Prob Severe Qualitatively Helpful

With a low shear, low mid-level lapse rate environment today...ProbSevere has not been very high, and most storms have not been severe. However with the severe storm we did have...Prob Severe only topped out at 25%. This may seem like a poor performance however, when looking at ProbSevere qualitatively and not quantitatively a survey of the storms at the time showed every other storm less than 10 percent. That means that for the severe storm, despite having a low value of ProbSevere, it was still 2.5 times that of any other storm in the area. This still is useful information given the environment and actually added confidence to the warning decision because it was the best we had seen all day.

-Jason Bourne

Lone storm in Arkansas

One lone storm in southern Arkansas along a sagging cold front jumped in ProbSevere values (40%->50%->74%->90%), due to a strong normalized satellite growth rate, and a jump in total flash rate. This storm was just outside the SPC's wind and hail outlook.

Fig. 1: Strong ProbSevere values on an isolated storm in southern Arkansas.

Fig. 2: Our storm in southern Arkansas with the western edge of the SPC severe wind outlook (5%).

OHX: Severe Thunderstorm Warning

Our first warning for the day was issued northeast of the city of Nashville, where a jump occurred with the pGLM data. The SVR TSRA polygon was overlayed show how the lightning jump was used in the preceding scan.

 

The 4 panel below shows the Sig Probs only rose to 21%, but a distinct velocity maximum of 50-55 kts was occurring and the 1-min visibility nicely shows the strong updrafts with rapid cooling in the IR imagery.  This storm was was likely capable of damaging winds. 

-Yodamaster777

1 Min Visible Sat Shows Convection Developing on Outflow Boundary

We noticed an outflow boundary moving into our CWA from the Huntsville CWA to our south. Convection is developing along this line and is producing several cells that are generating quite a bit of lightning activity.

 
We issued a severe thunderstorm warning on one of the cells for Coffee, Warren, and Grundy conties that produced a significant lightning jump. The flash extent density value reached around 40. -JP

PGLM and 1-min Overshooting top HUN

Captured a good shot of 1 minute imagery with an overshooting top in the warning box. This was a good example of a line of storms moving to the northeast that all looked fairly equal for a while. Then there was this overshooting top that corresponded with a jump in the flash rate the corresponed well. In the end there was a report of quarter size hail with this storm but it was in a county that was in the far southeastern flank of the warning polygon. This did not line up with the storm but the blowoff from the storm is travelling to the southeast. It could be possible that hail from this storm was blown southeastward out of the storm and thus landing in the adjacent county. Or it could have been a suspect report. -Jason Bourne

NUCAPS theta-e Cross Section

We recently gained the ability to create cross sections through the NUCAPS swaths. This will be helpful for diagnosing phenomena such as boundaries and convective instability. The first image below is a plan view display of theta-e at 660 mb across the region. Obvious is the much cooler, drier air behind the cold front (low theta-e) with moist, warmer air ahead of it to the east (high theta-e). Also plotted is a line, denoting the location for which the cross-section (image below) was taken, through the cold front. The cross-section depicts theta-e vertically through the atmosphere. This provides another perspective on the cold front, which is obvious in the image.

PGLM performance in HUN

I issued a warning for a storm in a line in the NW part of the HUN CWA. The PGLM performed very well showing higher flash densities as the storm grew. Overlaying my warning polygon on the storm you can see how the flash rate was increasing and was highest at the point of warning issuance. After that time the flash rate decreased until the warning was cancelled. I was not warning based on the PGLM but it was a confidence booster and even though the storm did not end up going severe...it certainly helped the warning process in both the decision to warn and the decision to cancel the warning earlier. As a cosmetic comment I prefer the original color table but either one of the top two color scales would be fine. I do not like either of the bottom two as they are not an intuitive progression. - Jason Bourne

May 12, 2016: NUCAPS sounding

The NUCAPS sounding used today from MetOp IASI satellite turned out a bit more unstable than expected when modified with a SFC T=78F and SFC Dewpt=68F. The SBCAPE was 2681 J/kg which was a little on the high side compared to the RAP and LAP data.  I think the problem today is the midlevel lapse rates are much more steeper compared to the 12Z KBNA sounding (6.8C/km).  This is the sounding below.

pLGM Lightning Data

We are finally in a CWA with pLGM lightning data today and the network is actually working! Convection hasn't been all that impressive so far today, but we did have one cell that produced a pea-sized hail report. The report was from Davidson County. The pLGM lightning flash extent imagery showed a spike to around 30-40 as the cell entered Davidson County. The LMA_1 color table highlighted this jump the best, I thought.

We also noticed the ProbSevere algorithm jumped rapidly from less than 10% up to 80% in less than 10 minutes around this time. This jump seemed to be mainly triggered by the increase in lightning activity. Luckily, we did not pull the trigger on a warning based on the ProbSevere algorithm, because the storm weakened again pretty quickly after this. The shear environment is fairly weak and does not seem able to support anything other than pulse cells at this point.

- JP

HUN Mesoanalysis 5/12/16

Soggy environment in place in HUN this afternoon as almost 1.5 inches of PW is in place and LAP layered PW shows that this pretty much extends through the entire column but the wettest levels below 700 mb. This is confirmed on both a RAP sounding in the area and the NUCAPS sounding. After modification...NUCAPS did very well despite the amount of moisture in the column. A comparison of the NUCAPS and the RAP soundings are below.

Looking at a wide angle of 1-minute satellite data...a cold front can be seen pushing through Arkansas with the wind shift ahead of it approaching the Mississippi border. This line should be the focus of convection later this afternoon with a more linear structure as this is where the highest shear values are. Further east...storms firing are more instability driven at this time with almost 3000 J/kg of cape across the area. That being said...low level lapse rates are very strong but mid levels are very weak and storms are having a hard time gaining any height. With the freezing level around 11 kft...severe hail will be hard to get in this scenario.

Looking at storms on the radar now...a storm over north central Alabama has had a flash density of 60 flashes / 2 min. Short term models have this first batch of convection exiting the area after 21z and then another line with the front coming through after 23z. Main threat of the day will be wet microbursts with some small hail possible. Not much shear in place at the moment so any kind of tornadic activity seems unlikely.
-Jason Bourne