Autumn storms on the Plains

An energetic, negatively-tilted shortwave trough traversed the middle of the country last week, bringing several bouts of severe weather to the Great Plains. By visualizing the accumulation of the ProbSevere output (storm centroids ≥ 50% [pink boxes]), the NWS warnings (orange and red polygons), and storm reports (blue, green, and red circles), we see that the ProbSevere model handled the first event on October 4th quite well, at least qualitatively.

Figure 1: A toggle between the 12Z 10/4 ->12Z 10/5 accumulation of NWS warnings and reports, with overlaid ProbSevere centroids greater than or equal to 50%.

A very long-lived storm that approached the Norman, OK area produced numerous hail reports. It was first identified at 23:47 UTC, with a probability exceeding 60%, due to high effective bulk shear, MUCAPE, and strong satellite growth rates. The MESH was 0.24" and total flash rate was only 1 fl/min. After quick jumps in the MESH (to ~0.5") and the flash rate (to almost 20 fl/min), the probability exceeded 80%, and was promptly warned at 23:52 UTC. We can see a time series of the ProbSevere probability (thick red line) and constituent predictors in the time series below. Note that the satellite growth rates were both 'Strong', and expired a little after 02:00Z. We can also see when NWS warnings were valid and when severe weather was reported in the lower subplot.

Figure 2: Time series of ProbSevere and its predictors (top), and NWS severe weather warnings and severe weather reports (bottom). The axes on the right are associated with the time series on the top subplot, while the legend in the lower right is associated with the bottom subplot.

The stunning time lapse of this storm was captured by Jim Ladue (NOAA/NWS/WDTD) as it approached from the west. The range of this video is from about 23:52 UTC (when the storm was first warned) to nearly 01:00 UTC.




A second short-wave ejected through the Plains on October 6-7 (as Hurricane Matthew threatened the southeastern seaboard), bringing with it another bout of storms, and this time, numerous tornadoes. ProbSevere again handled most storms quite well, with few false alarms and a couple of missed wind reports.

Figure 3: A toggle between the 12Z 10/6->12Z 10/7 accumulation of NWS warnings and reports, with overlaid ProbSevere centroids greater than or equal to 50%.

We can see the evolution of the storms and associated warnings below, over Kansas, Oklahoma, and far northern Texas. The animation is from 18:30UTC to 00:00UTC every 10 minutes. The pre-frontal storms and those close to the triple-point (in north central KS) remain discrete longer than those which are forced by the cold front. The red, orange, and green polygons denote NWS tornado, severe thunderstorm, and flash flood warnings.

Figure 4: Animation of ProbSevere contours, NWS polygons, and MRMS MergedReflectivity from 20161006-18:30Z to 20161007-00:00Z .

September storms near the Great Salt Lake

A number storms formed in the early afternoon of September 22nd in response to forcing associated with the North American Monsoon over the Intermountain West region of the U.S. The image below shows the accumulations of NWS severe weather warnings, storm reports from SPC, and the centroids of ProbSevere objects attaining 50%+, at each time. Each accumulation is over the timeframe of 12Z on 9/22 to 12Z on 9/23. You can get a quick-look at how the model performed using these accumulations (the previous day accumulations are here). On this day, the NOAA/CIMSS ProbSevere model performed reasonably well, with high probabilities corresponding to numerous wind, hail and tornado reports. There were a couple of false alarms to the south east of the Great Salt Lake and a couple wind reports missed to its west and south.

Figure 1: Accumulations of ProbSevere objects, reports, and NWS warnings for 9/22/2016.

The storm that produced hail, wind, and the one the tornado reported initiated well to the southwest of Salt Lake City. The time series below of its probability and constituent predictors in ProbSevere demonstrates its evolution.

The probability of severe is the thick red line, with the scale on the left. The six predictors in ProbSevere have varying scales on the right. The NWP predictors of effective bulk shear and MUCAPE are the dashed black and brown lines, respectively. The MESH is solid orange, and the total lightning flash rate is solid green. The lifetime max normalized satellite growth rate and glaciate rate are depicted by the solid blue and dashed cyan lines, respectively. Both satellite growth rates use the blue scale on the right with nominal 'Weak', 'Mod.' (moderate), and 'Strong' designations.

Figure 2: Time series of ProbSevere predictors and severe probability value for a long-lived storm affecting the Salt Lake City metro area.

We see that the normalized satellite growth rate from GOES-West was strong at 18:50Z, while the probability of severe jumped to 15%. The jump in MESH in a high shear environment also helped to jump the probability up to 50% at 19:12Z. Increasing MESH and flash rate helped the probability climb to over 90% by 19:50Z. The first severe thunderstorm warning was issued at 20:09Z. Golfball-sized hail was observed at the Antelope Island Marina at 21:37Z. The tornado in the city of Ogden was reported at 21:45Z, and left thousands without power.

2015 North American monsoonal storms

The NOAA/CIMSS ProbSevere model was reprocessed with total lightning data for several days in the fall of 2015 upon request from the National Weather Service. This post recaps a few of the interesting storms from these days.

