Major National Winter Storm Dec 25-28

[andyhb]

4/27/11 Produce plenty of tornadoes from midnight to 10AM. you don't need much daytime heating with that setup.

Also it is amazing how many large events have occurred between the 26-39th of the month in the last several years. Looks like a coincidence, but why can't the other 90% of the month get some action?

39th?

Also, there have been plenty of events over the past few years with dates outside the 20th-end of the month. 3/2/12, 4/14-16/11, 5/10/10, 4/14/12, 2/5/08, 4/10/09, 5/10/08, etc.

[Disc]

Alexandria, LA

[Amped]

Alexandria, LA

STP 13.1, CINH -22.2 Am I reading that right? Sounds nuts.

[Stebo]

That is quite a significant sounding that you posted Disc, several key things to note: Low LCLs, High EHI especially the 0-1km, High STP as well, and very high amounts of 0-3km CAPE almost approaching 200 J/kg. The most significant thing to note though is the strong clockwise turning in the lowest km of the atmosphere, really just an exceptional sounding.

[andyhb]

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1253 AM CST MON DEC 24 2012

VALID 251200Z - 261200Z

...THERE IS A MDT RISK OF SVR TSTMS ACROSS PARTS OF LA...SCNTRL MS

AND WCNTRL AL...

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE SRN

PLAINS...LOWER MS VALLEY...TN VALLEY...ERN GULF COAST STATES AND SRN

APPALACHIAN MTNS...

...SIGNIFICANT SEVERE WEATHER EVENT SETTING UP FOR CHRISTMAS DAY FOR

PARTS OF THE GULF COAST STATES...

...LOWER MS VALLEY/GULF COAST STATES...

AN IMPRESSIVE UPPER-LEVEL SYSTEM IS FORECAST TO MOVE ACROSS THE SRN

PLAINS INTO THE LOWER MS VALLEY ON TUESDAY. A POWERFUL MID-LEVEL JET

IN THE BASE OF THE UPPER-LEVEL SYSTEM WILL TRANSLATE QUICKLY EWD

OVER A MOISTENING WARM SECTOR RESULTING IN THE POTENTIAL FOR A

SIGNIFICANT SEVERE WEATHER EVENT ACROSS PARTS OF THE LOWER MS VALLEY

AND CNTRL GULF COAST STATES. AS THE UPPER-LEVEL TROUGH APPROACHES

THE REGION...MODEL FORECASTS MOVE A BAND OF STRONG LARGE-SCALE

ASCENT QUICKLY EWD ACROSS CNTRL TX EARLY TUESDAY MORNING WHICH

COINCIDES WITH A MOISTENING BOUNDARY LAYER ACROSS MUCH OF THE TX

COASTAL PLAINS. IN RESPONSE...THUNDERSTORM DEVELOPMENT SHOULD BE

UNDERWAY AT DAYBREAK ON TUESDAY WITH A CLUSTER OF STRONG TO

POTENTIALLY SEVERE STORMS MOVING ENEWD FROM SE TX INTO THE LOWER MS

VALLEY AROUND MIDDAY.

FORECAST SOUNDINGS AT VICKSBURG AND JACKSON MS ON TUESDAY GRADUALLY

DEVELOP MODERATE INSTABILITY WITH SFC DEWPOINTS REACHING THE LOWER

TO MID 60S F. THIS COMBINED WITH STRENGTHENING DEEP LAYER SHEAR AS

0-6 KM SHEAR APPROACHES 60 KT BY EARLY AFTERNOON SHOULD BE FAVORABLE

FOR SUPERCELLS AND TORNADOES. 0-3 KM STORM RELATIVE HELICITIES OF

350 TO 400 M2/S2 SHOULD BE SUPPORTIVE OF STRONG TORNADOES WITH THE

GREATEST THREAT ALONG A CORRIDOR FROM NEAR LAKE CHARLES LA TUESDAY

MORNING ACROSS SCNTRL MS INTO WCNTRL AL BY TUESDAY EVENING.

INSTABILITY...MOISTURE AND SHEAR PROFILES SUGGEST THAT ANY MCS THAT

BECOMES WELL-ORGANIZED COULD CONTAIN A TORNADO THREAT FOR AN

EXTENDED PERIOD. FOR THIS REASON...LONG TRACK TORNADOES COULD OCCUR

WITH SUPERCELLS THAT REMAIN DISCRETE AND MOVE ALONG THE NRN EDGE OF

MODERATE INSTABILITY. IN ADDITION...WIND DAMAGE WILL BE LIKELY WITH

LINE-SEGMENTS THAT ORGANIZE ON THE LEADING EDGE OF THE MCS. A THREAT

FOR WIDESPREAD WIND DAMAGE MAY DEVELOP ACROSS THE REGION TUESDAY

WITH WIND GUSTS OVER 70 KT POSSIBLE WITH BOWING SEGMENTS EMBEDDED IN

THE LINE. IF A WELL-DEVELOPED AND FAST MOVING SQUALL-LINE

ORGANIZES...THEN THE WIND DAMAGE THREAT COULD BECOME SUBSTANTIAL.

