2012

.UPDATE...
COLD AIR ADVECTION WITH A BREEZY NORTHWEST WIND WILL PREVAIL
TONIGHT THROUGH SATURDAY. NO CHANGES NEEDED TO THE PREVIOUS
FORECAST PACKAGE AS CLEAR SKIES WILL CONTINUE WITH THE MIN TEMPS
ON TRACK. STILL EXPECTING THE COLDEST NIGHT SO FAR THIS WINTER
SEASON WITH TEMPERATURES IN THE 20S. LOWEST TEMPS RECORDED SO FAR
AT OUR FOUR CLIMATE STATONS ARE...

ATL 30 ON NOV 25
AHN 27 ON NOV 25
CSG 30 ON NOV 25
MCN 25 ON NOV 26

After looking at the 2 meter temp's, I can now say that this 0Z GFS, as modeled, would appear to be a devastating ice storm for much of the CAD (cold air damming) areas possibly as far south as the ATL/AHN areas on 1/1. The 2 meter temp maps actually show 32 or colder after the precip, ~1.75" worth, has already fallen as far south as ~Gainseville. The details aren't important now, but they do show the potential of what could occur.

Everyone please note. The models will be going back and forth with solutions this far out. They will swing one way, and the next model run swing a different direction. But what you can start to get a feel for is the cold and the moisture. The details will only be known a few days in advance, if even then. Forecasting winter weather is MUCH more difficult than any other season of the year, as the timing of the cold and moisture have to be pretty close to perfect in order to get it right. So if you start hearing differing solutions right now, that is completely normal and expected. Every time a new model run comes out, weather weenies and meteorologist are all over them, frame by frame. In case you didn't know, the Euro and GFS have 4 model runs a day, at 00Z, 06Z, 12Z, and 18Z. Right now there is a 5 hour difference between our time and Z time, or Greenwich Mean Time.
So
00Z = 7pm
06Z = 1pm
12Z = 7am
18Z = 1pm

I even have a page that allows you to check on the NCEP model run processing. If you look at this page, you see that there are a LOT of models (and calculations) that basically run 24/7. http://www.daculaweather.com/4_model_run_status.php

WARNING... weather geek speak to follow...

Here's part one of the forecast a meteorologist made on December 13th. As you can see, he was spot on with this last storm and the current cold. My clarifications in ()

"With the MJO (http://www.daculaweather.com/4_mjo_phase_forecast.php) going into phase 1 and the 5-7 day lag for MJO induced waves to traverse the eastern US it would set us up for a wave to arrive in the east between Dec. 18-20 which is progged on the 180hr GFS and Euro moving across the upper Midwest or Great Lakes (this is what we just had). The same wave also ushers in relatively "colder" air (what we currently have) and plays nicely into the idea I spoke of the other day in my post about the pocket of relatively "colder" air arriving during the Dec. 21-23 time frame based off of GWO analogs for the eastern US. Using that same concept for the GWO I expect another batch of colder air Dec. 27-30. And following the spike in the 10 and 30 mb temps (stratospheric temps http://www.daculaweather.com/4_stratosphere_temp.php) that peaked back on Dec. 6 seems we should see a good shot of colder air Dec 30-Jan 3."

... continued.

... continued.

"I don't know if this wave was directly attributable to the MJO since some raise valid arguments that this wasn't a true MJO signal in the phase 1 region but we have our wave moving through the Great Lakes as discussed on Dec. 13 and temps for example in Detroit will be dropping from the upper 40's Thursday-Dec. 20 down to highs in the low 30's for Dec. 22. GFS and Euro both hinting at another wave moving through the Great Lakes Dec. 27-28 which would allow colder air to settle into the region for Dec. 28 by current models. Models indicate another pocket of cold air arriving to reinforce the prior batch with a short wave Jan 2-3 across the Eastern US which falls into the Dec. 30-Jan 3 date I laid out previously. The Euro and GFS both give credence to this time frame with a shortwave modeled off the coast of CA for 0z Dec. 30 giving it 4-5 days for it make it out into the Plains to the Great Lakes/East Coast. Although the GFS has been wavering in the magnitude of the cold air for this time frame, which I was expecting to be some of the coldest of the season guessing along the lines of -15C to -20C 850mb over southern Michigan and eastward. So the GFS is still not modeling that type of air mass as of right now I will leave that TBD as we get closer."

