103
FXUS21 KWNC 211759
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 21 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FORECAST TO MOVE INLAND ACROSS THE WESTERN
CONTIGUOUS U.S. (CONUS) EARLY IN WEEK-2. INCREASED CHANCES FOR HEAVY RAIN AND
HIGH WINDS ARE FORECAST TO LINGER INTO THE START OF THE PERIOD ALONG THE WEST
COAST. AS THIS MID-LEVEL LOW MOVES FURTHER EAST, SUBFREEZING TEMPERATURES ARE
POSSIBLE OVER PORTIONS OF THE ROCKIES AND GREAT BASIN. SURFACE LOW PRESSURE MAY
DEVELOP AND BRING ELEVATED CHANCES FOR HEAVY PRECIPITATION ACROSS PARTS OF THE
GREAT PLAINS AND MISSISSIPPI VALLEY BY THE MIDDLE OF THE PERIOD, ALTHOUGH
CONFIDENCE IS LOW GIVEN THE INCONSISTENCY IN THE MODELS. FURTHER EAST,
MID-LEVEL HIGH PRESSURE FAVORS ABOVE-NORMAL TEMPERATURES AND BELOW-NORMAL
PRECIPITATION, WITH THE GREATEST LIKELIHOOD OF ENHANCED RAINFALL OVER THE
SOUTHEAST AND MID-ATLANTIC NOW SHIFTING INTO THE LATE WEEK-1 TIMEFRAME. THESE
CONTINUED DRY CONDITIONS AND UNSEASONABLY WARM TEMPERATURES RESULT IN AN
ENHANCED RISK FOR RAPID ONSET DROUGHT (ROD) ACROSS PORTIONS OF THE SOUTHEAST
AND TENNESSEE VALLEY.
HAZARDS
SLIGHT RISK FOR HEAVY PRECIPITATION FOR WESTERN PORTIONS OF WASHINGTON, OREGON,
AND NORTHERN CALIFORNIA, FRI, SEP 29.
SLIGHT RISK FOR HIGH WINDS FOR COASTAL PORTIONS OF WASHINGTON, OREGON, AND
CALIFORNIA NORTH OF THE BAY AREA, FRI, SEP 29.
SLIGHT RISK FOR MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE GREAT
BASIN AND NORTHERN AND CENTRAL ROCKIES, SAT-MON, SEP 30-OCT 2.
RAPID ONSET DROUGHT RISK ACROSS MUCH OF ALABAMA AND ADJACENT AREAS OF
MISSISSIPPI, GEORGIA, AND TENNESSEE.
DETAILED SUMMARY
FOR SUNDAY SEPTEMBER 24 - THURSDAY SEPTEMBER 28:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY SEPTEMBER 29 - THURSDAY OCTOBER 05: AN AMPLIFIED 500-HPA HEIGHT
PATTERN IS FORECAST ACROSS NORTH AMERICA DURING WEEK-2, WITH TROUGHING MOVING
INLAND ACROSS THE WESTERN CONUS AND RIDGING OVER THE EAST. THE GEFS AND ECMWF
PROBABILISTIC EXTREMES TOOLS (PETS) CONTINUE TO INDICATE AT LEAST A 20 PERCENT
CHANCE 3-DAY PRECIPITATION TOTALS EXCEED THE 85TH CLIMATOLOGICAL PERCENTILE AND
1-INCH ACROSS PORTIONS OF THE PACIFIC NORTHWEST EARLY IN THE PERIOD. HOWEVER,
UNCALIBRATED GUIDANCE SUGGESTS THE HIGHEST CHANCES OF ENHANCED PRECIPITATION
OCCURRING LATE IN THE WEEK-1 PERIOD AND LINGERING INTO THE OUTSET OF WEEK-2,
WITH DIMINISHING CHANCES AS THE TROUGH MOVES FURTHER INLAND. THEREFORE, THE
SLIGHT RISK FOR HEAVY PRECIPITATION POSTED ACROSS COASTAL SECTIONS OF
WASHINGTON, OREGON, AND NORTHERN CALIFORNIA CONTINUES ON DAY-8 (SEP 29) ONLY,
ALONG WITH THE ACCOMPANYING SLIGHT RISK FOR HIGH WINDS. THE ECMWF PET STILL
DEPICTS AT LEAST A 20 PERCENT CHANCE WIND SPEEDS REACH THE 85TH PERCENTILE AND
25-MPH THROUGH THE BAY AREA OF CALIFORNIA, WITH THE GEFS CONFINING THESE
ENHANCED PROBABILITIES FARTHER NORTH ACROSS COASTAL WASHINGTON AND OREGON.
