Severe Weather to Ramp Down This Weekend, Temporarily

A brief break in our multi-day spate of U.S. severe weather appears to be forthcoming, but not until one more day of widespread severe storms. In its Day 1 outlook issued at 11:30 am CDT Thursday, the NOAA/NWS Storm Prediction Center placed a large swath from the Ohio Valley to the Gulf Coast under a slight risk of severe weather through Thursday night, with a pocket of enhanced risk over the lower Ohio Valley.

Thursday’s setup is looking less potent than its counterparts earlier this week, largely because of widespread clouds, showers, and thunderstorms already ongoing late Thursday morning. The weaker storms now covering much of Illinois and neighboring areas will tamp down on daytime heating toward the north end of the risk area. Meanwhile, a large thunderstorm complex centered in southeast Louisiana will impede Gulf moisture from flowing toward the enhanced risk area. Even though a compact, energetic upper-level trough is approaching the Mississippi Valley, there will be only limited instability for the trough to work with, given the factors above.


Figure 1. A GOES-16 infrared satellite image from 11:21 am CDT Thursday, March 30, 2017, shows the upper-level low centered near the KS/MO border, with extensive clouds and thunderstorms stretching ahead of it from the Great Lakes to the Gulf of Mexico. A small pocket of relatively clear air may allow for storm redevelopment on Thursday afternoon over the lower Ohio Valley. GOES-16 data are preliminary, non-operational, and still being tested. Image credit: NEXLAB/College of Dupage.


Figure 2. NOAA/SPC’s Day 1 outlook issued at 11:50 am CDT Thursday, March 30, 2017, includes an enhanced risk of severe weather for parts of southern Illinois and Indiana, western Kentucky, and far northwest Tennessee, surrounded by a much broader region of slight risk.

Tornado tallies for 2017: Still at a record clip
This week’s much-anticipated severe weather has produced plenty of large hail and high wind but thankfully, few tornadoes. SPC logged 7 preliminary tornado reports on Monday from Mississippi to Kentucky. A total of 17 reports came in from west Texas on Tuesday, with 4 reports from southeast Texas and central Arkansas on Wednesday. None of this week’s tornadoes thus far have been especially intense or destructive.

As of Thursday morning, the year to date had seen a total of 24 tornado-related fatalities in the U.S., all in January and February. The preliminary "inflation-adjusted" total of 349 tornadoes through Wednesday keeps 2017 at record levels for the largest number of tornadoes up to this point in the year in records going back to 1954.


Figure 3. Tara Shadoan takes her belongings before the insurance company tows her car damaged by a hailstorm the night before in Highland Village, Texas, on Monday, March 27, 2017. Image credit: Jae S. Lee/The Dallas Morning News via AP.

A pause, and then another burst next week?
Pockets of severe weather may continue through the weekend, but the fast pace of upper-level lows and troughs will make it difficult for adequate instability to get into the right place for a major outbreak. SPC is calling for a marginal risk of severe weather across parts of the mid-Atlantic on Friday, in association with the upper low now traversing the Ohio Valley, as well as a marginal risk over the southern High Plains ahead of the next system. This strong upper-level trough will become a cut-off low over the Southern Rockies late Friday into Saturday, then accelerate eastward across the Gulf Coast states from Sunday through Tuesday. It’s a safe bet that large clusters of intense storms will develop ahead of this trough, but these clusters may set up just offshore in the northern Gulf, reducing the risk of severe weather inland.

Yet another strong upper-level trough will enter the Plains by late Tuesday. This will be one to watch, as models project it to generate a very strong surface low, with powerful flow through the lowest few miles of the atmosphere leading to ample wind shear from the Southern Plains to the Midwest. The main question mark is how quickly moisture will return to the area.


Figure 4. March is often Colorado’s snowiest month. The snowstorm of March 17 - 19, 2003, was the most intense to strike the Denver area in over 80 years. In some parts of the Rocky Mountain foothills west of Boulder, more than 80 inches (2 meters) of snow fell. Image credit: UCAR, photo by Carlye Calvin.

End-of-month snow blitz for the Rockies
The upper low cutting off over the Southern Rockies on Friday and Saturday is on track to bring plenty of welcome precipitation to the Wyoming/Colorado mountains (as much as 1 to 2 feet of snow in favored areas) and on the adjacent high plains. Temperatures may be just cold enough for a wet snow along most of the Colorado Front Range, including Denver and Boulder, although periods of rain could cut back on the totals at some of the lower urban elevations.

Both Denver and Boulder have measured just a trace of snow in March thus far. That’s a notably meager total in what’s typically one of the snowiest months of the year. In Denver records going back to 1882, only two other Marches—2012 and 1995—got only a trace of snow for the entire month. It’s quite possible Denver will see measurable flakes by midnight Friday night. Boulder may have a better shot than Denver at a trace-only March, since its March 31 observation will be taken at the local COOP site at 6:00 pm. Friday.


Figure 5. The SREF ensemble-based modeling system projects a wide range of potential snowfall for Broomfield, CO (between Boulder and Denver). The big spread isn’t a surprise given that snow totals will hinge in large part on a very small temperature range. The ensemble average of around 12” by Saturday night (solid black line) seems plausible if temperatures get cold enough. Image credit: NOAA/SPC.

Three storm chasers killed on Tuesday in Texas
Three storm chasers died on Tuesday afternoon in a collision near the town of Spur, TX, east of Lubbock. Killed were Kelley Williamson and Randy Yarnall, contractors for the Weather Channel, and Corbin Jaeger, an Arizona-based chaser who filed reports on the website MadWX Chasing. Williamson and Yarnall were featured on TWC’s series “Storm Wranglers,” which debuted last fall. The Texas Department of Public Safety reported that Williamson and Yarnall were traveling north on a farm-to-market road on assignment with TWC when they ran a stop sign and collided with Jaeger’s vehicle, which was traveling west on another farm-to-market road.

“Kelley and Randy were beloved members of the weather community as well as true weather fans. We will miss them. Our thoughts and prayers are with the families and friends of all three victims as they mourn this loss,” said TWC’s Stephanie Abrams. “Corbin was one-of-a-kind and will be missed deeply,” noted a statement on the MadWx Facebook page. “Our thoughts and prayers go out to Corbin's friends and family, as well as the friends and family of Kelley and Randy, who also passed away in the accident.” For more, see the online video from TWC’s Ari Salsalari and a statement issued by the Weather Channel.

Bob Henson