Weather across America

(NATIONAL) March-like temperatures to engulf much of the eastern two-thirds of the Lower 48 (additional frosts/freezes expected), warmth in the Southwest, more showers and storms to develop in the South-Central U.S, damp and dreary conditions to linger in New England, and mountain snow and coastal and valley showers to spread from the Pacific Northwest to the northern and central Rockies. 

A large dome of Canadian high pressure has taken hold of the Nation's Heartland and will spread east into the Mid-South and Mid-Atlantic during the first half of the week. Accompanying this large high pressure system is an even more extensive area of below normal temperatures. Monday's daily low temperatures from the Middle Mississippi Valley on east into the Ohio/Tennessee Valleys and northern Mid-Atlantic will plunge near or below freezing, prompting the issuance of Freeze Warnings and Frost Advisories in some of these areas. A handful of daily record low minimum and maximum temperatures are possible as well. 

Temperatures east of the Rockies and into the Mid-Atlantic are likely to average 10-15 degrees below normal through mid-week, making it feel more like March outside rather than late April. The unusually chilly temperatures will be cold enough to where some precipitation in the Upper Great Lakes will fall in the form of snow at times on Monday, but little in the way of accumulation is expected. Farther west, an upper level ridge strengthening off the coast tomorrow and into mid-week will foster warmer than normal temperatures in California and the Desert Southwest. By Tuesday, portions of California's Great Valley will rise well into the 80s with desert areas of southern California and Arizona eclipsing the 90 degree mark.

One of the reasons for the cooler than normal temperatures regime in the eastern and central U.S. is a large, spiraling upper low over the Great Lakes and Ontario being blocked by a ridge of high pressure over Quebec and the Davis Strait. This ridge is forcing a storm system off the Northeast coast to become stationary, making for a dreary and unsettled first half of the work-week throughout much of New England. The upper low will also spark scattered showers and thunderstorms in the Great Lakes on Monday.

Farther south, rounds of strong storms are rapidly tracking south of Texas Sunday afternoon and drier conditions return Sunday night and into Monday. 

Meanwhile, a cold front over Florida will spawn strong-to-severe thunderstorms over The Sunshine State on Monday. The Storm Prediction Center does have a Marginal Risk for severe weather from Cape Canaveral on south to the Miami metro area. By Tuesday morning, a potent upper low tracking into northern Utah is set to be the next weather-maker in the South-Central U.S. as widespread showers and thunderstorms form along and north of a warm front positioned over northern Texas. 

The Storm Prediction Center has a Slight Risk for severe weather in southeast Texas, while the Weather Prediction Center has a Slight Risk for Excessive Rainfall in and around both the ArkLaTex and eastern Oklahoma. The lower elevations of the Colorado Front Range and the high plains of Colorado could also contend with Excessive Rainfall, as highlighted with the issuance of a Marginal Risk for Tuesday.

The aforementioned upper low responsible for the next round of severe weather and heavy rainfall in the South-Central U.S. on Tuesday will also be the reason for more mountain snow and valley/coastal rain from the Pacific Northwest Sunday night and Monday to the northern and central Rockies for Monday and Tuesday. 

As much as 6-12" of snowfall are forecast in the highest elevations of the Cascade Range and both the northern and central Rockies. There are Winter Storm Watches up for the highest elevations of southwest Montana that are in effect from Monday evening through Tuesday morning. The Colorado Rockies feature the best odds of seeing snowfall totals greater than 12" through Tuesday evening according to WPC snowfall probabilities (as high as 70% in the tallest peaks). 

Meanwhile, precipitation will be in the form of rain along the Pacific Northwest coast and in the valleys of the Intermountain West. In wake of this storm, the building ridge of high pressure along the West Coast will give the western third of the U.S. a short-lived reprieve from the active pattern to start the week and allow for drier and more tranquil weather to take shape for Wednesday.