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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made December 09, 2016

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

Valid Monday December 12, 2016 to Friday December 23, 2016

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 09 2016

Synopsis: As the forecast period begins, a surface low pressure system is anticipated to be shifting from the Great Lakes towards the Canadian Maritimes. Behind this system a cold front is expected to usher in a blast of arctic air as surface high pressure looks to build southward out of the Yukon into the Great Plains. Storm activity is expected to increase off the Pacific coast towards the middle of Week-1. Mid-level high pressure will potentially build across the Southeast in Week-2, limiting potential growth of the arctic air mass. The most bitterly cold air is expected to retreat into Canada by the latter half of Week-2.

Hazards Detailed Summary

For Monday December 12 - Friday December 16: A surface low pressure is forecast over the Great Lakes at the beginning of the outlook period, and is expected to lift east-northeast towards the Canadian Maritimes. On the northern periphery of this system a region of heavy snow (3" or more in 24 hours) is forecast for portions of upstate New York and northern New England on Mon, Dec 12. Behind the aforementioned system colder air is anticipated, along with 850-hPa flow out of the west and northwest over the next several days. This scenario, coupled with water temperatures in Great Lakes in the 40s and 50s, is expected to result in periodic locally heavy lake-effect snow in downwind coastal areas of the Great Lakes for Tue-Fri, Dec 13-16.

Hazardous much below-normal temperatures are anticipated to build across much of the U.S. during Week-1 associated with high pressure shifting southward out of the Yukon and into the central CONUS. Initially the coldest air is forecast to extend eastward from Washington state into the far western Great Lakes, and southward towards Nebraska for Mon-Tue, Dec 12-13. Much below-normal temperatures are thereafter anticipated for an even broader region from approximately Puget Sound eastward through the Appalachians, with a southern extent east of the Rockies through the Central Plains and Ohio Valley for the rest of Week-1, Wed-Fri, Dec 14-16. Associated with the aforementioned temperature hazards, minimum temperature anomalies of 12-25 degrees F below normal are possible.

Heavy precipitation (exceeding 3-5" liquid equivalent, with higher altitude snow and rain otherwise) is possible across portions of Northern California and Oregon eastward through Wyoming and Colorado on Wed-Fri, Dec 14-16. This precipitation is expected to be associated with onshore flow ahead of a surface low pressure system anticipated to be coming ashore over northern California late in Week-1. In the warm sector of this system high winds (exceeding 35 mph) out of the south are possible from southern California through West Texas on Fri, Dec 16.

Alaska is not expected to be spared from the anomalous cold that the CONUS is expecting, with much below-normal temperatures forecast for parts of the Alaska panhandle Mon-Tue, Dec 12-13. Here minimum temperature anomalies are forecast to exceed 16 degrees F below normal. Southerly flow is anticipated across the Bering and Chukchi Seas late in Week-1. This persistent flow could lead to significant waves exceeding 20' and potential associated coastal flooding for windward facing regions of western Alaska and the Aleutians Thu-Sun, Dec 15-18 (note this hazard extends two days into Week-2).

For Saturday December 17 - Friday December 23: Dynamical model guidance is in better agreement regarding the Week-2 500-hPa circulation relative to earlier in the week. The ECMWF ensembles have trended towards the colder GEFS solution featuring anomalous 500-hPa troughing across the west, while the GEFS has shifted closer to the warmer ECMWF forecast favoring anomalous ridging building across the Southeast throughout Week-2. The combined impact of these changes is a continuation of the cold-related hazards across the northern tier of the U.S. during Week-2, but a westward shift in the highest risk areas relative to outlooks from earlier in the week. A high risk of much below-normal temperatures is now forecast to extend from the Pacific Northwest through the Northern and Central Plains for Sat-Mon, Dec 17-19. An attendant area of moderate risk of much below-normal temperatures extends from Northern California through the Upper Mississippi Valley for Sat-Tue, Dec 17-20. A second area with a moderate risk of much below-normal temperatures is introduced for the Northeast Sat-Sun, Dec 17-18. Roughly the northern two-thirds of the CONUS is given a slight risk of much below-normal temperatures for the entirety of Week-2, Sat-Fri, Dec 17-23. The cold risk is expected to wane east of the Mississippi throughout Week-2 as anomalous mid-level ridging may build in the Southeast. The slight, moderate, and high risk areas correspond closely to the 20th, 40th, and 60th percent chances of being below the 15th climatological percentile of minimum temperature according to the GEFS.

Southern parts of the Alaska panhandle are also given a slight risk of much below-normal temperatures for Sat-Sun, Dec 17-18. Model guidance suggests storm activity could resume in the Gulf of Alaska late in the Week-2 timeframe. Storm activity is also indicated east of the Mississippi during Week-2, yet inconsistencies in time and space regarding forecast cyclones limit confidence in a specific hazard forecast.

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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