US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
251 PM EST Tue Dec 08 2020
 
Valid Friday December 11 2020 - Tuesday December 15 2020
 
Hazards:
- Heavy precipitation from portions of northern California into the Pacific Northwest, Sat-Sun, Dec 
12-Dec 13.
- Heavy precipitation across northwestern Washington, Tue, Dec 15.
- Heavy precipitation from portions of northern Illinois to lower Michigan, Sat, Dec 12.
- Heavy precipitation across portions of northern Maine, Sun, Dec 13.
- Heavy rain from across portions of lower Michigan down across the Midwest, the mid-Mississippi 
Valley, and to the west of the lower Mississippi Valley, Fri-Sat, Dec 11-Dec 12.
- Heavy snow across portions of the higher elevations of northeastern Washington, Idaho, and the 
northern Rockies, Sun-Mon, Dec 13-Dec 14.
- High winds across portions of interior Alaska, Sun-Tue, Dec 13-Dec 15.
 
Detailed Summary:

An active pattern in the northeastern Pacific will continue to send low pressure/frontal systems 
toward the Pacific Northwest into the medium range period (Friday, Dec 11 - Tuesday, Dec 15), with 
the next wave of precipitation likely reaching the coast by Friday night.  Lower-elevation rain and 
mountain snow will likely become heavy through much of the weekend as they will likely spread well 
inland across Idaho and into the northern Rockies such as the Wind River Mountains/Tetons on Sunday 
into Monday.  Generally higher amounts of precipitation should occur in the terrain of the Coastal 
Range and Cascades.  Gusty winds will likely accompany the passage of a warm front followed by a 
cold front across the Pacific Northwest this weekend into Monday.  By next Tuesday, there appears 
to be decent model agreement on the arrival of the next wave of moisture across northwestern 
Washington.

Meanwhile, an upper trough exiting into the central Plains will likely evolve into a deepening 
surface low pressure system that will track from the central Plains to the lower Great Lakes Friday 
into the weekend.  Ahead of the cold front associated with this low, Gulf of Mexico moisture will 
likely be lifted to result in a quick round of heavy rainfall Friday night to the west of the lower 
Mississippi Valley, and then across the Midwest toward the lower Great Lakes on Saturday.  In the 
mean time, cold air from Canada is forecast to settle southward and then wrap around the backside 
of the cyclone.  This will likely change the precipitation from rain to snow near and just north of 
the cyclone track.  A heavy precipitation area is indicated for this changeover where the 24-hour 
QPF amounts are an inch or higher.  There is still some uncertainty regarding the storm track but 
the models appear to have settled into the present forecast track.  Finally, it is worth mentioning 
that the ECMWF has introduced the possibility of a rapidly intensifying cyclone early next week 
across the Southeast to the Mid-Atlantic.  This scenario has not been incorporated into the current 
hazards map.

Warmer than average temperatures by 10 to 20 degrees are forecast to push from the central U.S. 
Friday into the eastern U.S. over the weekend ahead of the upper-level trough.  Near average 
temperatures (with maximum temperatures maybe slightly below) are expected for the Intermountain 
West through Saturday.

Over Alaska, a persistent high pressure system centered in the Arctic Ocean will be contrasted with 
deep cyclones moving across the northeastern Pacific to create a tight pressure gradient over 
mainland Alaska.  Interior central Alaska will likely see the highest threat for high winds late in 
the weekend into early next week.  The winds could cause blowing snow and lowering of visibility.

Kong