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Major National Winter Storm Dec 25-28


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I think I'm gonna go ahead and say that this event may have the highest ceiling of a winter event (before March 1st) since Super Tuesday 2008, all of the parameters tomorrow especially across the southern Gulf Coast states (and possibly extending further north eventually) are pretty scarily high, not only for a Winter event, but for many Spring events as well (with a pretty textbook upper level trough, no less). I seriously hope that people down there are aware of what might be about to happen over the next 48 hours...

And I agree that the second high risk for December on record may be indeed in the works, perhaps not with the first outlook, but once the mesoscale becomes clear, because a lot synoptically argues for it...across pretty much all of the models.

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The strength of the dry punch between 00-06Z Wednesday on the 00Z NAM is just incredible...indicating the intensity of the mid-level system. This allows the QLCS to remain mostly or entirely discrete supercells from extreme E LA / SW MS all the way to GA. In particular, it shows three rounds of tornado-producing supercells: a first round of storms starting 12Z Tuesday with elevated storms becoming surface-based in E/C LA; a second, protracted (meaning nocturnal) outbreak beginning at 21Z Tuesday in S/C MS; and a third, afternoon event at 21Z Wednesday in C AL extending into W GA. The orientation of the surface-700-mb bulk shear relative to the front, in particular, really helps to maintain discrete supercells for a long period of time. This almost reminds me of 04/27/2011, as frightening as it may be to invoke.

http://www.emc.ncep....yle/spcprod/00/

Given the trends, I am starting to think that a Day-1 High Risk for overall TOR and Severe Wind probabilities may be in order.

I wouldn't go so far as to make comparisons to -the day that cannot be speaketh- but I agree with you on the dry punch. This is an incredibly dynamic system and having the 100kt barb at H5 impinging on a moist and unstable warm sector after dark speaks of a very volatile situation.

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For what it's worth, I now think tomorrow deserves a high risk. Seeing the 4km NAM, SPC and NSSL WRF models show 1) the WAA rain shield pushing at a good pace towards the NE, leaving at least a several hour window for rapid destabilization to occur ahead of the cold front, and 2) a line of discrete cells as opposed to a solid squall line give me confidence that a major tornado outbreak will indeed occur tomorrow night.

At this point, the area I would include in a high risk area includes NE LA, south-central MS and south-central AL. This includes:

Baton Rouge, LA

Natchez, MS

Jackson, MS

Hattiesburg, MS

Meridian, MS

Tuscaloosa, AL

Birmingham, AL

Montgomery, AL

Hope everyone stays safe tomorrow night, as I'm not encouraged by what I see right now.

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The strength of the dry punch between 00-06Z Wednesday on the 00Z NAM is just incredible...indicating the intensity of the mid-level system. This allows the QLCS to remain mostly or entirely discrete supercells from extreme E LA / SW MS all the way to GA. In particular, it shows three rounds of tornado-producing supercells: a first round of storms starting 12Z Tuesday with elevated storms becoming surface-based in E/C LA; a second, prolonged (meaning nocturnal) outbreak beginning at 21Z Tuesday in S/C MS; and a third, afternoon event at 21Z Wednesday in C AL extending into W GA. The orientation of the surface-700-mb bulk shear relative to the front, in particular, really helps to maintain discrete supercells for a long period of time. This almost reminds me of 04/27/2011, as frightening as it may be to invoke.

http://www.emc.ncep....yle/spcprod/00/

Given the trends, I am starting to think that a Day-1 High Risk for overall TOR and Severe Wind probabilities may be in order.

This does actually remind me of a watered-down instability and slightly further south version of the 4/27/11 event. The shape and evolution of the trough and intensity of the wind fields are all quite similar. Even the rain shield associated with the warm front, and the rapid destabilization in its wake is similar, although in this case it'll happen after dark.

