Nocturnal storms wreak havoc in Tennessee

  A subtle 500-mb shortwave trough brought enough instability to pair with 60-70 kts of 0-6 km bulk shear to create some potent storms last night and early this morning. The Storm Prediction Center had issued a slight risk of storms, including tornadoes, for the mid-Mississippi Valley at 2000 UTC yesterday.

Fig. 1: SPC 2000Z categorical outlook with preliminary verification.

The severe storm activity began with one isolated storm that formed in southeast Missouri and traveled through Cairo, IL and southern Kentucky, dropping large hail. NOAA/CIMSS ProbSevere highlighted this storm early on, aided by a "strong" satellite growth rate from GOES-16 and a quickly-increasing total lightning flash rate, from ENTLN. The storm exhibited robust radar signatures by the time it was first warned.

Fig. 2: ProbSevere (contour) with MRMS MergedReflectivity and NWS severe thunderstorm warning (yellow polygon).

Below, we see the rapid increase in ProbHail and ProbWind around 22:00 UTC, while ProbTor increased much later in the storm's life (and produced a tornado report). You can see the ProbSevere predictor time series for this storm here.

Fig. 3: Time series of ProbSevere models' probabilities for an isolated storm in MO/IL/KY.

Numerous large hail reports (and several tornado reports, later on), were a result of convective storms in southeast Missouri.

Fig. 4: ProbSevere contours (outer contours are colored by the ProbTor value, present if ≥ 15%), MRMS MergedReflectivity, and NWS severe weather warnings.

Another storm formed in western Tennessee and was quickly warned by the NWS. This storm spawned several damaging and deadly tornadoes in Nashville and Cookeville, as well as hail up to the size of baseballs. ProbTor probabilities ramped up in response to increasing MRMS azimuthal (i.e., rotational) shear and total lightning density in an environment characterized by 40 kts of effective bulk shear, 50 kts of 1-3km AGL mean wind, and 400 J/kg of 0-1km AGL storm-relative helicity. More time series plots of ProbSevere predictors are saved here and here.

Fig. 5: ProbSevere contours (outer contours are colored by the ProbTor value; present if ≥ 15%), MRMS MergedReflectivity, and NWS severe weather warnings. 

Fig. 6: Time series of ProbSevere models' probabilities for a long-lived, deadly storm in Tennessee.

Fig. 7: Time series of ProbTor probabilities and constituent predictors for the long-lived, deadly storm in Tennessee.

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+).

Tornadoes in the desert

Early in the morning on the day after Thanksgiving, a long-wave upper-air trough with an embedded short wave disturbance and associated diffluent flow forced thunderstorms in the Phoenix, AZ region. There was enough low-level moisture return to provide adequate CAPE within a well-sheared kinematic environment, providing storm organization and maintenance.

ProbTor (from NOAA/CIMSS ProbSevere) captured the evolution of these storms, two of which spawned three tornadoes in the Phoenix metro area. The twisters uprooted trees, and caused damage to powerlines and roofs.

Fig. 1: ProbSevere contours (ProbTor is the outer contour), MRMS MergedRef, and NWS severe weather warnings. 

The tornado that traveled north of downtown Phoenix was rated EF1. From the time series below, ProbWind and ProbTor were about 30% when the NWS issued a severe thunderstorm warning. Then, ProbTor spiked to about 65% as the 0-2km MRMS AzShear increased markedly. The paltry lightning activity and weak 3-6km MRMS AzShear in this storm show that the rotation was shallow in the troposphere and this was not a supercellular storm. The 0-2km AzShear, along with very strong effective bulk shear and 1-3km mean wind helped the ProbTor values increase rapidly.

Fig. 2: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the northern tornadic storm. NWS warnings and preliminary local storm reports are plotted on the lower axes.

Fig. 3: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the northern tornadic storm. NWS warnings and preliminary local storm reports are plotted on the lower axes.

Further south along the line of storms, ProbTor values behaved in a more cyclic manner, first hitting 40% before dropping to 15%, then increasing rapidly to 30% (at the time of the first tornado LSR) and 60% (at the time of the second tornado LSR). ProbTor values then decreased to 20% and rebounded to 55%. This cyclic nature followed the 0-2km AzShear somewhat closely.

Fig. 4: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the southern storm. NWS warnings and preliminary local storm reports are plotted on the lower axes. 

