084  
FXUS21 KWNC 062038  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 06 2017  
 
SYNOPSIS: A STRONG AREA OF MID-LEVEL LOW (HIGH) PRESSURE IS LIKELY TO PERSIST  
OVER EASTERN (WESTERN) NORTH AMERICA THROUGH MID-DECEMBER. MULTIPLE COLD FRONTS  
ARE EXPECTED TO ADVANCE SOUTH FROM CANADA INTO THE EASTERN U.S., WHILE SURFACE  
HIGH PRESSURE PREVAILS ACROSS THE WESTERN U.S. ANOMALOUS SOUTHERLY FLOW AT THE  
SURFACE IS EXPECTED TO CONTINUE ACROSS MUCH OF ALASKA DURING THE NEXT TWO WEEKS.  
 
HAZARDS  
 
PERIODS OF MUCH BELOW-NORMAL TEMPERATURES ACROSS THE EASTERN U.S., SAT-WED, DEC  
9-13.  
 
PERIODS OF HEAVY, LAKE-EFFECT SNOW DOWNWIND OF THE GREAT LAKES, SAT-FRI, DEC  
9-15.  
 
HEAVY SNOW FOR PARTS OF THE CENTRAL AND SOUTHERN APPALACHIANS, TUE, DEC 12.  
 
MUCH ABOVE-NORMAL TEMPERATURES FOR INTERIOR MAINLAND ALASKA, FRI-WED, DEC 9-13.  
 
HEAVY PRECIPITATION FROM THE KENAI PENINSULA TO THE NORTHERN ALASKA PANHANDLE,  
SUN-MON, DEC 10-11.  
 
HIGH WINDS FOR THE ALASKA RANGE, MON, DEC 11.  
 
A HIGH RISK OF MUCH BELOW NORMAL TEMPERATURES FOR THE GREAT LAKES AND PARTS OF  
THE NORTHEAST, THU-SUN, DEC 14-17.  
 
A MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE EASTERN AND  
CENTRAL U.S., THU-SUN, DEC 14-17.  
 
A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE EASTERN U.S.  
AND NORTHERN GREAT PLAINS, THU-WED, DEC 14-20.  
 
A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARS OF THE SOUTHEAST,  
THU-FRI, DEC 14-15.  
 
A SLIGHT RISK OF HEAVY PRECIPITATION FROM THE ALASKA PENINSULA EAST TO THE  
NORTHERN ALASKA PANHANDLE, THU-MON, DEC 14-18.  
 
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT  
PLAINS, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR SATURDAY DECEMBER 09 - WEDNESDAY DECEMBER 13: A HIGHLY AMPLIFIED  
UPPER-LEVEL RIDGE IS FORECAST TO BECOME ANCHORED ALONG THE WEST COAST DURING  
THIS PERIOD. DOWNSTREAM OF THIS ANOMALOUS RIDGE, A HIGHLY AMPLIFIED UPPER-LEVEL  
TROUGH IS LIKELY TO DEVELOP WITH ANOMALOUS NORTHERLY FLOW USHERING IN A COUPLE  
OF OUTBREAKS OF ARCTIC AIR TO THE EASTERN U.S. PERIODS OF MUCH BELOW-NORMAL  
TEMPERATURES (DAILY NEGATIVE DEPARTURES OF 10 TO 15 DEGREES F) ARE LIKELY FROM  
THE GREAT LAKES SOUTH TO THE GULF COAST FROM DEC 9 TO 13. THE MOST ANOMALOUS  
COLD AIR IS EXPECTED TO OVERSPREAD THE EASTERN THIRD OF THE CONUS ON DEC 13  
WHEN MAXIMUM TEMPERATURES ARE FORECAST TO REMAIN AT OR BELOW FREEZING AS FAR  
SOUTH AS THE SOUTHERN APPALACHIANS AND MID-ATLANTIC.  
 
