Daylight Savings storms

As we mindlessly set our clocks ahead one hour, storms raged on in the southern U.S. A shortwave in the polar jet stream plunged southward over the Plains to force strong convection along a 500mb jet streak in eastern Oklahoma and western Arkansas. The NOAA/CIMSS ProbSevere model is moving toward hazard-specific statistical models using MRMS, GOES-ABI series, lightning, and NWP data. ProbHail, ProbWind, and ProbTor will be evaluated for a second year at the NSSL Hazardous Weather Testbed.

This storm northwest of Fort Smith, AR, shot up in ProbSevere value (the probability of severe is simply the maximum of the ProbHail, ProbWind, and ProbTor values) between 21:30Z and 21:40Z, going from 18% to about 50%. Golfball sized hail was recorded at 21:40Z. The increasing MESH (though still less than 1.0") spiked the ProbSevere value, while a jump in the total lightning flash rate increased the ProbSevere value from 65% to over 90% later on (see a time series of this storm's attributes in Fig 2). Interestingly, the 3-6km AzShear was very high before the lightning or MESH really took off, indicating strong mid-level rotation. The storm later recorded 2.0" hail east of Fort Smith. ProbHail had the highest probability for any of the models throughout the duration of the storm.

Fig. 1: ProbSevere contours, MRMS MergedReflectivityComposite for a storm in eastern OK / western AR. NWS severe weather warnings are also plotted.

Fig. 2: Time series of ProbSevere (max of ProbHail, ProbWind, and ProbTor) for the storm in OR/AR, with a subset of contributing predictors.

During the daylight savings time switch, another set of storms forced by a strong subtropical jet stream and moist southerly return flow from the Gulf of Mexico made their way from eastern Texas into Louisiana. The monster supercell in Figures 3 and 4 had a strong normalized satellite growth rate (4.1%/min observed by GOES-16) prior to 03:00Z. The growth rate brought the probability to about 20%, where the MESH and flash rate catapulted the ProbHail value from 20% to 90% in about 20 min. The storm had ProbHail and ProbWind exceeding 85-95%, recording numerous hail and wind reports, while ProbTor peaked at 20% (there were no tornado reports).

Fig. 3: ProbSevere, MRMS MergedReflectivityComposite and associated NWS severe weather warnings for a storm in east TX.

Fig. 4: Time series of ProbSevere (max of ProbHail, ProbWind, and ProbTor) for the supercell in east TX, with a subset of contributing predictors.