April 19th Severe Event

[Bob's Burgers]

Well that sure was fun.

[jrips27]

HRRR trending north with the front 

[CryHavoc]

...SUMMARY...
   An outbreak of tornadoes and severe thunderstorms is expected today
   from parts of the lower Mississippi Valley across much of the the
   Southeast. In addition to tornadoes, many of the storms will have
   very large hail and wind damage. The severe threat will be greatest
   from north-central Louisiana eastward to southern and central
   Mississippi, southern and central Alabama into south-central
   Georgia. Additional severe storms with a threat for tornadoes, wind
   damage and hail will be possible in parts of east Texas this morning
   and in the Carolinas tonight.


Seems like the strongest wording possible that wouldn't include a high risk for 4/19.

[Moderately Unstable]

I mean it is clearly a high risk writeup with a moderate forecast cone. It's almost comical to read. 

[Bob's Burgers]

   "The greatest chance for a regional
   cluster of tornadoes is forecast from just southeast of Jackson,
   Mississippi and Hattiesburg, Mississippi eastward across southwest
   and south-central Alabama."

 

 

 

[jojo762]

14 minutes ago, Moderately Unstable said:

I mean it is clearly a high risk writeup with a moderate forecast cone. It's almost comical to read. 

I thought the same thing. Vividly remembering Broyles writing style for High Risk outlooks in the past, and its like hmm... The part about the corridor of higher potential screams 30% TOR probs, but then its not evident in the actual probabilities (FWIW, this geographical area pretty much hugged the 00z HRRR solution).

Likewise, pick your model for tomorrow and stick with it... HRRR, et. al vs. WRF-NSSL, et. al.

Also, does anyone know in God's green earth why the HRRR keeps insisting on mixing-out surface moisture in S AL/MS tomorrow afternoon? Its the Deep south with unimpeded moisture transport, and not friggin western Kansas on a MRGL/SLGT dryline day.

[MUWX]

1 hour ago, yoda said:

Odd that there isn't even a mention of possible upgrade to HIGH at all...

That’s because they were going high until BMX talked them out of it. Just forgot to add that in there after the change. (Sarcasm.... maybe. Who knows lol) 

[cheese007]

Correct me if I'm wrong but isn't Broyles famous for being... "generous" with his risk forecasts?

[brianc33710]

Well, BMX has now pushed the 5 zones further south. Hamilton, Cullman & Gadsden has no risk at all. Jasper, Birmingham proper & Anniston are marginal. But the southern tip of Jefferson Co, most of Shelby Co, Tuscaloosa & Sylacauga are labeled slight. From Eutaw to Clanton to Alexander City is enhanced. And Demopolis, Selma, Montgomery & Auburn are still moderate. I guess we will just get everyday storms with a ton of rain, but that could cause flooding problems on its own.

[jpeters3]

5 hours ago, CryHavoc said:

I just don't get it. 

If we see 60+ tornadoes tomorrow, it's going to be two undersold predictions in a row, all in the name of trying to be careful.

Issuing a high risk last weekend would have been irresponsible given the uncertainty.  And issuing a high risk this morning also would have been irresponsible, regardless of the outcome.  

Yet another difficult forecast this morning.  There will be some regions of the warm sector with higher end parameters, but the CAM guidance is far from screaming "outbreak."  To me, it looks like we'll have another QLCS event with the chance of some rogue isolated supercells in the warm sector (though CAMS are all across the board in terms of the prevalence of the later).  QLCS tornado events are quite difficult to forecast.  Obviously last week's event was prolific, and there are some similarities this week (low-level saturation).  But there have been plenty events with high end parameters in the warm sector and a QLCS that plowed through the high end parameter region without producing many tornadoes.  Given these uncertainties (which to me seem a tad up from last weekend), it would have been totally irresponsible to issue a high risk.

[SRRTA22]

Someone is flying away with that storm southeast of Birmingham. Beautiful bow velocity signature

 

[janetjanet998]

Trend is slower....

 

10Z HRRR has the Houston Metro  and areas north  starting the action about 15-16z

 

[jpeters3]

One possibly large difference between this event and last week's is the roughly 10 kt difference in 0-6 km shear (weaker today than last Sunday).  Low-level shear is obviously most relevant to the immediate process of tornadogenesis through enhancing low-level upward dynamic pressure accelerations, but deep-layer shear (and the associated low-level storm-relative flow) have a strong influence on whether sustained supercells-like updrafts are possible.  The comparatively strong deep-layer shear last weekend may have facilitated the formation of sustained supercell-like updrafts within the QLCS, and facilitated the subsequent formation of numerous tornadoes because of the favorable low-level environment.  In this event, we will have a favorable low-level environment but not quite as much deep layer shear - this may make all the difference in terms of whether sustained supercell-like updrafts occur within the QLCS and subsequently whether a prolific QLCS event will happen.

