US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
324 PM EST Thu Dec 10 2020
 
Valid Sunday December 13 2020 - Thursday December 17 2020
 
Hazards:
- Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great 
Basin, Sun, Dec 13 and Tue-Thu, Dec 15-Dec 17.
- Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sun, Dec 13 and 
Tue-Thu, Dec 15-Dec 17.
- Heavy precipitation across portions of the Northeast, Sun, Dec 13.
- Heavy rain across portions of the Mid-Atlantic, Wed, Dec 16.
- Heavy precipitation across portions of the Central Appalachians, the Mid-Atlantic, and the 
Northeast, Wed-Thu, Dec 16-Dec 17.
- Heavy snow across portions of the Great Lakes, the Central Appalachians, the Mid-Atlantic, and 
the Northeast, Wed-Thu, Dec 16-Dec 17.
- High winds across portions of mainland Alaska, Sun-Tue, Dec 13-Dec 15.
 
Detailed Summary:

During the medium range period (Sunday, Dec 13 - Thursday, Dec 17), a series of low 
pressure/frontal systems are forecast to pass through the Northwest CONUS. This combined with 
periods of above average moisture should lead to periods of heavy precipitation there. The first 
round of precipitation should occur this weekend, with higher elevation snow and lower elevation 
rain in the Pacific Northwest, and generally the highest precipitation amounts are expected in the 
Coastal Ranges and the Cascades Sunday. Additionally, snow is likely in areas of the Northern 
Rockies. A lull in the precipitation amounts should occur Monday before additional rounds of 
moisture and precipitation come in come in Tuesday through Thursday, spreading into northern 
California/the Sierra Nevada by midweek. 

A low pressure system is forecast to deepen as it moves across the Midwest during the short range 
period. As the medium range period begins Sunday, this low is expected to track across southeastern 
Canada, and as such should spread precipitation mainly to northern Maine in the U.S., and the 
precipitation type could be changing between wintry precipitation and rain.

By Wednesday and Thursday of next week, low pressure systems in the central U.S. may consolidate as 
they move eastward and start to take a nor'easter track. Model guidance currently seems to have 
fairly good agreement (for 6-7 days out) with a low track near the Mid-Atlantic/Northeast coast. 
This could spread frozen precipitation on the backside of the surface low in the cold air, with 
snow possible inland with best chances west of the I-95 corridor. Small shifts in the track of this 
low could lead to sizable differences in the impacts, including the rain/frozen precipitation line 
of the system, and how far west the precipitation spreads. So this will continue to be monitored in 
the coming days. 

Elsewhere in the CONUS, a low pressure system is expected to move quickly across the south-central 
to southeastern U.S. Sunday and Monday, spreading rain to the Lower Mississippi Valley/Southeast 
and even the potential for some wintry weather across Oklahoma and the Southern Appalachians. This 
does not appear to be hazardous given low precipitation amounts due to the quick movement of the 
low. Anomalously hazardous temperatures are not expected.

Over Alaska, low pressure systems in the Gulf of Alaska and northeastern Pacific will be contrasted 
with a strong and persistent high pressure system north of the state for the first half of the 
week, leading to a tight pressure gradient across the mainland and the potential for high winds. 
The northwest coast of the state (Seward and Lisburne Peninsulas) and interior central Alaska 
should see the best chance for high winds, which could create blowing snow, reduce visibility, and 
cause travel hazards. As the week progresses, model uncertainty increases with the potential for 
another low pressure system and its track toward the Panhandle. There could be high winds and heavy 
precipitation associated with this low around midweek, but opted not to include hazard areas there 
for now given the model discrepancies in low track and precipitation amounts and timing.

Tate