THE STRONG COLD AIR ADVECTION IS EXPECTED TO RESULT IN PERIODS OF HEAVY,  
LAKE-EFFECT SNOW (6 INCHES OR MORE PER 24 HOURS) ACROSS THE FAVORED SNOWBELT  
AREAS DOWNWIND OF THE GREAT LAKES INTO NEXT WEEK. SURFACE TEMPERATURES ON LAKES  
ERIE AND ONTARIO ARE CURRENTLY NEAR 5 DEGREES C, WHILE 850-HPA TEMPERATURES ARE  
FORECAST TO FALL BELOW -15 DEGREES C. THE LARGE TEMPERATURE DIFFERENCE BETWEEN  
THE LAKE SURFACE AND LOW-LEVEL AIR PROMOTES A FAVORABLE ENVIRONMENT FOR  
ENHANCED LAKE-EFFECT SNOW.  
 
DETERMINISTIC CANADIAN, GFS, AND ECMWF MODELS AND THEIR ENSEMBLE MEMBERS  
CONTINUE TO INDICATE A WAVE OF LOW PRESSURE DEVELOPING ALONG A FRONT NEAR THE  
EAST COAST BY DEC 9. THIS SURFACE LOW PRESSURE SYSTEM COULD BRING ACCUMULATING  
SNOW FROM THE MID-ATLANTIC NORTH TO NEW ENGLAND THIS WEEKEND, BUT THE TRACK OF  
THE LOW PRESSURE SYSTEM AND ASSOCIATED SNOWFALL AMOUNTS REMAIN HIGHLY  
UNCERTAIN. THE PAST TWO RUNS OF THE CANADIAN MODEL PHASE THE NORTHERN AND  
SOUTHERN STREAMS, RESULTING IN A SWATH OF MODERATE TO HEAVY SNOW FROM THE  
SOUTHERN APPALACHIANS NORTHEAST TO SOUTHERN NEW ENGLAND. HOWEVER, THIS SOLUTION  
DOES NOT HAVE SUPPORT FROM THE DETERMINISTIC 12Z GFS OR 12Z ECMWF MODELS AT  
THIS TIME. ON DEC 12, UPSLOPE FLOW ASSOCIATED WITH RENEWED COLD AIR ADVECTION  
IS EXPECTED TO BRING MODERATE TO HEAVY SNOW (AROUND 4 INCHES) TO THE CENTRAL  
AND SOUTHERN APPALACHIANS.  
 
A PERSISTENT AREA OF SURFACE HIGH PRESSURE IS FORECAST TO MAINTAIN OFFSHORE  
SURFACE FLOW, ABOVE-NORMAL TEMPERATURES, AND LOW RELATIVE HUMIDITY VALUES  
ACROSS SOUTHERN CALIFORNIA. ALTHOUGH OFFSHORE WINDS MAY OCCASIONALLY GUST ABOVE  
40 MPH, THE STRONGEST WINDS ARE EXPECTED TO OCCUR PRIOR TO THIS PERIOD. THE  
STORM PREDICTION CENTER DISCONTINUES THE FORECAST OF CRITICAL FIRE WEATHER  
CONDITIONS AFTER DEC 8, BUT ELEVATED FIRE WEATHER CONDITIONS ARE LIKELY TO  
PERSIST THROUGH MID-DECEMBER.  
 
AN UPPER-LEVEL TROUGH OVER THE ALEUTIANS IS FORECAST TO LEAD TO ANOMALOUS  
SOUTHERLY FLOW AFFECTING THE KENAI PENINSULA, SOUTHEAST MAINLAND ALASKA, AND  
THE NORTHERN ALASKA PANHANDLE FROM DEC 9-13. THE MOST LIKELY TIME FOR HEAVY  
PRECIPITATION (2 INCHES OR MORE PER 24 HOURS, LIQUID EQUIVALENT) IS FORECAST ON  
DEC 10 AND 11. ANOMALOUS SOUTHERLY FLOW AT THE SURFACE IS FORECAST TO RESULT IN  
MUCH ABOVE-NORMAL TEMPERATURES (DAILY POSITIVE DEPARTURES FROM NORMAL OF MORE  
THAN 20 DEGREES F) ACROSS INTERIOR MAINLAND ALASKA. THESE MUCH ABOVE-NORMAL  
TEMPERATURES MAY LEAD TO ICY ROADS AND HAZARDOUS TRAVEL. WIDESPREAD CHINOOK  
WINDS (GUSTS ABOVE 50 MPH) ARE EXPECTED ACROSS THE ALASKA RANGE ON DEC 11.  
 