... continued.

... continued.

"With the newest GWO numbers today the latest analogs indicate a 15-20F warm up in surface temps Jan 3-Jan 6 compared to prior days highs across Midwest or NE (the Dec. 31-Jan 3 cold episode) and analogs suggest a stair stepped 15-20F temperature fall behind the warm up 1-2 days after that, meaning another spell of colder air Jan 5-8.

I extended my outlook through Jan 8 on Dec. 14. I still feel comfortable with those dates and with the newest GWO analogs and latest 10 and 30 mb temp forecast it appears the time frame between Jan 8-10 will see a warm up over the east with a short wave moving through the east Jan 10-11 with cooler air dropping in behind it for Jan 10-13 across the mid-west/Great Lakes and then pockets of cooler air arriving Jan 15-19 time frame for the east followed by a second reinforcement of cooler air Jan 20-24. My GWO analogs only take me out to Jan 9 so there may be another short wave or two to add to the 2 already expected for the Jan 9-24 time frame. I will update as the newest GWO numbers come out over the next week and half."

EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD
106 AM EST SAT DEC 22 2012

VALID 12Z TUE DEC 25 2012 - 12Z SAT DEC 29 2012

...CHANCES INCREASING FOR A MAJOR WINTER STORM FROM THE LOWER
MIDWEST TO THE EAST COAST...

THE 12Z/21 AND 00Z/22 GLOBAL NUMERICAL MODELS ARE CONVERGING ON A
SOLUTION THAT THREATENS THE NATION FROM THE CENTRAL MISSISSIPPI
VALLEY TO THE MID ATLANTIC REGION AND NORTHEAST WITH WIDESPREAD
SNOW AND ICE DAYS 4 AND 5. A LOW IS EXPECTED TO TRACK ACROSS THE
GULF STATES AND SOUTHERN APPALACHIANS EARLY IN THE PERIOD, THEN
REFORM ALONG THE MID ATLANTIC COAST AND DEEPEN RAPIDLY EARLY DAY
5. HOW CLOSE TO THE ATLANTIC COAST THE SNOW AND ICE GET WILL
DEPEND ON THE TRACK OF THE LOW AND THE SUPPLY OF DEEP COLD AIR TO THE NORTH AND WEST. THE REMAINDER OF THE PLAINS AND UPPER MIDWEST WILL BE DOMINATED BY COLD, DRY AIR THIS PERIOD. THE WEST WILL REMAIN UNSETTLED AS ENERGY POURS INTO THE REGION ON THE OTHER SIDE OF THE BLOCKING RIDGE AXIS ASTRIDE THE CONTINENTAL DIVIDE.

From my buddy Larry with my comments in (). His first comments are the Christmas system, The "one to follow" is the New Year's system, which I also believe will be our first significant winter precipitation event.

"With the 0Z CMC having gone a good bit NW, which would easily be enough to take the snow away from the bulk of N GA, with no CAD related wintry precipitation, with no model now giving sigificant wintry precip. to the bulk of N GA, and with run after run if both the GFS and Euro doing the same, I'm reducing the chance from 5% to 3% for that area and from 10% to 5% for far north GA (non-mtn areas) for significant wintry precip. The one to follow for these areas is the one near 1/1. Right now, I'm giving that storm a 25% chance of producing significant wintry precip. for the bulk of N GA ATL-AHN north."

