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Valid Friday December 15, 2017 to Tuesday December 26, 2017
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST December 12 2017Synopsis
: This outlook period begins with
the end of the amplified upper-level trough over the eastern U.S. The Week-1
period hazards are mostly attributed to a surface low-pressure system moving
from the Pacific Northwest southeast across the continental U.S. to the Gulf
Coast. Amplification to the upper-level pattern and an emerging upper-level
trough building into the northwest and central U.S. are likely to be
influential factors for Week-2. Persistent ridging in Alaska is expected to
weaken slightly toward the end of Week-1, only to re-intensify during Week-2.
Detailed Summary For Friday December 15 - Tuesday December 19:
- Heavy precipitation across portions of the
Pacific Northwest, Sat-Sun, Dec 16-Dec 17.
- Heavy rain across portions of the Lower Mississippi Valley and the
Tennessee Valley, Sun-Mon, Dec 17-Dec 18.
- Heavy snow across portions of the Northern Rockies and the Northern Great
Basin, Sat-Sun, Dec 16-Dec 17.
- High winds across portions of the Central Plains and the Southern Plains,
Fri-Sat, Dec 15-Dec 16.
- Much below normal temperatures across portions of the Northeast, Fri, Dec
- High winds across portions of mainland Alaska and the Aleutians, Fri-Mon,
Dec 15-Dec 18.
- Much above normal temperatures across portions of mainland Alaska, Fri-Sat,
Dec 15-Dec 16.
- High significant wave heights for coastal portions of the Aleutians,
Sun-Mon, Dec 17-Dec 18.
- Moderate risk of much below normal temperatures for portions of the Great
Lakes and the Northern Plains, Fri-Tue, Dec 22-Dec 26.
- Slight risk of much below normal temperatures for portions of the Northern
Plains, Northern Rockies and Pacific Northwest, Fri-Tue, Dec 22-Dec 26.
- Slight risk of much below normal temperatures for portions of the
Northeast, Wed-Thu, Dec 20-Dec 21.
- Moderate risk of much above normal temperatures for portions of mainland
Alaska, Fri-Tue, Dec 22-Dec 26.
- Slight risk of much above normal temperatures for portions of mainland
Alaska, Fri-Tue, Dec 22-Dec 26.
- Slight risk of heavy precipitation for portions of the Lower Mississippi
Valley and the Tennessee Valley, Fri-Sun, Dec 22-Dec 24.
- Severe Drought across the Lower Mississippi Valley, the Northern Plains,
Hawaii, the Northern Rockies, the Middle Mississippi Valley, the Southern
Plains, and the Southwest.
The day 3-7
period is likely to be a transition period in the upper-level pattern for much
of the continental U.S. To begin the period, much below normal temperatures are
likely for Dec 15 in the Northeast as an upper-level trough moves out of the
eastern U.S. northward to eastern Canada.
High winds are likely to form over the central and southern Plains Dec 15 -
16 as a cold front moves through the region. This area could also see an
elevated threat for fire weather, due to the high winds and low relative
humidity values. There is a possibility for a heavy rain event across the
western and central Gulf Coast for Dec 15 with the procession of the cold
front, though model guidance shows a lot of uncertainty.
Heavy precipitation is forecast in the Pacific Northwest for Dec 16-17,
following a low-pressure system bringing a cold front through the region. Among
the model guidance, the strength of this system has weakened compared to
previous model runs, so the threat for heavy precipitation (1 inch in 24 hours)
has been trimmed to areas of higher elevation. This frontal passage is also
likely to bring heavy snow (excess of 6 inches) to parts of northern Idaho and
western Montana for Dec 16-17.
As the surface low propagates eastward, it is expected to move southeast,
across the lower Mississippi Valley and Southeast. On Dec 17-18, heavy rain (>1
inches in 24 hours) is likely across the lower Mississippi Valley and Tennessee
Valley, as the associated cold front moves through.
An anomalously strong surface low is forecast to move over the Aleutian
Islands and along the Alaskan South Coast at the beginning of the period. This
is likely to bring high winds (gusts greater than 30 knots) to the Alaskan
Peninsula for Dec 15-16. As the low-pressure system moves into Gulf of Alaska,
there is a possibility for heavy precipitation along the southern Gulf Coast
and parts of the Panhandle.
Another low pressure system is likely to move east from the Bering Sea for
the latter half of Week-1, bringing a second threat of high winds (gusts
greater than 40 knots) and a threat for significant waves for to Aleutian
Islands for Dec 17-18. The upper-level ridge that has been present over central
Alaska will remain in the beginning of the period, continuing the likelihood
for much above normal temperatures for Dec 15-16. An upper-level trough may
move through in the middle of the period, bringing some moderation in
temperatures, but ridging is likely to return toward the end of
Week-1. For Wednesday December 20 -
Tuesday December 26:
Week-2 is likely to feature an amplification in the
upper-level pattern. An upper-level trough is expected to amplify over the
Great Lakes region and the north-central U.S. This pattern and teleconnections
upon a large 500-hpa positive height anomaly center over the Gulf of Alaska are
supportive of a cold outbreak over the northern U.S. A slight risk of much
below normal temperatures is posted across the northern tier of the US to the
Pacific Northwest toward the middle and end of Week-2, Dec 22-26. The moderate
risk of much below normal temperatures is forecast for the northern Plains and
western Great Lakes region where the GFS ensemble mean indicates temperatures
averaging near 10 degrees F below normal. This cold outbreak may extend the
much below normal temperatures further south through the central U.S. toward
the end of Week-2 and beyond.
A slight risk of heavy precipitation is forecast over the lower Mississippi
River Valley, resulting from a high-pressure system off the East Coast for Dec
22-24. Southerly return flow is likely to bring enhanced moisture into the
Persistent upper-level ridging over Alaska is likely to continue the
anomalous temperatures that have been affecting much of mainland Alaska for
most of the month of December. A slight risk for much above normal temperatures
is forecast throughout the Interior and western Alaska for Dec 22-26, with a
moderate risk of much above normal temperatures forecast for much of the
Interior of Alaska over the same time period.
The U.S. Drought Monitor, updated on Dec 5, shows an increase in areas of
exceptional drought (D2-D4) from 4.88% to 5.99%, a change of 1.11% over the
continental U.S. from the previous week.
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.