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Critical Weather Day Has Been Declared

START: 1200Z TUE DEC 25 2012 END: 1200Z FRI DEC 28 2012 TYPE: Regional WHO: NCEP, NWSTG, NCF, NWS SOUTHERN/EASTERN REGIONS REASON: Severe weather and tornado potential across portions of Southern Region Christmas Day and Christmas night evolving into a significant winter storm over Eastern Region Wednesday through early Friday.

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The 00Z soundings from KBRO and MMVR (Veracruz, Mexico) are coming in much more on the dry adiabatic side than even the 00Z NAM forecasted...indicating a very robust dry punch that will likely evolve much as the NAM and WRF indicate, perhaps even more significantly. This would cut down on morning convection tomorrow and really substantiate the impetus for a High Risk. As far as my comparison to 04/27/2011 goes...thanks to CUmet for realizing that I meant a watered-down (but still very severe) version. :santa: (It will certainly be bigger than 12/23/2002, which had no significant tornadoes.) Merry Christmas to all Dixie Alley residents...be ready for this.

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Most similar to 01/17/2010 in terms of shear-based parameters and the dry slot, but the upper-air pattern is more progressive, the instability far greater, and the dry slot more significant in this case. Vertical shear is also greater in the upcoming system. Few other analogs matched the precise upper-level pattern in this case; even the 01/17/2010 case does not depict as strong a Pacific NW shortwave and a pattern as progressive as in the upcoming case (in which the shortwave Rockies ridge will be less pronounced).

http://www.eas.slu.edu/CIPS/ANALOG/stats.php?reg=SE&model=NAM212&fhr=F036&flg=

http://www.eas.slu.edu/CIPS/ANALOG/event.php?reg=SE&fhr=F036&model=NAM212&dt=2010011712

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Moderate Risk D1:

day1otlk1200.gif

day1probotlk1200torn.gif

DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1200 AM CST TUE DEC 25 2012

VALID 251200Z - 261200Z

...THERE IS A MDT RISK OF SVR TSTMS ACROSS SOUTHEAST TX AND PORTIONS

OF LA/MS/AL/FL PANHANDLE...

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE GULF COAST/SOUTHEAST

STATES...

...A DANGEROUS CHRISTMAS DAY/NIGHT SEVERE WEATHER EPISODE IS LIKELY

TO UNFOLD...INCLUDING THE POTENTIAL FOR STRONG TORNADOES AND

WIDESPREAD DAMAGING WINDS...ACROSS PORTIONS OF THE GULF COAST REGION

AND LOWER MS RIVER VALLEY...

..SYNOPSIS

A PROGRESSIVE LARGE SCALE FLOW PATTERN WILL PREVAIL OVER THE CONUS

DURING THE PERIOD. THE PRIMARY SYSTEM OF FOCUS WITH RESPECT TO DEEP

CONVECTION WILL BE A VIGOROUS SHORTWAVE TROUGH /AND ASSOCIATED

STRONG UPPER JET STREAK/ AS IT SPREADS EASTWARD...AMPLIFIES...AND

THEN BECOMES INCREASINGLY CLOSED/NEGATIVELY TILTED TONIGHT AS IT

ADVANCES FROM THE SOUTHERN PLAINS TO THE LOWER MS VALLEY/TN VALLEY.

..GULF COAST REGION/LOWER MS VALLEY TO SOUTHEAST STATES

GIVEN THE LARGE SCALE SCENARIO AS PREVIOUSLY DESCRIBED...A SURFACE

LOW IS EXPECTED TO STEADILY DEEPEN AS IT SPREADS NORTHEASTWARD FROM

SOUTHEAST TX THIS MORNING TO THE TN VALLEY TONIGHT. AS THIS

OCCURS...AN INCREASINGLY MOIST LOW-LEVEL AIRMASS WILL DEVELOP

NORTHWARD IN TANDEM WITH A WEST-EAST ORIENTED WARM FRONT...WITH

LOWER 60S DEWPOINTS /AND SOME MIDDLE 60S F NEAR THE COAST/ BECOMING

INCREASINGLY PREVALENT WITHIN THE WARM SECTOR.

