Feb 18/19 Severe Thread

[andyhb]

Looking more and more like we could see a severe threat out of this system into this weekend, as it appears surface cyclogenesis (and potentially substantial cyclogenesis at that) will occur with a strong incoming mid/upper trough.

Considering the GFS is likely underdoing instability values and that moisture will not have to travel north far at all, this looks interesting for Southern AL/Southern GA/Northern FL and potentially the Carolinas.

Sfc Dews at 00z Sunday

H5 at 72 hrs (18z Saturday)

H5 at 78 hrs (00z Sunday)

Each time frame also has a 40-70 kt H85 jet feeding in off the Gulf.

NAM is slower and has more of a threat for LA and MS on Saturday and the aforementioned areas Sunday.

[metalicwx366]

Will this be a daytime threat or nightime threat for us?

[Buckeye05]

Looks interesting. Instability seems to be lacking on the 18th though, unless like you said, they are underplaying it.

[andyhb]

00z GFS/NAM have similar solutions to their 18z counterparts, we need to watch for trends north with the surface cyclogenesis, as this would give more time potentially for moisture to be advected out of the Gulf across the warm sector (and also for a more favorable LLJ configuration). Considering we're already seeing 65 dews across much of the area I mentioned in the first part of the post (S AL/GA/etc.), it looks fairly favorable even now.

Looks interesting. Instability seems to be lacking on the 18th though, unless like you said, they are underplaying it.

I'd be weary of trusting models' verbatims on instability three/four days out, as we've seen in the past that it can be significantly underdone even until the last minute, as can helicity (which doesn't seem to be lacking here, considering the types of wind profiles I'm seeing).

[tornadotony]

The GFS, NAM, and Euro are in agreement that this is going to likely be an extremely potent system for this part of the country. The questions lie in the track. I would not worry about instability. With the strong forcing that's going to be in place down there, along with very high levels of moisture, you will have enough of it. Add in the amount of bulk shear down there and you will get rotating updrafts. LCLs will most certainly not be a problem. We're 72 hours out and the somewhat atypical nature of this setup (though it really seems to be a hybrid of several severe patterns) lends me to wanting to be cautious, but if I were in the deep deep south, say srn AL, srn GA, SC, and nrn FL (including the nrn peninsula), I would be a bit concerned.

[andyhb]

Rather impressive day 3 from the SPC:

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0220 AM CST THU FEB 16 2012

VALID 181200Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS SAT AND SAT NIGHT ACROSS MUCH

OF THE CNTRL AND ERN GULF STATES THRU THE S ATLANTIC COAST STATES...

...SYNOPSIS...

MODELS CONTINUE TO DISPLAY CONSIDERABLE VARIABILITY CONCERNING SHORT

WAVE DEVELOPMENTS WITHIN AT LEAST A COUPLE OF DISTINCT BRANCHES OF

STRONGER FLOW THAT PERSIST ACROSS NORTH AMERICA. HOWEVER...A

SIGNIFICANT SHORT WAVE IMPULSE LIFTING OUT OF THE NORTHERN MEXICAN

PLATEAU ON FRIDAY APPEARS LIKELY TO CONTINUE ACCELERATING EAST

NORTHEASTWARD ACROSS THE GULF COAST STATES REGION ON SATURDAY. THIS

IS EXPECTED TO SIGNIFICANTLY SUPPRESS SUBTROPICAL RIDGING CURRENTLY

PRESENT...AND CONTRIBUTE TO SURFACE CYCLOGENESIS ALONG A STALLED

SURFACE FRONT ACROSS THE NORTHWESTERN GULF OF MEXICO BY 12Z

SATURDAY.

ALTHOUGH AT LEAST SUBTLE DIFFERENCES EXIST AMONG THE MODELS

CONCERNING THE SPECIFICS OF THIS EVOLUTION...PARTICULARLY TO THE LEE

OF THE SOUTHERN APPALACHIANS...MOST GUIDANCE GENERALLY INDICATES

THAT A DEEPENING SURFACE LOW WILL MIGRATE INLAND ACROSS THE CENTRAL

GULF COAST INTO NORTHERN/CENTRAL GEORGIA BY LATE SATURDAY EVENING.

