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Valid Monday November 20, 2017 to Friday December 01, 2017
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST November 17 2017Synopsis
: A series of low pressure systems
are likely to affect the northwestern U.S. early in the period, while a surface
low develops across the Gulf of Mexico. Arctic high pressure is forecast to
prevail across eastern mainland Alaska during the next week, while a couple of
low pressure systems track southeast from the Bering Sea to the Gulf of Alaska.
During Week-2, an area of mid-level low pressure is forecast along the East
Coast with more zonal flow developing across Alaska and the north Pacific.
Summary For Monday November 20 - Friday November
- Heavy precipitation across parts of northern
California and the Pacific Northwest, Mon-Wed, Nov 20-22.
- Much below-normal temperatures across eastern mainland Alaska, Mon-Fri, Nov
- High winds for the Alaska Peninsula and Kenai Peninsula, Mon, Nov 20.
- High winds for southeast mainland Alaska and the Alaska Panhandle, Mon-Wed,
- Slight risk of much below-normal temperatures for parts of the eastern
U.S., Sun-Tue, Nov 26-28.
- Severe Drought across parts of the lower and middle Mississippi Valley,
Great Plains, Northern Rockies, Arizona, and Hawaii.
A highly amplified ridge is forecast to remain anchored over the
Aleutians and Bering Sea with an anomalous trough offshore of the West Coast
through the early part of the period. This longwave pattern results in
continued hazardous precipitation (mostly rain) across the western U.S. and
much below-normal temperatures for parts of mainland Alaska.
An upper-level trough and onshore flow are likely to promote a series of
low pressure systems affecting the Pacific Northwest and northern California
from Nov 20 to 22. Therefore, periods of heavy precipitation (liquid equivalent
amounts of 1.5 inches or more per 24 hours) are forecast for these areas.
Southwesterly flow is likely to maintain high snow levels, above major pass
level. Any snowfall accumulations are expected to be limited to the highest
elevations of the Cascades.
Later in this period, deterministic model solutions on Nov 17 are in better
agreement with a shortwave trough separating from the westerlies as the polar
jet retreats north of the 40th parallel. This shortwave trough is expected to
progress southeast into the Gulf of Mexico and cause surface cyclogenesis along
a front. The heaviest rainfall, associated with this developing surface low, is
forecast to remain offshore of the Gulf Coast.
A moderately strong low pressure system (around 968-hpa) forecast across
the northern Gulf of Alaska on Nov 20, is expected to bring high winds to the
Alaska Peninsula and Kenai Peninsula. The surface low across the Gulf of Alaska
coupled with an arctic high over the Yukon is likely to promote another surge
of arctic air west and south into mainland Alaska. Based on the preferred 0Z
ECMWF model, much below-normal temperatures are forecast across eastern
mainland Alaska, as minimum temperature anomalies of -20 degrees F or greater
are forecast. The cold air pooling across the Alaska Range and other
mountainous areas in southern parts of the state is likely to give rise to the
potential for locally high winds in association with katabatic flow. A gap wind
event is also likely in the vicinity of Yakutat, where winds could gust to 100
mph or greater as the arctic air spills southward off of the Hubbard Glacier. A
high wind hazard is posted for parts of southeast mainland Alaska and the
northern Alaska Panhandle from Nov 20 to 22. For Saturday November 25 -
Friday December 01:
The low pressure system that forms in the Gulf of
Mexico prior to this period may interact with an upper-level trough along the
East Coast. Another possible outcome is for the northern stream to amplify and
induce cyclogenesis near New England. The axis of the longwave trough is
expected to be far enough to the east to prevent any major storminess along the
East Coast but the evolving pattern should be closely monitored.
The persistent 500-hpa ridge over the Bering Sea along with a building
500-hpa ridge centered at the Davis Strait is likely to result in at least a
brief period with a negative Arctic Oscillation (AO) index during mid-November.
The largely negative AO index is expected to be short-lived since model
solutions continue to indicate at least a temporary weakening of the high
amplitude ridge over the Bering Sea. However, GFS and ECMWF ensemble means
maintain the large positive 500-hpa anomaly center over the Davis Strait and
Greenland, which favors near- to below-normal temperatures across the eastern
U.S. during Week-2. Temperature guidance from the ECMWF model on Nov 17 along
with the evolving longwave pattern at the high latitudes supports a slight risk
of much below-normal temperatures across parts of the eastern U.S. Cold air
advection is expected to result in lake-effect snow downwind of the Great Lakes
early in Week-2 although amounts are uncertain.
Temperatures are likely to moderate across mainland Alaska during Week-2,
as the upstream ridge weakens and anomalous northeasterly flow ends. Therefore,
the much below-normal temperature hazard is discontinued in today's outlook.
The U.S. Drought Monitor, valid on Nov 14, indicates that severe to
exceptional (D2-D4) drought covers 3.52 percent of the continental U.S., which
is a slight increase from 2.90 percent last week. Severe drought has expanded
this month across Arkansas and Missouri.
Forecaster: Brad Pugh
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.