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FXUS21 KWNC 211847
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 21 2025
SYNOPSIS: MODEL ENSEMBLES FAVOR A SLOW DEAMPLIFICATION OF THE MID-LEVEL PATTERN
THROUGH WEEK-2, RESULTING IN FEW CONCERNS FROM A HAZARDOUS WEATHER PERSPECTIVE.
EARLY WEEK-2, MID-LEVEL LOW PRESSURE OVER THE NORTH PACIFIC MAY ENHANCE SURFACE
WINDS ALONG PART OF THE CALIFORNIA COAST EARLY IN THE PERIOD BEFORE THIS
FEATURE WEAKENS AND DRIFTS AWAY FROM THE AREA. A TROPICAL CYCLONE MAY MOVE
PARALLEL TO THE EAST COAST EARLY- TO MID-WEEK, BUT AT THIS TIME, IT LOOKS LIKE
ANYTHING THAT DEVELOPS WILL REMAIN A FEW HUNDRED MILES OFFSHORE. THIS SHOULD
LIMIT IMPACTS TO ROUGH SURF AND POTENTIALLY ISOLATED HIGH WIND GUSTS ON THE
IMMEDIATE COAST. MODERATE TO HEAVY PRECIPITATION IN THE NORTHWEST AND NORTHERN
CALIFORNIA IS NOT EXPECTED TO REACH HAZARDS THRESHOLDS, NOR ARE THE WET AND
WINDY CONDITIONS EXPECTED ACROSS SOUTH-CENTRAL AND SOUTHEASTERN ALASK
HAZARDS
SLIGHT RISK OF EPISODIC HIGH WINDS FOR COASTAL PORTIONS OF CALIFORNIA FROM CAPE
MENDOCINO TO POINT CONCEPTION, MON, SEP 29.
DETAILED SUMMARY
FOR WEDNESDAY SEPTEMBER 24 - SUNDAY SEPTEMBER 28:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR MONDAY SEPTEMBER 29 - SUNDAY OCTOBER 05: TODAY’S MODEL SOLUTIONS FOR
500-HPA HEIGHTS INDICATE A STEADY DEAMPLIFICATION OF THE 500-HPA HEIGHT PATTERN
THAT INITIALLY STRETCHES FROM THE NORTH PACIFIC ACROSS NORTH AMERICA. MODELS
ARE IN GOOD AGREEMENT ON THE LOCATION OF AMPLIFIED FEATURES EARLY WEEK-2, BUT
AS DEAMPLIFICATION BEGINS, THE MODELS BEGIN TO DIVERGE. BY THE END OF WEEK-2,
THE ENSEMBLE MEANS FORECAST 500-HPA HEIGHTS CLOSE TO NORMAL ACROSS ALL OF
CANADA AND THE CONTIGUOUS UNITED STATES (CONUS), BUT THIS IS AT LEAST PARTIALLY
DUE TO DIFFERENCES IN THE FORECAST PATTERN AMONG THE INDIVIDUAL MEMBERS USED TO
CALCULATE THE MEAN, WHICH TEND TO AVERAGE OUT TO NEAR NORMAL AT ANY GIVEN
LOCATION. SOME OF THE DETERMINISTIC MODELS DO NOT FLATTEN THE FLOW AS MUCH AS
THE ENSEMBLE MEANS, BUT THERE IS RUN-TO-RUN INCONSISTENCY IN FORECAST SOLUTIONS
AND DIFFERENCES BETWEEN THE DISPARATE MODELS, SO THERE IS LITTLE CONFIDENCE IN
ANY GIVEN SOLUTION. FOR THE LAST FEW MODEL RUNS, THE DETERMINISTIC ECMWF HAS
FORMED ONE OR MORE CLOSED 500-HPA LOWS OVER THE CONUS AND/OR THE MID-LATITUDES
OF THE OCEANS TO THE EAST AND WEST. THESE MEANDER AND PERSIST UNTIL THE END OF
WEEK-2, BUT THE FORECAST PLACEMENT AND TIMING OF THESE SYSTEMS SEEM TO CHANGE
ALMOST RANDOMLY FROM RUN-TO-RUN. THE GFS AND DETERMINISTIC CANADIAN HINT AT THE
POTENTIAL FOR ONE OR TWO WEAK CLOSED 500-HPA LOWS OVER OR NEAR THE CONUS NEAR
THE START OF THE PERIOD, BUT THESE DISSIPATE AND THE FLOW BECOMES FAIRLY ZONAL
ACROSS THE CONUS AFTER THAT, ALTHOUGH SOME AMPLIFICATION IS MAINTAINED OVER
CANADA AND THE NORTH PACIFIC.
