US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
321 PM EST Mon Dec 16 2019
 
Valid Thursday December 19 2019 - Monday December 23 2019
 
Hazards:
- Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great 
Basin, Thu-Sun, Dec 19-Dec 22.
- Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Thu-Fri, Dec 
19-Dec 20.
- Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Thu-Sat, Dec 
19-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 Great Lakes, the Mid-Atlantic, the 
Northeast, and the Central Appalachians, Thu-Fri, Dec 19-Dec 20.
- High significant wave heights for coastal portions of California and the Pacific Northwest, 
Thu-Fri, Dec 19-Dec 20.
- High winds across portions of the Aleutians, Sun, Dec 22.
- Much below normal temperatures across portions of mainland Alaska, Thu-Mon, Dec 19-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 Thursday and 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. The precipitation is currently forecast to break 
records for 24 hour precipitation amounts in and around Portland, Oregon on Friday. Additionally, 
the flow pattern will be such that significant waves are possible along the coast of Oregon into 
far northern California on Thursday night into Friday.

Thursday into Friday, the Lower Great Lakes region to Eastern Seaboard should see colder than 
average temperatures before moderating by the weekend. Warmer than average temperatures are 
expected in the central U.S. underneath upper-level ridging.

Another area to watch is a surface low off the Southeast U.S. coast, which could help cause locally 
heavy rainfall along the Atlantic coast of Florida and the Carolinas for the weekend. On the 
northern edge of the precipitation, light wintry precipitation could occur in the Southern/Central 
Appalachians, but these threats will continue to be monitored, as run-to-run model inconsistency 
remains high at this time.

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 this weekend 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 early next week there.

Tate