Huge Hail Pummels Denver; More Severe Weather on the Way

Windshields were shattered and roofs left tattered by a massive hailstorm that swept through the Denver area on Monday afternoon, wreaking damage that could end up in the hundreds of millions of dollars. Long nicknamed “Hail Alley,” the high plains of northeast Colorado just east of the Rocky Mountains are notorious for destructive hail. On Monday, a cold front with low-level upslope flow teamed up with unstable air and light south-southwest winds aloft to prime the Colorado Front Range for slow-moving severe weather.

Monday’s storm plowed across the northwest half of the Denver metro area around 3:00 pm MDT. It eventually merged with other storms into a mesoscale convective system that dumped more than 2” of rain in the Greeley, CO, area. The weak upper winds kept severe weather limited to the eastern High Plains of Colorado and New Mexico.  

Out of 60 hail reports from Monday collected by the NOAA/NWS Storm Prediction Center (SPC) as of midday Tuesday, the most impressive was 2.75” (baseball-sized) at a hospital in Wheat Ridge, just northwest of central Denver. (See this photo, tweeted from a nearby location.) Severe damage was reported to vehicles in Lakewood, another western suburb. The storm forced postponement till Tuesday of a Colorado Rockies-Chicago Cubs baseball game at Coors Field, where the field’s tarp was slathered with a coating of hail.                           

Hail damage in Denver could easily top $100 million

Even small hail can be surprisingly destructive if it comes down in a barrage. Monday’s storm in Denver featured a gamut of sizes, from pea to baseball, in the kind of onslaught that inflicts financial pain on both residents and insurers. Even as the storm unfolded, radar returns made it clear that big hail was likely falling (see this dramatic radar loop).

Insurers were in the field assessing damage on Tuesday. Past experience suggests that a hefty financial toll is possible. “It is entirely plausible that such an event can leave damage into the hundreds of millions, particularly when we get large hail like May 8 hitting densely populated areas,” said Steve Bowen (Aon Benfield).

Figure 1. Damage from the 11 costliest hailstorms along the Colorado Front Range, including the Denver area, over the last 25 years. None of these 11 storms occurred as early in the spring as Monday’s event, for which damages have yet to be assessed. Image credit: Rocky Mountain Insurance Information Association, courtesy Steve Bowen, Aon Benfield.

Rapid growth in Colorado’s urban areas over the last decade has greatly increased the potential for property damage from hail. Not counting Monday’s event, four major Colorado hailstorms in the last six years produced a combined total of more than $1 billion in damage (in 2015 dollars). Colorado ranked behind only Texas in the number of hail-related insurance claims filed in the 2013-2015 period, according to a 2016 report from the National Insurance Crime Bureau.
 

Figure 2. The number of insurance claims related to hail filed in each state from 2013 to 2015. Image credit: National Insurance Crime Bureau.

More hail in the queue?

Conditions are ripe for more hailstorms on Tuesday across eastern CO and NM into west TX. Within a slight-risk region for severe weather (see Figure 3), NOAA/SPC has flagged the high plains for a chance of significant hail (2” or larger). The storms should be more isolated toward NM and TX, and I wouldn’t be surprised to see a couple of impressive supercells over far west Texas that might generate a tornado or two (the risk of strong tornadoes is quite low) as well as giant hail. These cells may rumble well into the night, with the HRRR model suggesting that more extensive showers and storms will develop by morning across the Texas Panhandle.

Figure 3. Severe weather outlook for Tuesday, May 9, 2017, updated at 11:42 am CDT.

As a slow-moving upper low nudges into the Great Plains, a prototypical severe weather set-up is in the cards for Wednesday across parts of Texas, Oklahoma, and Kansas, shifting into parts of Arkansas and Louisiana by Thursday. Compared to Monday, upper-level winds will be considerably stronger and low-level moisture richer. It’s too soon to know the extent to which storms in the morning will hinder the odds of supercells later in the day, but if the atmosphere is relatively untouched, several tornadic supercells could well form—especially over western or central OK close to any surface boundaries left by earlier storms. Storms will be well-ventilated by upper-level winds that could exceed 110 knots (126 mph) at jet-stream level, about six miles above the ground.

Wednesday night’s storms are likely to segue into more garden-variety producers of heavy rain/wind/hail by Thursday, when there will be less of the directional wind shear (turning of winds with height) that helps foster rotating storms.

After Thursday, it could be next week before we see our next multi-day round of significant severe weather. The Plains will be heating and moistening, with highs well into the 80s and low 90s by Monday, and another strong upper low will be moving slowly toward the area. However, the GFS and ECMWF models disagree on how that low will evolve. Moreover, a substantial “cap” of warm, dry air several miles above the surface may keep a lid on widespread severe weather, at least initially. For the time being, NOAA/SPC is holding off on outlining the days and locations of next week’s severe weather.

Figure 4. Strong thunderstorms (shown in red) are increasing around the disturbance labeled 90E in this infrared satellite image taken at 1647Z (12:47 pm EDT) Tuesday, May 9, 2017. Image credit: NOAA/CSU RAMMB.

Invest 90E on the verge of becoming a tropical depression in East Pacific

What could become the earliest tropical storm on record in the Northeast Pacific Ocean is continuing to gradually organize in the open waters south of El Salvador and west of Costa Rica. In  a special Tropical Weather Outlook issued at 12:20 pm EDT Tuesday, the NOAA/NWS National Hurricane Center gave the disturbance dubbed 90E a 90% chance of development. Rotation was becoming more evident in satellite imagery of the cluster of showers and thunderstorms at the heart of 90E, and the system might already contain depression- or even storm-strength winds.

We have plenty of time to watch 90E. The system is located at the east end of a broad area of disturbed weather, and it will take a while to fully consolidate. Models agree that 90E will be moving slowly northwest well offshore from central America over the next few days. If the system develops into a tropical storm, it will be named Adrian. We’ll be back with an update on 90E by Wednesday morning.