FOR THURSDAY DECEMBER 14 - WEDNESDAY DECEMBER 20: THE HIGH AMPLITUDE LONGWAVE  
PATTERN OVER NORTH AMERICA IS LIKELY TO PERSIST INTO LATE DECEMBER. THE GFS AND  
ECMWF ENSEMBLE MEANS FEATURE A CORE OF NEGATIVE 500-HPA HEIGHT ANOMALIES NEAR  
200 METERS CENTERED OVER SOUTHEAST CANADA. THE HIGHLY ANOMALOUS PATTERN FAVORS  
A CONTINUATION OF MUCH BELOW-NORMAL TEMPERATURES ACROSS PARTS OF THE EASTERN  
U.S. THROUGH THE WEEK-2 PERIOD. A HIGH RISK (60 PERCENT CHANCE) OF MUCH  
BELOW-NORMAL TEMPERATURES IS FORECAST FOR THE GREAT LAKES AND PARTS OF THE  
NORTHEAST THROUGH DEC 17 WHERE THE GFS ENSEMBLE MEAN INDICATES TEMPERATURES  
AVERAGING MORE THAN 12 DEGREES F BELOW NORMAL. SUBZERO MINIMUM TEMPERATURES AND  
DANGEROUS WIND CHILL VALUES CAN BE EXPECTED ACROSS THE GREAT LAKES DURING  
MID-DECEMBER. THE SLIGHT RISK AREA ALONG THE GULF COAST AND FLORIDA IS  
DISCONTINUED AFTER DEC 15 AS 500-HPA HEIGHTS ARE FORECAST TO INCREASE ACROSS  
THIS REGION LATER IN WEEK-2.  
 
ANOTHER OUTBREAK OF ARCTIC IS EXPECTED TO RESULT IN A CONTINUATION OF HEAVY,  
LAKE-EFFECT SNOW THROUGH DEC 15. IF THE LONGWAVE TROUGH AXIS RETROGRADES, THEN  
THE EAST COAST WOULD BE VULNERABLE TO WINTER WEATHER HAZARDS DURING WEEK-2.  
 
THE MADDEN-JULIAN OSCILLATION (MJO) STRENGTHENED AT THE END OF NOVEMBER AND ITS  
ENHANCED PHASE IS PROPAGATING EAST FROM THE MARITIME CONTINENT TO THE WEST  
PACIFIC DURING EARLY DECEMBER. THIS MJO EVOLUTION IS EXPECTED TO CONSTRUCTIVELY  
INTERFERE WITH THE ONGOING HIGH AMPLITUDE LONGWAVE PATTERN OVER NORTH AMERICA  
AND PROLONG THE ANOMALOUS COLD AIR. THE FOCUS FOR MUCH BELOW NORMAL  
TEMPERATURES MAY BEGIN TO SHIFT INTO THE NORTHERN GREAT PLAINS LATER IN WEEK-2,  
GIVEN THE MJO EVOLUTION AND LA NINA CONDITIONS.  
 
ANOMALOUS SOUTHERLY FLOW IS LIKELY TO PERSIST ACROSS ALASKA INTO THE WEEK-2  
PERIOD WHICH SUPPORTS A SLIGHT RISK OF HEAVY PRECIPITATION FROM THE ALASKA  
PENINSULA EAST TO THE NORTHERN ALASKA PANHANDLE. BASED ON LOWER PROBABILITIES  
IN THE GEFS PRECIPITATION TOOL TODAY (20 TO 30 PERCENT), THE MODERATE TO HIGH  
RISK AREA IN YESTERDAY'S OUTLOOK WAS REMOVED.  
 
THE U.S. DROUGHT MONITOR, UPDATED ON NOV 28, SHOWS SEVERE TO EXCEPTIONAL  
(D2-D4) DROUGHT COVERING 4.88% OF THE CONTINENTAL UNITED STATES, A SLIGHT  
INCREASE OF 0.74% COVERAGE FROM THE PREVIOUS WEEK.  
 
FORECASTER: BRAD PUGH  
 

 
 
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