198  
FXUS21 KWNC 182101  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 18 2017  
 
SYNOPSIS: A LOW-PRESSURE SYSTEM AND ASSOCIATED COLD FRONT ARE FORECAST TO PUSH  
EASTWARD FROM THE CENTRAL TO THE EASTERN PARTS OF THE U.S. EARLY IN THE PERIOD.  
THE FRONT IS FORECAST TO STALL OVER THE EASTERN U.S. WHILE AN ARCTIC AIR MASS  
SPILLS SOUTHWARD INTO THE NORTHERN PLAINS. HAZARDOUS WINTRY WEATHER IS POSSIBLE  
ON THE COLD SIDE OF THE STALLED FRONTAL BOUNDARY OVER THE EASTERN U.S. ON OR  
NEAR CHRISTMAS DAY. ARCTIC HIGH PRESSURE IS FORECAST TO DOMINATE MUCH OF THE  
CONTIGUOUS U.S. BY THE END WEEK-1 AND INTO WEEK-2. A STRONG LOW-PRESSURE SYSTEM  
IS FORECAST TO AFFECT ALASKA LATER IN THE WEEK AND INTO THE WEEKEND, BEFORE THE  
PATTERN IS FORECAST TO RELAX.  
 
HAZARDS  
 
HEAVY PRECIPITATION FROM THE SOUTHERN PLAINS TO THE OHIO VALLEY AND NORTHWARD  
TO NEW ENGLAND, SUN-MON, DEC 24-DEC 25.  
 
HEAVY RAIN ACROSS PORTIONS OF THE LOWER AND MIDDLE MISSISSIPPI VALLEYS, THE  
OHIO VALLEY, AND THE TENNESSEE VALLEY, FRI, DEC 22.  
 
HEAVY SNOW ACROSS PORTIONS OF THE GREAT LAKES AND THE UPPER MISSISSIPPI VALLEY,  
THU-FRI, DEC 21-22.  
 
HIGH WINDS ACROSS PORTIONS OF SOUTHERN CALIFORNIA, THU-FRI, DEC 21-DEC 22.  
 
HIGH WINDS ACROSS PORTIONS OF THE CENTRAL GREAT BASIN, THE CENTRAL AND SOUTHERN  
ROCKIES, THE SOUTHWEST, THE GREAT PLAINS, AND THE UPPER MISSISSIPPI VALLEY ,  
THU, DEC 21.  
 
MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE CENTRAL PLAINS, THE  
CENTRAL ROCKIES, THE GREAT LAKES, THE CENTRAL GREAT BASIN, THE NORTHERN PLAINS,  
THE SOUTHERN PLAINS, THE NORTHERN GREAT BASIN, THE SOUTHERN ROCKIES, THE MIDDLE  
MISSISSIPPI VALLEY, CALIFORNIA, THE NORTHERN ROCKIES, THE UPPER MISSISSIPPI  
VALLEY, THE PACIFIC NORTHWEST, AND THE SOUTHWEST, SAT-MON, DEC 23-DEC 25.  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF MAINLAND ALASKA, THU-SAT, DEC 21-DEC 23.  
 
HIGH WINDS ACROSS PORTIONS OF THE ALASKA PANHANDLE, MAINLAND ALASKA, AND THE  
ALEUTIANS, THU-SUN, DEC 21-DEC 24.  
 
MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF MAINLAND ALASKA, THU-MON, DEC  
21-DEC 25.  
 
HIGH SIGNIFICANT WAVE HEIGHTS FOR COASTAL PORTIONS OF MAINLAND ALASKA, THU-SUN,  
DEC 21-DEC 24.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL  
PLAINS, THE NORTHEAST, THE CENTRAL GREAT BASIN, THE NORTHERN GREAT BASIN, THE  
MIDDLE MISSISSIPPI VALLEY, THE UPPER MISSISSIPPI VALLEY, THE CENTRAL  
APPALACHIANS, THE GREAT LAKES, THE SOUTHWEST, THE NORTHERN ROCKIES, THE CENTRAL  
ROCKIES, CALIFORNIA, THE SOUTHERN PLAINS, THE NORTHERN PLAINS, THE  
MID-ATLANTIC, THE PACIFIC NORTHWEST, THE LOWER MISSISSIPPI VALLEY, THE  
TENNESSEE VALLEY, THE SOUTHERN ROCKIES, THE SOUTHERN APPALACHIANS, THE  
SOUTHEAST, AND THE OHIO VALLEY, TUE-MON, DEC 26-JAN 1.  
 
SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES FOR PORTIONS OF MAINLAND ALASKA,  
TUE-WED, DEC 26-DEC 27.  
 
MODERATE RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL  
PLAINS, THE CENTRAL ROCKIES, THE MID-ATLANTIC, THE LOWER MISSISSIPPI VALLEY,  
THE CENTRAL APPALACHIANS, THE NORTHERN PLAINS, THE TENNESSEE VALLEY, THE GREAT  
LAKES, THE NORTHERN ROCKIES, THE SOUTHERN ROCKIES, THE MIDDLE MISSISSIPPI  
VALLEY, THE NORTHEAST, THE SOUTHERN APPALACHIANS, THE UPPER MISSISSIPPI VALLEY,  
THE SOUTHEAST, THE SOUTHERN PLAINS, AND THE OHIO VALLEY, TUE-FRI, DEC 26-DEC 29.  
 