Fig. 5: ProbSevere time series for ProbTor, ProbWind, and ProbHail for the southern storm. NWS warnings and preliminary local storm reports are plotted on the lower axes.

Fall severe weather outbreak over the Southern Plains

A strong, negatively tilted, diffluent short wave trough forced severe thunderstorms in the Southern Plains ahead of and along a potent cold front yesterday. Figure 1 shows a high-level evolution of the storms and ProbSevere v2 (PSv2) from discrete to more linear storm modes as the event proceeds.

Fig. 1: GOES-16 IR, MRMS MergedRef, ProbSevere storm contours, and NWS warnings.

One supercell that traveled through downtown Dallas, TX dropped a strong, EF3 tornado which produced much damage, which an NWS survey marked 01:58 UTC as the initial touchdown time. The storm went on to produce an EF1 tornado, starting at 02:36 UTC.


This storm was the right moving supercell after a left split (see Figure 2). Figure 3 shows the time series of PSv2 model output before the split, while Figure 4 shows the time series after the split, including NWS warnings and preliminary storm reports. The storm initially exhibited a strong satellite growth rate and a spike in MRMS MESH, which contributed to the rapid increase in ProbHail and ProbWind.

Fig. 2: ProbSevere, MRMS MergedRef, and NWS severe weather warnings in AWIPS2, depicting the storms affecting the DFW metro area.

You may find the time series of PSv2 model predictors for Figure 2 here and Figure 3 here. 

Fig. 3: Time series of ProbSevere models for a tornadic storm prior to it splitting. NWS warnings and preliminary storm reports are on the lower axis.

Fig. 4: Time series of ProbSevere models for a tornadic right-moving supercell after it split. NWS warnings and preliminary storm reports are on the lower axis (EDIT: tornado report times are the start times of tornadoes from an NWS survey).

An experimental convolutional neural network, which uses ABI channels 02 and 13, as well as flash extent density from the Geostationary Lightning Mapper, was deployed on this scene. The model produces an "Intense Convection Probability" (ICP). The 50% and 90% contours correspond well with robust satellite signatures, such as overshooting tops and enhanced-Vs. While there is also good correspondence with reports, probabilities of < 25% are present for some hail reports early in the event and some wind reports late in the event, showing that all severe weather is difficult to detect with a satellite-only approach. Regardless, such a model may be able to enhance ProbSevere, especially in regions with no radar coverage. See this CIMSS blog post for more information and examples from this model.

Convective Intensity Continues to Diminish in Eastern, ID

The Day Convection RGB generally shows weak to moderate convection persisting over eastern, ID.  There are occasional new updrafts that develop, but are short-lived.  Combining the satellite trends with radar reflectivity generally depict that the severe threat has become isolated over eastern, ID.

Day Convection RGB loop.

Convective Intensity Waning across Eastern, ID?

The Day Cloud Convection RGB still shows updrafts persisting over eastern, ID.  However, the anvils of the existing convection are spreading over the area and potentially inhibiting afternoon surface heating.  GLM data has also showed a diminishing trend in activity.   Note, the GLM data did not load for the last few frames.

Flash extent density overlayed on Day Cloud Convection RGB.

Tall cell in western GGW's area

Strong updraft in western GGW showed the red colors going over to greens...likely the updraft getting into the warmer stratosphere.
Day cloud phase distinction loop over the area shows a continued strong updraft with gravity waves radiating outward.



Quite a bit of flashes in that cell in GGW.

Charley

Storms Continue to track Northeast in Eastern, ID

The latest mesoanalysis shows values around 500 J/kg in portions of eastern, ID.  However, the NUCAPS modified sounding suggests that this could actually be in excess of 1600 J/kg by sampling a point in southern Fremont county.  Therefore, vigorous updrafts are expected to continue as these storms continue to track northeast this afternoon.


Latest SPC mesoanalysis of MLCAPE.


NUCAPS modified sounding.

Stronger Core

Continuing the SVR downstream, as yet another strong core pops up.  Colder cloud on IR and now prob severe is starting to jump up.  Blue color showing ProbHail and ProbWind in the 30s.
Cell also showing more signs of organization in the SRM...better chances for hail.



Also getting a pickup from the DMD (latency issue still for NMDA).



Charley