799  
FXUS21 KWNC 112017  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST DECEMBER 11 2017  
 
SYNOPSIS: THE HIGHLY AMPLIFIED RIDGE-TROUGH PATTERN OVER THE U.S. IS FORECAST  
TO CONTINUE INTO THE BEGINNING OF THE PERIOD BEFORE TRANSITIONING TO A FASTER  
WEST-TO-EAST FLOW REGIME LATER IN WEEK-1. ACTIVE WEATHER IS FORECAST LATE IN  
WEEK-1 OVER THE NORTHWEST DUE TO A BRIEF PERIOD OF STRONG ONSHORE FLOW. DURING  
WEEK-2, THE PATTERN IS FORECAST TO ONCE AGAIN BECOME MORE AMPLIFIED WITH  
UPPER-LEVEL HIGH PRESSURE BECOMING REESTABLISHED OVER NORTHWESTERN NORTH  
AMERICA, AND A MORE ACTIVE STORM TRACK OVER PARTS OF THE EAST-CENTRAL U.S.  
 
HAZARDS  
 
HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND THE NORTHERN  
GREAT BASIN, SAT-SUN, DEC 16-DEC 17.  
 
HEAVY RAIN ACROSS PORTIONS OF THE PACIFIC NORTHWEST, SAT-SUN, DEC 16-DEC 17.  
 
HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN ROCKIES AND THE NORTHERN GREAT  
BASIN, SAT-SUN, DEC 16-DEC 17.  
 
HIGH WINDS ACROSS PORTIONS OF SOUTHERN CALIFORNIA, THU, DEC 14.  
 
HIGH WINDS ACROSS PORTIONS OF THE SOUTHERN ROCKIES AND THE PLAINS, FRI-SAT, DEC  
15-DEC 16.  
 
MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE NORTHEAST AND THE GREAT  
LAKES, THU-FRI, DEC 14-DEC 15.  
 
MUCH ABOVE NORMAL TEMPERATURES ACROSS PORTIONS OF MAINLAND ALASKA, THU-FRI, DEC  
14-DEC 15.  
 
MODERATE RISK OF MUCH ABOVE NORMAL TEMPERATURES OVER PARTS OF MAINLAND ALASKA,  
THU-MON, DEC 21-DEC 25.  
 
SLIGHT RISK OF MUCH ABOVE NORMAL TEMPERATURES OVER PARTS OF MAINLAND ALASKA,  
THU-MON, DEC 21-DEC 25.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE CENTRAL AND  
NORTHERN PLAINS, THE NORTHERN AND CENTRAL ROCKIES, THE MIDDLE AND UPPER  
MISSISSIPPI VALLEYS, AND THE GREAT LAKES, SUN-MON, DEC 24-DEC 25.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PORTIONS OF THE MID-ATLANTIC,  
THE NORTHEAST, AND THE GREAT LAKES, WED-THU, DEC 20-DEC 21.  
 
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE MIDDLE MISSISSIPPI  
VALLEY, THE LOWER MISSISSIPPI VALLEY, THE SOUTHERN PLAINS, THE OHIO VALLEY, AND  
THE TENNESSEE VALLEY, THU-SAT, DEC 21-DEC 23.  
 
SEVERE DROUGHT ACROSS 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 14 - MONDAY DECEMBER 18: AN AMPLIFIED RIDGE-TROUGH  
PATTERN IS FORECAST TO PERSIST INTO THE BEGINNING OF THE PERIOD. MINIMUM  
TEMPERATURES RANGING BETWEEN 12 AND 16 DEGREES F BELOW NORMAL ARE FORECAST OVER  
PARTS OF THE GREAT LAKES AND THE NORTHEAST ON THURSDAY AND FRIDAY, BEFORE A  
MODERATING TREND BEGINS THIS WEEKEND.  
 
RECENT EXTREME FIRE WEATHER CONDITIONS OVER SOUTHERN CALIFORNIA COULD PERSIST  
INTO THURSDAY BEFORE THE THE UPPER-LEVEL RIDGE BEGINS TO WEAKEN. THERE IS STILL  
CONSIDERABLE UNCERTAINTY IN THE FORECAST, THOUGH GUSTY OFFSHORE WINDS ARE  
LIKELY TO CONTRIBUTE TO THE FIRE WEATHER RISK THROUGH DAY-3.  
 
