After a quiet period last week, the weather pattern in the Lower 48 is set to become much more active as a strong jet stream slices across the country. A surge of warmth in the East will be a sharp contrast to an impending shot of cold in the West.

The opposing air masses could bring severe storms to the nation’s central regions, with the threat of heavy downpours and flooding over the Ozarks and the Mississippi River Valley.

Meanwhile, a winter storm could spin up amid plummeting temperatures in the West. Winter storm watches were already up Monday in the Sierra Nevada and in Wyoming as disturbances begin to move east.

There are signs that the active weather pattern could last deep into March, with additional storm systems likely into the upcoming weeks.

Springlike warmth surges east

The week has begun with a preview of spring for just about everyone east of the Rockies, with the exception of the Northern Tier, northern Great Lakes and New England. A wedge of cool, dense air remained entrenched in places but will be rooted out gradually as warm air advances north this week.

Chicago was forecast to hit the lower 60s on Monday, the Windy City’s first 60-degree reading since Nov. 20. Chicagoans will have four days of mild conditions before a cold front swings through and sends temperatures tumbling by Friday.

Farther east, the warmth will be even more pronounced. D.C. is looking at temperatures in the mid-50s on Monday, but from Tuesday through Friday, highs could flirt with or exceed 70 degrees. The nation’s capital hasn’t hit 70 since Nov. 15.

New York City and Boston will also see some mild weather this week, though it won’t be as pronounced a warm-up as it will be in the Mid-Atlantic. For example, in New York City, Tuesday, Thursday and Friday are forecast to be in the 60s, though Wednesday and Saturday could hang in the 50s.

Across the South, 70s and a few lower 80s will dominate throughout the week.

Major cool-down grips the West

Behind the cold front, temperatures will plummet, causing any brief glimpse of spring to vanish. Highs in the 70s on the Front Range and High Plains will be replaced by 40s as this week wears on. Some locations may see a 24-hour temperature drop of nearly 40 degrees.

Casper, Wyo., is expected to be near 70 degrees Monday afternoon but is under a winter storm watch. Temperatures will fall into the teens there by late Wednesday night as bands of heavy snow pivot through the region.

Much of South Dakota will experience the same storm midweek, with a similar chance of light snow in eastern regions just a day after highs hit 70.

Across the Plains, dry air will allow for even more dramatic temperatures wings. McCook, Neb., for instance, could see a high of 80 degrees on Tuesday, but temperatures are expected to drop to 30 degrees by Wednesday night.

Severe storms likely for some

Ahead of the cold front, strong to severe thunderstorms are likely, with a multiday severe-weather event possible as air masses clash. There will be an uptick in thunderstorm chances Wednesday, when a few isolated storms producing gusty winds and hail are possible in Kansas between Wichita and Kansas City.

On Friday, much of Oklahoma could be in play for severe storms, with the chance of a few producing damaging winds, hail or even tornadoes.

The threat repeats in the same area Saturday, expanding to include parts of southern and central Kansas as well as the Dallas-Fort Worth area in Texas. On Sunday, eastern Oklahoma, southwestern Missouri, western Arkansas, northeastern Texas and northwestern Louisiana could see some strong storms.

While an approaching dip in the jet stream could induce spin in the upper atmosphere and help storms rotate, it’s unclear to what extent moisture and instability will be present in the lower atmosphere. That lends uncertainty to how widespread the severe weather will be.

Flooding potential increases

The upcoming pattern change favors areas of heavy rainfall in the next week to 10 days across portions of the central and southern Plains, the Ozarks and the Mississippi Valley. That’s where moisture pooling along the stalled west-to-east front could drop half a foot or more of rainfall by next Monday.

It’s not yet known where the axis of heaviest moisture will set up or exactly how much will come down, but a corridor of flooding is probable. The same thunderstorms that present a late-week severe-weather threat may merge each evening, with the dying complexes of storms producing repeated rains.

Largely because of upstream snow melt, flood warnings are in effect along parts of the Mississippi River, with minor inundation affecting a number of communities.

A major dose of winter in the Rockies

Bottled up within the low-pressure area overspreading the West is a disturbance that some computer models suggest could bring blockbuster snow to the Front Range this weekend. While that’s a long way off and highly uncertain, another system was already dropping out of the Columbia River Basin into the northern Rockies on Monday.

Three to six inches of mountain snow is slated to fall for most of western Montana, and the system could affect Wyoming, with half a foot or more in spots by Wednesday.

Thereafter, attention turns to the high-stakes late-week forecast.

“Now things get more interesting,” wrote the National Weather Service in Boulder, Colo., in reference to the evolving late-week forecast. “The main story is that there’s a pretty good chance of a prolonged period of at least light snow, and possibly a period of heavy snow, for most of our area.”

The greatest snow potential looks likely for Friday into Saturday, with accumulation potentially topping a foot in places. The forecast for Denver proper isn’t set in stone yet, but the possibility exists for a winter wallop there. While March might seem a bit late for such a major snowstorm, it’s typically the snowiest month in the Denver area.