INITIALLY...STRONG/POTENTIALLY SEVERE TSTMS ARE EXPECTED TO BE

ONGOING THIS MORNING...ESPECIALLY ACROSS SOUTHEAST TX INTO LA. THIS

INCLUDES THE POSSIBILITY OF ELEVATED SUPERCELLS NORTH OF THE WARM

FRONT /MAINLY CAPABLE OF HAIL/...ALONG WITH AN INCREASING SURFACE

BASED SEVERE THREAT NEAR/SOUTH OF THE WARM FRONT THIS MORNING ACROSS

THE UPPER TX COAST/SOUTHERN LA. WITHIN AN INCREASINGLY MOIST AND

HIGHLY SHEARED ENVIRONMENT...THIS INCLUDES THE POSSIBILITY OF AN

EARLY DAY TORNADO/WIND DAMAGE THREAT THIS MORNING/EARLY AFTERNOON

ACROSS SOUTHEAST TX INTO SOUTHERN LA.

WITH TIME...THE AIRMASS WILL FURTHER DESTABILIZE INTO THIS AFTERNOON

ESPECIALLY ACROSS CENTRAL/SOUTHERN LA AND SOUTHERN PORTIONS OF

MS/AL...WITH 1000+ J/KG MLCAPE FOR AREAS WITHIN 150-200 MILES OF THE

GULF COAST. LESSER...AND A BIT MORE UNCERTAIN...DESTABILIZATION IS

EXPECTED CLOSER TO THE EAST-NORTHEASTWARD SURFACE LOW LATER

TODAY/TONIGHT...LARGELY OWING TO PERSISTENT PRECIPITATION NEAR/NORTH

OF THE WARM FRONT AND A LESSER INFLUX OF LOW-LEVEL MOISTURE.

REGARDLESS...THIS DESTABILIZATION COUPLED WITH AN INCREASINGLY

STRONG DEEP LAYER WIND FIELD...WITH 70-100 KT AT 500 MB AND

110+ KT AT 250 MB...WILL RESULT IN DEEP LAYER SHEAR SUPPORTIVE OF

ROTATING UPDRAFTS/SUPERCELLS. ACCENTUATED BY THE DEEPENING PHASE OF

THE CYCLONE...EVENTUAL QLCS DEVELOPMENT SEEMS LIKELY TO OCCUR THIS

AFTERNOON INTO TONIGHT NEAR/AHEAD OF THE EASTWARD-ADVANCING COLD

FRONT...ALTHOUGH DISCRETE SUPERCELLS WILL REMAIN POSSIBLE WITHIN THE

WARM SECTOR AND NEAR/IMMEDIATELY AHEAD OF THE EVOLVING QLCS...WHICH

SEEMS LIKELY TO CROSS LA AND THE SOUTHERN HALVES OF MS/AL THROUGH

TONIGHT. POTENTIALLY WIDESPREAD DAMAGING WINDS WILL BE POSSIBLE WITH

SUSTAINED BOWING SEGMENTS. FURTHERMORE...A FEW STRONG/POTENTIALLY

LONG-LIVED TORNADOES WILL ALSO BE POSSIBLE WITH DISCRETE SUPERCELLS

AND/OR EVENTUAL EMBEDDED MESO-VORTICES LATER THIS

AFTERNOON/TONIGHT...ESPECIALLY GIVEN 200-500 M2/S2 0-1 KM SRH

COINCIDENT WITH A 50-70 KT LOW LEVEL JET LATE THIS

AFTERNOON/TONIGHT.

WHILE THE OVERALL SYSTEM STRENGTHENS...INSTABILITY WILL WANE LATE IN

THE PERIOD. EVEN SO...A SEVERE THREAT MAY REACH AS FAR EAST AS PARTS

OF AL...SOUTHEAST TN...AND WESTERN GA LATER TONIGHT...WITH SOME

POTENTIAL FOR AREAS INCLUDING COASTAL SC BY THE EARLY MORNING HOURS

OF WEDNESDAY.

