Wild weather weekend

Severe weather abounded this Memorial Day weekend, punctuated by a strong tornado in Dayton, Ohio. Tornadoes were observed in 13 states over the three days, from Idaho to Pennsylvania and from Texas to Minnesota. The Dayton storm emerged on the south end of a cluster of severe storms entering Ohio from Indiana (Figure 1), with nothing in its way from ingesting air parcels in an excellent environment (~1800 J/kg MLCAPE, ~55 kts eff. bulk shear, ~300 J/kg 0-1km SRH, ~40 kts 1-3km mean wind). This storm rapidly increased in ProbTor and produced its first tornado at 02:35 UTC, right after ProbTor exceeded 85%. The storm exceeded 90% ProbTor for almost an hour (see Figure 2), during which time the Dayton area suffered extensive damage. A tornado emergency was also issued for Dayton by the NWS at around 03:00 UTC. See the archived time series of ProbSevere products and predictors here and here (the storm merged with a storm to its north, prompting a change in ID).

Fig. 1: A cluster of storms with ProbSevere objects, MRMS MergedReflectivity, and NWS severe weather warnings. The Dayton storm is highlighted.

Fig. 2: Time series of the ProbSevere products in relation to severe LSRs and NWS severe weather warnings. 

A handy feature on the ProbSevere webpage is the "ProbSevere Accumulation" tab, which allows users to get a quick-look at the severe weather for a day and check out ProbSevere's performance. Once clicking the tab at the top, users can set the date using the UI on the left. Each day is considered a "convective day", starting at 12Z and going to 12Z the next day. For example, 2019-05-26 12:00 UTC is the start time and includes reports, NWS warnings, and ProbSevere objects from that time until 2019-05-27 11:59 UTC.

You can see the extent of the storms for the convective days 05-25, 05-26, and 05-27 in Figures 3 through 5. The green, blue, and red circles are preliminary LSRs for large hail, severe wind, and tornadoes, respectively. The orange and red polygons are NWS severe thunderstorm and tornado warnings, respectively. The white and pink boxes are centroids for ProbSevere objects at each valid time (every 2 min), colored by the probability of any severe value. In this way, a user or researcher can quickly see which areas ProbSevere 'hit' on, 'missed' on, or had 'false alarms' at the 50% threshold.

Fig. 3: An accumulation of severe LSRs (circles), NWS warnings (orange and red polygons), and ProbSevere centroids, colored by their probability of any severe value (boxes) for 05-25.

Fig. 4: An accumulation of severe LSRs (circles), NWS warnings (orange and red polygons), and ProbSevere centroids, colored by their probability of any severe value (boxes) for 05-26.

Fig. 5: An accumulation of severe LSRs (circles), NWS warnings (orange and red polygons), and ProbSevere centroids, colored by their probability of any severe value (boxes) for 05-27.

April and May have seen above average tornado activity nationwide, with the U.S. inflation adjusted number of tornadoes at 794 through yesterday, at about the 90th percentile for this time of year (Figure 6).

Figure 6: U.S. inflation adjusted cumulative annual distribution of tornadoes.

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.