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

Two Severes ongoing Fergus County

IR imagery from GOES-West shows good updrafts continuing to radiated waves outward from warned storms in Fergus County.



...but ProbSevere continues to run cold.  Unfortunately it will be very tough to verify my warning given the rural nature of the area.



MESH tracks followed nicely the cells in the warning...though primary thought was these were wind producers.  Still seeing very bright TOE on GOES-West with these cells.

Charley

MESH high ProbHail Moderate

At 2120z SFX radar depcited a potentially severe thunderstorm located SW of Pocatello.  At this time, nearly 50 dBZ returns were observed in excess of 35kft.  The -20C level was approximately 17kft.  The MESH readout in ProbSevere showed 1.59".  However, ProbHail was only 55%.  This appears to be a case where the ProbHail algorithm was reducing the probability based on the environment despite several aspects that support issuing in warning for severe hail.  A warning was issued for this storm at 2101z.

2120z SFX 0.5 radar.

EWX ProbHail Case

Had an interesting opportunity to watch a quickly intensifying cell relatively close to the radar.  While ProbHail did a decent job monitoring the trend in this cell, its (2 to 4 minute) latency in an environment conducive of rapidly intensifying cell could possibly lead to a reduction in Lead Time. By monitoring the cell via All-Tilts BR/BV/CC/ZDR and GOES-East Satellite Imagery via Mesosector, I was able to get roughly 5 additional minutes of lead time on my warning (ProbHail was less than 45% by warning issuance). A good reminder as to how the ProbSevere suite is best utilized as an additional tool, and not necessarily the deciding factor in warning ops, especially in freshly forming cells.



Quick update: The earlier warning did verify with quarter-sized hail. I did find some utility with the ProbHail when it came to making the decision to re-issue the warning or not, especially as the storm began to near the "cone of silence." Accessing the time series  which continued to show >95% ProbHail and a quick glance at a neighboring radar, I had enough forecasting confidence to continue the warning downstream.
---------------------------------------------------------------------------------------------------- Mountain Bone

Modified NUCAPS Sounding

Point in eastern TFX area shows higher CAPE on the latest RAP sounding from SPC, compared to the modified NUCAPS sounding. That area is popping now with new cells.

Charley

Potential Color Scale Change for GLM Products

An adjustment in the color scales for the GLM Avg Flash Area  and Minimum Flash Area to something of blue in the "cool" end to bright red in the "hot" end, signifying active and new convection, might help forecasters better interpret the data quicker.

SVR issued more because of previous reports

Had separate reports of 51 and 58 mph from earlier storms in the area.  ProbSevere wasn't very high in those cells, and neither was the lightning FED.  Given a high DCAPE environment, decided to pull the trigger a little earlier on some of the stronger cells going up.

Charley

All Sky LAP CAPE Too High?

Just prior to 20z the All Sky LAP CAPE shows 1000-1200 J/kg of CAPE across eastern ID.  However, most of this is in a cloudy or GFS layer depiction.  Looking at the GOES 16 derived CAPE, it only shows a narrow region a calculated data in eastern, ID that is about half (500-600 J/kg) of what is shown by the All Sky LAP.

All Sky LAP CAPE.

GOES 16 CAPE.

ProbSevere Seems to Handle Splitting Cells

Across the EWX CWA, we've had a number of splitting cells. The gif below includes 0.5 reflectivity with MESH & ProbSevere overlaid. As the cell begins to widen north-south, both MESH and the ProbSevere polygon elongate and split.

Convective Threat for Eastern, ID

The day convection RGB depicts stronger updrafts moving from northern UT to southern ID as of about 2030z.  This area is under a severe thunderstorm watch with no warnings in effect at this time.

Day Convection RGB loop.

Cell Near Twin Bridges, MT, still not severe, though growing

IR shows that cell as having the coldest cloud top.  Donovan heights are around 20 kft for 50dbz to get 1" hail.  This cell is near it, but ProbSevere is not too excited about it just yet.  MESH is under dime-sized and thus ProbHail is 7% now.  Will monitor for further development.  Issuing SPS for pea sized hail now.

Pocatello Storms

We're currently monitoring conditions in eastern Idaho. Right now I'm using various applications to analyze and initialize current conditions. Currently AllSky is showing cape around 1000kj, and PWATS around .80 inches.



That PWAT value on the morning sounding just west of the area was close to .64". Seeing how moisture levels are lower to west I can interpolate that the moisture values are similar to what's show on allsky. The allsky 900-700mb also seems to pick up on the relatively moist area in the lower half of the atmosphere.



Looking downstream we're noticing a line of developing convection using the Day land cloud RGB. It's helpful to analyze just how high the tops of the storms are getting combined with cloud tops.
Zooming in closer I can examine the few storms that have popped up more closely. The RBG help the differentiate between the ambient cirrus clouds from the growing storms. That might not have been as easy to see using a typical visible view.



The AFA and Minimum flash area maximums are hinting at growing convection just downwind. That growth leads to me to anticipate further growth as these storms move into a more favorable environment in eastern Idaho.

EWX Mesoscale Discussion

AllSky_LAP ML CAPE Forecast as of 1930z

19z Surface analysis shows the nearly stationary effective dryline boundary draped SW to NE across the CWA from roughly Del Rio through Briggs, TX. This boundary continues to play epicenter for semi-discrete super-cellular development this afternoon, with additional cold pool driven activity likely to form over the next few hours, especially as a back door cold front begins to slide southward through south-central TX. Latest AllSkyLAP CAPE analysis shows destabilization continues southeast of the dryline, with ML CAPE values nearing 3000 J/KG in and around San Antonio as of 19z. TPW values also remain relatively high southeast of the boundary, with 1.5 to 1.75 inch readings common place. Mid to upper-level kinematics remain favorable for some supercellular structures, so we'll continue to  keep large hail, damaging winds, and an isolated tornado threat in the forecast. Best chance for seeing a tornado (even with relatively week 0-1km shear values) will likely be through 7pm tonight in a cells that can remain discrete for awhile. Flash flooding concerns will continue thanks to the abnormally high PWAT values, especially in areas that see several rounds of convection this afternoon and evening.

----------------------------------------------------------------------------------------------------Mountain Bone

Lightning over the Mountains

Having the different lightning products from GLM as well as data from ENTLN allows you to pick out which cells have started producing lightning.  One flash up in northern WY on this image, the dot in the upper left, did show up in the FED data a couple minutes later.

Charley

Modified Sounding for RIW

Modified the RIW 12Z sounding for low 70s over around 40 in the southwest WY area, and got an MUCAPE of 655 over  A little higher than what AllSky is showing there...


...around 200 in the image above.

Charley

ProbSevere Values Affected by Shape/Size of Polygon

The following series of images shows the change in ProbHail/ProbWind values based on the size/shape of the polygon area. Initially, at 1806Z, ProbSevere was targeting the stronger cell to the south, indicating Hail/Wind 78%/85%. (At this point V was indicating around 20kt). At 1818Z, the northern cell intensified and ProbSevere grouped the two cells together, as a result, Hail/Wind both fell to 42%/61%. At 1827Z, after the southern cell split, the ProbSevere polygone became more filled with deep convection, thus Hail/Wind values increased to 59%/71%. Soon after, around 1834Z, ProbSevere polygons split and then indicated increased probs on the southern cell back to 69%/75% (Hail/Wind).

Incidentally, the larger the polygon, it becomes increasingly likely that ProbSevere values will decrease.