292  
FXUS21 KWNC 221913  
PMDTHR  
US HAZARDS OUTLOOK  
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD  
300 PM EST JANUARY 22 2021  
 
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FORECAST OVER THE NORTHEAST PACIFIC AT THE  
START OF THE PERIOD, ALONG WITH SURFACE LOW PRESSURE POSITIONED OVER THE GULF  
OF ALASKA. AS A RESULT, INCREASED NORTHERLY WIND SPEEDS ARE FAVORED ON THE  
BACKSIDE OF THIS SYSTEM, RESULTING IN A TREND TOWARD COLDER TEMPERATURES FOR  
MUCH OF ALASKA. INCREASED ONSHORE PACIFIC MOISTURE FLOW IS ALSO ANTICIPATED  
ACROSS PARTS OF NORTHERN CALIFORNIA AND THE PACIFIC NORTHWEST. DOWNSTREAM OVER  
THE REMAINDER OF THE CONUS, THE MID-LEVEL HEIGHT PATTERN IS FORECAST TO BE MORE  
ZONAL, ALTHOUGH INTERMITTENT SHORTWAVE DISTURBANCES MAY TRACK ACROSS THE  
CONTIGUOUS U.S., BRINGING ENHANCED PRECIPITATION CHANCES TO SOME AREAS  
THROUGHOUT THE PERIOD.  
 
HAZARDS  
 
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF NORTHERN CALIFORNIA AND THE  
PACIFIC NORTHWEST, SAT-WED, JAN 30-FEB 3.  
 
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF NORTHERN CALIFORNIA AND THE PACIFIC  
NORTHWEST, SAT-WED, JAN 30-FEB 3.  
 
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES ACROSS THE ALEUTIANS AND PARTS OF  
INTERIOR MAINLAND ALASKA, SAT-THU, JAN 30-FEB 4.  
 
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE ALEUTIANS AND SOUTHWESTERN  
MAINLAND ALASKA. SAT-SUN, JAN 30-31.  
 
DETAILED SUMMARY  
 
FOR MONDAY JANUARY 25 - FRIDAY JANUARY 29:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
FOR SATURDAY JANUARY 30 - FRIDAY FEBRUARY 05: ENSEMBLE MEANS FROM THE GEFS AND  
ECMWF BOTH FEATURE MID-LEVEL LOW PRESSURE OVER THE NORTHEAST PACIFIC DURING  
WEEK-2. THIS FAVORS ENHANCED FLOW OF PACIFIC AIR AND PERIODIC ATMOSPHERIC RIVER  
ACTIVITY INTO PORTIONS OF THE WEST. A SLIGHT RISK FOR HEAVY PRECIPITATION  
(GENERALLY RAIN, WITH HIGH-ELEVATION SNOW) IS FORECAST THROUGH 2/3 STRETCHING  
FROM THE PACIFIC NORTHWEST THROUGH JUST NORTH OF THE BAY AREA IN CALIFORNIA  
WHERE BOTH THE GEFS AND ECMWF REFORECAST TOOLS DEPICT AT LEAST A 20% CHANCE FOR  
3-DAY PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE AND 1.5 INCHES. GIVEN  
THE LACK OF SUBSTANTIAL COLD AIR IN PLACE, NO ASSOCIATED HEAVY SNOW HAZARD IS  
INCLUDED IN THE OUTLOOK, WITH SNOW LEVELS RISING TO LEVELS THAT ARE FAIRLY  
TYPICAL FOR THE REGION. A SLIGHT RISK OF HIGH WIND SPEEDS ACCOMPANIES THE  
ENHANCED ONSHORE PACIFIC FLOW, WHICH IS TYPICAL DURING ATMOSPHERIC RIVER  
EVENTS. THIS WIND HAZARD IS ALSO VALID THROUGH 2/3. DETERMINISTIC RUNS OF THE  
GFS HAVE SHOWN WIND SPEEDS EXCEEDING 40 MPH ALONG THE COASTLINE, BUT WITH  
LITTLE RUN TO RUN CONSISTENCY IN TERMS OF EXACT TIMING AND LOCATION OF THE  
STRONGEST WINDS. ONSHORE FLOW IS FORECAST TO GRADUALLY SHIFT TO THE NORTH AND  
WEAKEN LATER IN THE PERIOD AS THE TROUGH AXIS BECOMES FLATTER IN BOTH THE GEFS  
AND ECMWF ENSEMBLE GUIDANCE, AND SURFACE HIGH PRESSURE DEVELOPS OVER THE EAST  
PACIFIC. THIS IS REFLECTED IN THE GEFS AND ECMWF REFORECAST TOOLS WHICH BOTH  
SHOW DECREASED PROBABILITIES FOR HAZARDOUS PRECIPITATION RELATIVE TO EARLIER IN  
THE PERIOD, AND PROBABILITIES FOR FAVORABLE INTEGRATED VAPOR TRANSPORT ALONG  
THE COAST DECREASE AFTER 2/3 IN BOTH THE GEFS AND ECMWF ENSEMBLES. THEREFORE  
THE ASSOCIATED PRECIPITATION AND WIND HAZARDS ARE DISCONTINUED AFTER 2/3,  
ALTHOUGH SOME EPISODES OF ENHANCED PRECIPITATION ARE STILL POSSIBLE, ESPECIALLY  
CLOSER TO THE CANADIAN BORDER.  
 
