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andyhb

March 27th-29th Severe Weather Thread

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Seems a good time to start this given we now have a D3 slight for the eastern Plains, Ozarks and Mid/Lower MS Valleys for Thursday. In addition, it appears Friday into Saturday could hold potential as well for the Arklamiss and Mid MS Valley/perhaps approaching the TN Valley later in the day as a strong pool of instability (1500-3000+ J/kg on both the 00z Euro and GFS) interacts with another low amplitude and progressive s/w trough. Cyclogenesis and resulting wind profiles likely won't be as strong as they are on Thursday, but they should still be enough to cause a problem given storm development.

 

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DAY 3 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0230 AM CDT TUE MAR 25 2014
 
   VALID 271200Z - 281200Z
 
   ...THERE IS A SLGT RISK OF SVR TSTMS THURSDAY AFTERNOON AND EVENING
   ACROSS PARTS OF THE LOWER CENTRAL PLAINS INTO MIDDLE MISSISSIPPI
   VALLEY AND OZARK PLATEAU...
 
   ...SYNOPSIS...
   MODELS CONTINUE TO INDICATE THAT MID/UPPER FLOW WILL BECOME
   GENERALLY ZONAL ACROSS MUCH OF THE U.S. BY THURSDAY...WITH A BELT OF
   WESTERLIES EMERGING FROM THE MID LATITUDE PACIFIC MOST PROMINENT. 
   WITHIN THIS REGIME...THE REMNANTS OF AN INLAND ADVANCING LARGER
   SCALE TROUGH APPEAR LIKELY TO CONTINUE EASTWARD ACROSS THE INTERIOR
   U.S...INCLUDING ONE SIGNIFICANT IMPULSE THAT MAY CONTRIBUTE TO A
   DEVELOPING SURFACE LOW OVER THE CENTRAL PLAINS BY 12Z
   THURSDAY...BEFORE TRACKING TOWARD THE GREAT LAKES REGION THROUGH THE
   REMAINDER OF THE PERIOD.
 
   GIVEN THE PROGRESSIVE NATURE OF THIS SYSTEM NOW EVIDENT IN MOST
   MODEL GUIDANCE...IT STILL APPEARS MOST PROBABLE THAT STRONGER
   MID/UPPER FORCING FOR UPWARD VERTICAL MOTION AND DEEP LAYER MEAN
   FLOW FIELDS WILL BEGIN SHIFTING AWAY FROM THE GULF COAST REGION
   BEFORE GULF BOUNDARY LAYER MODIFICATION AND INLAND RETURN FLOW IS
   ABLE TO YIELD A SEASONABLY MOIST WARM SECTOR.  BUT A WINDOW OF
   OPPORTUNITY DOES APPEAR TO EXIST FOR MODEST WARM SECTOR MOISTENING
   AND DESTABILIZATION...WHICH SHOULD SUPPORT POTENTIAL FOR SEVERE
   STORM DEVELOPMENT...PRIMARILY FROM THE LOWER CENTRAL PLAINS THROUGH
   THE MIDDLE MISSISSIPPI VALLEY AND OZARK PLATEAU...FROM MIDDAY
   THURSDAY INTO THURSDAY EVENING.
 
   ...LWR CNTRL PLAINS INTO OZARK PLATEAU/MID MS VALLEY...
   DISCRETE SUPERCELL DEVELOPMENT APPEARS POSSIBLE BY AROUND 18Z NEAR
   THE SURFACE FRONTAL LOW ACROSS PARTS OF NORTHEAST KANSAS/WEST
   CENTRAL MISSOURI...SOUTHWARD ALONG A PRE-FRONTAL DRY LINE THROUGH
   EASTERN OKLAHOMA...PERHAPS NORTHEAST TEXAS.  SURFACE DEW POINTS MAY
   ONLY REACH THE MID 50S TO AROUND 60F ACROSS MUCH OF THIS
   REGION...EASTWARD INTO THE MISSISSIPPI VALLEY...BUT AT LEAST LARGE
   HAIL AND LOCALLY DAMAGING WIND GUSTS APPEAR POSSIBLE AS ACTIVITY
   DEVELOPS EASTWARD.  AND TORNADOES MAY NOT BE OUT OF THE
   QUESTION...BEFORE THE COLD FRONT OVERTAKES THE DRY LINE AND
   CONTRIBUTES TO THE EVOLUTION OF A BROKEN SQUALL LINE WITH DAMAGING
   WIND GUSTS BECOMING THE MAIN THREAT BEFORE ACTIVITY WEAKENS THURSDAY
   EVENING.
 
