Valid Monday, December 17, 2018 to Friday, December 28, 2018
Summary of Forecasts and Hazards
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST December 14 2018Synopsis
: Upper level low pressure located
off the west coast of the US will continue to result in multiple rounds of
heavy precipitation for coastal Washington and Oregon. At the end of the
period, 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 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. Hazards
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 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 will continue to see an onslaught of Pacific moisture over
the next several days as troughing in the east Pacific results in an
atmospheric river impacting the Oregon and Washington coast with heavy
precipitation (coastal rain and mountain snow). For our period of concern,
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
will be felt 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
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 (-15 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. The
00z deterministic ECMWF develops a significant surface cyclone along the east
coast. Yesterday's 12z ECMWF also showed coastal storm development, but had the
system come together further north with overall lesser impacts. Today's 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 CONUS, and show
cyclone development too far off the coast to have any meaningful impacts,
although some individual ensemble members do support a 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.
Until there is more model agreement in this regard, a hazard will not be
issued. For Saturday December 22 -
Friday December 28:
During the Week-2 period, an amplifying 500-hPa
circulation pattern is predicted, with a mean trough developing over eastern
North America, and a mean ridge over western North America. This forecasted
change in the 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 around Christmas. 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
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.