Severe Potential: April 22-25, 2015

[andyhb]

It appears that, driven by an active subtropical jet, the pattern will becoming conducive to increasing severe potential starting on Wednesday as a weak vort max moves through the ST jet in behind the northeastern upper low, spurring cyclogenesis over West Texas. Despite near neutral height falls/rather zonal flow, it appears a round of convection (possibly supercells given adequate deep layer shear) is possible over Oklahoma and Texas with this disturbance. I will add that despite Broyles' using the NAM soundings in the outlook (which he often does), the GFS has been showing some fairly impressive parameter spaces as well.

 

However, my attention is more drawn to Friday (24th), with the Euro/UK depicting a strong, rather low amplitude shortwave trough ejecting out of the SW with very strong shear and potentially moderate-strong instability over the Southern Plains. It will depend on how far south the front with the northeast upper low makes it and how far north the subsequent warm front with the lee cyclogenesis pushes. However, the Euro/UK solutions would suggest a possibly significant severe event is in the cards (and we have seen this multiple times with this type of system). The GFS has been wavering between a more favorable solution and problems with the warm front pushing north/timing issues (the 18z run was the former, 00z the latter). There is also a D4 slight for much of eastern Texas, western LA and nudging into the Arklatex/SE OK.

 

Here's the discussions from the D3 and D4-8 outlooks from tonight.

 

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK
0226 AM CDT MON APR 20 2015

VALID 221200Z - 231200Z

...THERE IS AN ENH RISK OF SVR TSTMS ACROSS PARTS OF CNTRL OK...SRN
OK AND NRN TX...

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE SRN
PLAINS...ARKLATEX AND LOWER MS VALLEY...

...THERE IS A MRGL RISK OF SVR TSTMS ACROSS PARTS OF THE CNTRL HIGH
PLAINS...SRN PLAINS...ARKLATEX AND LOWER TO MID MS VALLEY...

...SUMMARY...
THUNDERSTORMS WITH WIND DAMAGE AND LARGE HAIL ARE EXPECTED TO
DEVELOP FROM THE SOUTHERN PLAINS SOUTHEASTWARD ACROSS THE ARKLATEX
AND INTO THE LOWER MISSISSIPPI VALLEY. MARGINALLY SEVERE
THUNDERSTORMS MAY DEVELOP ACROSS PARTS OF THE SOUTHERN AND CENTRAL
HIGH PLAINS AND IN SOUTH CENTRAL TEXAS. THUNDERSTORM DEVELOPMENT MAY
ALSO OCCUR ACROSS PARTS OF THE CENTRAL TO NORTHERN ROCKIES AND
SIERRA NEVADA.

...SRN PLAINS/ARKLATEX
AN UPPER-LEVEL RIDGE IS FORECAST TO MOVE EWD ACROSS THE SRN ROCKIES
ON WEDNESDAY AS SFC LOW PRESSURE IN THE SRN HIGH PLAINS DEEPENS AND
MOVES SEWD. A WARM FRONT MAY BE PRESENT ACROSS SRN OK EXTENDING
ESEWD INTO AR. ALONG AND SOUTH OF THE BOUNDARY...SFC DEWPOINTS
SHOULD BE IN THE LOWER TO MID 60S WITH MODERATE INSTABILITY
DEVELOPING ACROSS MUCH OF THE WARM SECTOR BY AFTERNOON. AS A CAP
WEAKENS DURING THE DAY...THUNDERSTORM DEVELOPMENT APPEARS POSSIBLE
ALONG THE WARM FRONT AND FURTHER SOUTHEASTWARD ACROSS THE ARKLATEX
NEAR THE ERN EDGE OF THE STRONGEST INSTABILITY. AN MCS WILL BE
POSSIBLE ACROSS THE REGION LATE WEDNESDAY AFTERNOON WITH COLD-POOL
ORGANIZATION ALSO POSSIBLE BY EVENING.

