Location: Montana. Threat: severe hail/high winds. Soundings suggest potential for thermodynamically-driven downbursts over mountainous terrain. Question: will cloudtop cooling (CTC) rate be applicable to hail cores that could generate wet/dry microbursts?

CTC rates have been noted to have problems in mountainous regions such as the Rockies and even Appalachians. The rates were at or below 10C/15 minutes on the first few updrafts and only one storm briefly (one volume scan) reached 50 dbz through 20UTC. A storm entering Idaho County showed cooling rates of 12.6C/15 minutes at 2015 UTC, but this storm did not intensify until after 2130 UTC. This storm was not assigned a CTC rate after 2015 UTC because of masking by cirrus blowoff. Another storm that formed on the forward flank of this storm became marginally severe but was also never identified due to cirrus.

Possible sampling issues were noted in the MRMS sfc height of 60 dbz above -20C. The levels showed big scan to scan variability which may have been a function of distance from KMSO and sparse coverage of surrounding radars. As this storm got closer to the RDA, the scan to scan variability decreased.

SPC issued a severe thunderstorm watch at 21UTC with the main threats being large hail and high winds, with a secondary threat of a tornado.

– Ostuno

Despite moving atop an increasingly dry boundary layer (ref. to dewpoint readings in the mid to upper thirties), the strong to severe thunderstorms keep their strength. The GOES vertical theta-e difference (low-mid) product highlights the moist and unstable air mass nicely with 500-1000 J/kg (another product) present. In fact the thunderstorms really increased in strength when approaching and eventually crossing that plume of unstable air. For this event, the product by CIMMS helped a lot to get a feeling about how supportive the air mass stratification would be ahead of the thunderstorm despite decreasing boundary layer moisture. In the near future we should see a potential increase of a downburst risk with inverted-V profiles to the E/SE of the storm. A marginal hail risk continues with that storm.   Helge

Comparing the simulated satellite from NSSL-WRF/GOESR/CIRA on the right to the actual IR on the left at 23 UTC on May 12 depicts general good agreement.  The simulated satellite imagery overdeveloped isolated convection over southern Idaho, Montana, and Wyoming, and was a bit to quick/far to the east with the cooler/deeper moisture plume from far northern California to Alberta.  The simulated IR verified very well with post-frontal cold air cumulus depiction over western Washington and northwest Oregon. Michael Scotten

Finally have a storm with decent returns aloft that gives us a chance to investigate the new HSDA (Hail Size Discrimination Algorithm).  The image is a four panel from FSI.  Note the bottom left panel showing a cross section of the storm.  The red color is the usual dual-pol rain/hail mix signature.  The purple between 20 and 25 thousand feet is one of the new hail levels…in this case, indicating “large” (between 1 and 2 inch diameter) hail.  Of course, with the warm and dry surface layer across much of the area, this hail would likely have a hard time surviving to the surface.  We’ll see if we get any hail reports from this storm, but from what I saw to this point, I would be hard pressed to warn on this storm…perhaps an SPS for non-severe hail would be in order.

CL

We’re monitoring a few showers/weakly electrified storms over N-central Idaho. Using the cloud top cooling product, the focus for cooling was found to be along the northeastern fringe of that cluster. Using VIS imagery and keeping strong SW-erly shear in mind, the northeastward blowing cirrus shield (increasing in strength due to strengthening convective activity) caused the strongest signals for cooling (e.g. -20K/15min) along the NE fringe of that cluster. However, the most healthy looking cells eventually evolved just to the south of the cirrus shield, where some clearing occurred. It seems like this “messy situation” with rapidly clustering storms may mask local areas with better conditions for initiation.Reflectivity data beneath the cloud top cooling product.

Isolated to scattered high based storms are struggling to develop (with highest reflectivity cores 45-50 dBZ up to 25 kft MSL) as they move northeast across Idaho in a minima in 700-300 mb Precipitable Water Values (PW) according to CIMSS-NRW.  Wonder if the storms will intensify if they moved into higher PW values. Michael Scotten

Not expecting a gangbusters severe day.  The only area of real risk is across ID/MT today.  We are starting out in the Missoula CWA and I have been checking out the GOES-R SATCAST CI product.  There have been some weak cells already showing some modest CTC signals…that one in the upper right panel is around -10C/15 min.  The SATCAST product in the upper left has shown plenty of weak CI signals and a few stronger ones.  Of particular note are all the pink areas on the SATCAST product.  Those are “snow contaminated” areas…particularly widespread for this time of year!  I find it neat that one can use the SATCAST in concert with the visible image to discern the real signals from the snowy noise.  Continuing to wait for interesting storms.

CL