[janetjanet998]

ACUS11 KWNS 191222  
SWOMCD  
SPC MCD 191222   
LAZ000-TXZ000-191415-  
  
MESOSCALE DISCUSSION 0401  
NWS STORM PREDICTION CENTER NORMAN OK  
0722 AM CDT SUN APR 19 2020  
  
AREAS AFFECTED...PORTIONS OF CENTRAL AND SOUTHEASTERN TX INTO FAR  
WESTERN LA  
  
CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY   
  
VALID 191222Z - 191415Z  
  
PROBABILITY OF WATCH ISSUANCE...95 PERCENT  
  
SUMMARY...THUNDERSTORM ACTIVITY WILL INCREASE THROUGH THE MORNING  
FROM CENTRAL INTO SOUTHEASTERN TEXAS. VERY LARGE HAIL, A FEW  
TORNADOES AND DAMAGING WIND GUSTS ARE ALL POSSIBLE. A TORNADO WATCH  
WILL LIKELY BE NEEDED IN THE NEXT 1-2 HOURS.  
  
DISCUSSION...ELEVATED CONVECTION IS INCREASING THIS MORNING ACROSS  
PORTIONS OF CENTRAL TX WITH LARGE-SCALE ASCENT INCREASING AS A  
SHORTWAVE TROUGH BEGINS TO EJECT EASTWARD FROM NM INTO WESTERN TX.  
INITIAL CONVECTION WILL LIKELY REMAIN ELEVATED FOR A COUPLE OF HOURS  
UNTIL STRONGER HEATING AFTER SUNRISE ALLOWS SURFACE INHIBITION TO  
ERODE AND AS A WARM FRONT DRAPED ACROSS THE AREA CONTINUES TO LIFT  
NORTHWARD. NEVERTHELESS, VERY STEEP MIDLEVEL LAPSE RATES (7.5-8.5  
C/KM) ARE CONTRIBUTING TO MUCAPE VALUES AROUND 2000 J/KG PER 12Z  
MESOANALYSIS AND THE 12Z FWD RAOB. EFFECTIVE SHEAR IS SUFFICIENT FOR  
STORM ORGANIZATION CURRENTLY, AND WILL CONTINUE TO IMPROVE OVER THE  
NEXT FEW HOURS. LARGE HAIL (SOME POTENTIALLY GREATER THAN 2 INCHES  
IN DIAMETER) WILL BE THE MAIN CONCERN IN THE IMMEDIATE NEAR-TERM,  
ESPECIALLY WITH ANY DISCRETE CELLS.   
  
WITH TIME, CONVECTION WILL BECOME SURFACE-BASED AS IT APPROACHES FAR  
EASTERN TX AND THE SABINE RIVER VALLEY LATE THIS MORNING. AS THIS  
OCCURS, AN INCREASING THREAT FOR TORNADOES AND DAMAGING WINDS WILL  
ACCOMPANY CONVECTION. WHILE LOW LEVEL HODOGRAPHS WILL BECOME  
ENLARGED BY LATE MORNING INTO THE AFTERNOON, DEEP-LAYER WINDS ARE  
RATHER UNIDIRECTIONAL ABOVE 3KM. THIS COULD RESULT IN STORM  
INTERACTIONS AND CLUSTERING TOWARD MIDDAY, WITH A MIX OF  
SEMI-DISCRETE CELLS AND CLUSTERS/BOWING SEGMENTS. AS SUCH, ALL  
SEVERE HAZARDS ARE ANTICIPATED AND SOME LOCATIONS COULD SEE MORE  
THAN ONE ROUND OF SEVERE STORMS.  

[JasonOH]

I have a feeling the HRRR is really overdoing they mixing of the boundary layer. There is not any other model support for mixing of that strength and I find it pretty hard to believe it will happen with the quality of moisture. Some mixing will happen, but not as much as the HRRR is modeling.

[It's Always Sunny]

I like what the SPC has out but definitely going to monitor srn MS/srn AL with that MCS pretty much missing them this morning they are primed and ready to go this afternoon and tonight.

[janetjanet998]

DAY 1 CONVECTIVE OUTLOOK    
NWS STORM PREDICTION CENTER NORMAN OK  
0749 AM CDT SUN APR 19 2020  
  
VALID 191300Z - 201200Z  
  
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM THE  
TEXAS/LOUISIANA BORDER AREA TO SOUTHEASTERN GEORGIA...  
   