GFS is prolly my least favorite model, but it's always the model that "hypes" all the storms up. Then later the Euro comes out and says, "that GFS he's such a joker." LOL, anyways I'd rather skip any CAD stuff cause you can't have any fun w/sleet or ice. It's gotta be snow and just snow preferably, and hopefully there's something and not another rainstorm. At least a little bit of severe weather if we do get warm.

Starting to look like possibly some severe weather on Christmas.

BOTH GFS/ECMWF/NAM/SREF INDICATE THE COLD FRONT ASSOCIATED WITH THE CHRISTMAS EVE SYSTEM WILL NOT PUSH AS FAR INTO THE STATE AS PREVIOUS RUNS HAVE INDICATED. THIS IS A SIGNIFICANT CHANGE. THIS MEANS THE WARM GULF AIR WILL SPREAD INTO MOST OF NORTH GEORGIA CHRISTMAS
AFTERNOON. AS THE COLD FRONT MOVES INTO THE STATE CHRISTMAS
NIGHT...THE INSTABILITY AND SHEAR WILL INCREASE RAPIDLY ACROSS THE
ENTIRE CWA BRINGING THE THREAT OF SEVERE WEATHER TO THE ENTIRE CWA
CHRISTMAS NIGHT INTO EARLY WEDNESDAY MORNING. WRAP AROUND MOISTURE WILL OCCUR DURING THE DAY WEDNESDAY BUT SHOULD BE OUT OF THE CWA BY WEDNESDAY NIGHT REDUCING THE RISK OF SNOW SHOWERS TO A FEW FLURRIES WEDNESDAY NIGHT ACROSS THE MOUNTAINS.

It's now looking like a severe weather Christmas for us in Georgia. There will be a chance of thunderstorms and tornadoes during the day on Tuesday and into Tuesday night. A strong cold front will push into our area and the SPC has this to say.

FORECAST SOUNDINGS AT MIDDAY TUESDAY IN THE LOWER MS VALLEY DEVELOP AN IMPRESSIVE SHEAR ENVIRONMENT AS THE MID-LEVEL JET APPROACHES THE REGION FROM THE WEST. NAM FORECAST SOUNDINGS VALID TUESDAY AT 21Z JUST SOUTHWEST OF JACKSON MS SHOW MLCAPE VALUES AROUND 1500 J/KG WITH ABOUT 60 KT OF DEEP LAYER SHEAR. THIS COMBINED WITH 0-3 KM STORM RELATIVE HELICITIES AROUND 400 M2/S2 COULD BE FAVORABLE FOR SUPERCELLS AND TORNADOES. IF MANY CELLS CAN REMAIN DISCRETE... THEN CONDITIONS COULD ALSO BE FAVORABLE FOR STRONG TORNADOES AS THE LOW-LEVEL JET MOVES EWD ACROSS THE CNTRL GULF COAST STATES FROM TUESDAY MORNING INTO THE AFTERNOON. AT THIS POINT...THE POTENTIAL FOR WIDESPREAD WIND DAMAGE APPEARS JUST AS VIABLE.

THE MAGNITUDE OF THESE THREATS MAY DEPEND UPON THE DOMINANT STORM MODE. IF THE FAVORED MODE ENDS UP BEING LINEAR...THEN WIND DAMAGE COULD BE THE GREATER THREAT.

Mt Washington currently has steady hurricane force winds with gust to almost 120 mph
http://www.daculaweather.com/4_mt_washington.php

Everyone is becoming very concerned about the Christmas storm. The severe weather parameters are already not good and could be getting worse. Very warm moist air will be in place from the first system that passes through on Sunday/Monday, and couple that with deep level shear, and you end up with rotation in some of these storms and bring the chance for tornadoes and straight line winds.

Shear is the change in wind direction as you go up in height. If you have strong winds that are changing direction as they rise, then you have potential for tornadic development. The threat for us will even carry through Wednesday morning.

