The same parent storm system could bring a late-season snowstorm to parts of the Rockies and northern and western Plains. A foot or more may fall in a few locales, with blizzard conditions possible.
After that, there are signs that the remainder of April could be atypically turbulent, largely a symptom of a seasonal clash of the air masses.
The National Weather Service Storm Prediction Center logged more than 200 reports of tornadoes during the month of March, two and a half times the average of 80. Now the Center is warning that “large hail, damaging winds and tornadoes appear possible each day” between Monday and Wednesday — probably focused in the southern and eastern parts of the Plains. Portions of the Deep South, the mid Missouri Valley and the Ozarks could be in play as well as the week progresses.
A cooler, more stable air mass has spread over much of the central and eastern U.S. in the past several days, which will result in a dearth of severe weather on Friday and Saturday. That will begin to change on Sunday, when a dip in the jet stream, known as a trough, will allow cold air at the upper levels to spill south over the northwestern U.S.
Multiple “shortwaves,” or lobes of high altitude cold air, low pressure and spin, will swing down and around that jet stream dip, each bringing inclement weather and triggering storminess. The first will pass over the Sand Hills of western Nebraska on Sunday night into early Monday, with the next one arriving out of the Pacific Northwest and Columbia River Basin into Tuesday.
At the surface, a dryline will sharpen over Interstate 35 corridor in the southern Plains. That’s the boundary between arid air from the Desert Southwest and moisture-rich air from the Gulf of Mexico wafting in from the southeast. The clash between desert and tropical air juxtaposed in proximity will act as the triggering mechanism for multiple rounds of dangerous thunderstorms, with severe weather probable Monday, Tuesday and Wednesday. Wednesday may prove the worst day of the bunch as the jet stream races overhead and bolsters wind shear, or changing winds with height, that can encourage rotation within thunderstorms.
- Monday: The jury is still out the timing and specific track of the instigating storm systems that will give rise to severe thunderstorms, but confidence is sufficient that at least a few isolated severe thunderstorms will develop in central Oklahoma and Texas. Large destructive hail, perhaps bigger than golf balls, is a primary concern, but damaging straight-line winds and a few tornadoes are possible, too. The tornado risk will increase in the evening as the low-level jet stream, responsible for boosting wind shear, strengthens.
- Tuesday: Timing differences in the models paint differing pictures as to who may be most at risk for severe weather. The European (ECMWF) model has a wavier jet stream and tracks the parent storm system over the central High Plains, while the American (GFS) model takes it a bit farther north and east. In either case, severe thunderstorms are probable along the dryline across the southern and central Plains. Cities impacted might include Oklahoma City, Tulsa, Waco and Dallas.
- Wednesday: Wednesday features perhaps the greatest risk of severe weather, with plentiful shear supportive of rotating supercell thunderstorms producing hail, wind and tornadoes. The European model indicates a substantial severe weather threat for central and eastern Kansas, Oklahoma and Texas stretching east into the Mississippi Valley. The American model is a bit faster in its simulation, which would result in a slightly lesser (but still formidable) severe weather risk slightly farther east.
- Cities potentially affected include Oklahoma City; Tulsa, Waco, Dallas, and Lufkin, Tex.; Little Rock; Shreveport, La.; and Springfield, Mo.
- Thursday: Severe thunderstorms may be possible over parts of the Deep South, Tennessee Valley or Southeast depending on the evolution of the storm system.
Significant snowstorm expected
Meanwhile, the shortwave responsible for Tuesday’s storminess will give rise to a zone of low pressure that will eject out of the Colorado Front Range and pass through Nebraska and northern Iowa en route to the Upper Midwest by early Wednesday. Moisture drawn around the counterclockwise-spinning low will be tugged northwest, overlapping with a surge of subfreezing air dragged down from Canada.
Snow will be falling in much of Montana and separately across southern Wyoming and northern Colorado on Tuesday morning, the batches of wintry weather merging as the snowstorm gets going. By late Tuesday, an arc of snow will stretch across North Dakota, with heavy snow overnight into early Wednesday there and in northwestern Minnesota.
There may be a few pockets near the Canadian border that see winds gusting upward of 35 mph for a time on Wednesday. That could combine with falling snow or loft freshly fallen flakes to reduce visibility and produce blizzard conditions.
Weather models are bullish on dropping 5 to 10 inches of snow in much of Montana, and 8 to 12 inches in much of North Dakota and adjacent southern Manitoba or Saskatchewan.
There is a chance that South Dakota could be in play too, along with a more expansive swath of western Nebraska and northeast Colorado, if the European model’s simulations end up verifying.
“At this time range, we will begin to ‘sound the alarm’ for a potentially significant and rare storm,” writes the Weather Service office serving Boulder, Colo., and Denver. It says that if the European model forecast is correct, it would be “the ideal setup” for a major snowstorm and that blizzard conditions would be possible. “This would be the scenario with the most damaging impacts and it should be the scenario that preparations are made for despite the low confidence in the forecast,” it wrote. Other models present lower impact outcomes.
Ahead of the storm, warm, tranquil weather will prevail in the eastern U.S. with temperatures 10 to 20 degrees above normal midweek. In the storm’s wake along the West Coast, it will be rather chilly, an abrupt change from ongoing record warmth.