Valid Sunday December 11, 2016 to Thursday December 22, 2016
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EST December 08 2016Synopsis
: 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
Detailed Summary For Sunday December
11 - Thursday December 15:
- Locally heavy snow across portions of
the Great Lakes and the Ohio Valley, Sun, Dec 11.
- Locally heavy snow across portions of the Northeast and the Great Lakes,
Mon, Dec 12.
- Heavy lake-effect snow across portions downwind of the Great Lakess,
Tue-Fri, Dec 13-16.
- Heavy precipitation across portions of the Northern Great Basin, the
Northern Rockies, California, and the Pacific Northwest, Tue-Thu, Dec 13-15.
- Much below-normal temperatures across portions of the Upper Mississippi
Valley, the Northern Plains, and the Northern Rockies, Sun-Mon, Dec 11-Dec 12.
- Much below-normal temperatures across portions of the Great Plains, the
Northern Rockies, the Mississippi Valley, the Northern Great Basin, the Great
Lakes, and the Ohio and Tennessee Valley, the Central and Southern
Appalachians, and the Great Lakes Tue-Thu, Dec 13-15.
- Much below-normal temperatures across portions of the Alaska Panhandle and
southern mainland Alaska, Sun-Thu, Dec 11-15.
- High risk of much below-normal temperatures for portions of the Northern
and Central Plains, Upper Mississippi Valley, and the Northern Rockies,
Fri-Sun, Dec 16-18.
- Moderate risk of much below-normal temperatures for portions of the
Northern and Central Plains,the Northern Rockies, the Upper and Middle
Mississippi Valley, the Northern Great Basin, and the Great Lakes, Fri-Tue, Dec
- Slight risk of much below-normal temperatures for the Alaska panhandle and
near entirety of the lower-48 states with the exception of portions of the
Southwest, Fri-Thu, Dec 16-22.
- Moderate risk of much below-normal temperatures for the Alaska Panhandle
south of Yakutat, Fri-Sun, Dec 16-18.
- Severe drought across parts of the Eastern U.S., Great Plains, Missouri
River Valley, Central Rockies, Intermountain West, California and Hawaii.
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
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,
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
Forecaster: Daniel Harnos
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.