THE THREAT FOR SEVERE STORMS SHOULD CONTINUE THROUGH MUCH OF THE

OVERNIGHT PERIOD REACHING THE SRN APPALACHIAN MTNS AND ERN GULF

COAST STATES LATE TUESDAY NIGHT.

..BROYLES.. 12/24/2012

[Ground Scouring]

00Z soundings from TUS and other SW U.S. locations show that the dry punch is much more pronounced than what many of the short-term models like the NAM have been forecasting. Given the slower and deeper trend with the surface cylogenesis, the stronger dry punch over the warm sector would tend to offset or even reduce the high EL values that have been a potential kink in the outbreak potential. We would see even more potential for a broader-based tornado threat as the stronger, WSW-to-SW 850-mb vector component relative to the low-level moisture source will tend to advect higher Td's along a corridor from just N of KAEX->N of JAN->N of BMH. This would certainly offset concerns not only about junk convection, but also mixing as well. This event carries a narrow but high-risk (for significant tornadoes) zone in this area that appears to be growing in size by the hour as real-time trends continue. I am becoming increasingly concerned that we may be trending toward the upper range of the MDT hatched TOR probabilities--in regard to strong tornadoes--from E TX to the southern halves of MS / AL. I am also concerned about the risk for damaging downbursts in association with the MCS Wednesday from AL to GA.

[Ground Scouring]

I would have nudged the MDT a bit farther NW to just S of SHV, but otherwise I agree with the SPC probabilities. I am increasingly confident that a potential High Risk is not out of the question based not just on TOR but also on wind probabilities over a wide area. The prospects for a 24-hour-long MCS are also increasing.

[CUmet]

00Z soundings from TUS and other SW U.S. locations show that the dry punch is much more pronounced than what many of the short-term models like the NAM have been forecasting. Given the slower and deeper trend with the surface cylogenesis, the stronger dry punch over the warm sector would tend to offset or even reduce the high EL values that have been a potential kink in the outbreak potential. We would see even more potential for a broader-based tornado threat as the stronger, WSW-to-SW 850-mb vector component relative to the low-level moisture source will tend to advect higher Td's along a corridor from just N of KAEX->N of JAN->N of BMH. This would certainly offset concerns not only about junk convection, but also mixing as well. This event carries a narrow but high-risk (for significant tornadoes) zone in this area that appears to be growing in size by the hour as real-time trends continue. I am becoming increasingly concerned that we may be trending toward the upper range of the MDT hatched TOR probabilities--in regard to strong tornadoes--from E TX to the southern halves of MS / AL. I am also concerned about the risk for damaging downbursts in association with the MCS Wednesday from AL to GA.

I'm not so sure about the E TX/W LA part of that, because I think the low level jet will hold off on strengthening until the storms get east of there. Otherwise, I do agree that this setup looks quite ominous, and I wouldn't be surprised to see the MDT risk expanded slightly to the north and even towards the northeast as the surface low track becomes clearer.

It's good to see you posting around here again.

[Jim Marusak]

i looked at the bufkits near waco for later on this evening.... elevated convection, but could get nasty...

with the NAM bufkits, tt's 55+ starting 05Z, which is right near when midnight masses would be gathering and/or going on, and until 15Z, showalters peaking at -6 and TT's peaking near 61

as for the 4km bufkit soundings that penn state is now offering, even worse.. TT 55+ range same time-wise, but peaking at 66, with Showalter's peaking at -9 at 10Z

GFS looking at the same TT risk time period. SI's only near -3 or so.

both iterations are saying the layer of initiation will be ~ 900hPa

depending on when stuff blows up later tonight just west of i-35, this could also be a major safety concern between DFW/AUS to SHV/LCH, given how packed midnight masses usually are.

[tmagan]

Jackson, MS NAM BUFKIT forecast sounding for 02Z 26 December.

[QVectorman]

Alexandria, LA

Dang!!! And I don't say that much...but a LI of -5.2 with a CAPE of 1635 j/kg in Dec combined with STP of 13.1 Whooowee! 1km EHI 4 Impressive.

With those kind of numbers in Dec. if just someone farts a storm will pop and spin something up.

[andyhb]

D3, 30% sig-hatched for the Carolinas...