..GUYER/SMITH.. 12/25/2012

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SmartCast Analysis for 25 Dec/05Z to 26 Dec 05Z ( Base on my model output only) Update....

First the winter side of the storm system:

Currently Targeting the area of Tinker to McAlester this hour. My model showing the heaviest snow along this corridor.

McAlester Region: Tracking the snow to begin around 18Z, with periods of heavy snow, with snowfall rates of 1.3” per hour. In addition, winds look to increase to around 30MPH along the same timeframe, so look for blowing snow. Right now looking at 7.9” of snow in this area.

Tinker AFB/OKC Region: Showing a period of mixed precip from 06Z to 11Z, then changing to all snow by 12Z. Looking at visibilities down to less than ½ mile, and snowfall rates of up to 1.1” per hour possible. Right now looking max of 5.7” of snow accumulation in this area. This is down from 7.1” prediction on the 01Z run. In addition, looking at winds gusting to 35 to 40mph, which will create temporary blizzard conditions in this area.

Vance AFB Region: Looking to be on the fringe of the snow, but still looking at 3.5” of snow possible. Snow will begin around 09Z, and will be heaviest from 16Z to 20Z. In addition, winds will be gusting up to 35MPH during this time, so look for reduce visibility and blowing snow.

Dallas Region: Tracking some strong thunderstorms pushing through the area from 16Z to 21Z, then changing to snow, with a potential for localized 1-2” across the area. In addition, looking at winds gusting up to 44MPH from 22Z to 04Z.

Damaging Wind Potential:

Hobart, Lawton, Lubbock, Childress, and Clinton are all targeted for winds 45 to 50MPH winds during the next 24 hours. In addition, looking at snowfall from 1” to 3” possible. This will also cause tempo conditions of reduce vis and blowing snow.

Severe Wx Potential:

05Z SmartCast run is zoningin on the Lake Charles region, with a 76% of some tornadic event within the area. In addition, forecasted hail projected around 2.3” to 2.6” and convective winds of 55 to 68MPH Possible. In addition, heavy rainfall is possible in these areas with precip totals of 1.3 to 1.9” possible.

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Are there any factors re: the deepening rate of the surface low which may have prevented SPC from selecting a higher TOR probability than 15%? Perhaps a weaker cyclone is expected in the short term (06-12Z Tuesday) which would fail to mix out the boundary layer as efficiently as a stronger low-level wind profile would. I am just trying to account for the reasons as to why SPC would give more weight to morning junk convection in this case given the dry punch lurking to the south.

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Are there any factors re: the deepening rate of the surface low which may have prevented SPC from selecting a higher TOR probability than 15%? Perhaps a weaker cyclone is expected in the short term (06-12Z Tuesday) which would fail to mix out the boundary layer as efficiently as a stronger low-level wind profile would. I am just trying to account for the reasons as to why SPC would give more weight to morning junk convection in this case given the dry punch lurking to the south.

I just think they are trying to wait before mesoscale details are ironed out a bit further and, as you mentioned, the junk convection situation becomes more apparent until a possible upgrade, that was a pretty strongly worded moderate, although I am a bit surprised there was no mention of an upgrade to high (although that didn't happen on March 2nd this year so I suppose that doesn't mean that much).

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SPECIAL WEATHER STATEMENT

NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX

1247 AM CST TUE DEC 25 2012

TXZ177-198-199-212-250715-

WALLER TX-WALKER TX-MONTGOMERY TX-GRIMES TX-

1247 AM CST TUE DEC 25 2012

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN WALLER...

SOUTHWESTERN WALKER...NORTHWESTERN MONTGOMERY AND SOUTHEASTERN

GRIMES COUNTIES UNTIL 115 AM CST...

AT 1244 AM CST...NATIONAL WEATHER SERVICE DOPPLER RADAR WAS TRACKING

STRONG THUNDERSTORMS NEAR MAGNOLIA...OR 9 MILES NORTHWEST OF

PINEHURST...MOVING NORTHEAST AT 40 MPH.

