Severe storm outbreak in the Mississippi and Ohio River Valleys

A potent shortwave forced severe storms across the middle of the U.S. yesterday, wreaking havoc from Texas to Ohio. Below is the 850mb analysis from 00Z 04/04/2018 (Figure 1), showing well-defined warm and cold fronts emanating from the low, centered on Lake Michigan. We also see that moisture was well-established in the warm sector and the flow at 850mb was quite strong (30-40 kts).

Figure 1: 850mb analysis from 00Z 04/04/2018.

The NOAA/CIMSS ProbSevere model is experimenting with hazard-specific models: ProbHail, ProbWind, and ProbTor. The ProbHail model did fairly well to capture the initial hail threat along the cold front, with numerous storms exhibiting strong satellite growth from GOES-16.

These storms produced numerous hail reports between 1" and 1.5" in and around the Mark Twain National Forest.

Figure 2: ProbHail for storms in eastern MO, with MRMS MergedReflectivity and NWS severe weather warnings.

With sampling turned on in AWIPS2 (Figure 3), users of ProbHail can see the constituent predictors. Here, the strong satellite growth rate, high MRMS MESH (1.5"), and high Earth Networks® total lightning flash rate (40 fl/min) contributed to a ProbHail of 96%. NWP predictors of effective bulk shear, CAPE between -10C and -30C ("hail cape"), precipitable water (PWAT), and the lowest height of the wetbulb 0C isotherm are also used as inputs in ProbHail.

Figure 3: ProbHail readout for storms in eastern MO, with MRMS MergedReflectivity and NWS severe weather warnings.

Due to very favorable kinematics, the tornado threat was rather high, with the Storm Prediction Center issuing a wide swath of 10% probabilities for tornadoes within 25 mi of a location for their 2000Z outlook (Figure 4).

Figure 4: SPC tornado outlook and verification for 20Z and 12Z, respectively.

A storm that produced numerous reports of significant wind damage in the Dayton, OH metro area also produced a tornado east of the metro. ProbTor captured it's development well (Figure 5).

Figure 5: ProbSevere, MRMS MergedReflectivity, and NWS severe weather warnings for southwest OH.

The time series of this storm's attributes demonstrate that very high MRMS 0-2km AzShear (>20 * 0.001 /s), along with SRH of 300-400 m2/s2 in the 0-1km layer helped spike the ProbTor value to over 90%. In this case, the tornado report came slightly after the time of peak ProbTor. Several wind reports in the Dayton area were significant (e.g., "large debris cloud", "structural damage to homes"), around 20Z when ProbTor was in the 30-50% range. Thus, it's possible there may have been a tornado earlier (a NWS survey should confirm the presence or lack thereof).

Figure 6: Time series of ProbTor (red curve) for this storm with its predictors. The bottom axes show NWS warnings and reports.

The "ProbSevere" value simply uses the maximum of ProbHail, ProbWind, and ProbTor. For this long-lived storm, each model was the maximum at different times. Regardless, the ProbSevere values were mostly over 70% when there was reported severe weather. The MESH and lightning (pink and green curves) show the cycling of this storm nicely. The storm produced hail and wind reports in the Columbus, OH metro and later east of the metro.

Figure 7: Time series of ProbSevere (red curve) for this storm with its predictors. The bottom axes show NWS warnings and reports.

Storms later evolved into big wind-producers, evidenced by these storms in the Nashville, TN area. There were many reports of trees and power lines down in the metro area around 0200-0230Z.

Figure 8: ProbWind, MRMS MergedReflectivity, and NWS severe weather warnings for storms affecting the Nashville metro.

 ProbWind was in the 50-70% range for these storms, owing to a strong MeanWind in the 1-3km layer and moderate 0-2km MRMS AzShear. Note that the ProbWind readout in AWIPS2 now has a "weak", "moderate", and "strong" designation tag for the MRMS AzShear fields (e.g., Figure 9)

Figure 9: ProbWind readout for a storm south of Nashville.

At the end of each day, we make an "accumulation" product, plotting the centroids of storms colored by their ProbSevere value (or ProbTor value), along with the NWS severe weather warnings and SPC preliminary reports. From Figure 10, we see that ProbSevere corresponded with severe weather warnings and reports quite well in MO, AR, IL, IN, OH, KY, PA, and TN, and parts of TX and LA. However, many wind reports were missed (at the 50% threshold) in MS, AL, and GA. Many of these storms had meager reflectivity and lightning flash rates, and no discernible satellite growth. ProbWind was largely 10-30% for these storms, a small improvement over the legacy "all-in-one" ProbSevere product, which had < 10% for most of these storms. We are still working to improve upon probabilistic prediction of storms in this regime, and are open to ideas from the field.