917
FXUS21 KWNC 031804
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 03 2023
SYNOPSIS: EARLY IN WEEK-2, MID-LEVEL LOW PRESSURE IS FORECAST TO MOVE OFF THE
EASTERN SEABOARD FAVORING DECREASING CHANCES OF OVERNIGHT FROST ACROSS INTERIOR
PARTS OF THE EAST. ANOTHER AREA OF MID-LEVEL LOW PRESSURE IS FORECAST ACROSS
THE NORTHEAST PACIFIC LEADING TO ENHANCED ONSHORE FLOW AND INCREASED CHANCES
FOR ROUNDS OF HEAVY PRECIPITATION AND HIGH WINDS ACROSS THE PACIFIC NORTHWEST
AT THE START OF THE PERIOD. THIS FEATURE IS FAVORED TO RETROGRADE LATER IN THE
PERIOD, SHIFTING THE HIGHEST PRECIPITATION CHANCES TO SOUTHEAST ALASKA. RAPID
ONSET DROUGHT (ROD) RISK REMAINS ELEVATED ACROSS PARTS OF THE SOUTHEAST
QUADRANT OF THE CONUS DUE TO PROLONGED DRY CONDITIONS AND LITTLE RAINFALL
FORECAST IN WEEK-1, ALTHOUGH THERE ARE SOME INDICATIONS OF INCREASING CHANCES
FOR PRECIPITATION BY THE MIDDLE OF WEEK-2.
HAZARDS
SLIGHT RISK OF EPISODES OF HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC
NORTHWEST AND EXTREME NORTHERN CALIFORNIA, WED-THU, OCT 11-12.
SLIGHT RISK OF EPISODES OF HIGH WINDS ACROSS COASTAL PORTIONS OF THE PACIFIC
NORTHWEST AND EXTREME NORTHERN CALIFORNIA, WED-THU, OCT 11-12.
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF WESTERN
MAINLAND ALASKA AND THE ALASKA PENINSULA, WED-SAT, OCT 11-14.
RAPID ONSET DROUGHT (ROD) RISK OVER PARTS OF THE LOWER MISSISSIPPI AND
TENNESSEE VALLEYS, SOUTHEAST AND SOUTHERN APPALACHIANS.
DETAILED SUMMARY
FOR FRIDAY OCTOBER 06 - TUESDAY OCTOBER 10:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY OCTOBER 11 - TUESDAY OCTOBER 17: MID-LEVEL TROUGHING IS FORECAST
TO MOVE OFF THE EAST COAST OF THE CONUS EARLY IN WEEK-2. HOWEVER, THE UPSTREAM
PATTERN HAS BECOME MORE UNCERTAIN, WITH THE GEFS AND CANADIAN ENSEMBLES
INDICATING CYCLONIC FLOW AND WEAKLY NEGATIVE MID-LEVEL HEIGHTS PERSISTING OVER
THE EAST INTO THE MIDDLE OF THE PERIOD, WHILE THE ECMWF ENSEMBLE DEPICTS MORE
RIDGING OVER THE EAST-CENTRAL CONUS. SURFACE HIGH PRESSURE OVER THE EAST MAY
CONTINUE TO PROMOTE IDEAL RADIATIONAL COOLING CONDITIONS OVERNIGHT, WITH THE
ECMWF PROBABILISTIC EXTREMES TOOL (PET) INDICATING AT LEAST A 20 PERCENT CHANCE
MINIMUM TEMPERATURES FALL BELOW THE 15TH CLIMATOLOGICAL PERCENTILE AND 40 DEG F
EARLY IN WEEK-2. HOWEVER, THIS SIGNAL IS NON-EXISTENT IN THE GEFS PET AND
UNCALIBRATED MINIMUM TEMPERATURES IN THE ECMWF AND GEFS ENSEMBLE MEANS ARE
GENERALLY ABOVE 40 DEG F IN NEARLY ALL AREAS. THEREFORE, ANY FROST THREAT
APPEARS MORE LIKELY TO OCCUR IN THE LATE WEEK-1 PERIOD, WITH CHANCES
DIMINISHING BY WEEK-2. THEREFORE, THE SLIGHT RISK OF MUCH BELOW NORMAL
TEMPERATURES HAS BEEN DISCONTINUED ACROSS THE OHIO AND TENNESSEE VALLEYS AND
APPALACHIANS.
