U.S.: Arrival of large arctic air mass, widespread hazardous travel from west to Great Lakes
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First of all, snow will spread south and eastward across the West followed by the northern tier of the country into Wednesday as a trough deepens over the West as embedded shortwave energy helps to push multiple frontal systems across the region.
Ample moisture associated with the Pacific fronts will combine with increasingly warm and moist air streaming in from the Gulf of Mexico, and then clash with the arctic air surging in from Canada to fuel heavy snow rates.
Snow totals of 1-2 feet, locally higher, are expected for most of the mountain ranges across the West.
The heavy accumulating snow should be limited to higher elevations in the mountains, with a lighter rain/snow mix for the interior valleys.
Moderate to locally heavy rain as well as some thunderstorms are forecast for the lower elevation/coastal Pacific Northwest Tuesday.
Snow will begin to mix in with the rain as temperatures cool Tuesday night into Wednesday, and there is even a chance to see some light snow in the central California valleys.
In addition to the snow, a deepening low pressure system over the Great Basin and multiple fronts pushing through the West will bring widespread very strong, gusty winds of 50-60 mph, locally as high as 80 mph in favorable terrain locations, to most of the West and adjacent High Plains through Wednesday.
These winds will lead to blizzard conditions for portions of the northern/central Rockies, and areas of blowing dust across the interior Southwest to the southern High Plains.
Meanwhile, an elevated risk of fire weather is forecast to continue for the southern High Plains through Wednesday in stark contrast with the snowy and bitterly cold weather to the north.
An axis of heavy snow will expand eastward across the northern Plains and into the upper Midwest/Great Lakes and interior Northeast/New England through Wednesday as a strong low pressure system consolidates in the lee of the Rockies and moves northeastward across the Plains then into Midwest.
Some locations across the Upper Midwest/Great Lakes will have only a very brief reprieve from a clipper system exiting the region before this next round of heavy snow moves in.
There is a high probability of snow totals over 8" front South Dakota eastward through southern Minnesota, northern Wisconsin, and the Upper Peninsula/northern Lower Peninsula of Michigan.
Locally higher totals of 1-2 feet are possible, particularly across southern Minnesota.
Heavy snow rates of 1-2" per hour and gusty winds producing areas of blowing snow will lead to treacherous, potentially impossible travel conditions and possible power outages.
By Thursday, much of central New England will see snow streaming in from the southwest as a low pressure wave develops along the slowing warm front across the Mid-Atlantic.
A zone of freezing rain and/or sleet can also be expected farther south toward southern New England before a slow tapering off trend sets in during the day on Thursday.
Snow totals of 12" or more are also likely for higher elevations in the Interior Northeast including the Adirondacks and the Green and White Mountains, with lighter accumulations of around 2-4 inches at lower elevations across upstate New York and central New England.
A heavy wintry mix of snow, sleet, and freezing rain is expected along the southern end of the snow axis across the upper Mississippi Valley and into the lower Great Lakes.
Ice accumulations of 0.1-0.25", locally higher, are possible, especially across lower Great Lakes Wednesday into early Thursday.
Meanwhile, the upper Midwest should see snow and blustery conditions continuing as the center of the main low tracks across the lower Great Lakes.
Slow improvement of these conditions should gradually spread from west to east during the afternoon on Thursday.
Widespread showers and thunderstorms are forecast to develop near and north of the low pressure system's leading warm as a cold front transforms into a dry line structure across west of the Mississippi Valley late Tuesday night into Wednesday.
Intense low-level and upper-level dynamics as well as additional moisture streaming northward from the Gulf may lead to some locally heavy rain rates.
There is a Slight Risk of Excessive Rainfall (level 2/4) from northeast Missouri into northern Illinois, northern Indiana, and southern Michigan where there may be a few scattered instances of flash flooding.
It should be noted that uncertainty with the location of the warm front could shift the location where the heavier, convective rains transition to the heavy wintry mix expected just to the north.
Strong low-level, deep-layer shear and expected instability have also prompted a slight risk of severe weather (level 2/5) from the Storm Prediction Center from central Oklahoma northeastward through central Missouri for the chance of some damaging winds.
Besides the numerous weather hazards expected this week, another major story will be the significantly anomalous warm temperatures for February over the East and cold temperatures over the West.
Highs on Wednesday will be 20-30 degrees above average for many locations across the southern Plains, Midwest, and Southeast and 20-30 degrees below average over the northern/central Plains and much of the West.
Many record-tying/breaking highs can be expected particularly for the Ohio/Tennessee Valleys, where temperatures will be into the 70s, and closer to the Gulf Coast/Florida, where highs will be into the 80s.
Many record-tying/breaking minimum high temperatures will also be possible over the West, with highs in the 30s and 40s for the Pacific Northwest and in the 50s for California.
Bitterly cold wind chills 20-30 degrees below zero are expected for the Northern Plains.
These highly anomalous temperatures are forecast to continue later into the week.
Unsettled weather is forecast to continue spreading farther south down the West Coast on Thursday as additional upper-level energy digs southward off the coast.
Meanwhile, the Pacific Northwest will begin to clear out by then as the arctic air mass settles into the region with low temperatures dipping below freezing even at the coast.
California will see rain becoming more widespread during the day, with snow picking up in intensity over the mountains, including the Sierra Nevada.
Mixed rain and snow is also expected to reach into the higher elevations of the interior Southwest. ■