Intense storm front sweeping nation with snow and severe weather

A powerful storm front is charging across the Lower 48 states, bringing widespread snows in the Mountain West, the threat of fast-moving fires in the Plains and the potential for severe thunderstorms in the Midwest. It’s the first of two such storm systems set to sweep across the nation over the next week as weather patterns begin a transition to spring.

Over the Plains and heartland, an influx of hot, dry air near the front will sap the ground of moisture, sparking concerns for fast-moving fires Monday into Tuesday. Red flag warnings for dangerous fire weather stretch from the U.S.-Mexico border in Texas to just outside Chicago, affecting near 20 million people.

Ahead of the front, abnormally warm and humid air wafting north into the Midwest will allow thunderstorms to develop Tuesday, a few of which may produce large hail, damaging wind and a couple tornadoes. The area from Chicago to Detroit and south toward St. Louis is most at risk.

Behind the front, temperatures plummet. That’s why winter storm warnings and winter weather advisories blanket the high terrain of the Mountain West, affecting more than 5 million people. The National Weather Service is warning a vast area from Nevada and Montana to Colorado and Wyoming to expect “widespread mountain snow, high winds and dangerous travel.” As temperatures fall farther east into Tuesday, snow will also spread into North Dakota and northern Minnesota, where winter weather alerts are in effect.

Much of the active weather will be in the vicinity of the front, which currently stretches from Northern California to North Dakota. It is expected to progress to the east and south through midweek before exiting the East Coast.

Ahead of it, record-challenging warmth will spread north. Behind it, cold and dry air with bitter wind chills will crash south within a dip in the jet stream.

The colliding air masses will kick up severe thunderstorms over the Midwest along and ahead of the cold front, while wraparound moisture on the storm system’s back side will lead to snows. Dry air in the storm system’s “dry slot” — or wedge of arid air spiraling in from the southwest — will sweep over the Plains.

Thunderstorms, some severe, will form along and ahead of the cold front Tuesday in the Midwest. They will have a limited supply of fuel, but ample wind dynamics. With the jet stream slicing overhead, a dramatic change of wind speed and/or direction with height, known as shear, will be present.

A few rotating supercells will be possible, with a threat of large hail, damaging winds and tornadoes.

A Level 2 out of 5 slight risk of severe storms has been drawn by the Weather Service’s Storm Prediction Center. It stretches from south of Milwaukee to Detroit and south toward extreme southern Illinois and eastern Missouri. Chicago; Springfield, Ill; Indianapolis; Toledo; and St. Louis are all in the zone to watch.

As hot, windy weather surges north ahead of the front, dangerous fire weather is already present in West Texas, southeastern New Mexico and the Oklahoma and Texas Panhandles. That’s where the Storm Prediction Center has outlined a “critical” risk for wildfires through Monday. A lesser, but still significant, elevated wildfire risk extends into Nebraska and Iowa.

For the Plains, the main drivers of the fire weather are twofold:

◆ Temperatures are unusually warm, running 20 to 30 degrees above average. That means 80s and 90s in eastern New Mexico and Texas, with 60s and 70s for the remainder of the Plains — highly unusual for February. The hotter temperatures evaporate more humidity from the ground, sapping vegetation of moisture and desiccating the landscape.

◆ Downsloping winds, or air being pulled from high in the Rockies to a lower elevation over the Plains. A belt of strong westerly winds associated with the jet stream aloft is mixing down to the surface, and increasing strong westerly winds across the Rockies and western Plains. As air descends from higher elevations to lower elevations, it compresses, heats up and dries out. That brings very low humidity levels.

“Use caution if engaging in activities that may result in a fire starting,” the Weather Service advised.