Mike Smith and I tracked a convective cell cluster that was developing in the Allegheny Mountains of Virginia and West Virginia on Tuesday afternoon. We used the rapid scan GOES imagery to get a good look at developing overshooting tops and we were also looking for boundaries that could produce strong downbursts. At 1830Z, or 2:30pm local time, the surface observation data shows a buoyant environment to the east of the developing storm cells. The wind barbs also show a southeast flow in this area, and the obs to the west of the clouds were showing westerly and southwesterly winds. We were not able to look at radar data today in AWIPS as a verification tool, so we pulled up radar from the NWS’s site and looked at severe warnings as they were issued. This first image is from 1830Z, when the developing Cu aren’t looking too dramatic just yet. However, the GOES-R convective initiation product is showing a 90% probability for the middle Shenandoah Valley, indicated in red.
We decide to keep an eye on this area as the GOES-R CI was showing a strong signal for further development. In particular, Mike was looking for an outflow boundary to develop on the east side of this convective cell, which would produce a short-lived severe wind gust event. I was also following the northern edge of the clouds, in the area of the WV and MD panhandles. There was a large area of 60%-70% probability just to the northeast of the cloud. The cloud seemed to be growing in this direction, and I felt that having such a large area that the GOES-R CI was signaling on sort of increased the chance that something might happen in that area. I know that’s not the way the model is supposed to work, but larger spatial coverage just triggers higher confidence in my mind. So, I kept an eye on that area, as well.
Just 6 minutes later, at 1836Z, the CI product crashed down to an 8% reading for the cloud cluster in the Shenandoah Valley.
I wondered if this was an erroneous reading, or if the mountainous terrain might be challenging for the GOES-R CI algorithm. I know that the erratic percentage values are of particular interest in this experiment, so when I noticed that sudden change, it caught my attention. In the meantime, the yellow shading on the northern flank of the clouds has actually expanded in scope.
Skipping ahead about 45 minutes, the CI value has jumped back up in the cell in the Shenandoah Valley.
Just about 20 minutes later, a Flash Flood Warning was issued right over this area of erratic CI values. Later, it was confirmed via storm report that a creek overflowed its banks in the area. In the meantime, notice that the CI product has disappeared in the other area of interest in the WV/MD panhandles. At the same time, a SVR was issued for counties in West Virginia that are under that convective cloud shield. Later, a report of quarter-sized hail came in from the warned area, in Washington County, MD.
Obviously, the convective initiation product was not useful for the SVR development in WV because there was already convection happening in the warned area, hours before the storm became severe. It would have been interesting to see the ProbSevere product in this situation, to see if it picked up on any of the severe parameters, including MESH. Hindsight is 20/20!
The image above is a screen capture of radar and lightning data centered over the OHX (Nashville) CWA at 21:54z. Today we evaluated the Lightning Jump Detection Algorithm since the OHX CWA was within coverage of the Northern AL Psudo-Geostationary Lighting Mapper.
