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Valid Monday December 17, 2018 to Friday December 28, 2018
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST December 14 2018Synopsis
: Upper level low pressure
forecasted off the west coast of the US will continue to favor an onshore flow
and continued chances for heavy precipitation for coastal Washington and Oregon
throughout most of the day 3-7 period. By day-7, upper level high pressure is
forecast to build in allowing these regions to dry out. The eastern US is
projected to be mainly under the influence of upper level low pressure, with
some uncertainty toward the end of the 3-7 day period regarding the pattern
evolution in regards to surface low development. This uncertainty will continue
to dominate into week-2 as the troughing in the east may favor cyclone
development, but model signals remain inconsistent with respect to both timing
and magnitude, precluding any specific threats from being issued.
Detailed Summary For
Monday December 17 - Friday December 21:
- Heavy precipitation across portions of the
Pacific Northwest and the Northern Great Basin, Mon-Thu, Dec 17-Dec 20.
- Flooding possible across portions of the Southeast, the Middle Mississippi
Valley, the Mid-Atlantic, the Southern Plains, and the Ohio Valley.
- Flooding occurring or imminent across portions of the Southeast, the
Mid-Atlantic, and the Southern Plains.
- Flooding likely across portions of the Southeast.
- High winds across portions of the Pacific Northwest, Mon-Tue, Dec 17-Dec 18.
- High significant wave heights for coastal portions of the Pacific
Northwest, Mon-Tue, Dec 17-Dec 18.
- Much below normal temperatures across portions of southwest mainland
Alaska, Wed-Thu, Dec 19-Dec 20.
- Severe Drought across the Central Plains, the Central Rockies, the Central
Great Basin, the Southern Plains, the Northern Great Basin, the Southern
Rockies, California, the Alaska Panhandle, the Pacific Northwest, and the
On 12/17 surface low pressure is
forecast to be moving off the New England coast. In contrast to yesterday, the
deterministic run of today's 00z ECMWF now keeps the system closer to the coast
compared to the 06z GFS. The ECMWF solution would result in a period of
moderate to heavy snow for eastern Maine on 12/17. However, the ensemble
solutions from today's ECMWF are more in line with the GFS, keeping the
heaviest precipitation offshore. No wintry hazards are being posted on today's
map due to lack of model agreement, with the most favored scenario being
lighter snow showers and gusty winds on the back side of the departing surface
low. Lake effect snow showers due to a northerly fetch over Lakes Ontario and
Erie will also be possible on 12/17. Winds are forecast to be gusty in the
northeast in general early next week due to a tight pressure gradient
developing as surface high pressure builds in from the Midwest.
The west coast, particularly Oregon and Washington, is forecast to see an
onslaught of Pacific moisture throughout most of the 3-7 day period as
troughing in the east Pacific results in an atmospheric river event with heavy
precipitation (coastal rain and mountain snow). Surface low pressure and an
associated surge of moisture looks to arrive late on 12/17 into 12/18, and
linger into 12/20. For 12/17 and 12/18, more impacts are forecasted over
coastal Oregon, along with Washington, before shifting north to mainly just
Washington by 12/20. High significant wave heights of about 20-30 feet are
forecast along with high winds from the west-southwest (sustained 40 knots with
higher gusts) as the surface low moves ashore. Winds are favored to shift more
south-southwesterly by 12/20. The hazard for high winds and significant wave
heights will be discontinued after 12/18 as the threat becomes more marginal,
but some lingering impacts may remain through 12/20 especially farther north.
Model guidance continues to support 2"-5" of liquid with isolated higher totals
for the entire period. This is especially true for coastal Washington which is
forecast to have impacts through 12/20. By 12/21 ridging is forecast to build
over the west coast, ending the threat of heavy precipitation.
Much below normal temperatures are forecast for southwest mainland Alaska
for 12/19 and 12/20 as troughing over the state is forecast to lead to very
cold overnight low temperatures (-20 deg. F or lower possible) and subzero high
temperatures in some areas. There is less confidence on 12/21 as temperatures
may begin to slowly moderate as forecasted 500-hPa heights begin to increase.
On 12/20 and 12/21 attention will turn to the eastern US as a digging
trough may result in surface low development but confidence is very low.
Today's 00z and 12z deterministic ECMWF model runs develop a surface cyclone
along the east coast. The 00z Canadian also supports this system. In contrast,
the 06z and 12z deterministic GFS runs are not as amplified with the trough
over the eastern US, and show cyclone development too far off the coast to have
any meaningful impacts, although some individual ensemble members do support
the more amplified solution. The key component which needs to be resolved is
how quickly does the pattern amplify, as a more amplified pattern will tend to
favor a stronger cyclone. For now, no hazards are posted in association with
this threat but will be monitored for inclusion in next week's outlooks if the
potential for this system increases. For Saturday December 22 -
Friday December 28:
During the Week-2 period, an amplifying 500-hPa
circulation pattern is predicted, with a trough over eastern North America, and
a ridge over western North America. This forecasted pattern favors much drier
conditions for the west coast, bringing an end to the atmospheric river events
dominating the 3-7 day period.
Model guidance continues to waver in regards to threats during week-2, but
overall troughing pattern in the east may result in several opportunities for a
surface low pressure system to develop bringing increased precipitation chances
to the eastern US in week-2. However, due to no consistency among the models, a
threat is not being posted at this time.
The U.S. Drought Monitor, valid December 11th, indicates a slight decrease
in severe to exceptional drought (D2 to D4) coverage from 11.34 percent last
week to 11.20 percent this week. Improvement is noted across southern
California where extreme drought (D3) has been removed. However coverage of
severe drought (D3) has increased over Oregon and moderate drought (D1) has
expanded over southern Florida.
Forecaster: Thomas Collow
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
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