The 18 UTC run of the OUNWRF Wind Gust Speed for 21 UTC is depicted below.  From the model output, it appears that numerous wind gusts 30-40 kt will occur with a developing line of storms in Arkansas, just north of Little Rock.  Will it verify?  So far as of 2024 UTC, no significant wind gusts over 30 kt have been reported.

Update: There was a 50 mph report with downed power lines in Malvern southwest of Little Rock at 350 pm CDT (2050 UTC).  The OUNWRF Wind Gust Speed product suggested this potential for strong winds, however the location was way off.

Michael Scotten

The line of storms heading toward the DC area is showing up on the vLAPS model 800×800 grid. This is the forecast for max reflectivity at 19Z:

It shows moderate rainfall over DC itself, with heavy rainfall to the west. Max values on vLAPS (in red) were between 50-55 dBZ. At the time, the line of rain and storms was further to the west, and it was not raining in DC yet. However, some small convective showers were starting to  show up in front of the longer LCS line.

As you can see, the spatial coverage of the rain is way overdone on vLAPS at this point, and the line of convection is too far to the east as well. It might be the cloud cover that’s causing a problem for LAPS. You can see how heavy the cloud deck is on the rapid scan GOES visible imagery:

At some point, I’m not sure exactly when because I was on this side blogging :-)… but the model re-initialized and looked much better. This is the 20Z forecast for max reflectivity on vLAPS:

Notice how much further west the line of storms is in the frame above. It compares better with the actual base reflectivity:

… but now, it seems that the model has over-corrected. The storm line is now too far west by about 50 miles. Unfortunately, it looks like the vLAPS is not going to be initialized well enough to aid in severe forecasting today.

At our meeting earlier this afternoon, vLAPS developer Hong Le mentioned that it takes an hour or two for the model to completely switch over to its new environment once its coordinates have been moved. Was there still some bad data lingering in the system that caused the speedy FROPA earlier in the afternoon? Or, did the model need time to “catch up” to the new initial conditions in this 800x800km region? Or was there another reason altogether?

The top image below depicted a few marginal severe storms in western Arkansas,  The storm of interest is the one farthest west between the two other cells that were located slightly to the east.  ProbSevere gave this storm 9% chance of being severe in an environment with 898 J/kg, 27.4 kt of ENShear, MRMS MESH of 0.53 in, 1.27%/min normal vertical growth rate, and 0.03/in glaciation rate.

This storm produced quarter size (1 in diameter).  MESH underestimated the hail size in this case.  I (as well as the WFO Little Rock) did not issue a warning of this storm thinking the hail size was a bit smaller (closer to pennies – 0.75 in or nickels – 0.88 in).  The environment was characterized by the 12 UTC LZK (Little Rock) sounding below (bottom image) with a freezing level of 6579 ft AGL and -20C level at 15572 ft AGL, conducive for the development of hail.

The ProbSevere seemed to be too low in this case, only with a value of 9%. I would personally estimate the ProbSevere closer to 30 or 40% for this storm, especially considering that the reflectivities 16-17 kft AGL were 55-60 dbz (I typically use 60 dbz or greater at the -20C level as a proxy for issuing severe thunderstorm warnings for large hail.)

Michael Scotten

The CIMSS NearCast 900-700mb Precipitable Water product depicted a maximum of values 7 to 9 mm at 19 UTC on May 15 in southwest Arkansas (top image).  The next two images of this variable depicted at 22 UTC and 24 UTC have this minimum area moving east southeast into southeast Arkansas around 22 UTC then northwest Mississippi by 24 UTC.  I wonder if storm development will follow this minimum area for the next few hours.  If so, the NearCast forecast tool would be especially useful in near term forecasting (1-6 hr) and can give forecasters a heads up where and when convection occurs.

This post is very similar to the ThetaE difference post.

Michael Scotten

The CIMSS NearCast Vertical Theta-e Difference Mid-Low product depicted a minimum area of values -2 to -3 K at 19 UTC on May 15 in southwest Arkansas (top image).  The next two images of this variable depicted at 22 UTC and 24 UTC has this minimum area moving east southeast into southeast Arkansas around 22 UTC then northwest Mississippi by 24 UTC.  I wonder if storm development will follow this minima area for the next few hours.  If so, the NearCast forecast tool would be especially useful in near term forecasting (1-6 hr) and can give forecasters a heads up where and when convection occurs.

Michael Scotten

Here is an analysis of the OUN WRF and its ongoing performance.  It seems to be doing quite well with the ongoing convection!  As you can see in the picture above, there is a nice eastern line/area of convection that the model is picking up on quite well.  It has the storm mode correct too, with discrete storm development currently ongoing.  Will continue to monitor its performance through the day.

-Deitsch

Just a quick look at how the simulated satellite imagery is doing early this afternoon.  It appears to be doing pretty well!  It may be just slightly overdone with convection across Oklahoma, but it has a pretty good handle on the larger-scale features.  It definitely gives me some confidence in this model going forward through the day.

On Wednesday afternoon, a line of severe storms developed and began moving through western Pennsylvania. The LAPS 800×800 surface CAPE output was showing an interesting feature- relatively high CAPE levels (around 2000 J/kg; not bad for western PA).

30 minutes later, and the values in this area are progged to be very erratic, with strong gradients in CAPE developing along this same line.

The model output seems to produce a sort of streamline look, heading toward the center of this line. It sort of looks like a convergence zone. This is precisely the area that manifested as a line of severe thunderstorms. The first SVR for this line was issued at 22:54Z. This is what the radar looked like at about 23:30Z.

A whole bunch of convective cells, but oftentimes, you’ll get a line like this and it won’t turn severe. So it’s helpful to have guidance in the short term so see if these storms have true severe potential.

I also want to point out the storm cells along the PA/WV border, which had spawned severe warnings at the same time as the line to the north. The LAPS was showing high CAPE values (above 2000) in the vicinity of the developing cells, but the values were also high back behind the storm. So, the spatial coverage of the high CAPE values was too large to determine exactly where a strong thunderstorm would develop. But, you could see it on the super rapid scan GOES IR imagery:

The southern cell had a SVR issued about 10 minutes prior to the SVR for the northern line of storms. I noticed in later images of GOES IR (which I failed to capture… sorry) that the cloud tops had cooled significantly in the northern cell and a broad area of -60C cloud tops had developed.