EWP Blog

A handful of severe thunderstorm warnings were issued for the Pittsburgh CWA by our team today given upticks in the satellite signatures. A pea-sized hail report came in at 1951z and this allowed us to calibrate the satellite signatures to the intensity of the storms. Thus, given the uptick in both Gremlin and Octane, we were able to realize that it would take a bit stronger of a signal to warrant large hail and potential a downstream warning.

Figure 1: Gremlin East CONUS reveals a rapid uptick in dBZ just before the pea size hail report at 1951z.

Figure 2: OCTANE already revealed CTD present in the part of the line that produced the hail report. Additionally, the cloud tops seemed to cool more just before the hail report.

– Aurora Borealis

In addition to my colleagues above, while in warning Ops today, it was noted as a group that after some initial calibration of what was going on weatherwise and looking at the Octane Products, the Octane products look to have at least done a slightly better job at noting which convection was just weak enough to not produce hail or wind reports while radar data might have struggled slightly more with pointing out wind and hail threats overall and what storms were the strongest.

Figure 3: Cells on the southern end of the map in the (orange circle) had more significant cloud top cooling but was much shorter lived and cloud top divergence was also slightly weaker. These storms failed to produce more than pea sized hail and no wind reports. While storms noted further north (pink circle)  while they had slightly weaker cloud top cooling with a more sustained cloud top divergence was responsible for some wind reports and potentially a tornado.

Meanwhile focusing on one of the radar related four panel products I tend to use in warning ops when differentiating between what storms to look at, the farther south storms, tended to have higher Vertically integrated Ice, higher MESH values, and overall looked slightly more structured than storms at the far northwestern periphery of the area. However, the further north storms are the ones that ended up producing winds reports which correlated with the better divergence aloft signal found in figure 1.

Figure 4: (Panel Contents from Upper left to Lower left: Upper left panel is RALA, upper right panel is MESH, lower right panel is 30 min rotation tracks, and lower left panel is Vertically Integrated Ice.

So overall it appears at least in this instance, the trends in upper level divergence as well as cloud top cooling may have been a slightly better indicator for potential severe thresholds of storm activity across the PBZ CWA.

-Sting Jet

This is intended as a downstream follow-up to the first post of this name, which dealt with the issuance of an SPS for a cell north of North Platte, NE (Fig. 1).

Fig. 1: 1935Z OCTANE Speed Sandwich (top left) and OCTANE Cloud Top Cooling/Divergence for no smoothing (top right), low smoothing (bottom left), and high smoothing (bottom right) around time of SPS issuance.

As the cell continued southeastward, cutting across northern and eastern parts of Lincoln County, NE, the storm did increase modestly in satellite presentation. The OCTANE Speed Sandwich (Fig. 2, top left) showed some signs of strengthening in the form of displaying higher winds (oranges), indicating a storm growing a bit higher and tapping into stronger winds aloft. The OCTANE Cloud Top Divergence and Cooling views (Fig. 2, same as Fig. 1) was also intermittent cloudtop cooling (greens) over the expected updraft location, indicating continued upward motion and updraft maintenance, and the cloudtop divergence was showing pink to intermittent red, indicating moderate to occasionally strong divergence.

Fig. 2: 2014-2032Z OCTANE Speed Sandwich (top left) and OCTANE Cloud Top Cooling/Divergence for no smoothing (top right), low smoothing (bottom left), and high smoothing (bottom right) around time of SPS issuance.

As shown in Fig. 3, the GOES-East CONUS GREMLIN output was somewhat underwhelming, and the GOES GLM lightning data shows no appreciable increase, but the 10.35µm IR imagery showed clear signs of higher cloud tops.

Fig. 3: 2014-2032Z GOES-East GREMLIN (left) and 10.35µm IR imagery with GLM Flash Extent Density overlaid (right).

While the signs of strengthening could have been used to justify increasing the size of the hail on a follow-on SPS over the initial half inch forecast, in this forecaster’s opinion it would have been difficult to justify upgrading to a Severe Warning at this time.

Continuing on down the line, we see (Fig. 4) the OCTANE Speed Sandwich product show cloud top wind speeds at least maintaining, if not increasing a touch further, indicating at least maintenance of strength, if not a bit of strengthening. The Cloud Top Divergence continues to show moderate (purple) to intermittently strong (red) divergence, and the Cloud Top Cooling continued to show occasional upward bursts in the updraft region. These would indicate the storm is likely at least worthy of a high-end SPS, and may be approaching severe thresholds.

Fig. 4: 2039-2103Z OCTANE Speed Sandwich (top left) and OCTANE Cloud Top Cooling/Divergence for no smoothing (top right), low smoothing (bottom left), and high smoothing (bottom right) around time of SPS issuance.

The GREMLIN simulated radar output from the same time (Fig. 5, left) shows fairly consistent moderately strong returns, in the high 40s dBZ to near 50dBZ. And understanding that GREMLIN tends to underestimate real radar returns somewhat and applying a mental adjustment, we can assume the storm is on the strong side, and appears to be more consistently strong. While the GLM never got very high, during this time period it does finally perk up somewhat and show modestly higher values, indicating a bit stronger updraft and more cloud ice and mixed phase precip aloft.

Fig. 5: 2039-2103Z GOES-East GREMLIN (left) and 10.35µm IR imagery with GLM Flash Extent Density overlaid (right).

Again, it might be hard to justify a warning at this time based solely on the satellite and lightning data and the at-best marginal modeled environmental data (no radiosonde sounding data available). However, at 2058z there was a public report of 0.88” hail (nickle-sized), just below severe thresholds. Combined with the data showing a decently strong storm in the satellite data, and the inherent increased uncertainty in dealing with just satellite and lightning data in the absence of radar data, in this forecaster’s opinion receiving a hail report just below severe thresholds likely would have been enough to justify putting out a severe warning. The office did issue an LSR for a report of 1” hail from an NWS Employee just east of Maxwell at 2106Z.

– Marko Ramius