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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made December 08, 2016

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

Valid Sunday December 11, 2016 to Thursday December 22, 2016

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 08 2016

Synopsis: Arctic high pressure is forecast to expand south and east from the northern Rockies and northern Great Plains during Week-1. A low pressure system is expected to precede the arctic front and track across the upper Midwest into New England this weekend. Multiple waves of surface low pressure are expected to push inland into the western U.S. towards the end of Week-1 and into Week-2. Arctic surface high pressure is forecast to descend southward from the Yukon into the Central U.S. during Week-2.

Hazards Detailed Summary

For Sunday December 11 - Thursday December 15: As the forecast period begins a surface low pressure system is anticipated in the vicinity of the Quad Cities area. As this system is forecast to shift eastward, accompanying areas of heavy snow (exceeding 4" or more in 24 hours) along the northern periphery of the system are possible for southern parts of the Great Lakes on Sun, Dec 11. This system is forecast to continue into the Northeast, where a second region of heavy snow is possible in upstate New York and parts of New England on Mon, Dec 12. Both heavy snowfall regions are roughly coincident with where 6Z GEFS and 0Z ECMWF ensemble guidance support 0.25" or greater of precipitation within 24 hour periods and thermal profiles appear supportive for snowfall.

Arctic air is forecast to descend out of the Yukon and progress southward and eastward across the CONUS during Week-1. Initially, much below-normal temperatures are expected to be confined to the Northern Rockies and Northern Plains on Sun-Mon, Dec 11-12. The much below-normal temperature hazard is anticipated to extend Westward into Washington state while also expanding eastward and southward to additionally impact the Central Plains, Upper and Mid-Mississippi Valley, and Great Lakes on Tue-Wed, Dec 13-14. The most extensive day of much below-normal temperatures during Week-1 appears to be Thu, Dec 15 with impacted areas expected from the Northern Great Basin eastward through the Mid-Atlantic and southward through the Tennessee Valley. For each of the aforementioned much below-normal temperature hazards, daily minimum temperature anomalies are expected to be anywhere from 12 to 25 degrees F below normal.

Strong cold air advection associated with the aforementioned arctic outbreak is forecast to bring heavy lake-effect snows to the southern and eastern shores of the Great Lakes Tue-Fri, Dec 12-16 (note this hazard extends by one day into Week-2).

A 500-hPa trough is forecast to be approaching the western U.S. late in Week-1. This pattern favors persistent onshore flow from the Pacific, with periodic waves of accompanying waves of surface low pressure. This setup supports a region of heavy precipitation (exceeding 3-6" liquid equivalent for the full period, with higher elevation snow and rain otherwise) for Tue-Thu, Dec 13-15.

Arctic high pressure over western Canada and anomalous easterly flow is expected to result in much below-normal temperatures across the southern coast of mainland Alaska and the Alaska Panhandle from Sun-Thu, Dec 11-15. This hazard should be focused predominantly over the Panhandle from Dec 12 onward. Dynamical models generally forecast temperature anomalies here of 16-20 degrees F below normal.

For Friday December 16 - Thursday December 22: Dynamical model ensemble means continue to support anomalous 500-hPa ridging over the Bering Sea and western Alaska throughout Week-2. In this scenario, much below-normal temperatures could potentially impact much of the CONUS in addition to the Alaska Panhandle, with the exception of Southern California and parts of the Southwest. The slight, moderate, and high risk areas and periods of much below-normal temperatures are based generally on areas where the GEFS reforecast tool indicates daily minimum temperatures with a 20, 40, or 60 percent chance, respectively, of falling below the 15th percentile compared to climatology. These probabilities were subsequently adjusted slightly where ECMWF ensemble guidance provided additional or reduced support. While the 0Z ECMWF ensembles have trended cooler across the northern tier relative to yesterday its forecast would still result in more widespread warmth for the West relative to the markedly colder GEFS.

According to the latest U.S. Drought Monitor (USDM) valid on December 6, the coverage of severe or greater drought for the CONUS decreased by over 2.5% to 13.97%. Most of the improvements were focused over the southeastern quarter of the country where rains provided 1 to 2 class improvements in drought conditions while also helping with firefighting efforts tied to the long-term dryness. Some degradation of drought conditions was noted for the Great Plains.

Forecaster: Daniel Harnos

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Please consult local NWS Forecast Offices for short range forecasts and region-specific information.