October 18, 2015

A cluster of storms affected the Phoenix, AZ metro in the afternoon/evening of October 18, with one storm intensifying and producing severe weather in downtown Glendale. This storm only had weak satellite growth, but the good total lightning flash rate (up to 40 flashes/min) and strong MRMS MESH (1.08") generated a ProbSevere value of 51% at 22:48Z, despite rather weak effective bulk shear. The first wind report was at 22:50Z. So there wasn't much lead-time at all for this storm from the 50% threshold, but the ramp up in probabilities could have signaled to the forecaster that this was a storm to watch (3%-->11%-->17%-->38%-->41%-->51%), as well as how much higher the ProbSevere value was than neighboring storms. The storm produced multiple large hail (up to 1.25") and severe wind reports.

Figure 1: ProbSevere, MRMS composite reflectivity, and NWS warnings for storms near Phoenix, AZ.

Further southeast, northwest of Tucson, a very small storm produced big hail (1" diameter) at 21:04Z. A maximum MESH of 0.89", moderate satellite growth rate, and very low lightning (0-1 fl/min) combined for a max probability of only 21%, at the time of the first report. The very low lightning combined with modest effective bulk shear (~25 kts) certainly helped to keep the probability of severe low. This example shows that more training with western U.S. storms is necessary for the ProbSevere model, especially as far as total lightning is concerned.

Figure 2: ProbSevere and MRMS composite reflectivity for a small storm near Tucson. This storm had nearly zero observed lightning flashes (IC or CG). 

Strong storms also erupted in southeast California this day, with strong satellite growth rates, good total lightning flash rates, and strong MESH values, all combining to produce probabilities in excess of 90%. Only one storm was warned despite the high MESH values (as high as 1.45"), but no reports were recorded from these storms in the Mojave Desert region of California. The MESH might possibly have been biased due to rather sparse radar coverage in this region.

Figure 2: ProbSevere, MRMS composite reflectivity, and NWS warnings for storms in the Mojave Desert.

October 6, 2015

Numerous storms developed in southern Arizona in the early afternoon of October 6th. One storm stood out southwest of Phoenix, with a good flash rate (32 flashes/min), and good MESH (0.92"), but no satellite growth rates. The ProbSevere value ramped up from 17% to 66% in 10 min (from 18:58Z to 19:08Z). The probability then hovered in the 40-50% range before a tornado was reported at 19:34Z. Though ProbSevere doesn't have any predictors explicitly for tornadogenesis, this case demonstrates that it can highlight a strongly developing storm to the forecaster, which signals the need for him/her to further investigate it.

Figure 3: ProbSevere and MRMS composite reflectivity for a storm southwest of Phoenix, which produced a tornado. 

Further southeast in Tucson, a storm exhibited moderate glaciation and normalized satellite growth rates, modest lightning (< 20 fl/min), and modest MESH (lifetime max was 0.58"). The shear and MUCAPE were adequate (~35 kts and 1000 J/kg, respectively). The ProbSevere predictors all pointed to a garden variety thunderstorm (max probability was 24%), yet this warned storm went on to produce two 1" hail reports and a wind report in Tucson. The MESH may have been underestimated due to the storm being near the radar, and thus possibly partially in the "cone of silence". The next closest radar is in Phoenix, with it's lowest tilt being over 8,000 feet at the storm's location (possibly higher, depending on atmospheric conditions). It's also possible the storm may have been shallow, as well, with MESH not being as representative. The SPC mesoanalysis archive shows that the melting level was relatively low (~2500 m), which in the future might help correct the MESH in shallow storms.

Figure 4: A storm near Tucson, AZ, which produced severe hail and wind.

Finally, later in the afternoon, a storm quickly intensified (went from 10% at 21:50Z to 70% at 21:58Z), heading toward Casa Grande, AZ, and was promptly warned. The increasing MESH and total lightning caused the rapid increase in probability. The storm began producing golfball and silver dollar sized hail at 22:10Z.

Figure 5: A strong storm picked up by ProbSevere heading toward Casa Grande, AZ.

September 14, 2015

A couple of storms developed near the Phoenix, AZ metro area on the evening of Sept. 14th, with one storm producing multiple wind reports (e.g., trees and power poles down) in downtown Phoenix. The MUCAPE and effective bulk shear parameters for the wind-producing storm were good (~2200 J/kg and 30-35 kts, respectively). At 01:00Z, a moderate normalized satellite growth rate and MESH at 1.01" combined to generate a probability of 47% (the max in its lifetime). The flash rate was 7 flashes/min. About 10 min later, the storm diminished markedly, as the MESH went below 0.1" and flash rate below 5 fl/min. The ProbSevere values were in the single digits when the storm first began producing severe wind reports. So unless the radar operator was paying close attention to the probabilities nearly an hour prior, ProbSevere may not have helped much in this case. That being said, development is underway to incorporate other NWP and radar fields to better predict wet-microbursts. For instance, the low-level lapse rates were very good in this region (as shown by the SPC mesoanalysis archive), which helps in momentum transport. The ProbSevere developers will be investigating many fields, including low-level lapse rates to better predict severe wet-microbursts. The low ground-based total lightning also didn't help. It's not certain whether this is a detection efficiency or a meteorological cause, but it underscores the need for more training for western U.S. storms.

Figure 6: ProbSevere and MRMS composite reflectivity for a storm affecting the Phoenix metro.

These cases show that ProbSevere can help highlight storms for forecasters to watch and further interrogate, and that forecasters must also continue to bear in mind data problems (e.g., sparse radar coverage, possible low lightning detection efficiency), as well as environmental factors not captured in the ProbSevere model (e.g., shallow storms). We hope the ProbSevere model will constitute another piece of useful guidance to the forecaster and compliment the warning process.