..SUMMARY  
  
A SEVERE-WEATHER OUTBREAK IS EXPECTED ACROSS MUCH OF THE DEEP SOUTH  
THROUGH TONIGHT, WITH THE GREATEST THREAT FROM SOME OF EAST TEXAS  
AND THE DELTA REGION EASTWARD OVER PARTS OF ALABAMA AND GEORGIA.   
MULTIPLE EPISODES OF THUNDERSTORMS WILL RESULT IN TORNADOES (SOME  
STRONG), DAMAGING WIND AND LARGE HAIL.  
  
--- TECHNICAL DISCUSSION ---   
..SYNOPSIS  
  
IN MID/UPPER LEVELS, THE CONTINENTAL PATTERN WILL BE DOMINATED BY A  
VERY LARGE CYCLONE COVERING MUCH OF NORTHERN AND EASTERN CANADA,  
CENTERED NORTH OF HUDSON BAY.  ASSOCIATED CYCLONIC FLOW WILL EXTEND  
ACROSS THE GREAT LAKES.  THE LONG-LIVED RIDGE UPSTREAM -- INITIALLY  
POSITIONED OFF THE PACIFIC NORTHWEST TO THE MAIN BODY OF AK -- WILL  
DEAMPLIFY AND MOVE ASHORE FROM THE AK PANHANDLE TO INTERIOR WA/OR  
THROUGH THE PERIOD.  MEANWHILE, TO ITS SOUTH, AN UNBROKEN SOUTHERN-  
STREAM FETCH WILL CONTINUE FROM OFF THE CA COASTLINE ACROSS THE  
SOUTHWEST AND SOUTHERN PLAINS, BLENDING INTO A BROAD AREA OF NEARLY  
ZONAL FLOW FROM THE MISSISSIPPI VALLEY EASTWARD.  
  
AN EMBEDDED TROUGH -- INITIALLY EVIDENT IN MOISTURE-CHANNEL IMAGERY  
FROM THE TX PANHANDLE TO SOUTHEASTERN NM AND FAR WEST TX -- WILL BE  
THE MAIN SHORTWAVE INFLUENCE FOR THIS FORECAST.  AVAILABLE MORNING  
RAOBS AND RADAR-DERIVED WIND PROFILES ALREADY ARE STRENGTHENING  
AHEAD OF THIS FEATURE OVER CENTRAL AND NORTH TX.  THIS PROCESS --  
ALONG WITH INCREASING LARGE-SCALE LIFT -- WILL SPREAD EASTWARD  
ACROSS THE SOUTH IN STEP WITH THE TROUGH'S TRANSLATION EASTWARD TO  
PARTS OF AL/GA BY 12Z TOMORROW.  
  
AT THE SURFACE, 11Z ANALYSIS SHOWED A WAVY WARM FRONT FROM NEAR SSI  
ACROSS SOUTHERN GA, THE NORTHWESTERN FL PANHANDLE, TO NEAR MOB, HEZ,  
DRI, TO A WEAK LOW NEAR CLL.  A DIFFUSE COLD TO QUASISTATIONARY  
FRONT EXTENDED FROM THERE SOUTHWESTWARD ACROSS THE LRD AREA.  THE  
WARM FRONT IS EXPECTED TO SHIFT NORTHWARD THROUGH THE DAY, TO  
NORTHERN LA AND CENTRAL PORTIONS OF MS/AL/GA.  FURTHER POLEWARD  
PROGRESS OF THE FRONT EFFECTIVELY SHOULD BE STALLED BY ONGOING AND  
INCREASING CONVECTION TO ITS NORTH, BEFORE MCS ACTIVITY OVERTAKES IT  
IN A WEST-EAST MANNER.   
   
..EAST TX TO CAROLINAS  
  
MULTIPLE EPISODES OF SEVERE THUNDERSTORMS ARE EXPECTED TO CROSS THE  
OUTLINED SEVERE-RISK CORRIDOR THROUGH TOMORROW MORNING.  IN  
AGGREGATE, THIS ACTIVITY SHOULD YIELD A SWATH OF DAMAGING WIND,  
TORNADOES AND LARGE HAIL.  TORNADOES ARE POSSIBLE FROM SUPERCELLS  
NEAR THE WARM FRONT, IN THE WARM SECTOR, AND A COMBINATION OF  
LINE-EMBEDDED SUPERCELLS AND QLCS VORTICES WITH A DOMINANT MCS THAT  
IS FORECAST TO SWEEP ACROSS THE OUTLOOK AREA FROM WEST TO EAST.  THE  
FORECAST ENVIRONMENTAL PARAMETER SPACE IN THE WARM SECTOR SUPPORTS  
THE POTENTIAL FOR SIGNIFICANT (EF2+ DAMAGE-PRODUCING) TORNADOES,  
ESPECIALLY WITH ANY RELATIVELY SUSTAINED SUPERCELLS AND DEEP/INTENSE  
LINE-EMBEDDED CIRCULATIONS.  
  