   ...SOUTHEAST TEXAS INTO LOUISIANA...
   CONVECTIVE POTENTIAL IS A BIT MORE UNCERTAIN THAN FARTHER NORTH. 
   BUT A DISTINCTLY SEPARATE AREA OF CONVECTIVE DEVELOPMENT APPEARS
   POSSIBLE IN THE PRESENCE OF FORCING ASSOCIATED WITH LOW-LEVEL WARM
   ADVECTION BENEATH DIFLUENT AND...PROBABLY...INCREASINGLY DIVERGENT
   UPPER FLOW THURSDAY AFTERNOON.  LOW-LEVEL AND DEEP LAYER SHEAR WILL
   BE MORE MODERATE THAN AREAS TO THE NORTH...BUT SIZABLE BOUNDARY
   LAYER BASED CAPE APPEARS POSSIBLE IN THE PRESENCE OF BETTER MOISTURE
   INFLOW OFF THE GULF OF MEXICO. AT LEAST LOW PROBABILITIES EXIST FOR
   ISOLATED SUPERCELLS AND AN UPSCALE GROWING CONVECTIVE CLUSTER BEFORE
   ACTIVITY WEAKENS AND DEVELOPS EASTWARD THURSDAY NIGHT.
 
   ..KERR.. 03/25/2014

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A bit of a gripe with the D4-8 for Friday and Saturday, I don't really see any model indicating substantial Gulf convection except to the east, which isn't to the south of the main threat/unstable warm sector, and while the cyclogenesis is relatively weak, there have been a number of problematic MS Valley/western Dixie Alley events with a rather subtle shortwave trough like this. The Euro still puts a 50-60+ kt H5 max over the Memphis CWA at 00z Saturday, which would likely lead to fairly strong deep layer shear with substantial directional shear between 850 mb and 500 mb. This area is also located within the left entrance region of the ST jet streak.

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A bit of a gripe with the D4-8 for Friday and Saturday, I don't really see any model indicating substantial Gulf convection except to the east, which isn't to the south of the main threat/unstable warm sector, and while the cyclogenesis is relatively weak, there have been a number of problematic MS Valley/western Dixie Alley events with a rather subtle shortwave trough like this. The Euro still puts a 50-60+ kt H5 max over the Memphis CWA at 00z Saturday, which would likely lead to fairly strong deep layer shear with substantial directional shear between 850 mb and 500 mb. This area is also located within the left entrance region of the ST jet streak.

 

Sure 00z Saturday (Friday at 5PM central) has lots of potential. The GFS  and NAM both show plenty of CAPE in that area, the lower Mississippi Valley. Not sure why you are griping. The SPC just hasn't had the guts to issue a 30% risk for that day yet. That seems reasonable, I suppose.

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The 12z suite suggests a fairly good potential of a strong MCS Thursday mid day into the afternoon hours from near Austin extending ESE toward Houston and shifting E into Louisiana Thursday night. Locally we have been watching the MCS potential for about week. The trends suggest the current Slight Risk may be extended further S mainly for the damaging wind and hail threat.

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Sure 00z Saturday (Friday at 5PM central) has lots of potential. The GFS  and NAM both show plenty of CAPE in that area, the lower Mississippi Valley. Not sure why you are griping. The SPC just hasn't had the guts to issue a 30% risk for that day yet. That seems reasonable, I suppose.

 

Uh, I was saying some of the reasons they listed in there for a lower threat were a bit questionable.

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I'm seeing a strengthening trend with every run of the GFS/Euro for at least the last four model cycles for the 500 mb s/w on Friday, which, with the degree of instability that is in place, would likely lead to some problems.

 

The mid level lapse rates being suggested on Friday are impressive for east of the Plains, they are closer to dry adiabatic than moist adiabatic, which is rare once you move towards the MS Valley (obviously enhanced by a healthy EML that overspreads the area Friday morning and afternoon). 500 mb temps are very cold for a late March setup this far south, with -15˚C or lower temps overspreading a sizable chunk of the Arklamiss.