NAM FORECAST SOUNDINGS FROM OKLAHOMA CITY SEWD TO NEAR THE RED RIVER
AT 00Z/THU SHOW MLCAPE IN THE 2000 TO 2500 J/KG RANGE WITH 0-6 KM
SHEAR OF 45 TO 50 KT. THIS WOULD SUPPORT SEVERE STORMS WITH LARGE
HAIL AND WIND DAMAGE. THE MAIN QUESTION IS WHAT STORM MODE WILL
DOMINANT. IF CELLS REMAIN DISCRETE...THEN SUPERCELLS WOULD BE THE
MOST FAVORED STORM TYPE. HOWEVER...THE PATTERN WOULD SUGGEST
COLD-POOL ORGANIZATION WHICH WOULD FAVOR A LINEAR STRUCTURED MCS
WITH ENHANCED WIND DAMAGE POTENTIAL. EITHER WAY...SOME TORNADOES CAN
BE EXPECTED WITH ROTATING CELLS ASSOCIATED WITH THE BETTER ORGANIZED
STORM CLUSTERS. WILL ADD A 30 PERCENT SEVERE PROBABILITY FROM NEAR
AND SOUTH OF OKC SEWD TO NEAR AND EAST OF DALLAS.

FURTHER TO THE SOUTH ACROSS SE TX AND LA...FORECAST SOUNDINGS AT 21Z
ON WEDNESDAY SHOW MODERATE INSTABILITY AND STRONG DEEP-LAYER SHEAR.
DUE TO THIS ENVIRONMENT...CELLS THAT CAN INITIATE ALONG LOCALIZED
CONVERGENCE ZONES COULD ROTATE AND POSE A THREAT FOR WIND DAMAGE AND
HAIL. A SEVERE THREAT COULD ALSO DEVELOP DURING THE LATE EVENING OR
IN THE OVERNIGHT PERIOD AS AN MCS APPROACHES THE REGION FROM THE
NORTHWEST.

...ERN CO/SW KS/TX AND OK PANHANDLES...
AN UPPER-LEVEL RIDGE IS FORECAST TO MOVE EWD ACROSS THE SRN ROCKIES
WEDNESDAY AS SFC LOW PRESSURE DEEPENS ACROSS THE SRN HIGH PLAINS.
UPSLOPE FLOW IS FORECAST FROM NEAR THE SFC LOW EXTENDING NWD ACROSS
THE CNTRL HIGH PLAINS. THUNDERSTORM DEVELOPMENT MAY TAKE PLACE BY
AFTERNOON ALONG A SFC TROUGH IN THE TX PANHANDLE AND IN ERN CO.
FORECAST SOUNDINGS FOR LAMAR CO AND THE OK PANHANDLE AT 00Z/THU SHOW
MLCAPE AROUND 1500 J/KG WITH 0-6 KM SHEAR NEAR 40 KT. THIS COMBINED
WITH A RELATIVELY DRY BOUNDARY LAYER...SFC DEWPOINTS IN THE UPPER
40S F...WOULD SUPPORT MARGINALLY SEVERE STORMS WITH WIND DAMAGE AND
HAIL POSSIBLE MAINLY DURING THE LATE AFTERNOON AND EARLY EVENING.

..BROYLES.. 04/20/2015

 

DAY 4-8 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK
0343 AM CDT MON APR 20 2015