THIS IS EXPECTED TO BE ACCOMPANIED BY A STRENGTHENING SOUTHERLY 850

MB JET IN EXCESS OF 50 KT...ACROSS THE WARM SECTOR OF THE

CYCLONE...AS A STRONGLY DIFLUENT WEST/SOUTHWESTERLY 500 MB JET

STREAK...SHIFTING WITH THE UPPER TROUGH...STRENGTHENS IN EXCESS OF

90 KTS.

...CNTRL/ERN GULF STATES INTO THE S ATLANTIC COAST STATES...

DESPITE LINGERING UNCERTAINTY CONCERNING BOUNDARY LAYER

DESTABILIZATION EVIDENT IN THE MODEL DATA...GIVEN THE STRENGTH OF

THE DEVELOPING SYSTEM...IT APPEARS THAT A RELATIVELY MOIST BOUNDARY

LAYER PRESENT OVER THE GULF OF MEXICO SHOULD ADVECT INLAND WITHIN

THE WARM SECTOR OF THE LOW. EVEN IF MID-LEVEL LAPSE RATES DO NOT

STEEPEN SUBSTANTIALLY...AS COLDER MID-LEVEL AIR PROBABLY WILL REMAIN

DISPLACED TO THE WEST...SUFFICIENT CAPE IS EXPECTED TO SUPPORT

VIGOROUS CONVECTIVE DEVELOPMENT. IN THE PRESENCE OF WIND PROFILES

EXHIBITING LARGE CLOCKWISE CURVED LOW-LEVEL HODOGRAPHS AND STRONG

DEEP LAYER SHEAR...THIS SHOULD PROVE FAVORABLE FOR AN ORGANIZED

STORM CLUSTER...OR CLUSTERS...AS WELL AS DISCRETE SUPERCELLS.

IT CURRENTLY SEEMS MOST PROBABLE THAT ONE LARGE MESOSCALE CONVECTIVE

SYSTEM COULD EVOLVE NEAR OR JUST EAST OF THE LOWER MISSISSIPPI

VALLEY BY MID DAY SATURDAY...BEFORE ADVANCING EASTWARD ACROSS THE

EASTERN GULF STATES THROUGH THE REMAINDER OF THE AFTERNOON AND

EVENING. THIS PROBABLY WILL BE ACCOMPANIED BY THE RISK FOR DAMAGING

WINDS AND TORNADOES...WITH PERHAPS A MORE SIGNIFICANT TORNADIC

THREAT IN DISCRETE SUPERCELL DEVELOPMENT THAT APPEARS POSSIBLE JUST

AHEAD OF THE LINE.

SEVERE STORMS WILL REMAIN POSSIBLE SATURDAY NIGHT EASTWARD INTO THE

CENTRAL/EASTERN CAROLINAS...BUT THE DETAILS ARE LESS CLEAR AS THE

SURFACE CYCLONE EITHER MIGRATES NORTHWARD THROUGH THE PIEDMONT...OR

REDEVELOPS EASTWARD ACROSS THE COASTAL PLAIN.

..KERR.. 02/16/2012

[Buckeye05]

00z GFS/NAM have similar solutions to their 18z counterparts, we need to watch for trends north with the surface cyclogenesis, as this would give more time potentially for moisture to be advected out of the Gulf across the warm sector (and also for a more favorable LLJ configuration). Considering we're already seeing 65 dews across much of the area I mentioned in the first part of the post (S AL/GA/etc.), it looks fairly favorable even now.

I'd be weary of trusting models' verbatims on instability three/four days out, as we've seen in the past that it can be significantly underdone even until the last minute, as can helicity (which doesn't seem to be lacking here, considering the types of wind profiles I'm seeing).

