US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
302 PM EST Fri Dec 10 2021
 
Valid Monday December 13 2021 - Friday December 17 2021
 
Hazards:
- Heavy precipitation across portions of California, the Central Great Basin, and the Pacific 
Northwest, Mon, Dec 13.
- Heavy precipitation across portions of California, the Central Great Basin, the Pacific 
Northwest, the Northern Great Basin, and the Southwest, Tue, Dec 14.
- Heavy precipitation across portions of California, the Central Great Basin, and the Pacific 
Northwest, Wed-Thu, Dec 15-Dec 16.
- Heavy precipitation across portions of the Central Rockies, the Great Lakes, the Central Great 
Basin, and the Upper Mississippi Valley, Wed, Dec 15.
- Heavy precipitation across portions of the Central Great Basin, Tue-Wed, Dec 14-Dec 15.
- Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon, Dec 13.
- Heavy snow across portions of the Northern Rockies, Tue, Dec 14.
- Heavy snow across portions of the Central Rockies, Wed, Dec 15.
- High winds across portions of the Central Plains, the Central Great Basin, the Northern Plains, 
the Northern Rockies, the Central Rockies, the Upper Mississippi Valley, and the Great Lakes, 
Wed-Thu, Dec 15-Dec 16.
- Much below normal temperatures across portions of mainland Alaska and the Aleutians, Mon, Dec 13.
 
Detailed Summary:

The period begins with an amplified trough digging southward near the West Coast, ushering in heavy 
rain and mountain snows to parts of the Pacific Northwest and California. The Sierra Nevada will 
likely see big snowfall totals as this heavy precipitation event pushes further to the south into 
Tuesday. The precipitation will spread across the Great Basin and into Southern/Central Rockies by 
midweek as the frontal system moves onshore and inland across the West. This will increase the 
threat for heavy snowfall for the favored higher elevations with a rain/snow mix or all rain in the 
lower elevations. meanwhile tapering off its intensity behind the front.

Depending on the strength and timing of this trough, high winds may impact portions of the Central 
to Southern Rockies points east into the High Plains starting next Wednesday. There is potential 
for a robust low pressure system to develop and track across the Plains Wednesday into Thursday, 
which may yield a swath of wintry precipitation from the Northern/Central Rockies and across the 
northern U.S. although the specific details are uncertain at this time. Another moisture plume 
approaching the West Coast could bring the next round of heavy precipitation into northern 
California next Wednesday into Thursday.

An anomalous warm ridge of high pressure is expected to build over the central U.S. while the 
trough slowly progresses across the West. This will allow for temperatures to range 20 to 40 
degrees above seasonal normal for mid-December across the Midwest/Upper Midwest! While this is 
quite notable, it is not necessarily hazardous. It will help melt some of the recent snows across 
the north-central U.S. Behind the exiting low, a rather pronounced upper ridge is forecast to build 
and extend across the western U.S. toward the Great Plains. This pattern will encourage downslope 
winds to develop on the lee side of the Northern Rockies under strong pressure gradient during the 
weekend. Parts of eastern Wyoming will remain under an enhanced threat of strong Chinook winds on 
Sunday before subsiding Sunday night.

For Alaska, a large low pressure system near the southern coast of the state weaken as it continues 
to drift further south. Prior to its departure though, a large portion of the state will observed 
much below normal temperatures with many locations registering 20 to 30 degrees colder. The 
temperatures will be slowly trending to be less frigid by the end of the extended. The latest 
guidance is depicting another potent winter storm to enter the Bering Sea before impacting parts of 
the Aleutians with rain and strong gusty winds, followed by snowy and windy weather across 
southwestern Alaska. These conditions appear to linger into midweek with additional disturbances 
pushing into western Alaska.

Campbell/Kong