AS THIS TROUGH MOVES FURTHER INLAND, DECREASING MID-LEVEL HEIGHTS OVER THE
WESTERN CONUS FAVOR AN EXPANDING REGION OF BELOW-NORMAL TEMPERATURES. BECAUSE
IT IS EARLY IN THE SEASON, ACTUAL TEMPERATURES ARE GENERALLY NOT FORECAST TO BE
EXTREMELY COLD, BUT THE GEFS AND ECMWF PETS BOTH INDICATE AT LEAST A 20 PERCENT
CHANCE MINIMUM TEMPERATURES FALL BELOW THE 15TH PERCENTILE AND FREEZING OVER
PORTIONS OF THE INTERIOR WEST BEGINNING ON DAY-9 (SEP 30). VEGETATION IN THIS
AREA IS LIKELY TO BE VULNERABLE TO ANY FROST OR FREEZES THAT MAY OCCUR DURING
THE PERIOD. THE 0Z GEFS AND ECMWF ENSEMBLES BOTH DEPICT THE TROUGHING WEAKENING
BY THE SECOND HALF OF THE PERIOD, WITH THE 0Z CANADIAN ENSEMBLE BEING MUCH
QUICKER TO WEAKEN THE TROUGH. GIVEN THE DECENT AGREEMENT BETWEEN THE GEFS AND
ECMWF, ALONG WITH THE PETS, A SLIGHT RISK FOR MUCH BELOW NORMAL TEMPERATURES IS
HIGHLIGHTED ACROSS PORTIONS OF THE GREAT BASIN AND THE NORTHERN AND CENTRAL
ROCKIES FROM SEP 30 TO OCT 2. SNOW MAY ALSO BE POSSIBLE ACROSS THE HIGHER
ELEVATIONS OF THESE AREAS. THE GEFS SNOW WATER EQUIVALENT (SWE) PET INDICATES
ELEVATED CHANCES OF SWE EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE. HOWEVER,
CLIMATOLOGY IS STILL QUITE LOW AND NO CORRESPONDING RISK OF HEAVY SNOW IS
POSTED AT THIS TIME.
GIVEN THE INCREASED TROUGHING IN THE WEST, THERE ARE SOME INDICATIONS OF
SURFACE LOW PRESSURE DEVELOPING IN THE LEE OF THE ROCKIES AND BRINGING ENHANCED
PRECIPITATION INTO PORTIONS OF THE GREAT PLAINS AND MISSISSIPPI VALLEY TOWARD
THE MIDDLE OF WEEK-2. THE ECMWF PET IS SUPPORTIVE OF THIS SCENARIO, DEPICTING
AT LEAST A 40 PERCENT CHANCE OF 3-DAY PRECIPITATION EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE DURING THE OCT 1-3 TIMEFRAME OVER PARTS OF THESE
REGIONS. HOWEVER, PROBABILITIES IN THE GEFS PET ARE MUCH LESS, WITH MANY AREAS
NOT REACHING 20 PERCENT. ADDITIONALLY, THE UNCALIBRATED ENSEMBLE MEAN TOTALS
FROM THE 0Z GEFS, ECMWF, AND CANADIAN ARE FAIRLY LOW (DAILY TOTALS GENERALLY
LESS THAN 0.25 INCHES). DUE TO THE DISCREPANCY IN THE MODELS, NO RELATED HEAVY
PRECIPITATION HAZARD IS POSTED, AND THIS POTENTIAL WILL BE REEVALUATED AGAIN
TOMORROW. IT IS NOTEWORTHY THAT SEVERE TO EXCEPTIONAL DROUGHT CONTINUES ACROSS
MANY AREAS OF THE GREAT PLAINS AND MISSISSIPPI VALLEY, SO ANY PRECIPITATION
OVER THESE AREAS MAY END UP BEING MORE OF A WELCOME BENEFIT THAN A HAZARD.
