Severe Weather Returns to the Plains This Weekend

Supercells may again roam the southern and central Great Plains on Friday and Saturday, as they did last weekend. The trigger comes from a strong, cold upper-level low that’s delivering an unseasonably late dose of rain to parts of drought-plagued southern California. Major League Baseball was halted on Thursday night by the first rain delays in four years at San Diego and the first in 11 years in Los Angeles. Following 0.55” of rain for the month up through Wednesday, San Diego’s Lindbergh Field scored 1.51” in less than two hours on Thursday evening. The calendar-day total of 1.63” was a record for any day in May, and as of 10:00 am PDT Friday, Lindbergh Field has received 2.25” for the month thus far, with rain continuing. In records going back to 1850, the wettest May in San Diego was 2.54” in 1921.

Much like last week’s upper low, the current one will turn sharply northward as it pushes onto the plains on Saturday. The slow eastward progress, coupled with ample low-level moisture from the Gulf of Mexico, will foster multiple rounds of showers and thunderstorms, with cells darting northward as the entire system moves slowly eastward. Flash flooding may again be a threat, especially in places like far north-central Texas and central Oklahoma that are already waterlogged from upwards of 10” of rain over the last two weeks.


Figure 1. A strong upper low at the 300-mb level (about 30,000 feet) high will be centered over Nevada at 0000 GMT Saturday, according to the 0000 GMT Friday run of the GFS model. The low will bring jet-stream-level winds approaching 100 knots (115 mph) across parts of Kansas, Oklahoma, and Texas on Saturday, boosting the odds of severe weather. Image credit: WunderMap.

Friday’s action should be focused near a developing surface low in far northeast Colorado, sweeping into eastern Wyoming and western Nebraska. These storms may quickly turn severe by afternoon, but should affect only sparsely populated areas. The late arrival of the upper low may also trigger an isolated storm or two further south along a dry line in far western Kansas. Tornadoes are possible, but the more serious tornado threat will likely be on Saturday, as stronger high-level winds associated with the upper low boost the vertical wind shear across Kansas and Oklahoma. The latest outlooks from NOAA’s Storm Prediction Center show large slight-risk areas for both Friday and Saturday, with enhanced-risk pockets centered in western Nebraska on Friday and western KS/OK on Saturday.

The ingredients are lining up for a potential major outbreak on Saturday evening, but it’s possible that morning showers and storms could keep the pot from boiling. During some years, spring thunderstorms are strongly “capped” across the Southern Plains by a recurrent layer of very warm air one to two miles above the surface that moves across the region from New Mexico and west Texas. This spring, a strong low-latitude jet stream (likely aided by El Niño) has led to cool and wet conditions across NM and west TX and cut back on the strength of the cap. This allows showers and storms to develop more readily over the Plains, but it also hinders the buildup of explosive instability that can enhance the severity of late-day storms. A large complex of morning thunderstorms cooled the airmass and largely quashed severe weather last Saturday across much of Oklahoma and southern Kansas. It’s possible a similar scenario will play out again this Saturday, as suggested by the 1200 GMT Friday runs of both the GFS and NAM models. If the air mass does manage to warm and destabilize, there could be a significant tornado threat on Saturday afternoon in western KS and OK, heading toward the Oklahoma City and Wichita areas by evening.




New this month: Community-inspired WunderPosters
Many thanks to those of you who submitted images and ideas for the community-inspired phase of our ongoing WunderPoster series. We sifted through hundreds of submissions to make our final selections, and the first two installments created by the WU design team are now online!

Roll clouds sometimes appear this time of year when the cool outflow around large thunderstorm complexes encounters warm, moist air. A sea breeze can serve the same function as the outflow, so roll clouds may also be spotted near coastal areas—but they’re not extremely common, so catching one is a rare treat. This poster was designed by Jerimiah Brown and inspired by a photo submitted by @The_Yarniverous.

Some of the most delicate beauty of wintertime comes to us in the form of hoarfrost. The name is derived from an old English word that alludes to the white hair earned by age. If a night is especially clear, still, and cold, then water vapor may be deposited in crystalline form directly onto twigs, wires, and other surfaces, sometimes branching into intricate structures. An image submitted by @bee.herder served as the inspiration for this poster, which was designed by Lauren Moyer.

Each of the 15 WunderPosters produced to date can be downloaded in formats suitable for posters or postcards.

Have a great weekend, everyone!

Bob Henson


Figure 2. A classic thunderstorm-generated roll cloud rolls across western Kansas near Gem on June 26, 2014. “Not even the panorama did it justice...it was a true 180!” Image credit: wunderphotographer clkngrny.


Figure 3. Pine needles are coated with hoarfrost on February 7, 2014, in Zaporizhia, Ukraine. Image credit: wunderphotographer TanyaMass.