HAIL UP TO THE SIZE OF DIMES AND WIND GUSTS UP TO 40 MPH WILL BE

POSSIBLE WITH THIS STORM.

LOCATIONS IMPACTED INCLUDE...

PINEHURST...MAGNOLIA...NEW WAVERLY...MONTGOMERY AND TODD MISSION.

HGX_loop.gif

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MESOSCALE DISCUSSION 2203

NWS STORM PREDICTION CENTER NORMAN OK

0101 AM CST TUE DEC 25 2012

AREAS AFFECTED...PORTIONS OF CENTRAL/ERN TX...WRN LA

CONCERNING...SEVERE POTENTIAL...WATCH LIKELY

VALID 250701Z - 250800Z

PROBABILITY OF WATCH ISSUANCE...80 PERCENT

SUMMARY...TSTMS WILL CONTINUE TO DEVELOP/INTENSIFY NORTH OF A WARM

FRONT THROUGH THE EARLY MORNING HOURS. SUPERCELL TSTMS ARE LIKELY

WITH LARGE HAIL POSING THE PRIMARY THREAT. A WW IS LIKELY PRIOR TO

08Z.

DISCUSSION...TSTMS ARE DEVELOPING/INTENSIFYING NORTH OF A WARM FRONT

THAT WAS LOCATED FROM COT NEWD TO JUST NW OF HOU TO NORTH OF BPT AT

06Z. THIS DEVELOPMENT IS IN RESPONSE TO INCREASING WAA IN ADVANCE OF

AN INTENSE SEWD-MOVING SHORTWAVE APCHG W TX. THE ENVIRONMENT NORTH

OF THE WARM FRONT IS CHARACTERIZED BY 40-45 KT DEEP LAYER SHEAR THAT

IS FORECAST TO INCREASE TO 55-60 KT BY 12Z. MUCAPES OF 1000-1500

J/KG WILL SUPPORT ELEVATED SUPERCELLS WITH A THREAT OF LARGE HAIL.

STORMS ARE EXPECTED TO INCREASE IN COVERAGE AND INTENSITY OVER THE

NEXT FEW HOURS AND A SVR TSTM WATCH WILL LIKELY BE NEEDED BY 08Z.

..BUNTING/MEAD.. 12/25/2012

ATTN...WFO...LCH...SHV...HGX...FWD...EWX...

post-32-0-54044400-1356419111_thumb.gif

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The 00z 4km NMM is a lot more impressive than the 12z, too:

http://www.emc.ncep....yle/spcprod/00/

I think it has to do with the farther S low position. Get that low farther south and back the winds, you also change the direction of the bulk shear vector to a more zonal direction, meaning that it will be more perpendicular to the boundary.

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Jennings LA about to see a supercell with some good hail.

BULLETIN - EAS ACTIVATION REQUESTED

TORNADO WARNING

NATIONAL WEATHER SERVICE LAKE CHARLES LA

239 AM CST TUE DEC 25 2012

THE NATIONAL WEATHER SERVICE IN LAKE CHARLES HAS ISSUED A

* TORNADO WARNING FOR...

NORTHERN ACADIA PARISH IN SOUTHWEST LOUISIANA...

THIS INCLUDES...MERMENTAU...IOTA...

EAST CENTRAL JEFFERSON DAVIS PARISH IN SOUTHWEST LOUISIANA...

THIS INCLUDES JENNINGS...

* UNTIL 315 AM CST

* AT 236 AM CST...NATIONAL WEATHER SERVICE METEOROLOGISTS DETECTED A

SEVERE THUNDERSTORM CAPABLE OF PRODUCING A TORNADO NEAR JENNINGS...

MOVING NORTHEAST AT 30 MPH.

* THE TORNADO WILL BE NEAR...

MERMENTAU BY 245 AM CST...

EVANGELINE BY 250 AM CST...

EGAN BY 255 AM CST...

IOTA BY 305 AM CST...

MAXIE BY 310 AM CST...

MOWATA BY 315 AM CST...

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