Weather Prediction Center

National Oceanic and Atmospheric Administration

US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
333 PM EST Thu Dec 17 2020

Valid Sunday December 20 2020 - Thursday December 24 2020

Hazards:
- Heavy precipitation across portions of the Pacific Northwest, Sun-Mon, Dec 20-Dec 21.
- Heavy snow across portions of central Idaho into northeastern Oregon, Sun, Dec 20.
- Heavy snow across portions of the northern Rockies, Sun-Tue, Dec 20-Dec 22.
- Flooding possible across portions of western Washington.
- Flooding likely across portions of northwestern Washington.
- High winds across portions of western Montana, Sun, Dec 20.
- High winds across portions of southeastern Wyoming, Sun-Mon, Dec 20-Dec 21.
- Heavy precipitation across the Alaska Peninsula into southern Alaska, Mon-Wed, Dec 21-Dec 23.
- Heavy precipitation across portions of the Alaska Panhandle into southern Alaska, Wed, Dec 23.
- High winds across the Alaska Peninsula into southern Alaska, Mon-Tue, Dec 21-Dec 22.
- Much below normal temperatures across much of the lower elevations of eastern Alaska, Sun-Tue,
Dec 20-Dec 22.

Detailed Summary:

The active onshore flow pattern across the Pacific Northwest will continue into the medium range
period (Sunday, Dec 19 - Thursday, Dec 24) as additional moisture-laden and energetic frontal
systems are forecast to impact the area with heavy rain for the lower elevations and heavy snow
from the Coastal Ranges to the Oregon and Washington Cascades. Further inland, heavy snow is
expected across the northern Rockies in Idaho, northeastern Oregon, and western Montana late this
weekend into Monday, before tapering off later on Tuesday as a developing low pressure system exits
into the northern Plains. In the wake of the system, winds across parts of the eastern slopes of
the northern Rockies could be impacted by very strong Chinook winds on Sunday and into Monday. By
the middle of next week, it appears that the Pacific Northwest will catch a break from the active
weather as a strong ridge of high pressure aloft is forecast to build near the West Coast.

Along the Gulf Coast, moisture from the Gulf of Mexico is forecast to return by the middle of next
week. A cold front approaching from the Great Plains together with a developing low pressure wave
could increase the chance of heavy rainfall from portions of the Gulf Coast to the interior
Southeast, possibly up the Appalachians by next Wednesday into Thursday. The uncertainty is
currently too high to warrant a heavy rain area at this time.

Temperature-wise, the active zonal flow from the Pacific will result in above to well above normal
temperatures across the northern tier states into the central Plains with temperatures ranging
anywhere from 10 to 25 degrees above normal early in the medium range period. Though anomalous,
these temperatures are not expected to be hazardous. As the aforementioned ridge of high pressure
builds near the West Coast, colder air from central Canada will be encouraged to dip into the
Plains by next Wednesday into Thursday, resulting in below normal temperatures to return.

Meanwhile, the active synoptic pattern over the northeastern Pacific will continue to direct large
and energetic cyclones downstream toward Alaska during the medium range period. One such cyclone is
forecast to bring widespread gale to storm force winds and heavy precipitation across the Aleutians
on Sunday before spreading through the Alaska Panhandle on Monday. By Tuesday, much of the Alaska
Peninsula into the southern portion of mainland will likely be engulfed with very gusty south to
southeasterly winds along with heavy precipitation which should linger into Wednesday. In addition,
some of the heavy precipitation is forecast to reach the Alaska Panhandle on Wednesday. There could
be another cyclone approaching the Alaska Panhandle on Thursday but uncertainty remains high at
this time. Farther inland across the lower elevations of eastern Alaska, much below average high
and low temperatures are expected to continue into early next week, where temperatures are expected
to reach as low as -35 to -40F. However, a general warm-up is forecast spread from southwest to
northeast through Alaska from early to middle of next week.

Kong