AS THE PATTERN BECOMES MORE ZONAL AND PROGRESSIVE DURING THE PERIOD, STRONG  
WESTERLY SURFACE WINDS ARE FORECAST OVER PARTS OF THE CENTRAL AND SOUTHERN HIGH  
PLAINS. THIS HAZARD IS DEPICTED MAINLY TO RAISE AWARENESS TO THE POTENTIAL FOR  
CRITICAL FIRE WEATHER LATE THIS WEEK AND EARLY THIS WEEKEND.  
 
A PERIOD OF STRONG ONSHORE FLOW IS FORECAST OVER THE NORTHWEST THIS WEEKEND.  
HEAVY RAIN (>2 INCHES IN 24 HOURS) IS EXPECTED NEAR THE COAST, WITH HIGHER  
ELEVATION SNOW BECOMING MORE OF CONCERN IN THE CASCADES AND NORTHERN ROCKIES.  
AREAS WITHIN THE DEPICTED HAZARD ARE EXPECTED TO SEE IN EXCESS OF 6 INCHES OF  
SNOW, WITH SIGNIFICANTLY MORE LIKELY IN FAVORED LOCATIONS.  
 
MUCH ABOVE NORMAL TEMPERATURES (MAXIMUM TEMPERATURES NEAR 20 DEGREES F ABOVE  
NORMAL) ARE FORECAST FOR PARTS OF MAINLAND ALASKA IN FAVORED DOWNSLOPE REGIONS  
WITH SOUTHERLY WIND EARLY IN THE PERIOD. SOME MODERATION IS EXPECTED THIS  
WEEKEND.  
 
FOR TUESDAY DECEMBER 19 - MONDAY DECEMBER 25: THE OVERALL PATTERN IS FORECAST  
TO TRANSITION DURING THE WEEK-2 PERIOD FROM ONE FEATURING ANOMALOUS TROUGHING  
OVER THE NORTHEAST, TO ONE IN WHICH THE MEAN TROUGH POSITION IS FURTHER WEST.  
AT THE SAME TIME, THE VARIOUS DYNAMICAL MODEL GUIDANCE FAVORS THE DEVELOPMENT  
UPPER-LEVEL RIDGING OVER THE FAR NORTHEASTERN PACIFIC AND ALASKA BY THE MIDDLE  
OF WEEK-2.  
 
THERE IS A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES OVER THE NORTHEAST AND  
GREAT LAKES ON DEC 20-21 ASSOCIATED WITH A DEEP UPPER-TROUGH. AS THE PATTERN  
EVOLVES IN WEEK-2, THE THREAT OF ARCTIC AIR SURGING INTO THE PLAINS INCREASES,  
WITH A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR THE PLAINS AND UPPER  
MIDWEST ON DEC 24-25. AS ANOMALOUS RIDGING DEVELOPS OFF THE SOUTHEAST COAST,  
THE PATTERN BECOMES FAVORABLE FOR A STORM TRACK OVER THE INTERIOR SOUTHEAST,  
WHERE A SLIGHT RISK OF HEAVY PRECIPITATION IS DEPICTED DURING THE MIDDLE OF THE  
PERIOD.  
 
CONFIDENCE IS HIGHER OVER ALASKA OVERALL, WHERE THE DYNAMICAL MODELS ARE IN  
GOOD AGREEMENT AND THE ODDS OF MUCH ABOVE NORMAL TEMPERATURES FOR DAYS 10-14  
ARE SIGNIFICANTLY ELEVATED.  
 
THE U.S. DROUGHT MONITOR, UPDATED ON DEC 5, SHOWS AN INCREASE IN AREAS OF  
EXCEPTIONAL DROUGHT (D2-D4) FROM 4.88% TO 5.99%, A CHANGE OF 1.11% OVER THE  
CONTINENTAL U.S. FROM THE PREVIOUS WEEK.  
 
FORECASTER: STEPHEN BAXTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page