It’s Jonathan Case reporting from the Hazardous Weather Testbed (HWT) Experimental Forecast Program (EFP) in Norman, OK. At the EFP this year, teams of researchers and operational forecasters are coming together to examine cutting-edge, convection-allowing ensemble numerical model output, unique diagnostic model fields, and experimental observation platforms to forecast convective initiation and severe weather events. Each day, the participants are divided into sub-groups with one group forecasting severe weather over a pre-selected focus region while the other group forecasts convective initiation and coverage in the same general sub-domain.
On Monday, the focus region was over eastern New Mexico and the Texas Panhandle. Model guidance was pretty good at indicating storms developing over the higher terrain of NM, and then conglomerating into a SE-moving mesoscale complex over the TX Panhandle after 00z 22 May. During verification activities Tuesday morning, an interesting feature was seen in the visible satellite imagery over the western Texas Panhandle in the afternoon and early evening hours. A small area had a dearth in cumulus cloud development a little east of the NM border, as highlighted in FIG 1. At the time of the visible image in FIG 1 (2245 UTC), convective storms began developing on the NW edge of this cloud-free “patch”, ahead of the developing convection in NM. This convection intensified with time, as seen in the radar image in FIG 2. In fact, a tornado was reported at 0137 UTC 22 May not far from this location (http://www.spc.noaa.gov/climo/reports/120521_rpts.html), as the convection on the NW side of the “patch” continued to intensify into an isolated supercell over the next 2 hours (not shown).
FIG 3 is a 500-meter true-color image from MODIS, centered over the TX Panhandle at 1729 UTC 21 May, before cumulus cloud development occurred on the periphery of the “patch”. One can distinctly see the darker brown color of the “patch” compared to the lighter brown color of the surrounding landscape. Further investigation reveals that the darker brown color of the “patch” correlated very well with high antecedent rainfall from the previous day (FIG 4; 24-hour rainfall of ~0.5″ to 2.0″, ending the morning before the event of interest).
What likely happened is that the previous day’s rainfall moistened the top soil layer over this patch, thereby reducing the overall surface albedo making the land surface appear darker in the MODIS true color image. Because more incoming shortwave energy was partitioned into evaporating soil moisture, the surface heated more slowly than the surrounding landscape. This differential heating appears to have led to a small-scale “sea-breeze-like” circulation, enhancing the cumulus cloud development on the periphery of the moist patch by late afternoon. Additional research is required to determine whether this moist patch ultimately contributed to the tornado occurrence.