Prognostic Meteorological Discussion
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FXUS21 KWNC 082004
PMDTHR
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

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 16-20.

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.

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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