True, still learning how to read models. The helicity that day already, does look pretty striking. Latest discussion calling for a QLCS, with possible supercells ahead of it.

[Brick Tamland]

We've had more tornadoes than snow in the southeast this winter. Crazy weather.

[Buckeye05]

Setup not looking as impressive. Cloud cover looks like it will be a big limiting factor.

[andyhb]

500 mb trough is not as deep/energetic in the recent GFS/NAM runs, which could be an indication of this threat lessening, but I'd still remain cautious, and as I've said, watch for trends north with the surface cyclone.

Awaiting the 00z suite for fresh upper air data...

[Buckeye05]

When exactly is the next run?

[it*has*an*i]

When exactly is the next run?

NAM is running now and still has the decent squall line in Ms by 00z/6PM Saturday Night. It weakens it as it moves across Alabama. I don't think this has been the model of choice with this storm from what the NWS has been saying.

http://www.instantweathermaps.com/

[Ian]

[andyhb]

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1217 AM CST FRI FEB 17 2012

VALID 181200Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM EXTREME SERN TX THROUGH

SRN PORTIONS OF THE GULF COASTAL STATES...

...SYNOPSIS...

A PROGRESSIVE SPLIT FLOW REGIME WILL CHARACTERIZE THE SYNOPTIC

PATTERN SATURDAY. PRIMARY FEATURE OF INTEREST IS CUTOFF UPPER LOW

LOCATED OVER THE NRN GULF OF CALIFORNIA AS OF LATE THURSDAY EVENING.

THIS FEATURE WILL ACCELERATE EWD IN RESPONSE TO AN UPSTREAM WAVE

APPROACHING THE PACIFIC NW. MODEL CONSENSUS IS THAT THIS TROUGH AXIS

WILL BE LOCATED FROM CNTRL TX INTO NERN MEXICO EARLY SATURDAY AND

WILL SUBSEQUENTLY ADVANCE INTO THE CNTRL GULF COAST STATES SATURDAY

NIGHT. CYCLOGENESIS WILL COMMENCE OVER SERN TX SATURDAY MORNING IN

VICINITY OF BAROCLINIC ZONE AND WITHIN DIVERGENT EXIT REGION OF

UPPER JET ROUNDING BASE OF THE TROUGH. THIS LOW WILL LIKELY DEEPEN

AS IT DEVELOPS NEWD INTO THE SERN STATES...REACHING AL OR GA LATE IN

THE PERIOD.

...GULF COASTAL STATES...

OBSERVATIONAL DATA INDICATE A RESERVOIR OF UPPER 60S DEWPOINTS OVER

THE CNTRL GULF SOUTH OF A QUASISTATIONARY FRONT. RETREATING HIGH

PRESSURE AND STRENGTHENING LLJ ASSOCIATED WITH DEVELOPING CYCLONE

SHOULD CONTRIBUTE TO INLAND ADVANCE OF THE FRONT INTO SRN PORTIONS

OF THE GULF COAST STATES. HOWEVER...ISENTROPIC ASCENT NORTH OF THE

WARM FRONT WILL RESULT IN AREAS OF WIDESPREAD CLOUDS AND RAIN EARLY

IN THE PERIOD FROM ERN TX INTO THE LOWER MS VALLEY WHICH COULD

POTENTIALLY LIMIT BOUNDARY LAYER DESTABILIZATION AND SLOW THE NWD

PROGRESS OF THE FRONT. NEVERTHELESS...AT LEAST WEAK INSTABILITY WITH

300-500 J/KG SBCAPE SHOULD DEVELOP INLAND DUE PRIMARILY TO NWD

ADVECTION OF HIGHER DEWPOINTS.

