586
FXUS21 KWNC 201815
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 20 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE OFF THE WEST COAST IS FAVORED TO BRING PERIODS
OF POTENTIALLY HIGH WIND AND HEAVY RAIN TO COASTAL PORTIONS OF THE NORTHWEST
CONTIGUOUS U.S. (CONUS). A SURFACE LOW PRESSURE SYSTEM IS FORECAST TO FORM OFF
THE COAST OF THE SOUTHEAST U.S. LATE IN WEEK-1, BRINGING WITH IT THE POTENTIAL
FOR HEAVY PRECIPITATION FOR PORTIONS OF THE EAST COAST PERSISTING INTO EARLY
WEEK-2. CONTINUED DRY CONDITIONS AND UNSEASONABLY WARM TEMPERATURES RESULT IN A
RISK FOR RAPID ONSET DROUGHT (ROD) ACROSS PORTIONS OF THE LOWER MISSISSIPPI
VALLEY.
HAZARDS
SLIGHT RISK FOR HEAVY PRECIPITATION FOR WESTERN PORTIONS OF WASHINGTON, OREGON,
AND NORTHERN CALIFORNIA, THU-SAT, SEP 28-30.
SLIGHT RISK FOR HIGH WINDS FOR COASTAL PORTIONS OF WASHINGTON, OREGON, AND
CALIFORNIA NORTH OF THE BAY AREA, THU-SAT, SEP 28-30.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHEAST AND
MID-ATLANTIC, THU-SAT, SEP 28-30.
RAPID ONSET DROUGHT RISK ACROSS PORTIONS OF MISSISSIPPI, ALABAMA, AND SOUTHEAST
ARKANSAS.
DETAILED SUMMARY
FOR SATURDAY SEPTEMBER 23 - WEDNESDAY SEPTEMBER 27:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY SEPTEMBER 28 - WEDNESDAY OCTOBER 04: AT THE ONSET OF THE WEEK-2
PERIOD THE CONSENSUS AMONG ENSEMBLE FORECAST MODELS FOR 500-HPA HEIGHT
ANOMALIES DEPICT A DEEP ANOMALOUS TROUGH SITUATED OFF THE COAST OF THE PACIFIC
NORTHWEST. THIS TROUGH IS LIKELY TO PROGRESS INTO THE WESTERN CONUS AND WEAKEN
BY THE MIDDLE TO END OF THE FORECAST PERIOD. HOWEVER, FORECAST GUIDANCE
INDICATES THE POTENTIAL FOR A SERIES OF SHORTWAVE TROUGHS TO COME ASHORE NEAR
THE U.S.-CANADA BORDER EARLY IN THE PERIOD, RESULTING IN INCREASED CHANCES FOR
HEAVY RAIN AND HIGH WINDS. THE ECMWF, GEFS, AND CMCE PROBABILISTIC EXTREMES
TOOLS (PETS) ALL INDICATE PROBABILITIES OF AT LEAST 20% FOR THREE-DAY
PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE AND 1 INCH FOR WESTERN
PORTIONS OF WASHINGTON, OREGON, AND CALIFORNIA NORTH OF THE BAY AREA, THUS A
SLIGHT RISK FOR HEAVY PRECIPITATION IS POSTED FOR THESE AREAS FOR SEP 28-30.
ADDITIONALLY, THE PETS ALSO INDICATE AT LEAST A 20% CHANCE OF WIND SPEEDS
EXCEEDING THE 85TH PERCENTILE AND 25 MPH FOR COASTAL PORTIONS OF WASHINGTON,
OREGON, AND CALIFORNIA AS FAR SOUTH AS THE BAY AREA FOR THE SAME TIME PERIOD,
SEP 28-30, SUPPORTING A SLIGHT RISK FOR HIGH WINDS.
FURTHER INLAND ACROSS THE WESTERN CONUS, SNOW MAY BE POSSIBLE ACROSS THE HIGHER
ELEVATIONS OF THE CENTRAL AND NORTHERN ROCKIES. THE GEFS SNOW WATER EQUIVALENT
(SWE) PET IS INDICATING ELEVATED CHANCES OF EXCEEDING THE 85TH CLIMATOLOGICAL
PERCENTILE IN PARTS OF THESE REGIONS. HOWEVER, CLIMATOLOGY IS STILL QUITE LOW
AND NO CORRESPONDING RISK OF HEAVY SNOW IS POSTED AT THIS TIME.
