US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
233 PM EST Tue Dec 17 2019
 
Valid Friday December 20 2019 - Tuesday December 24 2019
 
Hazards:
- Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great 
Basin, Fri-Sun, Dec 20-Dec 22.
- Heavy rain across portions of the Southeast, Sat-Mon, Dec 21-Dec 23.
- Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Fri, Dec 20.
- Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Fri-Sat, Dec 
20-Dec 21.
- Flooding possible across portions of the Central Appalachians, the Pacific Northwest, and the 
Ohio Valley.
- Flooding occurring or imminent across portions of the Northern Plains.
- Much below normal temperatures across portions of the Lower Great Lakes, the Mid-Atlantic, the 
Northeast, and the Northern Appalachians, Fri, Dec 20.
- High significant wave heights for coastal portions of California and the Pacific Northwest, 
Fri-Sat, Dec 20-Dec 21.
- High winds across portions of the Aleutians, Sun-Mon, Dec 22-Dec 23.
- Much below normal temperatures across portions of mainland Alaska, Fri-Mon, Dec 20-Dec 23.
 
Detailed Summary:

The biggest threat in the medium range period (Thursday Dec 19 - Monday Dec 23) for the continental 
U.S. will be a persistent atmospheric river funneling moisture into the West.  The moisture plume 
is expected to focus in the Pacific Northwest Friday and slowly move south on Saturday into 
Northern California.  Heavy snow is forecast in higher elevations of the Cascades and Northern 
Rockies, with heavy rain in lower elevations.  Additionally, the flow pattern will be such that 
significant waves are possible along the coast of Oregon into far Northern California on Friday 
into Saturday.

On Friday, the Lower Great Lakes to Eastern Seaboard should see colder than average temperatures 
before moderating. Warmer than average temperatures are expected in the central U.S. underneath 
upper-level ridging even though the area is not depicted on the hazards map.

A surface low off the Southeast U.S. coast, will bring locally heavy rainfall along the Atlantic 
coast of Florida and the Carolinas on Saturday into Monday.  

A general upper-level troughing pattern over Alaska is expected to cause below average temperatures 
there, which has been unusual this year.  The largest cold temperature anomalies will be south of 
the Brooks Range into the central part of the state. Low pressure in the Gulf of Alaska will lead 
to precipitation for the Alaska Panhandle through the weekend, but rain and snow look to be lighter 
than hazards criteria. Then, there is potential for a strong low to cause high winds over 50 knots 
for the Aleutians. There is uncertainty with the track of this low, with the GFS model runs 
tracking north of the Aleutians with a deeper low, and the ECMWF/CMC tracking just south of the 
islands with a slightly weaker, yet still robust low. The latter was preferred for now, but 
regardless of the exact track, high winds seem like a possibility.

In Hawaii, strong trade winds are likely Friday and Saturday into Sunday morning as a anomalously 
strong surface high pressure system sets up north of the islands. Then there is the possibility of 
heavy rain coming in ahead of a cold front Tuesday, Christmas Eve, there.

Ziegenfelder