AN AREA OF LOW PRESSURE (NOW DESIGNATED AS POTENTIAL TROPICAL CYCLONE SIXTEEN)
IS FORECAST TO BRING HEAVY PRECIPITATION ACROSS PARTS OF THE EAST COAST OVER
THE NEXT FEW DAYS, WITH THE POSSIBILITY OF ANOTHER DISTURBANCE DEVELOPING OVER
PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC LATER IN WEEK-1. HOWEVER, ALL
INDICATIONS ARE PRECIPITATION CHANCES SHOULD BE DIMINISHING BY THE START OF
WEEK-2 OVER THE EASTERN CONUS AS RIDGING BECOMES ESTABLISHED OVER EASTERN NORTH
AMERICA, WITH MEAN SURFACE HIGH PRESSURE FORECAST OVER THE NORTHEAST DURING
WEEK-2. THEREFORE, THE SLIGHT RISK OF HEAVY PRECIPITATION IS DISCONTINUED ALONG
THE MID-ATLANTIC COAST, ALTHOUGH SOME LINGERING AREAS OF RAIN CANNOT BE RULED
OUT ESPECIALLY ON DAY-8 (SEP 29). RAPID ONSET DROUGHT (ROD) RISK REMAINS
ELEVATED ACROSS MUCH OF ALABAMA AND ADJACENT AREAS OF MISSISSIPPI, GEORGIA, AND
TENNESSEE. THESE AREAS ARE NOT FORECAST TO BE IMPACTED BY THE AFOREMENTIONED
DISTURBANCES OVER THE MID-ATLANTIC IN WEEK-1, AND ARE ALSO PREDICTED TO BE
DOWNSTREAM OF THE POTENTIAL SYSTEM THAT MAY IMPACT THE GREAT PLAINS AND
MISSISSIPPI VALLEY TOWARD THE MIDDLE OF WEEK-2. THEREFORE, NOT MUCH
PRECIPITATION IS FORECAST ACROSS THE HIGHLIGHTED ROD AREA DURING THE NEXT 2
WEEKS, ALONG WITH ABOVE-NORMAL TEMPERATURES, WHICH WOULD FAVOR DROUGHT
EXPANSION.
A TRANSITION TO MORE TROUGHING IS PREDICTED ACROSS ALASKA WHICH FAVORS
INCREASING CHANCES FOR HEAVY PRECIPITATION ACROSS SOUTHERN AND SOUTHEAST
ALASKA. ALTHOUGH, SEVERAL INCHES OF PRECIPITATION ARE POSSIBLE, WITH BOTH THE
GEFS AND ECMWF PETS DEPICTING AT LEAST A 20-40 PERCENT CHANCE FOR 3-DAY
PRECIPITATION TOTALS EXCEEDING 2-INCHES THROUGHOUT THE PERIOD, NO RELATED
PRECIPITATION HAZARD IS POSTED DUE TO LOWER PROBABILITIES BASED ON
CLIMATOLOGICAL PERCENTILES COINCIDING WITH THE EXPECTED INCREASE IN
PRECIPITATION OVER THIS REGION DURING THIS TIME OF YEAR.
FORECASTER: THOMAS COLLOW
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