Convection filling in

Our forecasters have begun the day in Shreveport, LA, Jackson, MS, and Birmingham, AL. A PDS-severe watch has been issued for Jackson's entire CWA. In Figure 1, you can see ProbSevere LightningCast capture the developing convection as it "fills in" between more mature storms. With forecasters concerned with severe, tornadic, and lightning threats (for DSS), LightningCast can help forecasters quickly discern new areas of concern.

Figure 1: LightningCast contours, GOES-16 ABI daytime cloud phase distinction RGB (background) and GOES-16 GLM flash-extent density (orange-yellow foreground).

ProbWind in northern Alabama

At the HWT, forecasters working the warning desk in Jackson, MS noted an arc of storms in northern Alabama where ProbWind v3 was much higher than its v2 counterpart. They shared that the greater ProbWind probabilities and the fact that they received sub-severe LSRs, (~40 kt gusts) gave them more confidence that there could be severe-level reports soon. The NWS in Huntsville, AL issued a severe thunderstorm warning at 21:18 UTC and there were indeed trees down at 21:49 UTC, near Athens, AL.

Figure 1: ProbSevere v3 contours, MRMS MergedRef, and official NWS severe weather warnings for storms in northern AL.

Figure 2: ProbSevere time series for the storm highlighted in Figure 1.

ProbWind v3 produced greater and more consistent probabilities than v2 for this storm. A post-mortem analysis showed that the top-5 contributing predictors were:

1. MRMS VIL (23.6 J/kg)
2. 0-3 km lapse rate (7.6 C/km)
3. MRMS 0-2 km AzShear (7 x 0.001 /s)
4. MRMS 3-6 km AzShear (7 x 0.001 /s)
   
5. ABI+GLM intense convection probability (ICP; 69%).

The ProbSevere team has been in active discussions with HWT forecasters regarding explaining and conveying model predictions in AWIPS in near-realtime. 

Tornadic thunderstorms menace Mississippi

Figure 1: SPC 1630Z outlook with 06Z verification.

A deep shortwave trough and strong 850mb jet brought ample low-level moisture and instability to a well-sheared environment in the Southeast U.S. yesterday. The NOAA SPC issued "Enhanced" outlook noting the potential for strong tornadoes.

ProbTor captured the tornadic threats in Mississippi during the afternoon and then during a second round of storms in the overnight hours. The animations in Figures 2 and 3 show outer contours colored by the ProbTor value (inner contours are colored by probability of any severe), which were configured to only appear when ProbTor ≥ 15%. NWS forecasters can configure this threshold using these instructions.

In the first bout of storms, tornadoes were observed for storms in Simpson and Smith counties (see storm time series of predictors), as well as Yazoo and Holmes counties (see storm time series), and Leake county.

Figure 2: ProbSevere/ProbTor contours with MRMS MergedReflectivity and NWS severe weather warnings for 19Z -- 23Z.

During the second round of storms, a potent thunderstorm dropped tornadoes in Jasper, Clarke, and Lauderdale counties (storm time series).

Figure 3: ProbSevere/ProbTor contours with MRMS MergedReflectivity and NWS severe weather warnings for 02Z -- 05:30Z.

Strong tornadoes in the Deep South

A seasonally strong shortwave trough tapped into abundant Gulf of Mexico moisture forcing severe and tornadic storms across Louisiana, Mississippi, and Alabama. The Storm Prediction Center issued a Moderate Risk outlook with tornadoes and strong wind gusts being the primary threats (see Figure 1).

Fig. 1: SPC outlook with 06Z 12/17 verification (dots).

ProbSevere/ProbTor models show the evolution of storms throughout the afternoon (Figure 2). The outer contours represent the ProbTor probability and are only present if ProbTor is ≥ 15%, here.

This environment was characterized by 1000 - 2500 J/kg of MLCAPE, 45-60 kts of effective bulk shear, and 1-3km AGL mean wind of 40-55 kts. The ProbSevere models generally track and discern the most dangerous threats well (see the NWS warning polygons). However, there are several storms with erroneously high ProbTor values (outer polygons with high probabilities) that quickly appear and disappear during the animation. Most of these false alarms are due to spurious MRMS azimuthal shear values which are produced by noisy Doppler velocity data. Work is ongoing to mitigate these errors in ProbTor.

Fig. 2: ProbSevere and ProbTor contours, MRMS MergedReflectivity, and NWS severe weather warnings.

A storm that formed in east Texas/west Louisiana spawned numerous tornadoes and prompted a tornado emergency for Alexandria, LA at 18:41 UTC. This storm was in a primed thermodynamic and kinematic environment, with 250-300 J/kg of 0-1km storm-relative helicity. The low-level and mid-level MRMS azimuthal shear values increased the ProbTor probabilities from 30% to 91% in about 20 minutes. You can see the saved time series of attributes for this storm here. Figure 3 shows how the ProbSevere products evolved for this storm in comparison to NWS severe weather warnings and local storm reports.

Fig. 3: Time series of ProbSevere probabilities for tornadic storm in Louisiana. The bottom axis plots durations for NWS severe weather warnings and times of preliminary LSRs.

At least one this this storm's tornadoes was expected to be rated as significant (EF3+).