The Storm Prediction Center has us in a Slight Risk area for severe weather, but it is possible for that to be changed as this system evolves. Stay tuned... I'll be providing updates as soon as they are available, and way before you'll hear it from your favorite TV weatherman! :-)

Steve - I thought I told you I wanted 74 and sunny for Christmas. ;)

Here's roughly how close we'll get to accumulating snow with this first system on Christmas: http://www.daculaweather.com/images/2012_12/snow.gif

Birmingham is getting very concerned

ON CHRISTMAS DAY THROUGH EARLY WEDNESDAY MORNING...THE THREAT OF TORNADOES AND DAMAGING STRAIGHT LINE WINDS LOOKS TO BE INCREASING AND POSSIBLE NOW FOR ALL OF CENTRAL ALABAMA. THE BEST TIME FRAME FOR POSSIBLE SEVERE WEATHER COULD START AS EARLY AS 2 PM CHRISTMAS AFTERNOON IN THE WEST WITH THE THREAT SPREADING EASTWARD THROUGH THE EVENING. THE THREAT COULD LAST THROUGH AT LEAST 2 AM WEDNESDAY MORNING IN THE EAST. ALL INTERESTS SHOULD CONTINUE TO MONITOR THE LATEST FORECASTS REGARDING THIS SYSTEM...AS CHRISTMAS DAY PLANS WILL BE IMPACTED.

.SPOTTER INFORMATION STATEMENT...

ACTIVATION OF STORM SPOTTERS AND EMERGENCY MANAGEMENT WILL BE
NECESSARY CHRISTMAS DAY THROUGH EARLY WEDNESDAY MORNING.

New Year storm talk from my buddy Larry:

"I'm definitely liking that the last three GFS runs have been centered on a weak, N GOM to ~Jacksonville track and consistently for 1/1-1/2. For N GA, this is about as optimal a track as can be had for the best shot at a sig. snow. So, to stay in the ballgame, I hope this continues. However, it isn't cold enough at 850 on two of the three runs. So, getting it cold enough might be a challenge. I'm sticking with a 25% chance for the bulk of N GA to get a significant wintry event (1"+ S/IP and/or 0.25"+ ZR) near 1/1-1/2. For 9-10 days out, those are pretty respectable odds."

Atlanta HWO released a few minutes ago...

...SEVERE THUNDERSTORMS WILL BE POSSIBLE CHRISTMAS NIGHT INTO EARLY WEDNESDAY MORNING...

A STRONG STORM SYSTEM WILL BE DEVELOPING OVER THE LOWER MISSISSIPPI VALLEY TUESDAY BRINGING WIDESPREAD RAIN AND A FEW THUNDERSTORMS TO MUCH OF NORTH AND CENTRAL GEORGIA CHRISTMAS DAY THROUGH WEDNESDAY MORNING. VERY STRONG SURFACE LOW PRESSURE WILL MOVE INTO NORTHERN MISSISSIPPI BY LATE CHRISTMAS DAY...AND INTO NORTHERN TENNESSEE BY WEDNESDAY MORNING. AN ASSOCIATED COLD FRONT WILL MOVE FROM WEST TO EAST ACROSS NORTH AND CENTRAL GEORGIA CHRISTMAS NIGHT. INSTABILITY AND VERTICAL WIND SHEAR WILL INCREASE ACROSS THE REGION CHRISTMAS NIGHT AND WILL BE ENOUGH FOR ISOLATED SEVERE THUNDERSTORMS TO DEVELOP...MAINLY ACROSS WESTERN GEORGIA BETWEEN 10PM AND 3AM LATE TUESDAY NIGHT...AND ACROSS EASTERN GEORGIA BETWEEN 3 AND 9 AM
WEDNESDAY MORNING.

AS THIS IS A DEVELOPING SITUATION PLEASE STAY TUNED TO THE LATEST UPDATES.