STORMS MAY INCREASE IN INTENSITY ALONG WARM CONVEYOR BELT AND

PRE-FRONTAL CONFLUENCE BANDS INITIALLY ACROSS SERN TX COASTAL AREA

WHERE WIND FIELDS AND VERTICAL SHEAR WILL STRENGTHEN WITH THE

APPROACH OF THE UPPER TROUGH. ACTIVITY WILL SUBSEQUENTLY DEVELOP

THROUGH THE GULF COASTAL STATES WARM SECTOR DURING THE DAY AND

OVERNIGHT WHERE MOISTENING BOUNDARY LAYER WILL CONTRIBUTE TO

POTENTIAL FOR NEAR SURFACE BASED CONVECTION. STRONG EFFECTIVE DEEP

SHEAR AND LARGE HODOGRAPHS ACCOMPANYING THE STRENGTHENING LLJ WILL

PROMOTE A RISK OF LEWP/BOWS AND EMBEDDED SUPERCELLS. DAMAGING WIND

AND ISOLATED TORNADOES WILL BE THE MAIN THREATS WITHIN THE EXPECTED

HIGH SHEAR/LOW CAPE ENVIRONMENT.

[Bryan Wood]

Setup not looking as impressive. Cloud cover looks like it will be a big limiting factor.

Early cloud cover will be moot point when the LLJ ramps up Saturday evening. Should bring plenty of moisture and shear.

[andyhb]

DAY 2 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

1125 AM CST FRI FEB 17 2012

VALID 181200Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE TX COAST...LOWER MS

VALLEY AND CNTRL TO ERN GULF COAST STATES...

...LOWER MS VALLEY/CNTRL AND ERN GULF STATES...

AN UPPER-LEVEL TROUGH IS FORECAST TO MOVE ACROSS SRN AND ERN TX ON

SATURDAY AS A SHARPLY DEFINED COLD FRONT ADVANCES INTO THE LOWER MS

VALLEY. NUMEROUS THUNDERSTORMS WILL LIKELY BE ONGOING ALONG AND

AHEAD OF THE COLD FRONT ON THE TX COAST AT THE BEGINNING OF THE

PERIOD. A LARGE MASS OF CONVECTION OR A LINEAR MCS SHOULD BE IN

PROGRESS AT 12Z MOVING EWD INTO THE LOWER MS VALLEY BY EARLY

SATURDAY AFTERNOON AND INTO THE CNTRL GULF COAST STATES SATURDAY

EVENING. A SEVERE THREAT WILL LIKELY EXIST AS THE MCS MOVES ENEWD

ACROSS THE SLIGHT RISK AREA SATURDAY AND SATURDAY NIGHT.

AS THE UPPER-LEVEL SYSTEM MOVES ACROSS SOUTH TX SATURDAY...A

WELL-DEVELOPED 65 TO 80 KT MID-LEVEL JET WILL MOVE OUT OF THE SRN

PLAINS AND OVERSPREAD THE LOWER MS VALLEY. THIS JET PATTERN SHOULD

SUPPORT THE DEVELOPMENT OF A SQUALL-LINE JUST TO THE RIGHT OF THE

MID-LEVEL JET MAX. FORECAST SOUNDINGS AT 21Z SATURDAY IN THE

VICINITY OF THE SQUALL-LINE IN SE LA SHOW AN IMPRESSIVE SHEAR

ENVIRONMENT WITH 0-6 KM SHEAR IN THE 50 TO 65 KT RANGE AND 0-3 KM

STORM RELATIVE HELICITY OF 300 TO 400 M2/S2. THIS SHOULD BE

FAVORABLE FOR ROTATING STORMS EMBEDDED IN THE SQUALL-LINE OR WITH

DISCRETE STORMS THAT DEVELOP OUT AHEAD OF THE LINE WITH A THREAT FOR

TORNADOES. THE GREATEST THREAT FOR TORNADOES SHOULD EXIST TOMORROW

AFTERNOON NEAR NEW ORLEANS AND IN THE MS DELTA WHERE MODEL FORECASTS

SHOW MODERATE INSTABILITY AND STRONG LOW-LEVEL SHEAR JUST TO THE

WEST OF THE LOW-LEVEL JET MAX. THIS SETUP MAY BE ENOUGH FOR AN

ISOLATED STRONG TORNADO OR TWO. THE TORNADO THREAT SHOULD CONTINUE

INTO THE EVENING ACROSS THE CNTRL GULF COAST AS A 50 TO 65 KT

LOW-LEVEL JET MOVES ENEWD ACROSS THE SLIGHT RISK AREA. THE

SQUALL-LINE WILL ALSO LIKELY HAVE A PERSISTENT WIND DAMAGE THREAT

FROM SRN LA LATE SATURDAY MORNING EXTENDING ENEWD TO CNTRL GA DURING

THE OVERNIGHT PERIOD.

