307
FXUS21 KWNC 151758
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 15 2023
SYNOPSIS: FORECAST MID-LEVEL HIGH PRESSURE OVER EASTERN CANADA AND THE SOUTHERN
TIER OF THE CONTIGUOUS U.S. (CONUS) IS EXPECTED TO BRING UNSEASONABLY WARM, BUT
NON-HAZARDOUS EARLY AUTUMN TEMPERATURES FOR MANY AREAS EAST OF THE ROCKIES.
MID-LEVEL LOW PRESSURE FAVORED UPSTREAM OVER THE WESTERN CONUS INCREASES THE
RISK OF HEAVY PRECIPITATION AND HIGH WINDS OVER THE CENTRAL CONUS EARLY IN
WEEK-2. CYCLONIC FLOW FAVORED UNDERNEATH A STRONG SURFACE HIGH IN THE
NORTHEAST ALSO SUPPORTS AN INCREASED RISK OF HEAVY PRECIPITATION OVER PARTS OF
THE SOUTHEAST AND MID-ATLANTIC. OVER ALASKA, WETTER THAN NORMAL CONDITIONS ARE
FAVORED FOR THE MAINLAND, BUT CONDITIONS ARE NOT EXPECTED TO REACH HAZARD
THRESHOLDS.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE GREAT PLAINS, MISSISSIPPI,
TENNESSEE AND OHIO VALLEYS, SAT-SUN, SEP 23-24.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHEAST AND
MID-ATLANTIC, SAT-MON, SEP 23-25.
SLIGHT RISK OF HIGH WINDS FOR PARTS OF THE GREAT PLAINS AND MISSISSIPPI VALLEY,
SAT-MON, SEP 23-25.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE LOWER MISSISSIPPI VALLEY AND
SOUTHEAST.
DETAILED SUMMARY
FOR MONDAY SEPTEMBER 18 - FRIDAY SEPTEMBER 22:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY SEPTEMBER 23 - FRIDAY SEPTEMBER 29: DYNAMICAL MODELS CONTINUE TO
ADVERTISE A SUBSTANTIAL BLOCKING RIDGE OVER THE HUDSON BAY OF CANADA, WITH
UPSTREAM MID-LEVEL TROUGHING OVER WESTERN NORTH AMERICA IN THEIR MEAN WEEK-2
PATTERNS. OF NOTE, BOTH THE LATEST GEFS AND ECMWF ENSEMBLES FAVOR STRONGER
SUBTROPICAL RIDGING ACROSS THE SOUTHERN TIER, WHERE COMBINED WITH A BROAD
COVERAGE OF POSITIVE HEIGHT DEPARTURES OVER THE NORTHEASTERN CONUS, IS LIKELY
TO LEAD TO WIDESPREAD, ABOVE NORMAL TEMPERATURES FOR MUCH OF THE COUNTRY MAINLY
EAST OF THE ROCKIES DURING WEEK-2. WHILE STRONG SUBTROPICAL RIDGING HAS BEEN
RESPONSIBLE FOR PROVIDING MUCH OF THE EXCESSIVE HEAT CONDITIONS ACROSS THE
SOUTHERN TIER THIS PAST SUMMER, ACTUAL MAXIMUM TEMPERATURES ARE NOT EXPECTED TO
REACH HAZARDS CRITERIA GIVEN THE TIME OF YEAR, AND NO CORRESPONDING HEAT
HAZARDS ARE ISSUED. FURTHER NORTH ACROSS THE EASTERN CONUS, THE UNSEASONABLY
WARM TEMPERATURES SHOULD FEEL QUITE PLEASANT FOR EARLY AUTUMN.