NCEP Operational Status Message

Sun Dec 23 15:01:00 2012 GMT
NOUS42 KWNO 231500
ADMNFD
SENIOR DUTY METEOROLOGIST NWS ADMINISTRATIVE MESSAGE
NWS NCEP CENTRAL OPERATIONS COLLEGE PARK MD
1457Z SUN DEC 23 2012

CRITICAL WEATHER DAY STATUS...
THERE IS A POTENTIAL FOR CWD BEGINNING 12Z TUE 12/25
THROUGH 12Z FRI 12/28 AS AN AREA OF LOW PRESSURE AND
ASSOCIATED PCPN/MOISTURE MOVES ACROSS SOUTHERN AND
EASTERN REGIONS.

A Critical Weather Day is defined as:

DEFINITION: A critical weather event is defined as any of the following occurring on/near the coast of or within the United States and its territories (including Puerto Rico, the Virgin Islands and Guam):

1. A widespread tornado outbreak;

2. A storm system causing a major precipitation event;

3. A severe cold-season storm affecting a large area with widespread heavy snow, freezing rain, or blizzard conditions; or

4. A tropical system threatening the coastline or producing excessive rainfall after moving inland.

Birmingham AFD...

THERE ARE MANY ELEMENTS THAT CAN INFLUENCE THE SEVERITY OF A
WEATHER EVENT...BUT FROM A SYNOPTIC STANDPOINT THIS SYSTEM IS
ABOUT AS CLASSIC AS IT GETS. ALL MODES AND TYPES OF SEVERE WEATHER ARE POSSIBLE...BUT TORNADOES AND DAMAGING STRAIGHT LINE WINDS ARE EXPECTED TO BE THE MAIN THREATS. THE THREAT COULD BEGIN DURING THE EARLY AFTERNOON ACROSS THE SOUTHWESTERN PART OF THE AREA WELL IN ADVANCE OF THE SURFACE LOW AS THE WARM FRONT LIFTS NORTHWARD AND INSTABILITY INCREASES. IN THIS AREA...THE BROAD NATURE OF THE DIFFLUENT UPPER JET WELL IN ADVANCE OF THE UPSTREAM JET MAX SUPPORTS THE IDEA OF SUPERCELL DEVELOPMENT. THE THREAT FOR SUPERCELLS MAY THEN BECOME MORE WIDESPREAD DURING THE EVENING HOURS AS THE NOSE OF UPPER JET STREAK CONTINUES TO APPROACH. THE PROSPECT OF SUPERCELL DEVELOPMENT IS ALSO SUPPORTED BY THE LOW AMPLITUDE NATURE OF THE MID LEVEL SHORTWAVE AS OPPOSED TO A SHARPER TROUGH AXIS WHICH WOULD SUGGEST MORE OF A LINEAR MODE OF CONVECTION. A TRANSITION TO BROKEN LINE OF SUPERCELLS AND BOWING SEGMENTS IS EXPECTED TOWARD 06Z EAST OF INTERSTATE 65 AS THE FORCING BECOMES MORE LINEAR IN ASSOCIATION WITH THE COLD FRONT.

And from the Atlanta NWS forecast discussion

REAL PROBLEM BEGINS TUES/TUES NIGHT AS MUCH STRONGER JET STREAK
DROPS INTO THE SRN PLAINS AND BOTTOMS OUT OVER THE LOWER MS VALLEY.
FAVORABLE DIFFLUENT PATTERN ALOFT AND STRONG WAA WILL ALLOW SHRA WITH A FEW EMBEDDED TSRA TO SPREAD NORTH IN THE AFTERNOON. AS FRONT APPROACHES TUES NIGHT...SHOULD SEE AXIS OF SBCAPE APPROACH 500 J/KG WITH VERY STRONG LOW LEVEL VERTICAL WIND SHEAR. 0-1KM BULK SHEAR VALUES OF 40-50KT NEAR THE STRONGEST WE'VE SEEN THIS COOL SEASON.
SURFACE LOW STILL DROPPING TO AROUND 995MB AS IT PASSES TO OUR NORTH. 12Z MODELS NOW SLOWER WITH THE TIMING OF THE FRONT... PARTICULARLY THE NAM. 18Z NAM JUST NOW COMING IN AND MAY BE EVEN SLOWER. ANYWAY FRONT SHOULD PUSH INTO THE STATE AROUND 09Z AND NOT EXIT THE SOUTHEAST CWA 16-18Z. WILL HAVE TO ADJUST OUR GRAPHICS AND HWO. IF ANY BOW ECHO OR LINEAR SEGMENTS CAN ORIENT THEMSELVES PERPENDICULAR TO THE MEAN FLOW...COULD EASILY SEE MESOVORTICES WITH WIND DAMAGE OR ISOLATED TORNADOES. TOO SOON TO PIN THIS DOWN BUT WILL CONTINUE TO PROVIDE OUR BEST INTERPRETATION.
STAY TUNED.