FROM A HAZARDS PERSPECTIVE, ANY CLOSED 500-HPA LOW AFFECTING PART OF THE CONUS
WILL BE WEAK, SPINNING UP SOUTH OF THE ANOMALOUSLY HIGH 500-HPA HEIGHTS OVER
CANADA. AS A RESULT, IF ANY SUCH SYSTEM FORMS, IT IS UNLIKELY TO BRING
PRECIPITATION OR WIND REACHING HAZARDOUS THRESHOLDS. THUS NO PRECIPITATION OR
WIND HAZARDS ARE POSTED AFTER DAY ONE (SEP 29). AT THE OUTSET OF WEEK-2, MOST
GUIDANCE CONTINUES TO DEPICT A FAIRLY DEEP TROUGH OFF THE WEST COAST, AND
TODAY’S EUROPEAN ENSEMBLE (ECENS) MEAN HAS COME INTO AGREEMENT. WITH SURFACE
HIGH PRESSURE EXPECTED OFF THE WEST COAST, AND ADJACENT SURFACE LOW PRESSURE
OVER THE INTERIOR WEST, A TIGHT PRESSURE GRADIENT AT THE SURFACE IS POSSIBLE,
INCREASING THE CHANCES OF HIGH WINDS ALONG PART OF THE IMMEDIATE WEST COAST. A
SLIGHT RISK OF EPISODIC HIGH WINDS IS POSTED FOR COASTAL PORTIONS OF CALIFORNIA
BETWEEN CAPE MENDOCINO AND POINT CONCEPTION, WHERE THE CHANCES FOR WIND SPEEDS
EXCEEDING BOTH THE 85TH PERCENTILE AND 25 MPH APPEAR MAXIMIZED. THIS HAZARD IS
UNCHANGED FROM YESTERDAY, ENDING AFTER THE FIRST DAY OF WEEK-2. THEREAFTER, THE
MID-LEVEL TROUGH SHOULD BE WEAKENING AND DRIFTING AWAY FROM THE AREA, WHICH
SHOULD WEAKEN THE SURFACE PRESSURE GRADIENT SUFFICIENTLY TO END THE RISK OF
HIGH WINDS. SOME MODELS HINT AT RENEWED POTENTIAL FOR HIGH WINDS LATER WEEK-2
ALONG AT LEAST PART OF THE WEST COAST, BUT THAT SOLUTION IS NOT FAVORED AT THIS
TIME GIVEN THE EXPECTED ZONAL FLOW, WITH OR WITHOUT A WEAK 500-HPA CLOSED LOW
NEARBY.
THIS MID-LEVEL TROUGH SHOULD ESTABLISH SIGNIFICANT MOISTURE TRANSPORT INTO THE
WESTERN CONUS, LIKELY RESULTING IN PERIODS OF ENHANCED PRECIPITATION FOR MUCH
OF THE CENTRAL AND NORTHERN WEST COAST REGION UNTIL THE MIDDLE OF WEEK-2. A FEW
INCHES OF PRECIPITATION MAY ACCUMULATE OVER BOTH COASTAL AND HIGHER ELEVATION
LOCATIONS IN WESTERN WASHINGTON AND NORTHWESTERN OREGON. HOWEVER, THIS IS A
FAVORED REGION FOR ENHANCED PRECIPITATION WHEN THERE IS DECENT MID-LEVEL
TROUGHING OFF THE NORTHWEST COAST, SO THESE AMOUNTS ARE NOT MARKEDLY ATYPICAL,
AND NO HAZARD IS POSTED. THE PROBABILISTIC EXTREMES TOOLS (PETS) FROM THE ECENS
AND GEFS BOTH SHOW THE HIGHEST CHANCES FOR PRECIPITATION TOTALS ABOVE THE 85TH
CLIMATOLOGICAL PERCENTILE IN NORTHERN CALIFORNIA, BUT NORMALS ARE FAIRLY LOW IN
THAT REGION THIS TIME OF YEAR, AND THE TWO PETS ONLY SHOW A 20 TO 40 PERCENT
CHANCE OF PRECIPITATION TOTALS TOPPING 0.5 INCH DURING THE FIRST FEW DAYS OF
THE PERIOD. GIVEN THE RELATIVELY LOW AMOUNT FORECAST, NO HAZARD IS POSTED THERE
AT THIS TIME.