VALID 231200Z - 281200Z

...DISCUSSION...
THE MEDIUM-RANGE MODELS INCLUDING THE ECMWF AND GFS SOLUTIONS BEGIN
THE 4 DAY 8 DAY PERIOD WITH AN UPPER-LEVEL RIDGE OVER THE SRN PLAINS
EXTENDING NNWWD INTO THE CNTRL HIGH PLAINS. THE MODELS MOVE THIS
FEATURE EWD ON THURSDAY/DAY 4 ACROSS A WARM SECTOR OVER THE ERN
TWO-THIRDS OF TX...SRN OK AND LA. ISOLATED TO SCATTERED THUNDERSTORM
DEVELOPMENT WILL BE POSSIBLE ACROSS THE WARM SECTOR WHERE MODERATE
INSTABILITY AND STRONG DEEP-LAYER SHEAR SHOULD MAKE CONDITIONS
FAVORABLE FOR SEVERE STORMS. AN ENHANCED THREAT FOR WIND DAMAGE AND
LARGE HAIL COULD DEVELOP ACROSS THE REGION THURSDAY AFTERNOON AND
EVENING WHERE A SLIGHT RISK HAS BEEN ADDED. MODEL SOLUTIONS DIVERGE
SHARPLY ON FRIDAY/DAY 5 WITH THE ECMWF MOVING AN NEGATIVELY TILTED
UPPER-LEVEL LOW INTO THE CNTRL HIGH PLAINS WHILE THE GFS MOVES A
POSITIVELY TILTED UPPER-LEVEL TROUGH OVER THE SRN HIGH PLAINS AND
CNTRL PLAINS. THE ECMWF SOLUTION WOULD BE FAVORABLE FOR SEVERE
STORMS IN THE SRN AND CNTRL PLAINS WHILE THE FASTER GFS SOLUTION
WOULD BE FAVORABLE FOR SEVERE STORMS OVER THE LOWER TO MID MS
VALLEY. AT THIS POINT...AM LEANING TOWARD THE SLOWER ECMWF SOLUTION
WHICH WOULD MEAN A POSSIBILITY FOR AN ENHANCED SEVERE THREAT IN FROM
NORTH TX NWD INTO SRN KS. LARGE HAIL...WIND DAMAGE AND TORNADOES
WOULD BE POSSIBLE FRIDAY AFTERNOON AND EVENING IN THE SRN PLAINS.
THE POTENTIAL EXISTS FOR AN OUTBREAK OF SEVERE STORMS IF THE ECMWF
SOLUTION VERIFIES. AT THIS POINT...THE LARGE SPREAD IN MODEL
SOLUTIONS IS TOO GREAT TO ADD AN ENHANCED SEVERE THREAT FOR FRIDAY.
ON SATURDAY/DAY 6...THE MODELS MOVE THE SYSTEM QUICKLY EWD ACROSS
THE MS VALLEY WITH A ZONAL FLOW PATTERN DEVELOPING IN THE GULF COAST
STATES. THIS PATTERN WOULD MAKE THUNDERSTORM DEVELOPMENT POSSIBLE IN
THE GULF COAST STATES WITH A SEVERE THREAT POSSIBLE THERE DURING THE
AFTERNOON. EVEN SO...PREDICTABILITY REMAINS QUITE LOW LATE IN THE
DAY 4 TO 8 PERIOD.

..BROYLES.. 04/20/2015

[kevinbente]

Yes have been looking at this today. Euro model seems to have DL a bit too far west IMO but those specifics will be worked out in the near future. Still a few days out so things can still change, but that run looked pretty hefty. 12z runs will be interesting. 

[Indystorm]

Monday's 12z GFS run also shows a potent 110-130 knot jet at 300mb nosing across OK at 21z this Friday. Gonna have to watch this for sure this week.

[Quincy]

Monday's 12z GFS run also shows a potent 110-130 knot jet at 300mb nosing across OK at 21z this Friday. Gonna have to watch this for sure this week.

It certainly has my attention in the upper levels and at the surface. Encouraging. We'll see how the 12z Euro looks. It's early, but seeing some agreement in the coming runs would be great.

Edit: FWIW the GFS depicts only modest instability at best up in OK.

[andyhb]

It certainly has my attention in the upper levels and at the surface. Encouraging. We'll see how the 12z Euro looks. It's early, but seeing some agreement in the coming runs would be great.

Edit: FWIW the GFS depicts only modest instability at best up in OK.

 

Since it is significantly faster than the rest of guidance.

[jhamps10]

So, if the Day 4-8 outlook has nothing for Friday, but the models show good reason to... why do we not have a risk area for Friday??