IN WEEK-1 AN AREA OF SURFACE LOW PRESSURE IS FAVORED TO FORM ALONG THE COAST OF
THE CAROLINAS AND PROGRESS NORTHWARD. THE NATIONAL HURRICANE CENTER IS
FORECASTING A 30% CHANCE OF A SUBTROPICAL CYCLONE DEVELOPING IN THIS AREA, BUT
REGARDLESS OF DEVELOPMENT, THE AREA OF LOW-PRESSURE IS LIKELY TO ENHANCE
CHANCES OF HEAVY PRECIPITATION ALONG THE EASTERN SEABOARD. A SECOND AREA OF
LOW-PRESSURE MAY DEVELOP ALONG THE SAME STATIONARY BOUNDARY LATER IN WEEK-1 OR
EARLY IN WEEK-2. THE ECMWF PET INDICATES AT LEAST A 20% CHANCE OF 3 DAY
PRECIPITATION EXCEEDING THE 85TH PERCENTILE AND 1 INCH OF PRECIPITATION FOR
PORTIONS OF THE CAROLINAS AND COASTAL GEORGIA AND SOUTHEAST VIRGINIA. WHILE THE
GEFS PET HEAVY PRECIPITATION TOTALS ARE NOT AS ANOMALOUS IN PERCENTILE SPACE,
IT CONTINUES TO INDICATE A 20% CHANCE OF 3 DAY PRECIPITATION TOTALS EXCEEDING
0.75 INCHES. THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR SEP
28-30 FOR PORTIONS OF VIRGINIA, THE CAROLINAS AND GEORGIA.
AS THE TROUGH ACROSS THE WEST COAST PUSHES EAST, A SHORTWAVE TROUGH AND SURFACE
LOW-PRESSURE MAY EJECT FROM THE ROCKY MOUNTAINS INTO THE NORTHERN PLAINS. THE
ECMWF PET TOOL INDICATES MODERATE CHANCES (>40%) FOR 3 DAY PRECIPITATION TOTALS
TO EXCEED THE 85TH PERCENTILE. THE GEFS AND CMCE PETS HAVE LOWER CHANCES (>20%)
IN THIS AREA. HOWEVER, RAW FORECAST GUIDANCE SHOWS SIGNIFICANTLY LOWER
PRECIPITATION TOTALS ACROSS THIS REGION, AND EVEN THE PETS DO NOT INDICATE
PRECIPITATION TOTALS EXCEEDING AN INCH OVER 3 DAYS. THEREFORE, NO RELATED HEAVY
PRECIPITATION HAZARD IS CURRENTLY POSTED FOR THIS REGION, BUT THE EVOLUTION OF
THE PATTERN BEARS WATCHING.
ANOMALOUS POSITIVE 500-HPA ANOMALIES OVER NORTHERN CANADA AND AN ASSOCIATED
RIDGE AXIS EXTENDING SOUTHWARD WELL INTO THE CONUS FAVOR ABOVE-NORMAL
TEMPERATURES ESPECIALLY FOR THE NORTHERN PLAINS AND THE GULF COAST.
CLIMATOLOGICAL VALUES DURING THIS TIME OF YEAR ARE MUCH LOWER THAN DURING THE
HEART OF SUMMER, SO WHILE TEMPERATURE ANOMALIES QUALIFY AS EXTREME IN
PERCENTILE SPACE, THEY ARE NOT LIKELY TO REACH HAZARDOUS CRITERIA IN ABSOLUTE
TERMS SO NO TEMPERATURE-RELATED HAZARDS ARE POSTED AT THIS TIME.
DROUGHT CONCERNS ARE FORECAST TO CONTINUE FOR PORTIONS OF THE LOWER MISSISSIPPI
VALLEY. ABOVE-NORMAL TEMPERATURES, ANTECEDENT DRYNESS FOR SOUTHERN MISSISSIPPI
AND ALABAMA IN PARTICULAR AND LITTLE PRECIPITATION IN THE FORECAST RESULT IN
THE CONTINUED RISK FOR ROD DEVELOPMENT FOR THIS REGION, SO NO CHANGES ARE MADE
TO THE ROD OUTLOOK TODAY.
FORECASTER: RYAN BOLT
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