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0231 AM CST MON DEC 24 2012

VALID 261200Z - 271200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE ERN GULF

COAST STATES...FL PENINSULA...CAROLINAS AND MID-ATLANTIC...

...CAROLINAS/ERN GULF COAST STATES/MID-ATLANTIC...

AN IMPRESSIVE NEGATIVELY-TILTED UPPER-LEVEL SYSTEM IS FORECAST TO

MOVE QUICKLY NEWD ACROSS THE ATLANTIC COASTAL REGION ON WEDNESDAY.

AN UPPER-LEVEL LOW OVER THE MID MS VALLEY AT DAYBREAK ON WEDNESDAY

SHOULD MOVE NEWD ACROSS THE OH VALLEY AS A SLAB OF VERY STRONG

LARGE-SCALE ASCENT MOVES FROM THE ERN GULF COAST STATES EWD ACROSS

THE CAROLINAS WEDNESDAY AFTERNOON. DUE TO THE STRONG FORCING...A

WELL-DEVELOPED SQUALL-LINE SHOULD BE ONGOING AT 12Z ON WEDNESDAY

WITH THIS LINEAR CONVECTIVE SYSTEM REMAINING INTACT ALL THE WAY TO

THE COAST.

FORECAST SOUNDINGS AT 21Z/WED AHEAD OF THE SQUALL-LINE IN ERN NC AND

NCNTRL SC SHOW INCREASING INSTABILITY DURING THE LATE MORNING AND

EARLY AFTERNOON WITH SFC DEWPOINTS REACHING THE LOWER 60S F. THIS

MAY ALLOW MODERATE INSTABILITY TO DEVELOP ACROSS PARTS OF THE ERN

CAROLINAS WEDNESDAY AFTERNOON. IN ADDITION...A WELL-DEVELOPED

LOW-LEVEL JET COMBINED WITH A POWERFUL MID-LEVEL JET WRAPPING AROUND

THE UPPER-LEVEL LOW WILL CREATE STRONG DEEP LAYER SHEAR PROFILES

FAVORABLE FOR SEVERE STORMS. ACROSS MUCH OF THE REGION...WINDS IN

THE BOUNDARY LAYER ARE FORECAST TO EXCEED 50 KT JUST ABOVE THE SFC

WHICH SHOULD ENABLE ANY LINE OF STORMS TO BE EFFICIENT AT PRODUCING

WIND DAMAGE. IF DISCRETE CELLS CAN INITIATE AHEAD OF THE APPROACHING

SQUALL-LINE...THEN A SUBSTANTIAL TORNADO THREAT COULD ALSO DEVELOP

WEDNESDAY AFTERNOON WITH THE POTENTIAL FOR A STRONG TORNADO.

CONSIDERING THE STRENGTH OF THE UPPER-LEVEL SYSTEM WHICH COMBINED

WITH MODERATE INSTABILITY AND STRONG DEEP LAYER SHEAR...ANY SEVERE

THREAT THAT DEVELOPS ACROSS THE CAROLINAS COULD BECOME ENHANCED. FOR

THIS REASON...A 30 PERCENT SEVERE PROBABILITY WITH A SIGNIFICANT

HATCHED AREA APPEARS WARRANTED.

FURTHER TO THE NORTH ACROSS THE MID-ATLANTIC...ANY SEVERE THREAT

WITH THE LINE OF STORMS SHOULD BE MORE ISOLATED DUE TO WEAKENING

INSTABILITY WITH NWD EXTENT. TO THE SOUTH ACROSS ERN GA AND NRN

FL...A SUBSTANTIAL WIND DAMAGE THREAT COULD EXIST BUT THE THREAT

SHOULD DECREASE WITH SWD EXTENT DUE TO WEAKER LARGE-SCALE ASCENT AND

DEEP LAYER SHEAR.

..BROYLES.. 12/24/2012

[andyhb]

To give an idea of the WAA we are dealing with here, there were freeze warnings for parts of the Gulf Coast last night, it is currently 65 degrees in Monroe, LA, at 3 in the morning...

[Srain]

