A seasonably potent, negatively tilted trough traversed the central U.S. with an embedded strong 850mb jet, spawning severe weather from Texas to Wisconsin.
Early in the day, storms surged through central Oklahoma, where NOAA's SPC had a 2% chance of tornadoes within 25 miles of a point (Figure 1). The tornadic nature of these storms was perhaps a bit of a surprise, as a tornado watch was issued at 13:50 UTC, after numerous twisters touched down in the OKC metro area. Structural damage to homes and overturned cars were reported.
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| Fig. 1: SPC Day 1 tornado outlook at 1300 UTC with tornado reports. |
The NOAA/CIMSS ProbSevere model (version2, v2) picked up on this tornado threat shortly before the first tornadoes touched down. In Figure 2, a new AWIPS-2 plug-in demonstrates how the probability of tornado product (ProbTor) is displayed with the probability of any severe. The outer contour is colored by the ProbTor value, whereas the inner contour is colored by the probability of any severe (i.e., the maximum of ProbHail, ProbWind, and ProbTor). Both contours use the same colorbar. The outer contour only appears if ProbTor is ≥ 3% by default, but this can be changed to any value by editing a menu file (note that this feature can also be turned off if the value is set to an impossible value, such as 101%). The developers hope that this outer contour will give forecasters a better visual and quantitative indication for a potential tornadic threat, while still being able to see the overall probability of severe.
In this line of storms, we can see that the two contours appear to have the same color, indicating that the ProbTor value is greater than both ProbHail and ProbWind. Note that by sampling the storm, you can still see the normal readout of probabilities and predictors for ProbSevere v2 (Figure 3).
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| Fig. 2: ProbSevere output, with ProbTor contours (outer contours), along with MRMS MergedReflectivity and NWS severe weather warnings. |
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| Figure 3: ProbSevere with sampling. |
Figure 4 shows the rapid increase in ProbTor from 22% at 13:20 UTC to 60% 6 minutes later. This was largely due to increasing MRMS azimuthal shear in an environment characterized by 45 kts of 1-3 km AGL mean windspeed. Interestingly, these storms had no lightning activity.
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| Fig. 4: Time series for ProbSevere v2 probabilities, with preliminary severe LSRs and NWS severe weather warnings. |
In the afternoon, storms spawned tornadoes in Missouri (Figure 5, Figure 6) and Iowa. A storm in Missouri exhibited large fluctuations of ProbTor before producing a tornado. The fluctuations in this case were tied to changes in the MRMS azimuthal shear. Sometimes noisy doppler velocity data contributes to rapid azimuthal shear increases or decreases, so forecasters should always be monitoring base velocity data as well.
In Iowa, numerous tornadoes were reported, but ProbTor was generally < 20% for many of them. Despite a conducive environment, azimuthal shear values were not very high for these storms.
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| Fig. 5: ProbSevere, MRMS MergedReflectivity, and NWS severe weather warnings for a storm in Missouri. |
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| Fig. 6: Time series of ProbSevere v2 probabilities for the highlighted storm in Figure 5. |
No tornadoes were reported in Wisconsin, but there were numerous wind and hail reports. The still image (Figure 7) and time series (Figure 8) show a strong satellite growth rate, moderate MRMS azimuthal shear, and a brisk 1-3 km AGL meanwind (~30 kts) contributing to a ProbWind of 53%, shortly before a barn and power lines were blown down in Iowa Co., WI. ProbWind later increased to over 60% and a tornado warning was issued (ProbTor = 3%), but no reports were received.
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| Fig. 7: ProbSevere, MRMS MergedReflectivity, and NWS severe weather warnings for storms in southwest Wisconsin. |
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| Fig. 8: Time series of ProbSevere v2 probabilities for the storm in Figure 7, in southwest WI. |