ANOTHER MID-LEVEL TROUGH IS FORECAST ACROSS THE NORTHEAST PACIFIC EARLY IN
WEEK-2, WHICH FAVORS INCREASING ONSHORE FLOW ACROSS THE PACIFIC NORTHWEST,
ALTHOUGH THE STRONGEST ATMOSPHERIC RIVER SIGNALS BASED ON THE GEFS INTEGRATED
VAPOR TRANSPORT TOOL ARE DURING THE LATE WEEK-1 TIMEFRAME. THE GEFS PET
CONTINUES TO DEPICT PROBABILITIES OF AT LEAST 20 PERCENT FOR 3-DAY
PRECIPITATION TOTALS TO EXCEED THE 85TH CLIMATOLOGICAL PERCENTILE AND 1-INCH
EARLY IN WEEK-2, WHILE THE ECMWF PET HAS DECREASED PROBABILITIES CONSIDERABLY
COMPARED TO YESTERDAY TO ABOUT 20-40 PERCENT. THE UNCALIBRATED ENSEMBLE MEANS
FROM THE GEFS AND ECMWF DEPICT THE HIGHEST PRECIPITATION CHANCES EARLY IN THE
PERIOD BEFORE THE TROUGH RETROGRADES, SHIFTING THE HEAVIEST PRECIPITATION
POTENTIAL TO SOUTHEAST ALASKA. THEREFORE, THE SLIGHT RISK FOR EPISODES OF HEAVY
PRECIPITATION IS POSTED ACROSS PORTIONS OF THE PACIFIC NORTHWEST FOR DAYS 8 AND
9 (OCT 11-12). AN ACCOMPANYING SLIGHT RISK OF EPISODES OF HIGH WINDS IS ALSO
POSTED ALONG COASTAL AREAS OF THE PACIFIC NORTHWEST DUE TO THE FORECAST
INCREASED PRESSURE GRADIENT. BOTH OF THESE RISK AREAS ARE CUT BACK OVER
NORTHERN CALIFORNIA COMPARED TO YESTERDAY GIVEN THE MORE NORTHWARD SHIFTED
SIGNAL IN THE PETS AND UNCALIBRATED GUIDANCE.
DRY SOILS AND LITTLE TO NO PRECIPITATION FORECAST DURING WEEK-1 CONTINUE TO
SUPPORT AN ELEVATED RAPID ONSET DROUGHT (ROD) RISK OVER PARTS OF THE LOWER
MISSISSIPPI AND TENNESSEE VALLEYS, SOUTHEAST AND SOUTHERN APPALACHIANS.
HOWEVER, BOTH THE GEFS AND ECMWF PETS INDICATE INCREASING PRECIPITATION CHANCES
ACROSS THE SOUTHEAST AND FLORIDA, DEPICTING SOME AREAS HAVING AT LEAST A 20
PERCENT CHANCE 3-DAY PRECIPITATION TOTALS EXCEED THE 85TH CLIMATOLOGICAL
PERCENTILE AND 1-INCH DUE TO AN INCREASING INFLUX OF TROPICAL MOISTURE. THE
GEFS PET KEEPS THESE ELEVATED PROBABILITIES CLOSE TO THE GULF OF MEXICO COAST
AND FLORIDA, WHILE THE ECMWF PET BRINGS THEM MORE INLAND ACROSS THE SOUTHEAST
AND LOWER MISSISSIPPI VALLEY BY THE MIDDLE OF THE PERIOD. THESE TRENDS WILL
CONTINUE TO BE MONITORED AND A SLIGHT RISK OF HEAVY PRECIPITATION MAY NEED TO
BE INTRODUCED IN TOMORROW’S OUTLOOK FOR SOME OF THESE AFOREMENTIONED AREAS.
PERSISTENT MID-LEVEL TROUGHING FORECAST ACROSS ALASKA FAVORS BELOW-NORMAL
TEMPERATURES ACROSS MUCH OF THE STATE. THE ECMWF AND GEFS PETS CONTINUE TO
INDICATE AT LEAST A 20 PERCENT CHANCE OF MINIMUM TEMPERATURES FALLING BELOW THE
15TH CLIMATOLOGICAL PERCENTILE AND 20 DEG F ACROSS MUCH OF WESTERN MAINLAND
ALASKA AND THE ALASKA PENINSULA, SUPPORTING A SLIGHT RISK OF MUCH BELOW NORMAL
TEMPERATURES, OCT 11-14. THIS COULD LEAD TO INCREASING FREEZE-UP OF RIVERS AND
LAKES. AS THIS TROUGH RETROGRADES LATER IN WEEK-2, CHANCES FOR HEAVY
PRECIPITATION ARE LIKELY TO INCREASE ACROSS SOUTHEAST ALASKA. HOWEVER, THE
ECMWF AND GEFS PET DO NOT INDICATE ANY AREAS WHERE PROBABILITIES EXCEED 20
PERCENT FOR HAVING 3-DAY PRECIPITATION TOTALS ABOVE THE 85TH CLIMATOLOGICAL
PERCENTILE. THEREFORE, NO RELATED HEAVY PRECIPITATION HAZARD IS POSTED ACROSS
ALASKA.
FORECASTER: THOMAS COLLOW
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