[on_wx]

NWS Norman's afternoon afd

FRIDAY MAY HOLD THE GREATEST SEVERE POTENTIAL THIS

WEEK...DEPENDING ON WHICH MODEL YOU CHOOSE TO BELIEVE. THE GFS

WOULD INDICATE A RAPID DRYLINE SURGE...DRY SLOTTING MUCH OF THE

REGION BY MID TO LATE AFTERNOON FRIDAY...AND RESULTING IN THE

GREATER SEVERE STORM POTENTIAL ACROSS THE LOWER/MID MISSISSIPPI

VALLEY REGION. THE EUROPEAN...HOWEVER...SHOWS A SLOWER PROGRESSION

OF THE UPPER TROUGH AND DRYLINE...AND WOULD FAVOR A POTENTIALLY

SIGNIFICANT SEVERE WEATHER EPISODE FROM SOUTHERN NEBRASKA DOWN

THROUGH WESTERN/CENTRAL KANSAS AND OKLAHOMA...AND INTO NORTH

TEXAS. EVERYONE WHO IS ANYONE SHOULD PAY VERY CLOSE ATTENTION TO

FRIDAY AS ADDITIONAL MODEL DATA BECOME AVAILABLE...AND THE

SITUATION BECOMES MORE CLEAR.

[jojo762]

So, if the Day 4-8 outlook has nothing for Friday, but the models show good reason to... why do we not have a risk area for Friday??

timing issues created by the GFS/EURO... The GFS depicts an area further to the east across the Mid/Lower Mississippi valley and definitely shows a less enhanced threat since it's is far more progressive than any other model... Meanwhile the EURO shows a far more significant threat area on either side of the I-35 corridor, and even areas north per the OUN AFD, as they mention even as far north as southern Nebraska. (prime chase territory due to relatively flat land and lack of trees.)

[jhamps10]

timing issues created by the GFS/EURO... The GFS depicts an area further to the east across the Mid/Lower Mississippi valley and definitely shows a less enhanced threat since it's is far more progressive than any other model... Meanwhile the EURO shows a far more significant threat area on either side of the I-35 corridor, and even areas north per the OUN AFD, as they mention even as far north as southern Nebraska. (prime chase territory due to relatively flat land and lack of trees.)

 

still makes sense to put out the risk area imo... the DFW news stations are already in alarming mode that frankly I don't recall hearing in the 2 years I've lived in the metro plex

[jojo762]

Honestly the 18Z GFS solution for Friday doesn't make much sense to me. At 7pm Friday it shows a 996MB low over northwestern OK and a dryline just blasting way east of it creating a very narrow warm sector. Doesn't seem like a dryline surges that far East of a sfc low that often. But correct me if I'm being dumb. The EURO and other models as outlined by Broyles seem way preferable on numerous accounts imo. On a basic bias that this range the ECMWF seems to depict events far better than the GFS ever does really... Also recently the GFS has been very progressive with several systems and it really hasn't proven itself right really, but that's just a personal biased really.

[NTXYankee]

still makes sense to put out the risk area imo... the DFW news stations are already in alarming mode that frankly I don't recall hearing in the 2 years I've lived in the metro plex

FWD seems far more focused on Wednesday and Thursday here with uncertainty regarding Friday. Wednesday and Thursday look impressive on NAM dynamics wise. TV stations will always hype it up imo probably for ratings. There are many details to be worked out for Friday, but GFS seems out to lunch to me too blasting dryline well east of main low in western Oklahoma.

[jhamps10]

FWD seems far more focused on Wednesday and Thursday here with uncertainty regarding Friday. Wednesday and Thursday look impressive on NAM dynamics wise. TV stations will always hype it up imo probably for ratings. There are many details to be worked out for Friday, but GFS seems out to lunch to me too blasting dryline well east of main low in western Oklahoma.

 

yes I agree on hype for ratings... but this seemed different to me... maybe it was the squall line Saturday night but something caused at least Rick and Henry to REALLY mention severe Friday over Wed even (while they did bring that up as well)

[brettjrob]

It's clear that Friday has the highest ceiling, even if it's less certain that anything substantial will affect the I-35 corridor than Wednesday. The media hype machine really is ramping up disproportionately early compared to similar threats so far this year (or even last year) on the guidance, though, as KFOR here already has a Friday graphic with SigTors.

