Sharp divide in Plains between dry and humid air will fuel fire risk and storms
A key weather boundary is setting up across the central United States. It’s called the “dryline” and is expected to fuel dramatic, impactful weather - in part driving fire risk and elsewhere fueling the chance of severe thunderstorms.
The dryline is the divide between bone-dry desert air to the west and moisture-rich Gulf of Mexico air to the east. The razor-sharp divide separates desiccating aridity from tropical humidity. And it makes for dramatic, impactful weather. On the western side of the boundary, hot, dry air is leading to dangerous fire weather. To the east, it’s soupy and sultry. And where the air masses clash, sporadic severe thunderstorms are expected. Some will be supercells, with damaging gusts, large hail and tornadoes.
This boundary will be the focal mechanism for storms though Friday, after which a pair of potent disturbances ejecting from the Rockies will stir up severe weather. Sunday and Monday could feature higher-end severe weather events with the potential for significant tornado activity.
Fire weather on Wednesday
Winds across the Plains were turning southerly on Wednesday. That’s ahead of the formation of dual low-pressure systems in the lee of the Rockies. One low is in southeastern Colorado, and the other is near the Canadian border. Both lows are pulling Gulf moisture northward ahead of them.
On their back sides, however, they’re inducing a process by which dry air is pulled out of the west. It descends down the Rockies and dries even more.
That means there’s a dividing line stretching south to north from the Texas Big Bend to northeastern Montana. On the western side, it’s exceptionally dry; to the east, it’s humid. On the dry side, gusty winds and extremely low relative humidity are making for critical fire weather concerns.
That’s especially true from Albuquerque to Denver to Casper, Wyoming, to Spearfish, South Dakota, to extreme southeastern Montana. Red-flag warnings, signifying dangerous fire weather concerns, cover most of the High Plains and include the Nebraska Sandhills and even much of Minnesota.
“A combination of strong winds, low relative humidity and warm temperatures can contribute to extreme fire behavior,” the National Weather Service wrote. “Any fires that ignite will spread rapidly and become difficult to control or suppress.”
Severe storm risk on Thursday and Friday
The same dryline is bringing severe weather threats. That’s because dry air is denser than moist air. As the dry air impinges on a humid air mass to the east, moist air rises and generates thunderstorms.
On Wednesday, only very isolated strong to marginally severe thunderstorms are expected across the High Plains. Moisture hasn’t quite fully arrived ahead of the dryline, meaning the environment is simply too dry for anything other than a rogue storm or two.
But Thursday will feature a greater severe weather risk. From Omaha to Minneapolis, including most of Iowa, a squall line will probably form by midevening. That squall will contain gusty to locally damaging straight-line winds and a bit of small hail. The tornado threat will be low.
But in eastern Kansas, the tornado risk is greater. That’s because of a secondary surface low forming in the northwestern part of the state. That surface low is inhaling winds out of the southeast, which contrasts against strong southwesterly winds aloft from the jet stream. Any thunderstorms that can blossom will be affected by those changing winds with height and rotate, yielding a tornado threat.
Thunderstorms that form in Kansas will be associated with a dryline arriving from the west. There’s also a volatile parameter space, or overlap of ingredients, to support a severe thunderstorm or two in Oklahoma. But there’s not much of a trigger, and there’s also a strong cap. That’s a lid of hot, dry air a mile above the ground that suppresses storm development by preventing surface air from rising. So it’s unlikely that any storms will form in Oklahoma, but if the cap breaks and a storm can form, it will pose the threat of very large hail, damaging gusts and tornadoes.
On Friday, there’s again a slight (Level 2 out of 5) risk of severe weather for eastern Oklahoma, northeastern Texas along Interstate 30 and most of Arkansas. A cold front, reinforced by cool air exhaust from Thursday night’s leftover storms, will sag south or southeast across the region. It may serve as a focal mechanism for the development of additional thunderstorms. Damaging gusts, hail and an isolated tornado are likely.