SNELSON

Severe discussion from the Atlanta NWS this morning, my comments in ():

...SEVERE WEATHER LIKELY CHRISTMAS NIGHT...

EXTENDED PERIOD INITIALIZES QUITE ACTIVE FOR CHRISTMAS NIGHT WITH
THE INCREASING LIKELIHOOD OF SEVERE WX ACROSS THE AREA. A MODERATE RISK OF SEVERE STORMS HAS NOW BEEN POSTED BY SPC (http://www.daculaweather.com/4_spc_day2_outlook.php) WITH SLIGHT RISK ENCOMPASSING MOST OF WEST GEORGIA FOR DAY 2. MODELS IN EXCELLENT AGREEMENT IN SHOWING A DEVELOPING 995 LOW OVER THE TENNESSEE VALLEY PROGRESSING NNE...LEAVING THE LOCAL AREA IN THE WARM SECTOR BY 06Z WED WITH DEWPOINTS LIKELY IN THE UPPER 50S TO LOWER 60S (lots of energy to work with). ALTHOUGH RESULTANT CAPE VALUES WILL NOT BE AS HIGH AS LOCATIONS TO THE WEST...EXPECT DYNAMICS OF SYSTEM ALONE INCLUDING IMPRESSIVE DIFFLUENT PATTERN ALOFT (the diffluent winds aloft will cause lift) TO COMPENSATE LACK OF CONVECTIVE FORCING. SHEAR WILL NOT BE AN ISSUE (lots of it... especially at the lower levels. Shear causes rotation) WITH THIS SYSTEM WITH A 60 KT 850MB JET TRAVERSING THE AREA AND 0-1KM HELICITY OF 400 M2/S2 TO 600 M2/S2.

IN FACT ANY TORNADOES ABLE TO FORM WILL BE CAPABLE OF ACHIEVING F2 STATUS OR PERHAPS GREATER AND EXHIBIT LONG LIFE CYCLES GIVEN STP OVER 2 INDICATED BY MOST MODELS. TOOK A LOOK AT THE CIPS ANALOG SITE FOR THIS EVENT AND SEEMS MOST SIMILAR EVENTS DID PRODUCE SEVERE WEATHER OVER THE AREA INCLUDING TORNADOES WITH THE EMPHASIS ON THE SOUTHERN CWA.

One thing I find interesting is that the low that will cause the severe weather tomorrow, hasn't even formed yet. The piece of energy is still over the Pacific NW.

This will be one of those systems that forms through cyclogenesis. A lee-side trough or low is caused by the stretching of a column of air as it descends on the lee-ward side of highly elevated terrain such as the Rocky Mountains. This stretching causes a spin-up of vorticity and a resulting upper level divergence. Lee-side cyclones can intensify over time as they move with the upper level flow and can initiate cyclogenesis. A downslope flow will be most intense if the wind flow is perpendicular to the mountain range and the winds have a high velocity.

Here are two maps that show the potential track of the low pressure systems.
http://www.daculaweather.com/4_hpc_sig_lows.php