National Weather Service United States Department of Commerce

Overview

A rare nocturnal hail event impacted El Paso County on Wednesday June 13th, 2018.  The origins of the storm stem from storms that occurred earlier in the evening, near the border of Colorado and New Mexico/Oklahoma. The storms produced an outflow boundary that traveled up the Arkansas River Valley, through Colorado Springs and finally up Cheyenne Mountain where the severe thunderstorm developed. The outflow boundary seemed to travel directly up Cheyenne Mountain, which makes it an efficient precipitation producer.  The air mass before the outflow boundary propagated over recorded an air temperature of 72F and a dew point temperature of 45F at Colorado Springs Airport (KCOS), and the air mass after the outflow boundary propagated over recorded an air temperature of 69F with a dew point temperature of 52F at KCOS, as well. To put in perspective how moist the air is, the average maximum precipitable water (PWAT) for June 13th in Boulder, Colorado (the most representative point available) is 1.01”, whereas the resolved PWAT value over the thunderstorm initiation point was 1.1”.  The Theta-E maps attached in the “Environment” section does a good job illustrating the origins of the moisture. MUCAPE, which happened to be SBCAPE, was around 2000 J/kg during initiation, due to mid-level lapse rates being dry adiabatic and the increase of low level moisture. Shear values were modest at best. Sfc-1km shear values were around 20 m2/s2, sfc-6km values were around 40 m2/s2, and the effective bulk shear values were around 55 knots. Synoptic scale support was difficult to find for this event, but what was found while digging through archived Storm Prediction Center mesoanalysis data was that there was diffluent flow analyzed at 300hPa (divergence aloft), upward ageostrophic accelerations due to various wind maxima at 300hPa, and a steady positive differential vorticity advection over the thunderstorms location.

Storm Evolution:
When the storm was developing initially, the 0.5 degree radar volume scans were not able to see anything, but the 6.4 degree radar volume scan did see the storm developing, which means that storm initially developed aloft. The original updraft split in to a left moving supercell and a right moving supercell, but what was odd was that the left moving supercell was cyclonically rotating and the right moving supercell was moving anticyclonically. The northeastern moving supercell quickly moved into Boulder’s region, dropping 1.25” hail over Black Forest, Colorado on its way out.  The left moving supercell, remained anchored to the boundary (the mountains) and propagated over Fort Carson dropping 2.5” hail, as reported by a spotter. While the storm was over Fort Carson, a mid-altitude radial convergence (MARC) signature was observed, 170 knot storm top divergence values were observed, and a slew of dual-polarization products supported that there was large hail. The main updraft then disconnected to its mountain boundary, started rotating cyclonically and intensified over Fountain, Colorado where 3.0” hail was reported. The storm remained severe until around 2:45 AM.
 

Model Performance:
Model guidance was not very useful for this storm. 18Z/00Z NAM was too far north ant too quick with convective development in the evening. The HRRR varied greatly from run to run, with a few mid-evening runs capturing some convection over El Paso County. The HRRR runs right before the event showed no convection developing at all.

 

Statistics of the Warning for the Hail Event:
False Alarm Ratio: 0.2 (4 out of 5 warnings verified)
Probability of Detection: 0.82
Threat Score: 0.68
Average Lead Time for the First Event: 10.5 Minutes
Average Lead Time for all Events: 17.1 Minutes
Max Lead Time: 45 Minutes
Minimum Lead Time: 0 minutes

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