 

There are lots of factors keeping me from getting too excited about Friday at this point. From a synoptic standpoint, the combination of a downstream eastern US trough and this being a short-wavelength disturbance make me really question the likelihood of an apocalyptic outcome in April. This strikes me as a scenario where several different details have to come together correctly (particularly in terms of timing) to reach anywhere near the ceiling, such that within a hypothetical ensemble of reasonable solutions, less than 50% would be a big outbreak for the I-35 corridor westward. If the ECMWF is even 25% too slow on timing relative to the GFS, it probably shoves the main threat into the jungle and E of the major cities. But if the ECMWF holds its ground by tomorrow night's run and the GFS starts caving, I certainly see the potential for the biggest day so far this spring, given both a negative-tilt trough and ample low-level moisture.

[Chinook]

The 18z GFS has some 80kt 500mb winds over Texas on Friday. This seems unusual. You don't see much over 60kts. If it verifies, that's some crazy high 0-6km shear over a warm moist area.

[Quincy]

The 18z GFS has some 80kt 500mb winds over Texas on Friday. This seems unusual. You don't see much over 60kts. If it verifies, that's some crazy high 0-6km shear over a warm moist area.

Even the Euro has some 80kt winds at h5 over the panhandle. Let's see how subsequent model runs look.

[kevinbente]

Certainly seeing 60+ knots at H5 over 3000 cape is a big deal. Lets see if low levels trend in the right (for chasers) direction. Usually for a 996 low you usually see a mass response. Me thinks low levels are being underplayed a bit. Could just be the optimistic bias I have lol.

[Quincy]

The NAM keeps Wednesday in play too, but let's see if it hangs on once we're inside of 36 hours.

[1900hurricane]

Yeah, NAM has been all over Wednesday for several runs, spitting out some impressive parameters in the middle Brazos and upper Trinity basins. Depending on how the forecast evolves, I may be scrambling to try and clear Wednesday afternoon and take a drive up 6. Very interested in seeing the SPC Day 2.

 

Friday is already a circled chase day for me. No class or work for me that day and a wedding in Dallas the next day means I have way more range than I usually would. Hoping it delivers!

[andyhb]

WPC model diagnostic discussion through tonight's 00z NAM/GFS runs:

 

TROUGHING IN THE SOUTHWEST WEDNESDAY INTO FRIDAY  

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: CANADIAN/ECMWF COMPROMISE  
CONFIDENCE: AVERAGE  
 
THE 00Z NAM/00Z GFS/12Z UKMET ARE QUICKER TO PROGRESS THIS SYSTEM  
EASTWARD THAN THE OTHER GUIDANCE. IT APPEARS THE AMPLITUDE OF A  
SHORTWAVE APPROACHING CA IS PARTIALLY TO BLAME HERE, WITH WHICH  
THE GFS IS STRONGEST. THE 21Z SREF AND 12Z ECMWF ENSEMBLE MEANS  
SUPPORT A SLOWER PROGRESSION, WHICH TENDS TO WORK OUT WITH  
SOUTHWESTERN SYSTEMS. A COMPROMISE OF THE 12Z ECMWF/12Z CANADIAN  
 

IS PREFERRED WITH AVERAGE CONFIDENCE.   

[RCNYILWX]

WPC model diagnostic discussion through tonight's 00z NAM/GFS runs:

TROUGHING IN THE SOUTHWEST WEDNESDAY INTO FRIDAY

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

PREFERENCE: CANADIAN/ECMWF COMPROMISE

CONFIDENCE: AVERAGE

THE 00Z NAM/00Z GFS/12Z UKMET ARE QUICKER TO PROGRESS THIS SYSTEM

EASTWARD THAN THE OTHER GUIDANCE. IT APPEARS THE AMPLITUDE OF A

SHORTWAVE APPROACHING CA IS PARTIALLY TO BLAME HERE, WITH WHICH

THE GFS IS STRONGEST. THE 21Z SREF AND 12Z ECMWF ENSEMBLE MEANS

SUPPORT A SLOWER PROGRESSION, WHICH TENDS TO WORK OUT WITH

SOUTHWESTERN SYSTEMS. A COMPROMISE OF THE 12Z ECMWF/12Z CANADIAN

IS PREFERRED WITH AVERAGE CONFIDENCE.

