007  
FXUS21 KWNC 041939  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 04 2017  
 
SYNOPSIS: THE ONSET OF A MAJOR PATTERN CHANGE IS FORECAST LATER THIS WEEK AS A  
STRONG AREA OF MID-LEVEL LOW (HIGH) PRESSURE BECOMES ESTABLISHED OVER EASTERN  
(WESTERN) NORTH AMERICA. THIS PATTERN IS EXPECTED TO PERSIST THROUGH AT LEAST  
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. AN AREA OF MID-LEVEL LOW PRESSURE IS FORECAST TO REMAIN OVER THE NORTH  
PACIFIC DURING THE NEXT TWO WEEKS.  
 
HAZARDS  
 
MUCH BELOW-NORMAL TEMPERATURES EXPANDING ACROSS THE EASTERN U.S., FRI-MON, DEC  
8-11.  
 
PERIODS OF HEAVY, LAKE-EFFECT SNOW DOWNWIND OF THE GREAT LAKES, THU-WED, DEC  
7-13.  
 
ENHANCED WILDFIRE RISK FOR SOUTHERN CALIFORNIA, THU, DEC 7.  
 
HIGH WINDS FOR SOUTHERN CALIFORNIA, THU, DEC 7.  
 
MUCH ABOVE-NORMAL TEMPERATURES FOR INTERIOR MAINLAND ALASKA, THU-MON, DEC 7-11.  
 
PERIODS OF HEAVY PRECIPITATION FROM THE KENAI PENINSULA TO THE NORTHERN ALASKA  
PANHANDLE, FRI-MON, DEC 8-11.  
 
HIGH WINDS FOR THE ALASKA RANGE, SUN-MON, DEC 10-11.  
 
A HIGH RISK OF MUCH BELOW NORMAL TEMPERATURES FOR THE GREAT LAKES AND OHIO  
VALLEY, TUE-SUN, DEC 12-17.  
 
A SLIGHT TO MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR THE EASTERN  
THIRD OF THE CONTINENTAL U.S., TUE-MON, DEC 12-18.  
 
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT  
PLAINS, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR THURSDAY DECEMBER 07 - MONDAY DECEMBER 11: 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 AN OUTBREAK OF  
ARCTIC AIR TO THE EASTERN U.S. MUCH BELOW-NORMAL TEMPERATURES (DAILY NEGATIVE  
DEPARTURES FROM NORMAL OF 10 TO 20 DEGREES F) ARE LIKELY FROM THE GREAT LAKES  
SOUTH TO THE GULF COAST, BEGINNING ON DEC 8. THE COLDEST MORNING ACROSS THE  
SOUTHEAST IS EXPECTED TO OCCUR ON DEC 10 WHEN FROST OR A LIGHT FREEZE COULD  
EXTEND AS FAR SOUTH AS THE NATURE COAST OF FLORIDA, WHILE MINIMUM TEMPERATURES  
IN THE TEENS ARE LIKELY ACROSS THE TENNESSEE VALLEY AND SOUTHERN APPALACHIANS.  
SUBZERO MINIMUM TEMPERATURES ARE FORECAST TO SHIFT SOUTH INTO THE UPPER  
MISSISSIPPI VALLEY AND GREAT LAKES BY DEC 11.  
 
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, BEGINNING ON DEC 7. SURFACE TEMPERATURES ON  
LAKES ERIE AND ONTARIO ARE CURRENTLY AT OR ABOVE 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.  
 
RECENT DETERMINISTIC GFS AND ECMWF MODELS AND THEIR ENSEMBLE MEMBERS CONTINUE  
TO INDICATE THE POTENTIAL FOR A WAVE OF LOW PRESSURE TO DEVELOP ALONG A FRONT  
NEAR THE EAST COAST ON DEC 8 OR 9. THIS SURFACE LOW PRESSURE SYSTEM COULD BRING  
ACCUMULATING SNOW TO THE MID-ATLANTIC AND NORTHEAST THIS WEEKEND, BUT HEAVY  
SNOW IS NOT EXPECTED AT THIS TIME.  
 