SHORT RANGE FORECAST DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

222 AM EST MON DEC 24 2012

VALID 12Z MON DEC 24 2012 - 12Z WED DEC 26 2012

***LIGHT RAIN AND SNOW TO AFFECT THE NORTHEAST THROUGH TUESDAY***

***BAD WEATHER FOR THE DEEP SOUTH ON CHRISTMAS DAY***

***BITTERLY COLD OVER THE NORTHERN PLAINS AND UPPER MIDWEST***

***MORE RAIN AND MOUNTAIN SNOW FOR THE WEST COAST***

AFTER A QUIET WEATHER DAY ON SUNDAY, CLOUDS WILL BE ON THE

INCREASE ACROSS MUCH OF THE EASTERN U.S. ON MONDAY AS ANOTHER

STORM SYSTEM APPROACHES. THIS FEATURE WILL BE CONSIDERABLY WEAKER

THAN THE ONE LATE LAST WEEK. HOWEVER, IT IS EXPECTED TO BRING A

ROUND OF MAINLY LIGHT PRECIPITATION FROM THE DEEP SOUTH TO NEW

ENGLAND ON MONDAY EVENING AND EXTENDING INTO EARLY CHRISTMAS

MORNING. MIXED WINTRY PRECIPITATION IS A GOOD BET IN THE CENTRAL

APPALACHIANS AND INTO INTERIOR PARTS OF THE NORTHEAST, AND

TRAVELERS SHOULD ALLOW EXTRA TIME TO REACH THEIR DESTINATIONS.

OVER THE DEEP SOUTH AND INTO TEXAS, A DEVELOPING LOW PRESSURE

SYSTEM IS EXPECTED TO FORM BY MONDAY EVENING AND WIDESPREAD SHOWER

AND THUNDERSTORM ACTIVITY IS LIKELY AHEAD OF THIS LOW. THERE IS A

GOOD CHANCE FOR SEVERE THUNDERSTORMS TO DEVELOP CHRISTMAS DAY OVER

PARTS OF THE DEEP SOUTH AS WARM AND HUMID AIR IS ADVECTED

NORTHWARD TOWARDS A DEEPENING SURFACE LOW IN A HIGH WIND SHEAR

ENVIRONMENT. ANOTHER FACET OF THIS STORM SYSTEM IS THE SNOW THAT

IS FORECAST TO FALL ON THE NORTHWEST SIDE OF THE LOW TRACK FOR

CHRISTMAS AND EXTENDING INTO WEDNESDAY FROM EASTERN OKLAHOMA TO

SOUTHERN MISSOURI, AND EVENTUALLY INTO INDIANA.

OVER THE WEST COAST STATES, MORE RAIN AND MOUNTAIN SNOW IS

EXPECTED FOR CHRISTMAS DAY AND BEYOND AS ANOTHER PACIFIC FRONT

APPROACHES THE COAST. THE HEAVIEST PRECIPITATION IS LIKELY FOR

WESTERN OREGON AND WASHINGTON STATE, AND LIGHTER AMOUNTS FOR THE

INTERIOR VALLEYS AND INLAND MOUNTAIN RANGES EAST OF THE CASCADES.

ELSEWHERE, BITTERLY COLD AIR WILL CONTINUE FOR THE NORTHERN PLAINS

AND UPPER MIDWEST THROUGH THE FIRST HALF OF THE WEEK.

HAMRICK

[Srain]

DAY 1 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1029 AM CST MON DEC 24 2012

VALID 241630Z - 251200Z

...THERE IS A SLGT RISK OF SVR TSTMS OVERNIGHT FOR PORTIONS

CENTRAL-E TX AND WRN LA...

...SYNOPSIS...

PROGRESSIVE UPPER-AIR PATTERN WAS ANALYZED THIS MORNING OVER

CONUS...FEATURING TWO SHORTWAVE TROUGHS OF PRIMARY PERTINENCE TO

THIS FCST INITIALLY OVER...

1. MID-SOUTH REGION TO MS/AL COAST. THIS FEATURE WILL MOVE EWD AND

DEAMPLIFY SLIGHTLY BEFORE MOVING OFFSHORE SC AROUND 03Z.

2. INLAND PAC NW SSEWD ACROSS NV. PERTURBATION IS FCST TO

STRENGTHEN THROUGH PERIOD AND MOVE SEWD ACROSS CENTRAL/SRN

ROCKIES...REACHING CENTRAL/SRN HIGH PLAINS BY 12Z. EMBEDDED/500-MB

CLOSED LOW SHOULD FORM BY 12Z OVER TX PANHANDLE/SOUTH-PLAINS

REGION...WHILE MID-UPPER HEIGHT GRADIENT INTENSIFIES MARKEDLY TO ITS

SW-SE. BY 12Z...EXPECT 500-MB JET MAX 90-100 KT OVER SERN NM/TX

PERMIAN BASIN REGION...WITH FLOW STRENGTHENING TO 60-70 KT RANGE

OVER SFC WARM SECTOR IN SE TX.

