990
FXUS21 KWNC 121830
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 12 2023
SYNOPSIS: COLD WEATHER IN PARTS OF THE EASTERN CONTIGUOUS U.S. (CONUS) IS
EXPECTED TO END LATE IN WEEK-1 AS SURFACE HIGH PRESSURE PUSHES OFF THE COAST,
LEAVING MILD WEATHER OVER MOST OF THE CONUS FOR MUCH OF WEEK-2. UNSEASONABLY
WARM WEATHER IS EXPECTED IN CALIFORNIA AND THE SOUTHWEST EARLY IN WEEK-2, BUT
TEMPERATURES ARE NOT EXPECTED TO REACH HAZARDS THRESHOLDS. THE PASSAGE OF A
FRONTAL BOUNDARY MAY BRING MODERATE RAIN TO PARTS OF THE SOUTHERN PLAINS AND
THE LOWER MISSISSIPPI VALLEY EARLY IN WEEK-2, BUT MODELS SOLUTIONS ARE
INCONSISTENT, AND GENERALLY DRIER THAN YESTERDAY. SURFACE LOW PRESSURE IN THE
GULF OF ALASKA SUPPORTS INCREASED CHANCES FOR ENHANCED PRECIPITATION AND HIGH
WINDS ACROSS PARTS OF THE SOUTH-CENTRAL MAINLAND AND THE SOUTHEAST AT THE
BEGINNING OF THE PERIOD.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION IN PORTIONS OF THE SOUTH-CENTRAL MAINLAND
AND THE SOUTHEAST,, FRI-SAT, OCT 20-21.
SLIGHT RISK OF HIGH WINDS ACROSS COASTAL PORTIONS OF THE SOUTH-CENTRAL MAINLAND
AND THE SOUTHEAST, FRI-SAT, OCT 20-21.
DETAILED SUMMARY
FOR SUNDAY OCTOBER 15 - THURSDAY OCTOBER 19:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY OCTOBER 20 - THURSDAY OCTOBER 26: A FAIRLY AMPLIFIED 500-HPA HEIGHT
PATTERN IS FORECAST ACROSS NORTH AMERICA FOR WEEK-2, WITH ANOMALOUS POSITIVE
(NEGATIVE) MID-LEVEL HEIGHTS ACROSS THE WESTERN (EASTERN) CONUS. FOR THE MEAN
WEEK-2 PATTERN, THERE IS GOOD AGREEMENT AMONG MULTIPLE MODELS REGARDING A
MID-LEVEL TROUGH CENTERED OVER THE CENTRAL NORTH PACIFIC, WITH ABOVE-NORMAL
500-HPA HEIGHTS FARTHER NORTH AND EAST ACROSS ALASKA WITH DOWNSTREAM RIDGING
OVER WESTERN NORTH AMERICA. FARTHER EAST, A MID-LEVEL TROUGH IS DRAPED ALONG
THE EASTERN SEABOARD. A STRONG ANOMALOUS MID-LEVEL RIDGE IS EXPECTED TO BUILD
INTO ALASKA LATER WEEK-2, INDUCING SOME DOWNSTREAM PROGRESSION OF MID-LEVEL
FEATURES. BY THE END OF WEEK-2, THE CENTRAL NORTH PACIFIC RIDGE IS EXPECTED TO
BE CLOSER TO THE WESTERN NORTH AMERICAN COAST, LOWERING 500-HPA HEIGHTS IN THE
WESTERN CONUS AND BRINGING MODERATE RAINS INTO THE PACIFIC NORTHWEST.
MEANWHILE, A RELATIVELY WEAK MID-LEVEL RIDGE IS ESTABLISHED IN THE CENTRAL
CONUS, AND SUBNORMAL 500-HPA HEIGHTS PERSIST IN THE EAST AS THE TROUGH AXIS
DRIFTS EASTWARD.
A MID-LEVEL TROUGH OVER THE EASTERN CONUS THROUGH DAY 7 FAVORS UNSEASONABLY
COLD AIR ACROSS PORTIONS OF THE REGION, BUT TEMPERATURES ARE EXPECTED TO
REBOUND AS WEEK-2 STARTS, PROMPTING THE REMOVAL OF THE COLD WEATHER HAZARD IN
THE SOUTHEAST AND ADJACENT AREAS. THE PROBABILISTIC EXTREMES TOOLS (PETS) STILL
SHOW ELEVATED CHANCES FOR SUBNORMAL TEMPERATURES, BUT THE ODDS FOR HAZARDOUS
COLD ON DAY 8 (OCT 20) ARE LOWER THAN INDICATED YESTERDAY. THE ECMWF PET TOOL
IS INDICATING VERY LOW CHANCES FOR TEMPERATURES DROPPING INTO THE COLDEST 5TH
PERCENTILE CLIMATOLOGICALLY ON OCT 20 ACROSS FLORIDA. YESTERDAY, THE SAME TOOL
SHOWED ODDS OF OVER 20 PERCENT THAT TEMPERATURES WOULD FALL INTO THE COLDEST
5TH PERCENTILE.