THE AFOREMENTIONED EVOLUTION OF THE LONGWAVE PATTERN OVER THE PACIFIC ALSO HAS  
IMPLICATIONS FOR ALASKA DURING WEEK-2. THE TROUGH OVER THE NORTHEAST PACIFIC IS  
FORECAST TO BE ASSOCIATED WITH SURFACE LOW PRESSURE ACROSS THE GULF OF ALASKA,  
WITH INCREASED WIND SPEEDS FAVORED ON THE BACKSIDE OF THIS FEATURE UNDERNEATH A  
TIGHT PRESSURE GRADIENT. A SLIGHT RISK OF HIGH WINDS IS POSTED FOR THE  
ALEUTIANS AND PARTS OF THE SOUTHWESTERN MAINLAND THROUGH 1/31. THE LAST SEVERAL  
RUNS OF THE DETERMINISTIC GFS INDICATE WIND SPEEDS POTENTIALLY EXCEEDING 40 MPH  
ACROSS THE HIGHLIGHTED AREAS. THESE STRONG NORTHERLY WINDS ARE ALSO LIKELY TO  
USHER IN COLDER AIR, RESULTING IN A SLIGHT RISK OF MUCH BELOW NORMAL  
TEMPERATURES FOR THE ALEUTIANS ALONG WITH INTERIOR PORTIONS OF MAINLAND ALASKA  
THROUGH 2/4. THE GEFS AND ECMWF REFORECAST TOOLS INDICATE AT LEAST A 20% CHANCE  
THAT TEMPERATURES FALL BELOW THE 10TH PERCENTILE OVER THESE AREAS, WITH THE  
HIGHEST PROBABILITIES (30-40%) OVER THE ALEUTIANS. ACTUAL TEMPERATURES IN THE  
ALEUTIANS ARE FORECAST TO RANGE FROM THE TEENS FURTHER WEST TO NEAR 0 DEG F  
CLOSER TO THE ALASKA PENINSULA. TEMPERATURES MAY FALL BELOW -20 DEG F ACROSS  
PORTIONS OF THE SOUTHWEST MAINLAND, AND -40 DEG F OVER THE EAST-CENTRAL  
INTERIOR MAINLAND AS ENHANCED EASTERLY FLOW ADVECTS COLDER AIR OVER WESTERN  
CANADA INTO THE STATE. PROBABILITIES FOR HAZARDOUS COLD ARE FORECAST TO  
DIMINISH BY THE END OF THE PERIOD, WITH THE MUCH BELOW NORMAL TEMPERATURES  
HAZARD DISCONTINUED AFTER 2/4.  
 
THE INCREASED PACIFIC FLOW INTO THE CONUS SUGGESTS A FAIRLY MILD PATTERN DURING  
WEEK-2 FOR THE CONTIGUOUS U.S, ALTHOUGH THERE IS SOME SIGNAL FOR TROUGHING  
DEVELOPING OVER THE NORTH-CENTRAL CONUS LATER IN THE PERIOD, WHICH COULD BRING  
SOME COOLER CONDITIONS TO THE NORTHERN PLAINS AND ROCKIES. THE NORTH ATLANTIC  
OSCILLATION IS FORECAST TO SHIFT BACK TOWARD A NEUTRAL OR SLIGHTLY POSITIVE  
PHASE, WHICH WOULD SUPPORT A TREND TOWARD WARMER TEMPERATURES IN THE EAST.  
INTERMITTENT SHORTWAVE DISTURBANCES ARE FAVORED TO EJECT OUT OF THE MEAN TROUGH  
AXIS OVER THE NORTHEAST PACIFIC. THESE SYSTEMS HAVE SOME POTENTIAL TO AMPLIFY  
AS THEY TRACK ACROSS THE CONUS, BUT THERE IS LITTLE MODEL AGREEMENT AND  
CONSISTENCY IN TERMS OF THE TIMING AND LOCATION OF ANY ASSOCIATED IMPACTS, AND  
THEREFORE NO RELATED HAZARDS ARE POSTED EAST OF THE ROCKIES AT THIS TIME.  
 
FORECASTER: THOMAS COLLOW  
 

 
 
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