MID-LEVEL TROUGHING NEAR OR SOUTH OF ALASKA IS EXPECTED TO PRODUCE ONE OR MORE
SURFACE STORMS NEAR THE ALEUTIANS AND/OR INTO THE GULF OF ALASKA DURING WEEK-2
WHICH MAY MEANDER CLOSE TO THE SOUTHEAST ALASKA COAST. WITH HIGH SURFACE
PRESSURE EXPECTED WEST OF THE CONTIGUOUS UNITED STATES (CONUS), THIS SET-UP
WOULD FAVOR INCREASED SURFACE WINDS AND ABUNDANT MOISTURE TRANSPORT FROM THE
NORTH PACIFIC INTO SOUTH-CENTRAL AND SOUTHEASTERN ALASKA, WHICH IN TURN WOULD
ENHANCE CHANCES FOR HEAVY PRECIPITATION ACROSS THIS REGION. THE ECENS, CANENS,
AND GEFS MEANS ALL SHOW OVER 4 INCHES OF RAIN FALLING ON A LARGE PART OF THIS
AREA DURING WEEK-2, ALONG WITH PERIODS OF GUSTY WINDS. HOWEVER, THIS IS THE
WETTEST TIME OF THE YEAR IN THE REGION, AND THE FORECAST PATTERN IS NOT TOO
UNUSUAL. WEEKLY NORMALS EXCEED 2 INCHES ALMOST REGION-WIDE NOW, REACHING AS
HIGH AS 6 INCHES IN CENTRAL SOUTHEASTERN ALASKA ALONG THE IMMEDIATE COAST. THE
PATTERN ALSO FAVORS STRONGER WINDS THAN ARE TYPICAL FOR MOST OTHER PARTS OF THE
CONTINENT. GIVEN THE CLIMATOLOGY, NEITHER THE FORECAST PRECIPITATION TOTALS NOR
WIND GUSTS ARE EXPECTED TO REACH HAZARDS THRESHOLDS.
MANY OF THE DETERMINISTIC MODELS - IN PARTICULAR THE ECMWF AIFS - BRING A
TROPICAL CYCLONE NORTHWARD IN THE WESTERN NORTH ATLANTIC PARALLEL TO THE EAST
COAST DURING THE EARLY TO MIDDLE PART OF THE PERIOD. ALL MODELS KEEP ANY
DEVELOPMENT SEVERAL HUNDRED MILES OFFSHORE, SO IT’S LIKELY THAT ANY IMPACTS ON
THE CONUS WILL BE LIMITED TO ROUGH SURF AND POSSIBLY ISOLATED OCCURRENCES OF
STRONG WIND GUSTS ALONG THE IMMEDIATE COASTLINE. THERE ARE A FEW INDIVIDUAL
MEMBERS OF ALL THE ENSEMBLES THAT SHOW TROPICAL CYCLONE DEVELOPMENT IN THE
WESTERN ATLANTIC CLOSER TO THE EAST COAST, OR IN THE GULF, BUT THESE ARE MARKED
OUTLIERS AND THERE IS NO RUN-TO-RUN CONSISTENCY ON PLACEMENT OR STRENGTH. THIS
POTENTIAL WILL BE MONITORED, BUT IT IS NOT THE FAVORED SOLUTION AT THIS TIME
GIVEN THE OVERWHELMING MAJORITY OF ENSEMBLE MEMBERS DO NOT FORECAST A TROPICAL
CYCLONE NEAR THE CONUS DURING WEEK-2.
FORECASTER: RICH TINKER
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