HIGH RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT LAKES,  
THE UPPER MISSISSIPPI VALLEY, AND THE NORTHERN PLAINS, TUE-WED, DEC 26-DEC 27.  
 
SEVERE DROUGHT ACROSS THE CENTRAL PLAINS, THE LOWER MISSISSIPPI VALLEY, THE  
NORTHERN PLAINS, HAWAII, THE NORTHERN ROCKIES, THE MIDDLE MISSISSIPPI VALLEY,  
THE SOUTHERN PLAINS, AND THE SOUTHWEST.  
 
DETAILED SUMMARY  
 
FOR THURSDAY DECEMBER 21 - MONDAY DECEMBER 25: A LOW-PRESSURE SYSTEM IS  
FORECAST TO PROGRESS EASTWARD THROUGH THE GREAT LAKES THURSDAY NIGHT AND  
FRIDAY. A SWATH OF MODERATE TO HEAVY SNOW (6+ INCHES) IS LIKELY ON THE  
NORTHWEST SIDE OF THE CYCLONE TRACK. LOCALLY HEAVY RAIN (>1 INCH IN 24 HOURS)  
IS FORECAST IN THE WARM SECTOR AHEAD OF THE COLD FRONT ON FRIDAY.  
 
IN THE WAKE OF THE COLD FRONT, ARCTIC HIGH PRESSURE IS EXPECTED EXERT ITS  
INFLUENCE OVER MUCH OF THE NORTHERN CONUS, PUSHING SOUTH AND EAST WITH TIME.  
MUCH BELOW NORMAL TEMPERATURES ARE DEPICTED FOR MUCH OF THE WEST-CENTRAL CONUS,  
WHERE MINIMUM TEMPERATURES RANGING FROM 12 TO 24 DEGREES F BELOW NORMAL ARE  
FORECAST.  
 
A PERIOD OF HIGH WINDS OVER SOUTHERN CALIFORNIA IS LIKELY LATE THIS WEEK.  
MODELS ARE SHOWING WIND SPEEDS OF AT LEAST 30 MPH, WHICH IMPLIES EVEN HIGHER  
VALUES IN THE LOCATIONS WHERE TERRAIN FUNNELING CAN INCREASE WIND SPEEDS. HIGH  
WINDS (>35 MPH) ARE ALSO INDICATED FOR PARTS OF THE PLAINS AND THE SOUTHWEST AT  
THE START OF THE PERIOD ASSOCIATED WITH A DEEP UPPER-LEVEL TROUGH.  
 
LATE THIS WEEKEND AND EARLY NEXT WEEK, WINTRY WEATHER IS LIKELY ON THE COLD  
SIDE OF A STALLED FRONT OVER THE EASTERN CONUS. GIVEN THE INCREASED VISIBILITY  
ASSOCIATED WITH THE CHRISTMAS DAY HOLIDAY, A BROAD SHAPE FOR DEC 24-25 IS  
DEPICTED FOR THOSE REGIONS WHERE SNOW, SLEET, OR FREEZING RAIN COULD IMPACT  
TRAVEL.  
 
AN ACTIVE PATTERN IS FORECAST FOR SOUTHERN ALASKA, WITH HIGH WINDS (IN EXCESS  
OF 45 MPH) FOR SOUTHERN AND WESTERN ALASKA, AND HIGH SIGNIFICANT WAVE HEIGHTS  
(EXCEEDING 20 FEET) FOR SOUTHERN ALASKA LATE THIS WEEK INTO THE WEEKEND. IN THE  
WAKE OF THIS ACTIVE WEATHER, SURFACE RIDGING IS FORECAST TO BUILD IN. INTERIOR  
ALASKA IS LIKELY TO EXPERIENCE TEMPERATURES MUCH ABOVE NORMAL FOR AN EXTENDED  
PERIOD OF TIME.  
 
FOR TUESDAY DECEMBER 26 - MONDAY JANUARY 01: WEEK-2 IS LIKELY TO FEATURE A  
CONTINUATION OF THE WARMTH OVER ALASKA, THOUGH THIS IS EXPECTED TO TIME OFF  
QUICKLY AS THE MEAN RIDGE POSITION RETROGRADES. DOWNSTREAM OVER CENTRAL NORTH  
AMERICA, THE COLDEST AIR MASS OF THE SEASON THUS FAR IS FORECAST TO AFFECT THE  
NORTHERN PLAINS, EXTENDING SOUTHWARD AND EASTWARD AT TIMES. SLIGHT, MODERATE,  
AND HIGH RISKS OF MUCH BELOW NORMAL TEMPERATURES ARE DEPICTED FOR THE WEEK  
UNDER ARCTIC HIGH PRESSURE. A HIGH RISK IMPLIES AT LEAST A 60 PERCENT CHANCE OF  
MINIMUM TEMPERATURES BEING IN THE LOWEST 15TH PERCENTILE.  
 
THE U.S. DROUGHT MONITOR, UPDATED ON DEC 12, SHOWS AN INCREASE IN AREAS OF  
EXCEPTIONAL DROUGHT (D2-D4) TO 6.49% FROM 5.99%. MOST OF THE NEWLY INDICATED  
SEVERE DROUGHT WAS OVER THE SOUTHERN PLAINS.  
 
FORECASTER: STEPHEN BAXTER  
 

 
 
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