743  
FXUS21 KWNC 051908  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 05 2017  
 
SYNOPSIS: A MAJOR PATTERN CHANGE IS FORECAST BY THE BEGINNING OF THE OUTLOOK  
PERIOD 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. ANOMALOUS SOUTHERLY FLOW AT THE SURFACE IS EXPECTED TO  
PERSIST ACROSS MUCH OF ALASKA DURING THE NEXT TWO WEEKS.  
 
HAZARDS  
 
MUCH BELOW-NORMAL TEMPERATURES EXPANDING ACROSS THE EASTERN U.S., FRI-MON, DEC  
8-12.  
 
PERIODS OF HEAVY, LAKE-EFFECT SNOW DOWNWIND OF THE GREAT LAKES, FRI-WED, DEC  
8-13.  
 
MUCH BELOW-NORMAL TEMPERATURES FOR THE RIO GRANDE VALLEY AND GULF COAST OF  
TEXAS, FRI, DEC 8.  
 
HEAVY SNOW FOR PARTS OF THE CENTRAL AND SOUTHERN APPALACHIANS, TUE, DEC 12.  
 
MUCH ABOVE-NORMAL TEMPERATURES FOR INTERIOR MAINLAND ALASKA, FRI-TUE, DEC 8-12.  
 
PERIODS OF HEAVY PRECIPITATION FROM THE KENAI PENINSULA TO THE NORTHERN ALASKA  
PANHANDLE, FRI-TUE, DEC 8-12.  
 
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, WED-SAT, DEC 13-16.  
 
A MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE EASTERN  
U.S., WED-SAT, DEC 13-16.  
 
A SLIGHT OF MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE EASTERN U.S.,  
WED-TUE, DEC 13-19.  
 
A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARS OF THE SOUTHEAST,  
WED-FRI, DEC 13-15.  
 
A HIGH RISK OF HEAVY PRECIPITATION FOR THE KENAI PENINSULA, WED-FRI, DEC 13-15  
 
A MODERATE RISK OF HEAVY PRECIPITATION FOR SOUTHERN COASTAL MAINLAND ALASKA,  
WED-FRI, DEC 13-15.  
 
A SLIGHT RISK OF HEAVY PRECIPITATION FROM THE ALASKA PENINSULA EAST TO THE  
NORTHERN ALASKA PANHANDLE, WED-SUN, DEC 13-17.  
 
SEVERE DROUGHT ACROSS PARTS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEY, GREAT  
PLAINS, ARIZONA, AND HAWAII.  
 
DETAILED SUMMARY  
 
FOR FRIDAY DECEMBER 08 - TUESDAY DECEMBER 12: 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. 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 FROM DEC 8 TO 12. SUBZERO TEMPERATURES  
ARE FORECAST TO SHIFT SOUTH INTO THE UPPER MISSISSIPPI VALLEY ON DEC 11 AND 12.  
 
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 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.  
 
SURFACE HIGH PRESSURE IS EXPECTED TO BECOME CENTERED OVER THE RIO GRANDE VALLEY  
ON DEC 8 WHEN MUCH BELOW-NORMAL TEMPERATURES ARE FORECAST FOR THE RIO GRANDE  
VALLEY AND GULF COAST OF TEXAS. HARD FREEZES ARE EXPECTED FOR THE BIG BEND  
REGION OF TEXAS ON DEC 8.  
 
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 AND 9. THIS SURFACE LOW PRESSURE SYSTEM COULD  
BRING ACCUMULATING SNOW TO THE MID-ATLANTIC AND NORTHEAST THIS WEEKEND, BUT  
WIDESPREAD, HEAVY SNOW IS NOT EXPECTED AT THIS TIME.  
 
A PERSISTENT AREA OF SURFACE HIGH PRESSURE IS FORECAST TO MAINTAIN OFFSHORE  
FLOW, ABOVE-NORMAL TEMPERATURES, AND LOW RELATIVE HUMIDITY VALUES ACROSS  
SOUTHERN CALIFORNIA FROM DEC 8-12. ALTHOUGH OFFSHORE WINDS MAY OCCASIONALLY  
GUST ABOVE 40 MPH, THE STRONGEST WINDS ARE EXPECTED TO OCCUR PRIOR TO THIS  
PERIOD. ACCORDING TO THE STORM PREDICTION, CRITICAL FIRE WEATHER CONDITIONS ARE  
FORECAST THROUGH DEC 7.  
 
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 8-12. THE 0Z ECMWF MODEL INDICATES  
120-HOUR TOTAL PRECIPITATION AMOUNTS (LIQUID EQUIVALENT) OF NEARLY 10 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 11.  
 
FOR WEDNESDAY DECEMBER 13 - TUESDAY DECEMBER 19: 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 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 16. THE GFS ENSEMBLE MEAN INDICATES TEMPERATURES AVERAGING MORE THAN 12  
DEGREES F BELOW NORMAL ACROSS THESE AREAS. 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 AIR NEXT WEEK 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. THE FOCUS FOR MUCH BELOW  
NORMAL TEMPERATURES MAY BEGIN TO SHIFT INTO THE NORTHERN GREAT PLAINS BY THE  
END OF WEEK-2, GIVEN THE MJO AND LA NINA CONDITIONS.  
 
ANOMALOUS SOUTHERLY FLOW IS LIKELY TO PERSIST ACROSS ALASKA THROUGH AT LEAST  
DAY 10 WHICH SUPPORTS A SLIGHT (DEC 13 - 17) TO HIGH (DEC 13 - 15) RISK OF  
HEAVY PRECIPITATION FROM THE ALASKA PENINSULA EAST TO THE NORTHERN ALASKA  
PANHANDLE. THE HIGH RISK AREA IS BASED ON THE GEFS TOOL WHICH INDICATES THAT  
THERE IS MORE THAN A 80 PERCENT CHANCE THAT PRECIPITATION AMOUNTS EXCEED THE  
85TH PERCENTILE FROM DEC 13 - 15.  
 
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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