INITIAL SEVERE THREATS THIS MORNING SHOULD EVOLVE OUT OF ACTIVITY IN  
THE EASTERN AND WESTERN ENDS OF THE OUTLOOK SWATH.  AN ONGOING AREA  
OF MOSTLY NON-SEVERE, ELEVATED THUNDERSTORMS OVER PARTS OF MS/AL MAY  
CONTINUE TO OFFER ISOLATED, SPORADIC LARGE-HAIL POTENTIAL, WHILE ITS  
SOUTHERN END ACCEPTS PROGRESSIVELY MORE-UNSTABLE INFLOW ROOTED  
NEARER TO THE SURFACE WITH TIME IN EASTERN AL AND GA.  OTHER  
THUNDERSTORMS MAY DEVELOP OVER PARTS OF CENTRAL/SOUTHERN GA, MOVING  
INTO A DIURNALLY DESTABILIZING AIR MASS CLOSER TO THE GA COAST.  
  
MEANWHILE, AHEAD OF THE SHORTWAVE TROUGH, STRENGTHENING DEEP-LAYER  
LIFT WILL SUPPORT DEVELOPMENT AND UPSCALE GROWTH OF CONVECTION FROM  
CENTRAL/EAST TX EASTWARD THIS MORNING INTO AFTERNOON, AND PERHAPS  
FROM SOUTHEASTERN OK/NORTHEAST TX EASTWARD THIS AFTERNOON INTO  
OVERNIGHT.  SIGNIFICANT-HAIL RISK SHOULD BE GREATEST RELATIVELY  
EARLY IN EITHER ASSOCIATED MCS-ORGANIZATION PROCESS, OR WHATEVER  
WARM-SECTOR SUPERCELLS FORM, IN A REGIME OF STEEP LOW/MIDDLE-LEVEL  
LAPSE RATES AND FAVORABLE DEEP SHEAR.  AS THIS ACTIVITY SHIFTS  
QUICKLY EASTWARD, IT WILL ENCOUNTER RELATIVELY MAXIMIZED LOW-LEVEL  
SHEAR/LIFT ALONG THE WARM FRONT, AS WELL AS A WARM SECTOR  
CHARACTERIZED BY UPPER 60S TO LOW 70S F SURFACE DEW POINTS, GREATER  
DIURNAL HEATING, 2000-3000 J/KG MLCAPE, AND 40-55-KT EFFECTIVE-SHEAR  
MAGNITUDES.  THOUGH WESTERLY SURFACE-WIND COMPONENTS WILL LIMIT  
HODOGRAPH SIZE SOMEWHAT, EFFECTIVE SRH OF 200-400 J/KG IS POSSIBLE  
IN THE WARM SECTOR, LARGER IN THE WARM-FRONTAL ZONE.   
  
STRONG-SEVERE THUNDERSTORMS SHOULD TRAIN ALONG AND NORTH OF THE  
FRONT AS FAR EAST AS COASTAL SC, UNTIL THE MCS ACTIVITY ARRIVES LATE  
TONIGHT INTO TOMORROW MORNING WITH A PREDOMINANT WIND/TORNADO  
THREAT.   
  
THE MAIN UNCERTAINTIES PRECLUDING GREATER PROBABILITIES AT THIS TIME  
ARE TWO-FOLD:  
1.  COVERAGE AND DURATION OF ANY SUPERCELLS IN THE FREE WARM SECTOR,  
ABOUT WHICH MOST SYNOPTIC AND CONVECTION-ALLOWING PROGS APPEAR  
SOMEWHAT RESERVED, IN THE ABSENCE OF STRONGER DEEP-LAYER/PRE-MCS  
FORCING, AND  
2.  MESSY NATURE OF THE DOMINANT STORM MODE:  WHAT MAY BECOME ONE OR  
TWO LARGELY WIND-DOMINANT MCSS SWEEPING EASTWARD, WITH OCCASIONAL,  
EMBEDDED, SHORTER-LIVED (BUT STILL POTENTIALLY DESTRUCTIVE) TORNADIC  
MESOCIRCULATIONS.  

[Wmsptwx]

Sounding like storm mode is big concern.