MEANWHILE, POSITIVE 500-HPA HEIGHT ANOMALIES FROM APPROXIMATELY THE ROCKIES
WESTWARD FAVOR ABOVE-NORMAL TEMPERATURES INTO LATE WEEK-2. THE ECMWF AND GEFS
ENSEMBLE MEANS DEPICT TEMPERATURES AVERAGING MORE THAN 10 DEG. F ABOVE NORMAL
FOR WEEK-2 AS A WHOLE IN SOME PARTS OF THE WESTERN FOUR CORNERS STATES, GREAT
BASIN, DESERT SOUTHWEST, AND CALIFORNIA. THE GEFS PET SHOWS AT LEAST A 20
PERCENT CHANCE OF TEMPERATURES EXCEEDING 90 DEG. F IN THE CALIFORNIA VALLEYS
AND WESTWARD INTO PARTS OF THE GREATER SAN FRANCISCO AND LOS ANGELES AREAS ON
DAY 8 (OCT 20). THE ECMWF AND CANADIAN PETS ARE NOT AS ROBUST, BUT BOTH SHOW
NONTRIVIAL CHANCES FOR TEMPERATURES OVER 90 DEG. F IN THE CALIFORNIA VALLEYS.
GIVEN THE OCTOBER CLIMATOLOGY, NO HAZARD FOR ABOVE-NORMAL TEMPERATURES IS
POSTED AT THIS TIME, BUT THE EXPECTED SYNOPTIC SITUATION IMPLIES THE POTENTIAL
FOR PERIODS OF DRY, GUSTY WINDS FROM THE CENTRAL CALIFORNIA VALLEYS SOUTHWARD.
THE SET-UP FOR SUCH CONDITIONS IS NOT IDEAL, BUT GIVEN THE TIME OF YEAR AND THE
TYPICAL LACK OF SURFACE MOISTURE IN OCTOBER, ANY UNUSUALLY DRY AND/OR HOT
WEATHER MAY INCREASE THE THREAT OF LARGE AND RAPIDLY-EXPANDING WILDFIRES.
AS THE CENTRAL NORTH PACIFIC RIDGE DRIFTS EASTWARD, 500-HPA HEIGHTS ARE LOWERED
IN AND NEAR THE PACIFIC NORTHWEST. THIS INCREASES PRECIPITATION PROBABILITIES
IN THE PACIFIC NORTHWEST LATER WEEK-2, BUT ANY HAZARDOUS PRECIPITATION IS
EXPECTED TO FALL FARTHER NORTH IN SOUTHWESTERN CANADA AND SOUTHEAST ALASKA.
GUIDANCE SHOWS ODDS FOR HEAVY PRECIPITATION PROGRESSIVELY TRENDING DOWNWARD FOR
THE PAST COUPLE OF DAYS, SO NO RELATED HAZARD IS POSTED HERE.
THERE IS SOME POTENTIAL FOR HEAVY PRECIPITATION ACROSS PORTIONS OF THE
SOUTH-CENTRAL/SOUTHEASTERN CONUS AT THE BEGINNING OF THE PERIOD ASSOCIATED WITH
A FRONTAL PASSAGE, BUT MOST GUIDANCE IS DRIER THAN YESTERDAY. THE CANADIAN
ENSEMBLE PET STILL SHOWS AT LEAST A 20% CHANCE OF 3-DAY RAINFALL TOTALS
EXCEEDING THE 85TH PERCENTILE AND ONE INCH ACROSS NORTHEASTERN TEXAS AND MOST
OF LOUISIANA, BUT THE GEFS AND ECMWF COUNTERPARTS DO NOT REACH THESE
THRESHOLDS. DUE TO THIS HIGH UNCERTAINTY AND A GENERAL TREND TOWARD LESSER
PRECIPITATION, NO RELATED HAZARD IS NOT POSTED.
SUPER TYPHOON BOLAVEN, WITH SUSTAINED WINDS REACHING 180 MPH, IS ABOUT 2000
MILES EAST-NORTHEAST OF THE PHILIPPINES, HEADED NORTHWARD OVER OPEN WATERS IN
THE NORTH PACIFIC. THE REMNANTS OF THIS SYSTEM MAY INTERACT WITH A MID-LEVEL
TROUGH AND SURFACE LOW PRESSURE SYSTEM TO BRING ENHANCED PRECIPITATION AND HIGH
WINDS TO PORTIONS OF SOUTH-CENTRAL AND SOUTHEASTERN ALASKA NEAR THE START OF
WEEK-2. THE ECMWF AND CANADIAN PETS SHOW SOME NEAR-COASTAL AREAS ACROSS THE
KENAI PENINSULA AND SOUTHEASTERN ALASKA REACHING AT LEAST A 20% CHANCE OF 3-DAY
MEAN WINDS EXCEEDING THE 85TH PERCENTILE. ALL THREE PETS SHOW AT LEAST 20
PERCENT CHANCES FOR 3-DAY PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE
CLIMATOLOGICALLY FOR MUCH OF THE AREA, BUT ONLY A FEW LOCALIZED AREAS HAVE
SIGNIFICANT ODDS FOR TOTALS TO EXCEED 3 INCHES. THIS WOULD TYPICALLY PRECLUDE A
BROAD PRECIPITATION HAZARD FROM BEING DESIGNATED IN THIS CLIMATOLOGICALLY-WET
REGION, BUT HISTORICALLY, MODELS AND DERIVED TOOLS HAVE DIFFICULTY ADDRESSING
INTERACTIONS BETWEEN TROPICAL AND MIDLATITUDE STORM SYSTEMS. THIS INCREASES
FORECAST UNCERTAINTY, BUT THE POTENTIAL EXISTS FOR HEAVIER PRECIPITATION THAN
WHAT IS INDICATED BY ENSEMBLE MEANS AND THE PETS. THEREFORE, A SLIGHT RISK
HAZARD FOR HEAVY PRECIPITATION IS POSTED FOR SOUTH-CENTRAL AND SOUTHEASTERN
ALASKA EARLY IN WEEK-2, ALONG WITH A SLIGHT CHANCE OF HIGH WINDS NEAR THE
COAST, BOTH VALID FOR OCT 20-21.
FORECASTER: RICH TINKER
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