In association with a high amplitude closed upper low over the Mississsippi Valley, and a retreating wedge front at the surface, a mainly late night outbreak of wind damage and tornadoes occurred in a classic high shear, low CAPE (HSLC) environment, beginning late evening on May 4 and lasting beyond 8am on May 5. There were a few tornado reports earlier in the day farther south that may not have occurred technically in low enough shear to meet the original CSTAR-defined thresholds for SBCAPE and MLCAPE, but most if not all in northern SC, and all of NC and VA did. This review will just focus on a couple of these tornadoes that were rated EF1 and not on the EF0s or the numerous reports determined to be straight line winds.
Synoptic U/A maps:
Fig 1. 500 hPa Hgt/Temp 0000 UTC 05 May 2017.
Fig 2. 850 hPa Hgt/Temp/DewPt 0000 05 May 2017
Surface frontal and large scale radar evolution:
Fig 3. WPC surface analysis and radar mosaic, 0000 UTC 05 May 2017.
Fig 4. Same as Fig 3 but for 0900 UTC.
SPC summary of national severe reports:
Fig. 5. SPC reports from 1200 UTC 04 May 2017 – 1200 05 May 2017 (left); and same for 05 May – 06 May (right).
Tornadoes that occurred between 8pm (May 4) and 8am (May 5) almost all appeared to occur in HSLC environments based on the Sherbun and Parker (2014) definition (SBCAPE of 500 J/kg or less, MUCAPE of 1000 J/kg or less, and 0-6km bulk wind difference of 18 ms-1 or more), when eyeballing the SPC mesoanalysis regional images. Maps of the tornadoes (with all other reports removed) between 8pm – 8am are shown below.
Fig. 6. All tornado reports between 0000 UTC (8pm) 04 May 2017 and 1200 UTC (8am) 05 May 2017.
Fig. 7. Same as Fig 6 but zoomed in on northern NC and southeastern VA, and with the EF1 tornadoes highlighted along with time of touchdown and approx. path length labelled.
Following are some SPC mesoanalysis fields and radar images associated with the Rockingham Co NC EF1 at around 3am (the far SW tornado in Fig. 7 above):
Fig. 8. MLCAPE at 0700 UTC 05 May 2017. Approximate location of Rockingham Co NC EF1 indicated by purple star.
Fig. 9. Same as above but with 0-6km Bulk Shear Vector and magnitude.
Fig. 10. Same as above but for 0-1km Storm Relative Helicity (SRH).
Fig. 11. Same as above but for Sig Tor Parameter (STP).
Fig. 12. Same as above but for SHERBE parameter.
Fig. 13. Same as above but for Modified SHERBE (MOSHE).
Summarizing the above for the Rockingham Co NC EF1, the STP and SHERBE, while both showing underwhelming values in the location where the tornado occurred, at least indicated this was near the nose of a ridge for these parameters, but the MOSHE more clearly showed a maximum with values of at least 2.5.
Radar images for Rockingham Co NC EF1:
Fig. 14. KFCX Z/SRM images at 0703 UTC (about 10 min before tornado touchdown) near Eden. Top images are the 0.5 deg slice, bottom ones are the 1.3 deg slice. Storm is about 45 nm from radar, and radar beam at 0.5 is about 6,000ft AGL. Radar is to the northwest.
Fig. 15. Same as above but for 0711 UTC (about the time of touchdown near Eden).
Following are similar mesoanalysis fields as shown above but for 6am (was not able to capture 7am for most of these) and with the locations of the two Dinwiddie Co VA EF1s (a little south of Richmond) and the Orange Co VA EF1 (northeast of Charlottesville).
Fig 16. MLCAPE at 1000 UTC (6am) May 05 2017. Approximate location of the two Dinwiddie Co EF1s (southern-most purple star) and of the Orange Co VA EF1 (northern-most purple star).
Fig. 17. Same as above but for 0-1km SRH.
Fig. 18. Same as above but for STP.
Fig. 19. Same as above but for SHERBE.
Fig 20a. Same as above but for MOSHE.
Fig 20b. Same as above but for 1100 UTC (7am). [For some reason this was available at 7am but none the other fields were.]
Radar from Dinwiddie Co VA storm that produced two EF1 touchdowns, these are associated with initial touchdown. Radar configuration suggests more of a bow echo with circulation near comma head of bow, which was a common storm mode/configuration for several of the other tornadoes early this morning.
Fig. 21. KAKQ radar at 1045 UTC, about 6 min before EF1 showing circulation coincident with comma-head region of bow echo. This location is just under 40nm from radar. Upper left panel is 0.5 deg, upper right 0.5 SRM, lower right is 0.5 NROT, and lower left is 1.3 Z.
Fig. 22. Same as above but at 1051 UTC (right at time of tornado touchdown), and lower left image is now CC showing subtle tornado debris signature (just to the left of the “M” in McKenney).
Note that a second tornado (also EF1) touched down in northern Dinwiddie Co. less than 20 min later with the same storm, and this was just after the bow echo signature went through a “Broken-S” evolution, the circulation briefly tightened again, and another TDS was observed as well (not shown).
This is just a sampling of a few of the tornadoes associated with a classic High Shear Low CAPE (HSLC) environment, that were part of a nighttime outbreak of tornadoes and widespread severe weather on 4-5 May, 2017, and in association with a deep amplitude nearly vertically stacked upper-level trough and retreating wedge front at the surface. Most of the tornadoes (if not all) that occurred between 8pm May 4 and 8am May 5 appeared to form in environments that easily fit the HSLC criteria from Sherbun and Parker (2014). A closer near-storm environmental analysis using surface observations and modified RAP soundings may be needed for each of these tornadoes in order to confirm this.
A closer look at the EF1 tornadoes in NC and VA showed that while they occurred on the northern edge of ridges in the analyses of traditional composite parameters (such as STP, and even the SHERBE), the Modified SHERBE (or MOSHE) seemed to show a better signal at the location of these tornadoes with values well above 1.0 in most cases. The caveat here is that the time shown for most of the analyses (1000 UTC) is about an hour before the tornadoes that are overlaid with them, but yet the 1100 UTC MOSHE analysis (which was available) fits pretty well with the tornadoes that occurred around that time.
Radar analysis of the storms associated with these tornadoes showed very shallow reflectivity signatures (and in fact, lightning activity was generally non-existent with them). In fact, in most cases there were only subtle signatures suggestive of a tornadic threat , such as small-scale bow echoes and in one or two cases evolving through “Broken-S” signature. Velocity fields did show weak to moderate circulations with many storms before the tornadoes touched down, but including some storms that did not produce known tornadoes. Many of the tornadic storms were within 30-40nm of the nearest radar, but even so these signatures were often subtle, especially in terms of reflectivity structures.