Severe Thunderstorms North and South of Rapid City

Clusters of severe thunderstorms developed north and south of Rapid City, SD. Eventually, a line of thunderstorms formed once the clusters began to move east-northeastward. The line filled in between the clusters and became severe at times, although the storms wee persistently severe at the northern and southern ends of the line.

The ProbSevere data clearly highlighted the hail threat, with the wind threat also increasing as the line formed and a well defined outflow moved east of the storms. I did notice an increase in the ProbTor percentages for a while with the northern cluster; subsequent analysis showed a brief period of significant rotation at about 6000 feet AGL with lesser rotation on a few scan levels above the 0.5 degree scan. The rotation weakened in about 15 minutes, and the ProbTor levels fell quickly. No tornado was reported nor was a warning issued.

GLM lightning data showed definite flash clusters near the strongest updrafts and overshooting cloud tops, although the parallax in western South Dakota is pretty significant from GOES-16, so I had to use the existing lightning network data and NEXRAD data to get a better idea of storm placement. It will be interesting to see how the eventual GOES-17 data would improve things in that part of the country.

Gary Cannalte

ProbSevere way too large

We talked briefly about using the MRMS isothermal (-10C, I think) reflectivity to differentiate cells for the ProbSevere algorithm. This would certain improve the smearing of echoes that occurs when storms are nearby. In this particular example, At least 4-5 separate updrafts were observed but the ProbSevere included all as one cell, basically covering much of 2 counties, plus a few more. Certainly the algorithm is useful for the strongest storm, but it doesn't provide much spatial value like this. I feel a longer processing time is useful if it can prevent this from occurring.

Fig 1. Multi-county ProbSevere on a non-linear cluster of storms.

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Local "Higher" Flash Extent Density on Low Lightning Day

The majority of the storms across the AMA forecast area today did not have very impressive lightning signatures/numbers on GLM, despite having storms with hail reported as large as half dollars. Most of the storms were slow developing intense cores. However, starting around 2232Z, one storm rapidly intensified and developed a forward speed more than double the average of storms so far today. During this ramp-up in intensity, the Flash Extent Density also spiked up. While not an especially high value (max 14 flashes on 2 minute sampling/1 minute update product), it was the most impressive seen today.

0.5 Degree AMA Reflectivity 2222Z-2253Z

GLM Flash Extent Density (2 minute sample/1 minute update) 2232Z-2254Z

Alex Brown

Flash Extent Density and Flash Centroids

Here's another comparison of GLM flash extent density with ENI flashes and pulses. GLM parallax displacement is again apparent with the line of storms extending southwestward across western SD into eastern WY. Allowing for that, there is good general agreement between the two data sources regarding lightning location and concentration. The GLM FED clearly gives a more complete picture of the area extent of lightning.

The next image shows GLM flash centroid density (blue boxes) instead of FED. Most of the values are 1 indicating a single flash centroid. (This product is probably not destined for operations.) Interestingly, these GLM flash centroids match up fairly well with the ENI flashes and also with the MRMS RALA reflectivity cores. A.Cope

Supercell SW of Rapid City

Thunderstorm development began shortly after we began. Cells north of Rapid City became severe, with hail as the main threat.

Shortly after 2000Z, a supercell thunderstorm developed rapidly along the Wyoming/South Dakota state line. Visible satellite imagery showed a nice flanking line of towering cumulus feeding into the southern part of the supercell, and the storm was showing signs of rotation in the lower levels, although the radar beam was at about 6500 feet AGL, and about 10,000 ASL.

In monitoring the 2-minute (1-minute update) GLM event density lightning, there was a noticable increase in flash events in the 10 minutes or so leading to where low-level rotation was observed, with a quick drop off in lightning from that point on for the next 20 minutes as the storm probably began producing large hail.

In checking the ProbSevere products, the hail showed a large increase into the 85% range (the MESH hail size was around 1.25") and a smaller increase into the 30% range of wind. Interestingly, even with the low leve rotation and some evidence of a hook echo, the ProbTor probabilities stayed less than 2%.

Gary Cannalte

AllSky SFC-900mb PW Nicely Captures Dryline

The AllSky SFC-900mb PW nicely captured the eastward advance of the dryline into the AMA forecast area this afternoon.

Alex Brown

CI Probs Along WY-SD Border

Another example of the GOES-16 general convective initiation probabilities. Loop below shows CI probs overlaid on VIS imagery. From 1700Z to 1900Z, storms developed N-S along the WY-SD border. Prior to deep convection developing, the CI probs were no higher than about 30 percent. Seems like they should be higher than that. A. Cope

NUCAPS Lapse Rates Not Steep Enough

The afternoon NUCAPS 700-500mb lapse rates were about 1-2C/km too low across the AMA forecast area this afternoon when compared to models. This was also confirmed by the 18Z AMA RAOB.

19Z NUCAPS 4 Panel 700-500mb Lapse Rate Procedure

18Z AMA RAOB

Alex Brown

Parallax error on CWA border

We are all familiar with the parallax issues that arise with the satellite data. Sometimes that error can be in a location that may confuse the forecaster if this isn't in the front of his/her mind. I posted this mainly as a suggestion that we really hit the parallax issue hard in the training for operational staff, especially since lightning location is a significant aspect of IDSS.

Fig 1. Storm located over Parmer County, TX (in LUB CWA). GLM indicates lightning extent into southern Deaf Smith County (AMA CWA) due to parallax issue.

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Terrain showing in Layered PWAT

The image below shows the AllSkyLAP total PWAT and lower/upper layer PWAT, plus topo image in the lower left. Interesting that higher terrain, such as the Black Hills and mountains of CO/WY, can be seen in the PWAT images, even the upper-level 700-300mb image. A.Cope