The timing issue is also important downstream on Saturday, as the 12z European synoptically was likely supportive of significant severe weather in Dixie Alley on Saturday.

[andyhb]

The timing issue is also important downstream on Saturday, as the 12z European synoptically was likely supportive of significant severe weather in Dixie Alley on Saturday.

 

Yes, I was looking over some of the indices from a pay site for that and it popped my eyes, even with SSW near-surface flow, that's one of those areas where you get away with a strong speed shear/comparatively subtle directional shear combo (although the directional shear wasn't lacking either, to be fair). There's a couple of events that came to mind for the 12z Euro's setup there and they produced some rather ugly results.

[Smokey_mountain_vols]

I noticed that on Wednesday the southern counties in Oklahoma are gonna prolly get hit. Good thing I live in the Northern part of the state. Still I'll prolly get a bit of a storm Wednesday and Friday.

[brettjrob]

SWODY2 maintains ENH risk for the Red River Valley, with the expectation of late afternoon initiation along the warm front. This would certainly lead to some monster sups. However, every convection-allowing model I've seen tonight smothers this region in morning storms, shunting the boundary well south and essentially ruining the entire setup (other than perhaps some potential across the I-20 corridor, but true warm sector initiation seems highly questionable if that scenario comes to pass).

 

Going to be an interesting model battle, as none of the coarser models are nearly as harsh with the stabilizing effects of morning convection. I'd lean toward the crapvection-ruination scenario for now.

[Smokey_mountain_vols]

SWODY2 maintains ENH risk for the Red River Valley, with the expectation of late afternoon initiation along the warm front. This would certainly lead to some monster sups. However, every convection-allowing model I've seen tonight smothers this region in morning storms, shunting the boundary well south and essentially ruining the entire setup (other than perhaps some potential across the I-20 corridor, but true warm sector initiation seems highly questionable if that scenario comes to pass).

Going to be an interesting model battle, as none of the coarser models are nearly as harsh with the stabilizing effects of morning convection. I'd lean toward the crapvection-ruination scenario for now.

I think Abilene to Dallas to McAllister Oklahoma gets pummeled by storms along with OKC and Moore.

[Roy]

Not liking the morning convection today. Hopefully that won't occur tomorrow, or that will really lower risks.

[kevinbente]

12z nam has Sigtors in the 6-8 range, but it's the NAM in fantasy land. Still interesting. 

[Quincy]

Have a few concerns about Friday with several pieces to the puzzle not lining up quite right. With that said, 12z Euro looks at least slightly better than 00z in some aspects, but not by much. 85kt wind max at 500mb swinging through north Texas, but at 18z. Modest to moderate instability up into central/eastern OK by 00z. If this whole thing could slow down just a touch, there would be more concern for significant severe.

[1900hurricane]

Rich Thompson talked about tomorrow's threat in his last Tornado Forecast Workshop video tonight.

 

[Smokey_mountain_vols]

Have a few concerns about Friday with several pieces to the puzzle not lining up quite right. With that said, 12z Euro looks at least slightly better than 00z in some aspects, but not by much. 85kt wind max at 500mb swinging through north Texas, but at 18z. Modest to moderate instability up into central/eastern OK by 00z. If this whole thing could slow down just a touch, there would be more concern for significant severe.

On a scale of one to boned how boned is Tulsa? Lol

[David Reimer]

On a scale of one to boned how boned is Tulsa? Lol

 

I'd say Tulsa is gonna get a good raking on Friday. The highest tornado potential may be up in SE KS depending on low level winds - but severe is a good bet IMHO.