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Jim Martin

Severe Weather Threat in the ArkLaTex - Sunday March 12

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Here we go...

 

00_NAM_069_33.13,-93.58_severe_ml.png

 

Posted the wrong time of sounding there. Here we go. Near Shreveport, Louisiana late Sunday afternoon off of the 0z NAM. My bad.

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A small, but fairly potent, shortwave transverses from the southern Plains toward the Mississippi Valley on Sunday. There has been good model agreement with the overall setup, with the Euro/GFS/NAM showing a corridor of 1000+ J/kg SBCAPE developing by afternoon from northern Louisiana into Arkansas. Despite cold temperatures aloft, there are signs of some capping to start the event, but height falls and steepening lapse rates should allow for thunderstorm initiation by mid to late afternoon. Storm mode looks to be discrete/semi-discrete early on with a large hail threat. The wind fields may support some damaging wind gusts too. Surface winds are not ideal for a tornado threat, however, the models suggest winds remain modestly backed (SSW to S) early on, particularly near the Mississippi River.

 

My thinking is that the initially isolated storms merge into a line segment or small MCS as they move into northern Mississippi and perhaps western Tennessee. Any tornado threat will depend upon low-level wind fields and how substantial the moisture return is. Right now, the consensus seems to be for mid-50s to near 60-degree dew-points in the threat area.

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I noticed the large hail threat as well Quincy given the lapse rates. Some of the analogs on the sounding I posted were interesting to note as well.

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The NAM has been stepping up this threat, gradually. 4km now squeaking into range and the setup is concerning for Arkansas. Let's see if GFS/Euro today are on board.

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The 12z 12km and 4km NAM certainly raise some concern.

 

12_NAM_060_34.03,-91.85_severe_ml.png

The NAM has been stepping up this threat, gradually. 4km now squeaking into range and the setup is concerning for Arkansas. Let's see if GFS/Euro today are on board.

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Euro has stepped up instability a bit in the warm sector, with 1000+ J/kg in central to southern Arkansas and initiation in roughly the 21-00z time-frame. Assuming the forecast doesn't significantly change, I could see this ending up as an ENH risk event. The Euro maintains a S (180) wind direction ahead of the storm system, while the 4km NAM shows SSE to SE flow in central Arkansas. The tornado threat could be a bit higher than I had initially thought, but it still appears isolated and not outbreakish.

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Where were you able to see the Euro parameters? 

Euro has stepped up instability a bit in the warm sector, with 1000+ J/kg in central to southern Arkansas and initiation in roughly the 21-00z time-frame. Assuming the forecast doesn't significantly change, I could see this ending up as an ENH risk event. The Euro maintains a S (180) wind direction ahead of the storm system, while the 4km NAM shows SSE to SE flow in central Arkansas. The tornado threat could be a bit higher than I had initially thought, but it still appears isolated and not outbreakish.

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00_NAM_048_35.38,-91.07_severe_ml.png

 

Atmosphere is starting to look ripe in Northeast Arkansas for Sunday Evening. While not pictured, the 0z 4k NAM is popping fast moving supercell thunderstorms across Central Arkansas Sunday Evening as well.

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4 km has a couple of really nasty looking cells ongoing at 00z there. Haven't seen it popping convective signals like that in awhile.

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In Southeast Arkansas Sunday Evening... Holy cow.

 

This sounding is convectively contaminated. Note how deep the moist layer is compared to other soundings in the area and also the SRH is much higher. It's a result of the model's response to convective mixing/overturning.

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From the Storm Prediction Center Day 2 Outlook issued a short time ago...

 

 ...ARKLATEX AND PORTIONS OF THE LOWER MS VALLEY...
 
   DETERMINISTIC GUIDANCE HAS SHOWN A SLIGHT NORTHWARD SHIFT IN THE
   TRACK OF THE UPPER SHORTWAVE TROUGH AND SURFACE LOW OVER THE LAST
   COUPLE OF RUNS. AS SUCH...THE MARGINAL AND SLIGHT RISK AREAS HAVE
   BEEN ADJUSTED TO REFLECT CURRENT TRENDS. OTHERWISE...FORECAST
   PHILOSOPHY REMAINS SIMILAR TO THE PREVIOUS DAY 2 OUTLOOK IN THAT
   ISOLATED TO WIDELY SCATTERED DISCRETE SUPERCELLS ARE FORECAST TO
   FIRST DEVELOP ALONG THE PSEUDO-DRYLINE FROM FAR EASTERN OK INTO
   WESTERN/CENTRAL AR DURING THE AFTERNOON. THESE CELLS WILL TRACK
   EASTWARD THROUGH THE AFTERNOON TOWARD THE MS RIVER AND INTO
   SOUTHWEST TN/NORTHWEST MS BY THE 00Z-03Z TIME FRAME. IT IS DURING
   THAT TIME...FROM CENTRAL AR INTO SOUTHWEST TN/NORTHWEST MS THAT THE
   TORNADO THREAT WILL INCREASE. HERE...BETTER LOW LEVEL MOISTURE
   /DEWPOINTS APPROACHING MID 60S/ WILL OVERLAP BACKED LOW LEVEL WINDS
   BENEATH NEARLY 50 KTS SOUTHWESTERLY MIDLEVEL FLOW...RESULTING IN
   INCREASING LOW LEVEL HODOGRAPHS AND LOWER LCL/S COMPARED TO FURTHER
   WEST. ADDITIONALLY...STEEPENING MIDLEVEL LAPSE RATES NEAR 7.5 DEG
   C/KM AND MLCAPE VALUES AS HIGH AS 1000-1500 J/KG WILL SUPPORT LARGE
   HAIL...SOME OF WHICH COULD APPROACH 2+ INCHES. HOWEVER...CONFIDENCE
   IN SIG HAIL IS LOW AT THIS TIME IN THE ABSENCE OF GREATER
   INSTABILITY.