WITH MEAN MID-LEVEL TROUGHING FAVORED ACROSS THE WESTERN CONUS, THERE IS GOOD
MODEL AGREEMENT FEATURING SHORTWAVE ENERGY EJECTING FROM THE ROCKIES AND
SURFACE LOW FORMATION ACROSS THE GREAT PLAINS. BASED ON THE WEATHER PREDICTION
CENTER'S (WPC) DAY 6 AND DAY 7 QPF, INCREASED AMOUNTS OF PRECIPITATION ARE
FAVORED LATE IN WEEK-1 ACROSS THE COUNTRY'S MID-SECTION, AND APPEAR LIKELY TO
LINGER INTO EARLY WEEK-2. THE SYNOPTIC PATTERN FAVORED APPEARS CONDUCIVE FOR
AN INCREASED RISK OF HEAVY PRECIPITATION WITH THUNDERSTORM ACTIVITY REMAINING
POSSIBLE GIVEN WARM, MOIST GULF OF MEXICO RETURN FLOW FAVORED OVER PORTIONS OF
THE GREAT PLAINS AND MISSISSIPPI VALLEY. A SLIGHT RISK OF HEAVY PRECIPITATION
REMAINS ISSUED (SEP 23-24), AND IS EXPANDED EASTWARD IN THE PARTS OF THE OHIO
AND TENNESSEE VALLEYS WHERE THERE IS INCREASED SUPPORT IN THE PROBABILISTIC
EXTREME TOOLS (PETS) FOR 3-DAY AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1
INCH. FOR AREAS THAT DO NOT RECEIVE LOCALLY HEAVY PRECIPITATION, THE WET
CONDITIONS FAVORED ARE HOWEVER EXPECTED TO PROVIDE MOISTURE RELIEF FOR AREAS IN
THE MIDWEST AFFECTED BY DROUGHT. IN ADDITION, STRENGTHENING PRESSURE GRADIENTS
REMAIN POSSIBLE ASSOCIATED WITH THE DEEPENING SURFACE LOW, AND A SLIGHT RISK OF
HIGH WINDS REMAINS ISSUED FOR SEP 23-25. THE SLIGHT RISK AREA NOW INCLUDES THE
NORTHERN PLAINS GIVEN THE POSITION OF THE MEAN SURFACE LOW FAVORED AND
INCREASED WIND SIGNALS IN THE ECMWF AND CANADIAN PETS.
LATER IN WEEK-2, ENSEMBLES FAVOR THE POTENTIAL FOR A RELOADING TROUGH WITH
NEGATIVE HEIGHT DEPARTURES OVERSPREADING THE NORTHEASTERN PACIFIC AND
NORTHWESTERN CONUS. WHILE THE GEFS FAVORS A MORE WESTERLY SOLUTION OF THIS
MID-LEVEL FEATURE, THERE IS BETTER SUPPORT IN BOTH THE ECMWF AND CANADIAN
DEPICTING THE TROUGH AXIS BECOMING ESTABLISHED NEAR OR OVER THE WEST COAST TO
PROMOTE ENHANCED ONSHORE FLOW AND INCREASED PRECIPITATION AMOUNTS. BASED ON
RAW AND PET GUIDANCE, NO CORRESPONDING PRECIPITATION HAZARDS ARE ISSUED BUT
THIS POTENTIAL WILL CONTINUE TO BE MONITORED IN UPCOMING OUTLOOKS.
UNDERNEATH THE STOUT MID-LEVEL RIDGING FAVORED OVER EASTERN CANADA AND THE
NORTHEASTERN CONUS, THERE ARE STRONGER INDICATIONS IN THE ENSEMBLES FEATURING
CYCLONIC FLOW WITH INCREASED AMOUNTS OF PRECIPITATION OVER THE LOWER HALF OF
THE EASTERN SEABOARD. UNCALIBRATED PRECIPITATION TOOLS SHOW THE HIGHEST DAILY
AMOUNTS REMAINING OFFSHORE, HOWEVER, THERE IS BETTER SUPPORT IN THE PETS
INDICATING INCREASED CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE 85TH PERCENTILE
AND 1 INCH. THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION IS ISSUED ACROSS
PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC FOR SEP 23-25.
WITH ABOVE-NORMAL TEMPERATURES PREDOMINATELY FAVORED THROUGHOUT THE
SOUTH-CENTRAL AND SOUTHEASTERN CONUS, A RAPID ONSET DROUGHT (ROD) HAZARD
REMAINS POSTED OVER PARTS OF THE LOWER MISSISSIPPI VALLEY AND THE GULF COAST
REGION. MANY AREAS WITHIN THE HIGHLIGHTED REGION HAVE RECEIVED LESS THAN HALF
THEIR NORMAL RAINFALL ACCUMULATION DURING THE PAST 30 DAYS, AND REMAIN FAVORED
TO RECEIVE LOW PRECIPITATION AMOUNTS FAVORED DURING THE NEXT TWO WEEKS.
OVER THE HIGHER LATITUDES, AMPLIFIED 500-HPA RIDGING OVER THE ALEUTIANS AND
NEAR TO BELOW NORMAL HEIGHTS ARE FAVORED OVER THE BERING SEA EARLY IN THE
PERIOD IS LIKELY TO BRING MOIST SOUTHWESTERLY FLOW AND ABOVE-NORMAL
PRECIPITATION INTO MAINLAND. PETS SHOW INCREASED CHANCES FOR 3-DAY AMOUNTS
EXCEEDING THE 85TH PERCENTILE, BUT AMOUNTS ARE EXPECTED TO BE BELOW HAZARD
THRESHOLDS.
FORECASTER: NICK NOVELLA
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