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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made November 17, 2017

 Days 3-7Days 8-14Prob. Days 8-14
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Categorical Outlooks
Experimental Probabilistic Outlooks (Information)

Valid Monday November 20, 2017 to Friday December 01, 2017

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST November 17 2017

Synopsis: 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.

Hazards Detailed Summary

For Monday November 20 - Friday November 24: 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

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