April 1st storms fool nobody

Two regions of storms traversed the central part of the U.S. on Wednesday, forced by a cold front in IA/NE/KS, and in a region of moist, unstable air ahead of an MCV in the TX/OK/AR/MO area. Marginal and Slight risk SPC outlook regions corresponded with most of the severe hail and wind reports.

NOAA/CIMSS ProbSevere performed well in a number of cases where satellite growth was observed. The statistical model also tends do perform decently in the spring months, when melting levels are lower, and the MRMS MESH values are more accurate (perhaps due to less melting in the lower troposphere).

Fort Worth, TX had to contend with a slow moving storm, in a very unstable, moderately sheared environment (~3000 J/kg MUCAPE and ~18 kts effective bulk shear). While only weak normalized vertical growth rate was observed by GOES-East at 20:28 UTC, the probability shot up from 27% to 61% at 20:30, when a strong vertical growth rate and a moderate glaciation rate were observed. The MESH also increased to almost 0.75". The strong satellite growth was still evident despite some mid-level cloud cover, and in part thanks to the call for rapid-scan operation of GOES-East. The probability fluctuated a little bit as the MESH oscillated, but was first warned at 20:49 UTC. This storm would go on to produce tennis-ball (2.5") hail in the Fort Worth metro.

Further south, another storm popped up in Brown Co., TX, with moderate normalized satellite growth, and very strong MESH (1-1.5"). The first probability ≥ 70% came at 21:02 UTC, and 1" hail was observed at 21:25 UTC near the Brownwood Regional Airport. No warning was issued for this storm.

A small storm initiated quickly around 7pm CDT in northern Arkansas, in a moderately unstable, moderately sheared environment. Very strong satellite growth was observed at about 23:30 UTC, despite some high cirrus cloud from convection to its west. The probability of severe was 90% at 00:04 UTC, with MESH=0.90". The probability then fluctuated (as low as 39%, briefly) with the MESH, before taking off again. The first severe thunderstorm warning was issued at 00:44 UTC, with golf ball hail reported 20 min later.

One question we ask to forecasters is how can/do you use the probability information when it seems to fluctuate a bit. Most forecasters seem to prefer to see some sustained strength before issuing a warning. Though the type of storm environment may change that inclination, especially for environs with extreme instability.

Lastly, here is a loop of convection along the cold front in the central Plains. Moderate vertical growth was observed for these storms, whereas only weak glaciation was evident, owing to the mid-to upper-level cloud cover during initiation. Lead-times of 6, 10, and 0 minutes to the first NWS severe thunderstorm warnings was achieved by ProbSevere for these first three clusters of storms. In this case, ProbSevere may have been a handy tool to increase confidence for warning, and perhaps increase lead-time by a NEXRAD scan or two.

GOES-East IR brightness temperature with MRMS composite reflectivity, ProbSevere, and NWS warnings overlaid. Reflectivity less than 25 dBZ was masked out, to better see the satellite growth in the IR band.

John Cintineo
UW-CIMSS