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
300 PM EST December 14 2018

Synopsis: 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

• 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 Southwest.

Detailed Summary

For Monday December 17 - Friday December 21: 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 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 (-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.