[Orangeburgwx]

Sounding like storm mode is big concern.

It is since the HRRR has a stick up its rear end and keeps mixing out too much low level moisture... Lot of people are going to be in for one hell of a nasty wake up call

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[Indystorm]

 URGENT - IMMEDIATE BROADCAST REQUESTED
   Tornado Watch Number 120
   NWS Storm Prediction Center Norman OK
   840 AM CDT Sun Apr 19 2020

   The NWS Storm Prediction Center has issued a

   * Tornado Watch for portions of 
     Western Louisiana
     East Texas
     Coastal Waters

   * Effective this Sunday morning and afternoon from 840 AM until
     300 PM CDT.

   * Primary threats include...
     A few tornadoes likely with a couple intense tornadoes possible
     Widespread large hail and isolated very large hail events to 2.5
       inches in diameter likely
     Scattered damaging wind gusts to 70 mph likely

   SUMMARY...Thunderstorms are expected to intensify this morning
   across east Texas and spread across the watch area through the
   afternoon.  Large hail is the primary threat this morning.  But
   increasing winds aloft will promote a risk of tornadoes by early
   afternoon.

   The tornado watch area is approximately along and 105 statute miles
   east and west of a line from 20 miles north of Shreveport LA to 55
   miles southeast of Angleton TX. For a complete depiction of the
   watch see the associated watch outline update (WOUS64 KWNS WOU0).
 

[MississippiWx]

3k NAM is coming in much different in regard to location of thunderstorms. It shows several storms/clusters across the warm sector in South MS/LA. 
 

 

[Quincy]

2 minutes ago, MississippiWx said:

3k NAM is coming in much different in regard to location of thunderstorms. It shows several storms/clusters across the warm sector in South MS/LA. 
 

 

Similar to the HRRR depictions. 

With regard to the western half of the risk area, pay close attention to how convection in East Texas evolves over the next 1-3 hours as it approaches western Louisiana. Even with a messy storm mode, you can get embedded tornadic supercells, but it may temper the threat somewhat. 

[Orangeburgwx]

Much different across SC, wonder why the change? 12z keeps the state clear of the SIG TOR

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[janetjanet998]

9 minutes ago, Orangeburgwx said:

Much different across SC, wonder why the change? 12z keeps the state cl

it looks like outlfows keep the best moisture south...

some models have two warm fronts...the one with the best moisture remains south until very late

if the model radar is correct then pattern recognition is for training outflow dominated sagging storms along the warm front with flooding and wind threats

the exceptions may be early over east Texas and in to LA before they form a blob and late tonight over GA

 

time will tell

 

 

 

[MississippiWx]

14 minutes ago, Quincy said:

Similar to the HRRR depictions. 

With regard to the western half of the risk area, pay close attention to how convection in East Texas evolves over the next 1-3 hours as it approaches western Louisiana. Even with a messy storm mode, you can get embedded tornadic supercells, but it may temper the threat somewhat. 

Yeah, as a resident of South Ms, I’m hoping the clusters get too messy and close to one another. However, I will be concerned if they can remain semi-discrete. 

[Orangeburgwx]

it looks like outlfows keep the best moisture south...
some models have two warm fronts...the one with the best moisture remains south until very late
if the model radar is correct then pattern recognition is for training outflow dominated sagging storms along the warm front with flooding and wind threats
the exceptions may be early over east Texas and in to LA before they form a blob and late tonight 
 
 
 

So you saying that the outflows will soare SC? or could there still be tornados?

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[Dunkman]

8 minutes ago, Orangeburgwx said:

So you saying that the outflows will soare SC? or could there still be tornados?

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You're in a hatched 10% tornado prob. Wouldn't put all my stock in a run of the NAM.

[janetjanet998]

3 minutes ago, Orangeburgwx said:

So you saying that the outflows will soare SC? or could there still be tornados?

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It will be a NOWCAST thing of course

you will have to wait until the low strengthens late tonight and pushes the front north

NAM nest finally gets the moisture into the area around 12z just in time

 

 

[MississippiWx]

Coastal convection this morning is interesting. It’s probably not enough to limit moisture transport today, but wondering if that’s the reason the HRRR has been showing lower dew points in the warm sector later today.

[Quincy]

5 minutes ago, MississippiWx said:

Coastal convection this morning is interesting. It’s probably not enough to limit moisture transport today, but wondering if that’s the reason the HRRR has been showing lower dew points in the warm sector later today.

No shortage of moisture sampled at LIX/Baton Rogue:

https://www.spc.noaa.gov/exper/soundings/20041912_OBS/LIX.gif

Dews around 72-73F along I-10 across Louisiana.