A PERSISTENT AREA OF SURFACE HIGH PRESSURE (AROUND 1040-HPA) IS FORECAST TO  
MAINTAIN OFFSHORE FLOW, WARM TEMPERATURES, AND LOW RELATIVE HUMIDITY VALUES  
ACROSS SOUTHERN CALIFORNIA FROM DEC 7-11. THE HIGHEST WINDS (GUSTS ABOVE 50  
MPH) DURING THIS PERIOD ACROSS SOUTHERN CALIFORNIA ARE MOST LIKELY ON DEC 7.  
THE GUSTY WINDS RESULT IN ENHANCED WILDFIRE RISK ACROSS SOUTHERN CALIFORNIA ON  
DEC 7 AND POTENTIALLY LONGER.  
 
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 8-11. THE 0Z ECMWF MODEL INDICATES  
96-HOUR TOTAL PRECIPITATION AMOUNTS (LIQUID EQUIVALENT) EXCEEDING 9 INCHES  
ACROSS THE NORTHERN ALASKA PANHANDLE. 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 10 AND 11.  
 
FOR TUESDAY DECEMBER 12 - MONDAY DECEMBER 18: THE HIGH AMPLITUDE LONGWAVE  
PATTERN OVER THE NORTHERN HEMISPHERE IS LIKELY TO REMAIN STABLE THROUGH  
MID-DECEMBER. THE GFS AND ECMWF ENSEMBLE MEANS FEATURE LARGE 500-HPA HEIGHT  
ANOMALIES OF MORE THAN 200 METERS WITH THE POSITIVE (NEGATIVE) CENTER OVER THE  
ALASKA PANHANDLE (NORTHEASTERN U.S.) DURING WEEK-2. THIS 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 OHIO VALLEY  
THROUGH DEC 17. THE GFS ENSEMBLE MEAN INDICATES TEMPERATURES AVERAGING AS MUCH  
AS 16 DEGREES F BELOW NORMAL ACROSS THESE AREAS. SUBZERO MINIMUM TEMPERATURES  
AND DANGEROUS WIND CHILL VALUES CAN BE EXPECTED ACROSS THE HIGH RISK AREA  
DURING MID-DECEMBER. BROADER REGIONS WITH A SLIGHT AND MODERATE RISK OF MUCH  
BELOW-NORMAL TEMPERATURES EXTEND ACROSS THE EASTERN U.S. THROUGH WEEK-2.  
 
THE DETERMINISTIC MODEL SOLUTIONS ARE IN EXCELLENT AGREEMENT THAT A SECOND  
OUTBREAK OF ARCTIC AIR IS LIKELY TO OVERSPREAD THE UPPER MISSISSIPPI VALLEY AND  
GREAT LAKES EARLY NEXT WEEK. 850-HPA TEMPERATURES ARE FORECAST TO FALL BELOW  
-20 DEGREES C BY DEC 12 ACROSS THESE AREAS. THE COLD AIR ADVECTION, USHERING IN  
THIS ANOMALOUSLY COLD AIR, IS EXPECTED TO RESULT IN A CONTINUATION OF HEAVY,  
LAKE-EFFECT SNOW THROUGH AT LEAST DEC 13. 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 WITH  
ITS ENHANCED PHASE PROPAGATING EAST TO THE MARITIME CONTINENT DURING THE  
BEGINNING OF DECEMBER. THE MJO IS FORECAST TO CONTINUE ITS EASTWARD PROPAGATION  
TO THE WEST PACIFIC DURING THE NEXT WEEK. THIS MJO EVOLUTION IS EXPECTED TO  
CONSTRUCTIVELY INTERFERE WITH THE ONGOING HIGH AMPLITUDE LONGWAVE PATTERN OVER  
NORTH AMERICA AND PROLONG THE ANOMALOUS COLD AIR ACROSS THE EAST-CENTRAL U.S.  
THROUGH WEEK-2.  
 
THE U.S. DROUGHT MONITOR, UPDATED ON NOV 28, SHOWS SEVERE TO EXCEPTIONAL  
(D2-D4) DROUGHT COVERS 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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