AT SFC...WEAK LOW NOW OVER NRN KY IS FCST TO PROCEED NEWD UP OH

VALLEY THROUGH REMAINDER DAYLIGHT HOURS AND WEAKEN...WITH OCCLUSION

TRIPLE-POINT LOW POSSIBLY FORMING INVOF SERN NY COASTAL WATERS

AROUND 06Z. ACCOMPANYING SFC COLD FRONT WAS ANALYZED AT 15Z FROM

LOW SSWWD ACROSS NRN AL...SWRN MS...SWRN LA...AND S-CENTRAL TX. TX

PORTION OF BOUNDARY IS DECELERATING...AND IS FCST TO MOVE NWD ACROSS

PORTIONS CENTRAL/SE TX OVERNIGHT AS WARM FRONT. MEANWHILE...LEE

CYCLONE IS FCST TO DEEPEN CONSIDERABLY TODAY ACROSS NERN NM AND SERN

CO REGION...WITH APCH OF AMPLIFYING TROUGH ALOFT. DURING LAST 6

HOURS OF PERIOD...SFC CYCLONE WILL TRANSITION TO FRONTAL-WAVE

FEATURE AND MOVE/REPOSITION SEWD ALONG ZONE OF

FRONTOGENESIS...REACHING POSITION OVER CENTRAL TX BY 12Z.

...TX/LA AREA OVERNIGHT...

CONVECTIVE POTENTIAL WILL INCREASE OVERNIGHT...FIRST N OF SFC FRONT

LATE EVENING THROUGH PRE-DAWN HOURS WITH MOST INTENSE CELLS POSING

HAIL THREAT. ACTIVITY MAY DEVELOP PROGRESSIVELY CLOSER TO FRONT

THROUGH END OF PERIOD...EVENTUALLY BECOMING SFC-BASED WITH

SUPERCELLS AND BOW/LEWP FEATURES POSSIBLE. AS SUCH...THREAT EXISTS

DURING LAST FEW HOURS OF PERIOD FOR SVR HAIL/GUSTS. TORNADO RISK

EXISTS FROM ANY SFC-BASED AND RELATIVELY DISCRETE CELLS IN WHAT

SHOULD BE VERY FAVORABLE SHEAR ENVIRONMENT...BUT COVERAGE OF

SFC-BASED TSTMS IS TOO UNCERTAIN ATTM FOR MORE THAN 5% UNCONDITIONAL

TORNADO RISK.

DEEP SHEAR AND LOW-LEVEL HODOGRAPHS EACH ARE EXPECTED TO STRENGTHEN

OVER RETURNING WARM SECTOR TONIGHT...AS 60S F SFC DEW POINTS

CONTRIBUTE TO 1000-1500 J/KG MLCAPE BY 12Z ALONG AND S OF FRONT OVER

SE TX. LARGEST LOW-LEVEL HODOGRAPHS WILL BE ALONG FRONT...WHERE

BACKED SFC WINDS WILL CONTRIBUTE TO EFFECTIVE SRH 300-500 J/KG. SRH

ALSO WILL BE FAVORABLE FARTHER S OVER COASTAL AREAS IN WARM

SECTOR...HOWEVER FCST SOUNDINGS SUGGEST PRONOUNCED EML-BASE

INVERSION BETWEEN 850-700 MB THAT MAY RESTRICT TSTM COVERAGE AWAY

FROM BOUNDARIES.

...CENTRAL-ERN GULF COASTAL PLAIN...NEXT 4-6 HOURS...

BULK OF CONVECTION IS EXPECTED TO OUTRUN LOW-LEVEL DESTABILIZATION

OVER THIS REGION THROUGH REMAINDER DAYLIGHT HOURS. HOWEVER...AT

LEAST ISOLATED TSTMS STILL MAY DEVELOP OVER THIS AREA THROUGH AROUND

21Z...SOME OF WHICH MAY BECOME SFC-BASED. DAMAGING WIND AND TORNADO

RISKS ARE NONZERO BUT VERY CONDITIONAL. DESTABILIZATION WILL BE

SLOWED BY ONGOING CLOUDS/PRECIP...WHICH WILL RESTRICT PACE OF

DIURNAL WARMING. LOW-LEVEL THETAE ADVECTION SHOULD RESULT IN

POTENTIAL INFLOW PARCELS EFFECTIVELY BECOMING SFC-BASED ACROSS THIS

REGION THROUGH MID-AFTN...WHILE DEEP-LAYER ASCENT WEAKENS. THESE

OFFSETTING INFLUENCES WILL AFFECT TSTM COVERAGE...BUT ANY

SUSTAINED/SFC-BASED CONVECTION HAS POTENTIAL TO BECOME

SUPERCELLULAR. AS SUCH...WILL MAINTAIN MRGL UNCONDITIONAL SVR

PROBABILITIES FOR THIS OUTLOOK.