THE MODELS ARE IN GOOD AGREEMENT CONCERNING THE SPEED OF THE SYSTEM

SO CONFIDENCE IS FAIRLY HIGH THAT A NARROW CORRIDOR FOR NUMEROUS

SEVERE WEATHER EVENTS WILL EXIST ACROSS THE LOWER MS VALLEY. THE

MAIN UNCERTAINTY CONCERNS INSTABILITY. IF MORE INSTABILITY IS

REALIZED AHEAD OF THE SQUALL-LINE THAN IS CURRENTLY FORECAST...THEN

A MORE SUBSTANTIAL THREAT FOR WIND DAMAGE WILL BE POSSIBLE. THIS

POSSIBILITY JUSTIFIES A 30 PERCENT SEVERE PROBABILITY ALONG THE

PROJECTED TRACK OF THE LINEAR MCS.

..BROYLES.. 02/17/2012

[David Reimer]

I don't care how much wind shear there will be tomorrow because with these instability values, you won't get an updraft that will be able to use that wind shear to do anything meaningful. Maybe if we had some backed winds along that warm front/instability axis in LA we could get an isolated downburst or tornado, but that's just picking at straws. I honestly don't see much happening tomorrow. If instability values increase, so will the severe weather threat. At this point I can't find a model supporting instability values over 750 J/Kg very far onshore. I'm surprised to see such strong wording from SPC.

[it*has*an*i]

I don't care how much wind shear there will be tomorrow because with these instability values, you won't get an updraft that will be able to use that wind shear to do anything meaningful. Maybe if we had some backed winds along that warm front/instability axis in LA we could get an isolated downburst or tornado, but that's just picking at straws. I honestly don't see much happening tomorrow. If instability values increase, so will the severe weather threat. At this point I can't find a model supporting instability values over 750 J/Kg very far onshore. I'm surprised to see such strong wording from SPC.

Hopefully you are right. Tallahassee says there have been cases in which it has happened though. This hasn't been a normal winter though so does the cool season thing apply in this area this late in Feb?

http://forecast.weat...on=3&glossary=1

AREA FORECAST DISCUSSION

NATIONAL WEATHER SERVICE TALLAHASSEE FL

357 AM EST Fri Feb 17 2012

Given the other very favorable factors, it would be unwise to downplay

this too much at this stage. While instability is not forecast to

be particularly large, it doesn`t have to be with 0-1 km shear

values at 40 knots. The local TAE cool season significant tornado

climatology has several cases in it that have SBCAPE values well

under 500 j/kg with low level shear values around 40 knots.

[Buckeye05]

Yeah the "strong tornado" mention IS pretty surprising.

[David Reimer]

Hopefully you are right. Tallahassee says there have been cases in which it has happened though. This hasn't been a normal winter though so does the cool season thing apply in this area this late in Feb?

http://forecast.weat...on=3&glossary=1

Sure it can happen, but a ton of mesoscale factors have to play into it. How much of that instability is located within the lowest 3 KM? What is saturation factor of the atmosphere? What are lapse rates looking like? I'm speaking from a chasing perspective. We'll see what data does later tonight.

[tornadotony]

Sure it can happen, but a ton of mesoscale factors have to play into it. How much of that instability is located within the lowest 3 KM? What is saturation factor of the atmosphere? What are lapse rates looking like? I'm speaking from a chasing perspective. We'll see what data does later tonight.

This isn't a chase setup. This is a rain-wrapped EF1-EF3 that no one ever sees coming setup.

