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Massive Heat Wave; Severe Storms and Heavy Rain

A massive heat wave is consuming much of the Midwest into the Southern tier of the U.S. with hazy, hot and humid conditions. These dangerous conditions will likely persist through Thursday as widespread excessive heat warnings and heat advisories are in effect. North and east of the heat and area of high pressure, thunderstorms with damaging and severe winds, large hail and heavy rain are likely. Read More >


Radar/Photo Radar/Photo
Overview of the Severe Thunderstorm Warnings Issued Throughout the Event Overview of the Severe Thunderstorm Warnings Issued Throughout the Event
Satellite View of the Prolific Hail Event


The general synoptic set-up shows that the day was going to be a monsoonal thunderstorm day. A high pressure system was centered over the desert southwest of the U.S. The low-level flow was from the southeast, advecting moisture from the Gulf of Mexico towards southern Colorado. Generally speaking, during a monsoonal pattern like this, the upper-level winds tend to be weak, but during the August 6th, 2018 event, there were anomalously strong winds aloft, creating higher wind shear magnitudes, as analyzed in the mesoanalysis images, located underneath the “Environment” tab. The surface dew point temperatures were 58°F accompanied by a weak 5 knot southeasterly wind. Before convective initiation at 12:00 PM MDT across El Paso County was around 2000 J/kg (Fig. 9) with effective bulk wind shear value of 45–50 kts (Fig. 5), which are values high enough to produce substantial severe weather. To further dive in to the thermodynamics of the afternoon, the lifted index (LI) was -7, the mid-level lapse rates were around 8.5° C/km (Fig. 6), which is common for the lee of the Rocky Mountains, the low-level lapse rates were around 7.5°C/km (Fig. 7), lifted condesation levels (LCL) were 1250–1500 m AGL (Fig. 8), and there was a theta-e “tongue”, with values of around 354 Kelvin (Fig 10), reaching all the way up the point of where convective initiation occurred, as well as calculated theta-e advection against the lee of the Rocky Mountains. As for the lifting mechanism that sparked the thunderstorm in Park County, the overall dynamics were weak, but the 700hPa analysis (Fig. 2) illustrates a shortwave trough propagating over the region, 700hPa frontogenesis charts (Fig. 3) illustrates a magnitude maximum located over Park County, which is likely due to orographic effects, and the 300hPa jet circulation chart (Fig. 4) indicated a region of divergence over Park County.

Storm Evolution:
The storm initiation occurred over eastern Park County, quickly developing in to Teller County and becoming severe. The first Severe Thunderstorm Warning (for the supercell in discussion) was issued at 1:19PM MDT, followed by the second Severe Thunderstorm Warning at 1:45PM MDT. The first local storm report (LSR) was 2.75 inch hail at 1400 LDT, 3 miles south of Colorado Springs. At that point the thunderstorm had been warned for 41 minutes. At that point, the upper level divergence values, via the KPUX radar, were around 144 kts, which according to literature, corresponds to 2.00”–2.50 ” hail. MESH was in agreement, estimating hails size around 2.63”. The storm moved over Cheyenne Mountain Zoo at around 2:20PM MDT, at which the storm had been warned on for 1 hour. Upper level divergence values were again around 143 kts, which corresponds to 2.00–2.50” hail. LSRs confirm that hail stones were around 2.75” in diameter once more. The supercell continued to propagate southeasterly, eventually over Fountain, CO, in which 2.5” diameter hail was observed. A large hail spike was observed, starting at 3:14PM, during that time there was 3.0” diameter hail located around north of the Pueblo / El Paso County line.

Warning Statistics (12PM - 6PM):
False Alarm Ratio = 0.27
Probability of Detection = 0.88
Threat Score = 0.66
Average Lead Time = 25.5 Mintutes
Max Lead Time = 45.0 Minutes
Min Lead Time  = 0.0 Minutes

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