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Looks pretty likely that Friday will feature a decent risk for large hail/damaging winds over portions of Texas/Arkansas/Louisiana. The 12z ECMWF beefed up instability considerably from the 00z run and also has a more interesting UL config.

 

As Andy stated, the past couple model cycles have ticked instability up in this region, with some areas of 3500J of SBCAPE showing up on the NAM and ECMWF.

 

At this point, I'll take any kind of severe weather that we can squeeze out of this abysmal pattern. 

 

Should see a Slight Risk introduced on the next outlook:

 

 

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If anything, this setup would favour discrete convection given the low amplitude of the waves involved, even after dark.

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If anything, this setup would favour discrete convection given the low amplitude of the waves involved, even after dark.

 

Which day we talking about here? Each day or just Friday?

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Which day we talking about here? Each day or just Friday?

 

I'd argue both days, although it was mostly in response to Superstorm's post regarding Friday.

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Disclaimers about the NAM apply, but the 00z run is looking quite threatening for far Eastern OK into MO and AR on Thursday afternoon/evening. It shows a rather broad axis of 1000-2000 J/kg of CAPE with supercells initiating sometime between 21z and 00z and moving across a strongly sheared region on the west side of a 50+ kt LLJ. There is 60 kts of 0-6 km shear in place as well and across the instability axis as well.

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Disclaimers about the NAM apply, but the 00z run is looking quite threatening for far Eastern OK into MO and AR on Thursday afternoon/evening. It shows a rather broad axis of 1000-2000 J/kg of CAPE with supercells initiating sometime between 21z and 00z and moving across a strongly sheared region on the west side of a 50+ kt LLJ. There is 60 kts of 0-6 km shear in place as well across the instability axis as well.

 

Agreed. Very unfortunate location for chasing, but verbatim it would probably justify a 10% hatched tornado threat for much of the Ozarks. GFS is far less impressive, with the ECMWF straddling in between. Tough SWODY2 coming up.

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D2 essentially keeps the same look as yesterday for Thursday. Discrete supercells seem to be the favored convective mode, which is supported by guidance.

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The NAM 4km busts out some convection in central Missouri, going down to NW Arkansas, and even to SE Oklahoma. Looks like some of the thunderstorms could move over to St Louis at nighttime. This is tomorrow, 21z, 00z, and 03z forecast times.

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If there was better/quality moisture across E. KS/MO/S. IA tomorrow I'd probably chase, but <55F DP's isn't going to get me out the door.

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when will they roll out the Marginal/Slight/Enhanced/Moderate/High system of products?  April 1?  A few days after that?

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15% hatched on the new Day 2 over me. Not fun.

 

I saw that JoMo.

 

What is the timing of initiation tomorrow afternoon/evening?

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I saw that JoMo.

 

What is the timing of initiation tomorrow afternoon/evening?

 

After 3-4 PM I think. 

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Tulsa does not seem quite as bullish as the SPC on this one.  Question on this: Is the same criteria used by Tulsa and the SPC to derive the risk percentage depicted on their respective maps?

 

post-742-0-74174700-1395872817_thumb.jpg

 

post-742-0-74980700-1395873097_thumb.jpg

 

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Best overlay of CAPE and shear looks to be from eastern Texas into Arkansas. Here's a NAM-based forecast sounding for KASL. Not as much CAPE as further southwest, but shear has my attention.

post-533-0-78980200-1395881344_thumb.jpg

Below is MUCAPE/Shear from GFS:

post-533-0-74635900-1395881364_thumb.jpg

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Kansas City to St Joseph looks slightly more intense than southwest Missouri. Fortunately for the public this is only a 40/20 probability tornado watch either way.

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Overall threat today seems to be on the low end, and confined to areas near the front.

 

There's a fairly narrow axis of clearing/warmer temps/insitibility/higher DP's(Still <55F) near the front. Winds along the front veer to the SSW as well.

 

DP's in portions of the tor watch in C. Missouri are still in the mid-upper 40's.

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Updrafts having some problems becoming isolated. I would think this would be less of a problem later on to the south, if storm development occurs in E OK into NW AR and SW MO.

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