A man checks his mobile device while walking from a gym to his car on Monday in Denver as a winter storm drifts over the Intermountain West. (David Zalubowski/AP)

From sudden temperature drops, to disruptive ice and snow, to severe storms and flooding rain, a major storm will make its mark as it charges across the Lower 48 states this week.

The powerful storm system has already sent temperatures tumbling in the Rockies, shifting springlike conditions back to winter in a matter of hours. On Sunday, Boulder and Denver were in the 70s. By Monday, they had ice and snow.

The Southern Plains will next endure such a radical transition. and snow is expected deep into Texas, in places unaccustomed to it, such as San Angelo and Lubbock from Tuesday night into Wednesday. In some areas farther north into Oklahoma, “the potential exists for 4 to 8 inches of snow,” according to the National Weather Service.

By the time the storm reaches the eastern United States, between late Wednesday and early Friday, it will draw abundant heat and moisture from the Gulf of Mexico, fueling thunderstorms in the Southeast and heavy rainfall through the Mid-Atlantic and southern New England. The Weather Service is warning of “multiple rounds of heavy rain and thunderstorms, especially from the central Gulf Coast to the Ohio Valley and central Appalachians.”

The snow and crashing temperatures

As the instigating energy rolled through the High Plains on Monday, it brought with it heavy snow, ice and a nearly 60-degree temperature plunge in just 30 hours.

Denver’s temperature plummeted from a daily-record-tying high of 74 at 2 p.m. Sunday to 16 degrees by 8 p.m. Monday, a wild 58-degree swing in just 30 hours. As the thermometer went into a free fall overnight Sunday into Monday, freezing drizzle quickly formed a glaze of ice on Denver-area roadways early on Monday, leading to a nasty and deadly morning commute. Ice-covered roadways led to at least one weather-related fatality on Interstate 25 just outside of Denver on Monday morning.

But that was just the beginning of a long day of winter weather across the Denver area. By Monday night, the ice had switched to snow, stacking two layers of frozen precipitation on top of one another and leading to 17 car accidents in Denver over just 30 minutes.

As of Tuesday morning, 2.8 inches of snow had officially been reported at Denver’s official climate observation site at Denver International Airport, but significantly higher totals were reported just west and north of the city. Nearby Boulder saw between 10 to 12 inches of snowfall, and another Denver suburb, Genesee, saw 10.6 inches, according to the Weather Service.

This week’s snow perhaps also offered Denver a not-so-subtle reminder that it’s still February after all, and in classic Colorado fashion: with several inches of snow, just hours after springlike warmth.

European model simulation of precipitation Wednesday morning. (Weatherbell)

The storm system is organizing as it plunges south from the Four Corners region into eastern New Mexico. Winter storm watches and warnings stretch from the U.S.-Mexico border to northeastern Oklahoma, with advisories stretching north beyond that. Snow is on the docket in an area not terribly well acquainted with it.

The National Weather Service's forecast for how much snow could fall through Thursday. (Weatherbell)

San Angelo, Tex., will climb into the upper 60s on Tuesday, but by late Tuesday night, the mercury will crash into the 20s. Accumulating snow and possibly sleet is anticipated, with up to two inches possible. It may not sound like a big deal, but around there, it is.

“The last time we had measurable snowfall was December 27, 2015,” said Matt Groh, a meteorologist at the Weather Service in San Angelo. “People are excited to see snow, since we haven’t seen it in a few years in this part of Texas. But it will have an impact on very limited snow removal equipment. Any snow or ice that accumulates on the roads will be very impactful on travel.”

Snow will fall in San Angelo beginning late Tuesday night and persisting through Wednesday. One to two inches is possible there, but Texas Big Country and parts of the Concho Valley could see three to seven inches.

Snow will fall, light to moderate at times, Wednesday along the cold front from the Red River in Oklahoma to near the Great Lakes. Oklahoma City’s temperature will drop like a rock behind the front. The high on Tuesday will be confined to the mid-30s after hitting 77 on Sunday and 69 on Monday.

Four to six inches of snow are predicted on Tuesday night and Wednesday in Oklahoma City, which is under a winter storm warning.

Severe thunderstorm potential

At the same time, a severe weather threat will present itself on the warm side of the system ahead of the approaching cold front. A marginal risk of severe weather exists Tuesday from extreme East Texas through northern Louisiana/southern Arkansas and across the Mississippi River toward western Tennessee. The ingredients for severe thunderstorms will become less favorable as the day wears on, so that risk should wane with time. However, there will remain the threat of a very isolated strong wind gust.

On Wednesday, the area to watch for severe storms encompasses eastern Louisiana, much of Mississippi and Alabama, and the Florida Panhandle. Eventually, cells and clusters Wednesday will form into a broken line that will translate east through the Southeast on Thursday.

Although there is little “instability” to foster rising motion in the atmosphere, the winds aloft will be very strong. In environments such as that, gusty winds are the primary concern in any heavier elements of storminess that develop, but a brief tornado can’t be ruled out.

The heavy rain

The German ICON model's depiction of heavy rain anticipated to fall from the Deep South to the East Coast between Wednesday and Friday. (Weatherbell)

Whether thunderstorms become severe, they are expected to generate heavy rain over a large area of the South. Some places are forecast to see upward of three inches by the end of the week. The heaviest will fall in Alabama and north Georgia, where a few pockets approaching or even exceeding four inches are possible.

“We’re looking at two to four inches in central Alabama over a three-day period,” said Nathan Owen, a meteorologist the Weather Service in Birmingham. “That doesn’t exactly stand out for flash flooding, since we’d need to get the rain in a shorter period of time. But we could see some local rivers and creeks rise.”

The heavy rain will then shift northeast.

Virtually the entire East Coast looks to get drenched Thursday into Friday. More than two inches of rain are possible in places such as Washington, D.C.; Philadelphia; Baltimore, and New York City. Boston, Providence and Hartford could top 1.5 inches of precipitation. Localized flooding with some three-to-four-inch amounts is possible.

Atmospheric moisture content as simulated by the European model for Friday morning. Note the deep moisture stretching from the tropics all the way northeast of Newfoundland. (Weatherbell)

As the stream of moisture exits to the east on Friday, it will be tightening into an “atmospheric river” over the Atlantic — illustrative of just how anomalously humid the lowest levels of the atmosphere will be beneath its influence. The same plume of moisture set to deluge the eastern United States will even lap at Newfoundland and the Canadian Maritimes on Friday. The juicy air it transports originates from near the Yucatán Peninsula in the tropics.

Winter weather potential in the interior Northeast

Much of the moisture that falls initially in New England will come down as sleet, freezing rain or snow. Areas in western Maine and northern Vermont/New Hampshire are most favored to see substantial accumulations, but parts of the interior high terrain of even Connecticut/Massachusetts could deal with a healthy dose of Thursday morning wintry slop.

Warm air will eventually stream up the East Coast, changing precipitation to rain in most areas (except far northern Maine) by Thursday night. By Friday, however, many areas in the interior Northeast, especially toward northern New England, will see precipitation change back to snow before the storm exits Friday night.

Chris Bianchi reported from Denver.