ProbSevere in Montana

The generally quiet weather continued over much of the US on Monday.  Isolated convection developed over Montana in an environment marginally supportive for severe thunderstorms.  The SPC outlook probabilities for severe wind and hail were both less than 5% over much of Montana.  The NOAA/CIMSS ProbSevere model had probabilities in the 20-40% range from ~2220 UTC onward for a developing storm and then jumped over 50% at 2250 UTC (Figure 1).  The NWS office in Billings issued a severe thunderstorm warning for this cell at 2253 UTC.  The ProbSevere model probabilities continued to increase (to near 80% at 2310 UTC, Figure 2) as the radar estimated storm intensity increased.  The first severe hail report associated with this storm was received at 2335 UTC (1.00"; 1 mile SW Hardin, MT)--good lead-time for both the ProbSevere model and NWS warning.

Figure 1.  NOAA/CIMSS ProbSevere contours valid at 2250 UTC 5 May 2014 (no radar was displayed as the HWT radar domain was being shifted to encompass these storms.)

Figure 2.  NOAA/CIMSS ProbSevere and MRMS composite reflectivity at 2310 UTC 5 May 2014.  The ProbSevere output continued to increase to near 80% as the radar intensity increased.

-------UPDATE-----

Further west at 2328 UTC 5 May 2014, the ProbSevere model produced probability of 50% on a storm near Laurel, MT (Figure 3).  The storm continued to evolve with probabilities between 50 and 60%.  The storm was not warned, but a severe hail report (1.00" 2 miles S Laurel, MT) was received at 2341 UTC--13 minutes after the initial probability in excess of 50%.

Figure 3.  NOAA/CIMSS ProbSevere output and MRMS composite reflectivity highlighting the western storm near Laurel, MT valid at 2328 UTC 5 May 2014.

-Sieglaff