An isolated and long-lived thunderstorm developed along the Raton Mesa during the late afternoon of 13 September 2019 in an environment characterized by weak forcing but favorable SBCAPE (~1500 j/kg) and EBSHEAR (~35 knots). While probability was low that a storm would develop, any storm that did develop had the potential to be strong to severe. Surface southerly to southeasterly upslope flow onto the Raton Mesa and low level convergence with the deepening surface lee trough would be the drivers for initiation. Early day HRRR runs hinted at the potential for very isolated storm development in this area as well.

Clear skies prior to cumulus cloud field development and deep convective initiation (CI) allowed for the analysis of the split window difference early in the day. There were no obvious signs of moisture pooling or boundaries in the area of future CI. However, upon crunching a linear grayscale colortable, a subtle moisture maximum corridor is apparent along the northern edge of the Raton Mesa between KTAD and K4V1, east of KVTP. This would imply relatively high amounts of low level moisture, maybe low-level moisture convergence/pooling, and an opportunity area for CI. Additionally, there appears to be a surge eastward out of La Veta Pass (KVTP) prior to CI, possibly enhancing low level moisture convergence up the northern portion of the Raton Mesa. These signs, along with what the HRRR had been showing, should give a forecaster increasing confidence that this particular area probably has the best chance for CI.

Convergence along the leading edge of this surge is apparent in radar imagery progressing toward the moisture maximum just prior to CI.

GOES-16 5-min Sandwich RGB imagery provided some helpful insight during the evolution of this event. During the early stages, clumping cu were becoming more apparent northwest of KTAD after 20Z, with an orphan anvil developing around 21Z off of the easternmost towering cu (fig 3). This feature indicates a failed convective initiation attempt and that a capping inversion is still present but may be on the verge of breaking. Convection successfully initiated from the same area shortly thereafter.

After convective initiation, the IR component to the sandwich appears and rapid cooling is apparent. Pretty soon after CI, an overshooting top is diagnosed, and an associated above anvil cirrus plume shortly thereafter. It appears as though plume generation likely began between 2220Z and 2230Z, at least 15 minutes prior to Maximum Estimated Size of Hail (MESH) first exceeding 1″. One-minute imagery would have made detection and tracking of this feature easier. Figure 4 zooms in on the area of interest and ends on a time when both the OT and plume are obvious in the sandwich imagery.

GOES-16 sandwich RGB imagery for the full duration of the storm through sunset and with no annotations is included in Figure 5. The storm remained over rural areas so warnings were not verified. MESH algorithm indicated hail larger than 1″ in diameter for much of the time between 2245Z and 0055Z, with a peak over 2″ around 0030Z (end of the animation).

Bill Line, NWS