..EDWARDS/COHEN.. 12/24/2012

[Ground Scouring]

Models, including the NAM and GFS, have shifted the track of the surface low farther south across E TX and LA overnight Tuesday-Wednesday, with the warm front making little northward progress through 36 hours beyond a line along the JAN-KMGM corridor. This would restrict the initial severe threat to the buoyancy-laden Gulf Coast along a line from KGLS->KLCH->KMSY. Given the forecasted ascent, there is a risk of greater-than-expected convective feedback associated with the initial wave passage between 02-06Z Tuesday, though the strong dry punch would keep precip. coverage to a minimum. The proximity to the Gulf Coast and historic modular tendency to underestimate junk convection worry me a bit. However, the cooling associated with the initial convection might well encourage cool outflow boundaries which would serve as a focal point for supercellular development as convection transitions to more surface-based coverage beginning 06Z and later Tuesday. In all, my concern is that we could see unusually intense supercells for a large-scale system so close to the Gulf, in part due to the mid-level cold pool spreading into the warm sector and combining with high 0-3-km EHI, strong sfc.-850-mb bulk shear, and low LCL heights to favor strong updrafts (latest hodographs for GLS 12Z Tuesday morning are as impressive as ever). While I may be getting ready to shift the significant tornado risk farther south through 24 hours, my concern for strong tornadoes is as high as ever, given the environment any supercell that develops--and there will almost certainly be several high-impact ones--will have to work with, even though they will be restricted to SE TX/S LA (KGLS->KLCH->KMSY).

[Srain]

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1122 AM CST MON DEC 24 2012

VALID 251200Z - 261200Z

...THERE IS A MDT RISK OF SVR TSTMS OVER PARTS OF LA...CENTRAL/SRN

MS...AND WRN AL...

...THERE IS A SLGT RISK OF SVR TSTMS FROM ERN TX ACROSS THE GULF

COAST/LOWER MS VALLEY AND SRN APPALACHIANS...

...SIGNIFICANT SEVERE WEATHER EVENT EXPECTED FOR CHRISTMAS DAY OVER

PARTS OF THE GULF COAST STATES...

...SYNOPSIS...

AN ENERGETIC FLOW PATTERN IS EVIDENT IN SATELLITE IMAGERY FROM THE

EASTERN PACIFIC ACROSS THE CONUS AS A SERIES OF VIGOROUS UPPER

TROUGHS PROGRESS EWD. THE PRIMARY SYSTEM ASSOCIATED WITH A MAJOR

SEVERE WEATHER THREAT TOMORROW IS CURRENTLY OVER THE GREAT BASIN.

WITH LATEST UPPER AIR DATA INDICATING THE STRONGEST MID/UPPER LEVEL

WINDS DIGGING INTO THE SWRN PORTION OF THE TROUGH...MODEL CONSENSUS

INDICATES THE SYSTEM WILL AMPLIFY/DEVELOP INTO A PROGRESSIVE CLOSED

LOW MOVING ACROSS THE SRN PLAINS DURING THE DAY REACHING THE MIDDLE

MS VALLEY BY 26/12Z. AT THE SURFACE...A BROAD LOW PRESSURE REGION

OVER CENTRAL/ERN TX EARLY IN THE PERIOD IS FORECAST TO CONSOLIDATE

INTO A DEEPENING LOW MOVING ACROSS NRN LA DURING THE AFTERNOON AND

CONTINUING INTO MIDDLE TN BY THE END OF THE PERIOD. AN EAST-WEST

ORIENTED WARM FRONT FROM SERN TX ACROSS THE SRN PARTS OF MS/AL AND

GA WILL SLOWLY LIFT NWD AS THE LOW MOVES NEWD...WHILE A COLD FRONT

EXTENDING SWWD FROM THE LOW IS EXPECTED TO ACCELERATE EWD

/ESPECIALLY DURING THE NIGHT/ ACROSS THE LOWER MS VALLEY REACHING AL

BY 26/12Z.

...GULF COAST REGION/LOWER MS VALLEY...

AREAS OF STRONG/SEVERE THUNDERSTORMS ARE EXPECTED TO BE ONGOING FROM

ERN TX INTO THE LOWER MS VALLEY AT THE START OF THE PERIOD AS

INTENSE LARGE SCALE ASCENT IN THE LEFT EXIT REGION OF THE UPPER

LEVEL JET COINCIDES WITH FOCUSED LOW LEVEL CONVERGENCE/WARM

ADVECTION ALONG/NORTH OF THE WARM FRONT. SOUTH OF THE WARM

FRONT...SURFACE DEW POINTS IN THE LOW/MID 60S F WILL BE RETURNING

NWD ACROSS A BROAD WARM SECTOR EXTENDING ACROSS MUCH OF THE GULF

COAST REGION. THIS WILL CONTRIBUTE TO AN UNSTABLE ENVIRONMENT WITH

MLCAPE RANGING FROM 1000-1250 J/KG NEAR THE COAST TO AOB 500 J/KG

NORTH OF THE WARM FRONT.

