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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page