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Could definitely see a few big hailers given the cold air aloft/steep lapse rates. If anything, there may be a very slight trend west/slower, meaning storms may initiate in eastern Oklahoma. Note that although parameters (SCP, EHI, CAPE) are more impressive on some guidance over Louisiana, initiation outside the far northern portion of the state appears unlikely based on lack of forcing and a warmer layer from ~900-800mb. The models, overall, are in very good agreement with a discrete or at least semi-discrete storm mode for several hours to start the event.

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Could definitely see a few big hailers given the cold air aloft/steep lapse rates. If anything, there may be a very slight trend west/slower, meaning storms may initiate in eastern Oklahoma. Note that although parameters (SCP, EHI, CAPE) are more impressive on some guidance over Louisiana, initiation outside the far northern portion of the state appears unlikely based on lack of forcing and a warmer layer from ~900-800mb. The models, overall, are in very good agreement with a discrete or at least semi-discrete storm mode for several hours to start the event.

 

Agreed with your thoughts. Louisiana wouldn't be chaseable regardless due to the catastrophic flood event underway. The slower trend may allow storms to mature as they approach the Little Rock Metro and I-530 down to Pine Bluff or Highway 167 up to Searcy. The terrain along/east of those points is much more chase-friendly. Instability seems to be trending up the past couple of runs. Winds at the surface should be out of the southwest with a quick veer to the southwest by 850 millibars. The hodo should have a decent 'kink' in the lowest kilometer. The discrete mode should promote an unusually impressive hail threat for early March. Any outflow boundaries or topographical effects will definitely help locally enhance tornado potential. I just hope storms will approach the chase-able terrain by sunset. FWIW Sunset in Little Rock is just after 7:15 PM tomorrow night. Probably will see a small ENH for hail in the SWODY1 tomorrow. 

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Sounding off the 18z NAM E of PBF.

 

nam_2016031218_030_34.17--91.83.png

 

I'm impressed by the vort max with this system that was evident in the 12z upper air analysis and also the PV+ anomaly evident in the DT fields (guessing we'll see quite a pronounced tropopause fold in the cross sections across the S Plains tomorrow morning/afternoon). The NAM's moisture fields don't seem to be that unrealistic this time around and frankly the Bunkers right motion there is pretty chaseable. Would probably set up somewhere in the LZK-SRC-SGT-PBF quadrilateral as it sits now, I like the prospects for classic supercells tomorrow given the strong bulk shear and less saturated profiles.

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Haven't had a ton of time today to analyze the runs, but this setup should yield a few tornadoes, and definitely large hail. The 18z 4KNAM has a massive updraft track just south of little rock. That shows a definite possibility for large hail. If the shear environment is as good as the nam advertises, it could result in tornadoes.

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Probably going to set up just east or southeast of Little Rock and hope the storms produce east of the Little Rock metro. Don't like the prospects of chasing storms around town during rush hour. Terrain is much better east too. 

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Yeah that 00z NAM run is really impressive. Looks like an increase in low level shear from 21z to 00z from previous runs with plenty of low level moisture for a tornado threat for that whole period.

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With that said, one potential limiting factor I am seeing could be the potential for this to grow upscale into an MCS as it moves into SE Arkansas. 4km NAM seems like it might want to hint at that somewhat. 

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To me, the threat is shaping up this way...

 

- Very Large Hail (2+ inches in diameter)

- Tornadoes

- Lesser threat of damaging straight line winds

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HRRR out to 16z SUN more or less lined up with the 00z 4km NAM. The general model agreement has been very solid, but now we'll see if the storm mode is as discrete/isolated as the models suggest. I suspect this could get upgraded to ENH risk, as mentioned before, a narrow significant hail probability zone may be added in AR.

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