STRONG WINDS IN THE MID AND UPPER LEVELS /70-90 KT AT 500 MB AND

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

ROTATING UPDRAFTS...INCLUDING ELEVATED SUPERCELLS NORTH OF THE WARM

FRONT. AS THE DAY PROGRESSES...FORCED ASCENT ALONG THE ADVANCING

COLD FRONT IS EXPECTED TO RESULT IN QLCS DEVELOPMENT CONTAINING

EMBEDDED FAST-MOVING BOW ECHOES AND POSSIBLE

SUPERCELLS/MESO-VORTICES AS IT MOVES EWD ACROSS LA AND THE SRN HALF

OF SRN MS. THERE WILL ALSO BE CONTINUED POTENTIAL FOR A FEW

DISCRETE SUPERCELLS IN THE WARM SECTOR AND ALONG THE WARM FRONT

AHEAD OF THE QLCS. LOW LCL HEIGHTS AND CLOCKWISE TURNING LOW LEVEL

HODOGRAPHS /SRH VALUES OF 200-400 M2 PER S2/ INDICATE POTENTIAL FOR

TORNADIC SUPERCELLS. THE POSSIBILITY EXISTS FOR SEVERAL LONG-TRACK

STRONG TORNADOES ASSOCIATED WITH ANY LONG-LIVED FAST-MOVING

SUPERCELLS...ALONG WITH MORE WIDESPREAD DAMAGING WINDS ALONG THE

MOST INTENSE PARTS OF THE BOWING QLCS. DESPITE GRADUALLY DECREASING

INSTABILITY AS BOUNDARY LAYER COOLING OCCURS AFTER DARK...A SEVERE

THREAT IS EXPECTED TO SPREAD EWD/NEWD ACROSS PARTS OF AL...SRN TN

AND WRN GA DURING THE NIGHT AS THE SYSTEM CONTINUES TO STRENGTHEN.

..WEISS.. 12/24/2012

[Ground Scouring]

NAM projected 18Z sounding for areas in and near JAN shows insane turning in the 900-850-mb column with strong SRH just above that.

[andyhb]

NAM projected 18Z sounding for areas in and near JAN shows insane turning in the 900-850-mb column with strong SRH just above that.

Yeah there are some pretty incredible soundings coming out of that area tomorrow afternoon, JAN is going to have a very busy day in all likelihood.

That overnight destabilization into AL is concerning as well...

[Ellinwood]

The cold-air damming signal continues to improve with the 12z model runs. The CAD is now supportive of 1-3+ inches of snow north and west of DC up into the higher terrain at the start of the event:

(Note: precip is only shaded at 0.05" or greater, with no 0.01" contour)

[Hoosier]

Blizzard Watch has been issued by the Paducah, KY NWS.

[Ground Scouring]

Is the 18Z NAM overdoing boundary-layer mixing between 18Z Tuesday and 12Z Wednesday in MS/AL? Given the low three-hour QPF and the absence of significant saturation, the SBCAPE appears to be too low given the projected instability based upon composite indices. Only a narrow, spotty sliver of 1000 j/kg along the QLCS advancing through MS/AL? That seems too low.

[andyhb]

Is the 18Z NAM overdoing boundary-layer mixing between 18Z Tuesday and 12Z Wednesday in MS/AL? The SBCAPE appears to be too low given the projected instability based upon composite indices.

I'm gonna say yes, although there is actually more instability than the 12z run across the southern part where the MDT risk is. Especially considering the WF has well cleared the area, the chances for significant overnight destabilization are increasing by the hour across Eastern MS and all of AL. This would not be good, as shear gets even more off the hook after 00z as the LLJ intensifies to nearly 70 kts out of the S/SSW.

[brettjrob]

I'm fairly skeptical of the sigtor potential outside of immediate coastal areas of MS/AL and S LA, possibly into extreme SE TX. Looking farther N within the MDT area toward JAN-BHM, forecast soundings are moist adiabatic through the troposphere with pitifully small LI values. I know it doesn't take much CAPE to get things rolling in this part of the country, but the buoyancy values through the column are simply miniscule. If these same forecast 500-1000 J/kg SBCAPE values were distributed through a shorter column (fatter CAPE), I'd be a lot more interested.

With off-the-charts SRH and incredible hodographs, the risk for embedded spinups in this area will still be noteworthy, but I'm skeptical of long-trackers along the I-20 corridor as things stand. The 12z WRF-NMM shows no discrete activity ahead of the line, FWIW.

[SmokeEater]

Any chance we can seperate the severe and winter sides to this storm? Being both sides could be significant?