[WeatherNC]

This isn't a chase setup. This is a rain-wrapped EF1-EF3 that no one ever sees coming setup.

So where are you thinking Tony? Southern half of MS into SE AL, maybe propagating through GA south of Macon...

[tornadotony]

So where are you thinking Tony? Southern half of MS into SE AL, maybe propagating through GA south of Macon...

Last couple runs of the NAM indicate we might have to go past US 80 in AL and GA...we'll see...

[WeatherNC]

Last couple runs of the NAM indicate we might have to go past US 80 in AL and GA...we'll see...

My best guess at this range is likely south of there, but I strongly agree on the variety and sig of the threat, unfortunately there is going to be a couple EF2 life takers, hopefully the local media keep the areas abrest today and tomorrow as it will get hairy for a period. I don't see the warm front getting much further north than central GA. Probably a 1002mb low sitting just south and west of Birmingham Sunday morning, transferring to somewhere offshore between CHS and ILM. That places the southern half of MS, areas SE of Birmingham, likely the area of greatest concern, and the southern 1/3 of GA inside the triple point. High confidence this will be a sig svr wx event and will warrant a mod hatch, likely a good case study to add to other low cape events this time of year, but insane shear with a potent Gulf low coming inland.

[andyhb]

I don't care how much wind shear there will be tomorrow because with these instability values, you won't get an updraft that will be able to use that wind shear to do anything meaningful. Maybe if we had some backed winds along that warm front/instability axis in LA we could get an isolated downburst or tornado, but that's just picking at straws. I honestly don't see much happening tomorrow. If instability values increase, so will the severe weather threat. At this point I can't find a model supporting instability values over 750 J/Kg very far onshore. I'm surprised to see such strong wording from SPC.

Maybe the fact that it is Broyles has something to do with it?

[it*has*an*i]

How bad is a 1-3 on the STP? It seems like it was a lot higher in past big events more like 6+ but I keep seeing the map posted. Will it warrant a hatched area likely?

http://wxcaster4.com...OS_STP_42HR.gif

[andyhb]

How bad is a 1-3 on the STP? It seems like it was a lot higher in past big events more like 6+ but I keep seeing the map posted. Will it warrant a hatched area likely?

http://wxcaster4.com...OS_STP_42HR.gif

Probably not.

[Buckeye05]

My best guess at this range is likely south of there, but I strongly agree on the variety and sig of the threat, unfortunately there is going to be a couple EF2 life takers, hopefully the local media keep the areas abrest today and tomorrow as it will get hairy for a period. I don't see the warm front getting much further north than central GA. Probably a 1002mb low sitting just south and west of Birmingham Sunday morning, transferring to somewhere offshore between CHS and ILM. That places the southern half of MS, areas SE of Birmingham, likely the area of greatest concern, and the southern 1/3 of GA inside the triple point. High confidence this will be a sig svr wx event and will warrant a mod hatch, likely a good case study to add to other low cape events this time of year, but insane shear with a potent Gulf low coming inland.

New to this, but what exactly is indicating the possibility of strong tornadoes now? The models are showing a marginal setup IMO.

[it*has*an*i]

New to this, but what exactly is indicating the possibility of strong tornadoes now? The models are showing a marginal setup IMO.

According to NWS Mobile

http://forecast.weather.gov/product.php?site=NWS&issuedby=MOB&product=AFD&format=CI&version=1&glossary=1

THE LATEST SREF RUN HAS "LOCKED IN" ON SOME OF THE BETTER PERFORMING DISCRIMINATING TORNADO PARAMETERS AND THEY SUGGEST THAT UP TO A LOCALIZED EF-2 INTENSITY TORNADO IS POSSIBLE WITHIN THE WARM SECTOR.

Also New Orleans NWS infographic says "Isolated tornadoes could be strong"

http://www.srh.noaa.gov/lix/

[on_wx]

Just for the record...I'm a little bit jealous of my family who are in New Orleans right now