Weather across America

(NATIONAL) A stagnant upper-level jet stream pattern across the country will continue to result in a very broad and rather deep trough of low pressure across the West with a ridge of high pressure focused across much of the South and portions of the East as we head into the final week of 2021. 

Significant snowfall will continue for portions of West Coast mountain ranges and the Intermountain West; along with record cold for parts of the West Coast. Record warm temperatures are expected for broad areas of the South through the early part of the week while bitterly cold air expands across the Northern Plains. And heavy snow is expected in the Northern Plains and Upper Midwest Sunday night with some light snow and ice accumulations possible in the northern Mid-Atlantic and interior Northeast Monday morning.

The most impactful weather remains out West as a steady fetch of Pacific moisture and anomalously cold temperatures through the atmospheric column will support heavy snowfall rates. The snow will continue to blanket the mountain ranges of the West, with an emphasis on areas from the Sierra Nevada to the central Rockies. 

The Sierra Nevada, which has already picked up locally several feet of snow over the last few days, is expected to see an additional 2 to 5 feet of snow going through Tuesday. Meanwhile, other ranges such as the Shasta, the Oregon Cascades, the Wasatch, Tetons, and Colorado Rockies can expect anywhere from 1 to 2 feet of additional snow. 

Hazardous travel conditions are expected across these ranges, prompting the issuance of Winter Storm Warnings that extend as far south as the Transverse Range of Southern California. Measurable and disruptive snowfall accumulations are also on tap again in the Pacific Northwest, including in both the Portland and Seattle metro areas. 

This stretch of wintry weather is able to occur thanks to frigid temperatures, some of which may tie or break record cold highs and lows going through Tuesday. 

On the flip side, remarkably warm temperatures remain anchored over the southern U.S. and will continue into the first half of the upcoming week. The mixing of unusually warm temperatures, low humidity levels, and windy conditions has prompted the Storm Prediction Center to issue a Critical Risk of fire weather across portions of the central and southern High Plains, including the Texas and Oklahoma Panhandles. 

Temperature departures for Monday and Tuesday will be greatest stretching from the southern Plains to the Mid-South where temperatures reaching well into the 70s and 80s will equate to departures of 25 to 35 degrees above normal. A passing cold front will cool down part of the Mid-Atlantic on Monday, but the very warm temperature off the southwest will begin to encroach on parts of the Ohio Valley and the Mid-Atlantic again by the middle of the week.  

The one exception to the anomalous warmth (aside from the cooler, stormy set-up along the West Coast) is the Northern Plains, where bitterly cold Canadian air will be filtering southeast behind a strong cold front. Morning lows are forecast to be sub-zero over the northern Plains of Montana and North Dakota with daytime highs struggling to get above zero and through the single digits to start the new week. In fact, low temperatures Monday morning and Tuesday morning are locally forecast to be as much 20 to 30 degrees below zero.

Elsewhere, the first in a series of storm systems to exit the Intermountain West is set to produce heavy snow in the Northern Plains and Upper Midwest through Sunday night and early Monday morning. This snow will advance across the northern Great Lakes on Monday before tapering off Monday evening. Snow totals through Monday night are likely to range between 6 to 12 inches with locally higher totals possible, especially in the Minnesota Arrowhead. 

This same storm will usher in light snow and freezing rain accumulations to the northern Mid-Atlantic and interior Northeast Monday morning. While totals look to be light, slick travel conditions and delays are possible in these areas on Monday.