[Ground Scouring]

This event between 06-18Z Tuesday closely resembles the 500-mb shortwave progression of Nov. 7-8, 1957, another significant tornado outbreak in SE TX and S LA. The difference is that the connection with the STJ is not as strong in the current situation and that the secondary low formation will be much farther south, along the TX Gulf coast--as opposed to over AR in 1957. In the 1957 case, however, a strong low-level jet crossed SE TX/S LA as the mid-/upper-level system became negatively tilted, much like the present case, with very high 0-3-km helicity and major 0-3-km SRH juxtaposed over an unstable warm sector. The difference is that in 1957, the low was not as near the Gulf Coast and there was no pronounced dry punch as in the current case, so the source of the tornadoes was a bit different in 1957. The current case will feature a strong dry punch to allow discrete formation, whereas in 1957 the STJ combined with a cold pool helped break an inversion which we will not see in this case.

[andyhb]

For those unfamiliar...

http://www.srh.noaa.gov/lch/?n=e110757

[andyhb]

BMX with a very strongly worded discussion:

SUPERCELLS ARE POSSIBLE SOUTH OF THE WARM FRONT DURING THE AFTERNOON

ACROSS WEST CENTRAL AND SOUTHWEST ALABAMA WITH STRONG DEEP LAYER

SHEAR AND 500-1000 J/KG CAPE EXPECTED. THE IDEA OF DISCRETE

SUPERCELLS IS SUPPORTED BY A DIFFLUENT UPPER JET OVERSPREADING THE

REGION DURING THE LATE AFTERNOON AND EVENING HOURS WELL AHEAD OF THE

COLD FRONT. A RAPIDLY STRENGTHENING LOW LEVEL JET WITH 0-1 KM

SHEAR OF AT LEAST 45 KT AND 0-1 KM HELICITY VALUES GREATER THAN 400

M2/S2 WILL SUPPORT STRONG TORNADOES AND DAMAGING STRAIGHT LINE WINDS

WITH PERHAPS THE GREATEST THREAT FOR STRONG TORNADOES BETWEEN 00Z

AND 06Z FOR THE SOUTHWESTERN QUADRANT OF THE FORECAST AREA.

THE THREAT WILL OVERSPREAD ALL OF CENTRAL ALABAMA DURING THE EVENING

AND OVERNIGHT AS THE WARM FRONT RETREATS AND A SURFACE LOW DEEPENS

OVER EXTREME NORTHWEST ALABAMA TO AROUND 994 MB. A SQUALL LINE OR A

BROKEN LINE OF STORMS SHOULD DEVELOP ALONG THE COLD FRONT...BUT

SUPERCELLS ARE ALSO POSSIBLE OVERNIGHT IN ADVANCE OF THE MAIN LINE.

THE LOW LEVEL JET WILL CONTINUE TO STRENGTHEN DURING THE OVERNIGHT

CREATING AN ENVIRONMENT CAPABLE OF PRODUCING NUMEROUS FAST MOVING

TORNADOES AND AN ENHANCED DAMAGING STRAIGHT LINE WIND POTENTIAL IF

NEAR SURFACE BASED CAPE VALUES CAN REMAIN IN THE 400-700 J/KG RANGE

AS EXPECTED. THE THREAT SHOULD END AS THE COLD FRONT PUSHES INTO

WESTERN GEORGIA AROUND 09Z.

[Srain]

The warm front has begun retreating N across the Texas Coast and the approaching short wave is nearing ABQ. The latest ensembles update suggest a bit more southern track of the low pressure system and cyclogenesis should begin in earnest overnight into tomorrow morning across the Permian Basin (N Central TX) and wrap up into one heck of a Christmas Storm. Concern grows in the warm sector as a surface low develops along the warm front near or just NW of Austin/San Antonio and heads ENE. Further E in Louisiana/Mississippi/Alabama, the severe parameters are very concerning and certainly lend credibility to the Slight Risk across E TX/W LA tonight into tomorrow and the Moderate Risk further E in the Northern Gulf Coastal States into the Carolinas. In the cold sector, 6-10 inches of snow with possible higher amounts are likely from Oklahoma City to Tulsa, Fort Smith/Fayetteville Arkansas area with lesser amounts along the Red River Valley down to Dallas/Ft Worth/Shreveport/Little Rock. Blizzard conditions are likely with up to a foot of snow across portions of Western Kentucky/Southern Illinois/SW Indiana.

[Ed Lizard]

First time I have tried using RAP/HRRR page in weeks, and I'm getting error messages. 502 proxy errors or

(Data not available for hrrr_jet, full, -999)

Perhaps it has a critical need sensor. First chance at interesting weather IMBY in weeks, and no short range high res guidance.