155
FXUS21 KWNC 131814
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 13 2024
SYNOPSIS: ONE OR TWO LOW PRESSURE SYSTEMS MAY DEVELOP NEAR THE SOUTHEAST OR
MID-ATLANTIC COAST DURING MID TO LATE SEPTEMBER. THERE IS ALSO AN INCREASING
CHANCE OF TROPICAL CYCLONE DEVELOPMENT ACROSS THE WESTERN CARIBBEAN SEA LATER
IN WEEK-2. BY SEPTEMBER 21, A LOW PRESSURE SYSTEM AND TRAILING COLD FRONT ARE
EXPECTED TO PROGRESS EASTWARD FROM THE GREAT PLAINS TO THE MISSISSIPPI VALLEY.
ENHANCED, ONSHORE FLOW IS FORECAST TO PERSIST ACROSS SOUTHEASTERN ALASKA
THROUGH SEPTEMBER 24.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE MID-ATLANTIC AND
SOUTHEAST, SAT-MON, SEP 21-23.
SLIGHT RISK OF HIGH WINDS FOR THE COASTAL MID-ATLANTIC AND SOUTHEAST, SAT-MON,
SEP 21-23.
SLIGHT RISK OF HEAVY PRECIPITATION FOR CENTRAL AND SOUTH FLORIDA, SAT-FRI, SEP
21-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE GREAT PLAINS AND
MISSISSIPPI VALLEY, SAT, SEP 21.
SLIGHT RISK OF HIGH WINDS FOR PARTS OF THE ROCKIES AND NORTHERN TO CENTRAL
GREAT PLAINS, SAT, SEP 21.
SLIGHT RISK OF HEAVY PRECIPITATION FOR SOUTHEASTERN ALASKA, SAT-TUE, SEP 21-24.
DETAILED SUMMARY
FOR MONDAY SEPTEMBER 16 - FRIDAY SEPTEMBER 20:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY SEPTEMBER 21 - FRIDAY SEPTEMBER 27: A MID-LEVEL TROUGH MAY RESULT
IN THE DEVELOPMENT OF MORE THAN ONE LOW PRESSURE SYSTEM NEAR THE SOUTHEAST OR
MID-ATLANTIC COAST DURING MID TO LATE SEPTEMBER. AS OF 2PM EDT ON SEPTEMBER
13, THE NATIONAL HURRICANE CENTER STATES THAT THERE IS A 40 PERCENT CHANCE OF A
NON-TROPICAL LOW TO FORM AND BECOME A TROPICAL CYCLONE ACROSS THE WESTERN
ATLANTIC. THIS INITIAL LOW PRESSURE SYSTEM WOULD LIKELY TRACK INLAND BEFORE THE
START OF WEEK-2. HOWEVER, A NUMBER OF GEFS AND ECMWF ENSEMBLE MEMBERS CONTINUE
TO DEPICT A SECOND LOW FORMING NEAR THE SOUTHEAST COAST EARLY IN WEEK-2. THE
UNCALIBRATED ECMWF MODEL ALONG WITH THE GEFS AND ECMWF PROBABILISTIC EXTREMES
TOOL (PET) SUPPORT A 20 TO 40 PERCENT CHANCE OF HEAVY PRECIPITATION FOR
PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC FROM SEPTEMBER 21 TO 23. REGARDLESS
OF ANY SURFACE LOW DEVELOPMENT, ENHANCED ONSHORE FLOW LEADS TO A SLIGHT RISK OF
HIGH WINDS (SEPTEMBER 21-23) FROM COASTAL NORTHEASTERN FLORIDA TO THE
CHESAPEAKE BAY. DUE TO RECENT GEFS AND ECMWF ENSEMBLE MODEL GUIDANCE FEATURING
AN INCREASING CHANCE OF TROPICAL CYCLONE DEVELOPMENT ACROSS THE SOUTHERN GULF
OF MEXICO OR WESTERN CARIBBEAN SEA LATER IN WEEK-2, THE SLIGHT RISK OF HEAVY
PRECIPITATION FOR SOUTH TO CENTRAL FLORIDA IS VALID FOR THE ENTIRETY OF WEEK-2.
THIS IS ALSO CONSISTENT WITH THE UNCALIBRATED ECMWF MODEL OUTPUT.
THE GEFS AND ECMWF ENSEMBLE MEANS ARE IN GOOD AGREEMENT AND REMAIN CONSISTENT
INDICATING A LONGWAVE TROUGH SHIFTS EASTWARD FROM THE WEST BY THE BEGINNING OF
WEEK-2. THIS LONGWAVE TROUGH IS LIKELY TO RESULT IN A LOW PRESSURE SYSTEM AND
TRAILING COLD FRONT MOVING EASTWARD ACROSS THE GREAT PLAINS AND MISSISSIPPI
VALLEY. A SLIGHT RISK OF HEAVY PRECIPITATION (LOCALLY MORE THAN 1 INCH) IS
MAINTAINED ON SEPTEMBER 21 FOR PARTS OF THOSE AREAS. GIVEN THE ENHANCED
LOW-LEVEL MOISTURE AND INSTABILITY, THE COLD FRONT COULD INITIATE THUNDERSTORMS
WITHIN THE SPECIFIED HEAVY PRECIPITATION HAZARD. THE SLIGHT RISK OF HIGH WINDS
WAS REDUCED FROM THE PREVIOUS DAY AND LIMITED TO THE NORTHERN AND CENTRAL GREAT
PLAINS WHERE THERE IS AN ELEVATED CHANCE OF GUSTY POSTFRONTAL WINDS.
AN ANOMALOUS 500-HPA TROUGH IS LIKELY TO PROMOTE ENHANCED, ONSHORE FLOW ACROSS
SOUTHEASTERN ALASKA. THE GEFS AND ECMWF ENSEMBLE MEANS DEPICT MULTIPLE LOW
PRESSURE SYSTEMS TRACKING FROM THE BERING SEA TO THE NORTHERN GULF OF ALASKA
THROUGH THE EARLY PART OF WEEK-2. A SLIGHT RISK OF HEAVY PRECIPITATION (VALID
SEPTEMBER 21-24) IS POSTED FOR SOUTHEASTERN ALASKA WHERE THE GEFS AND ECMWF
PETS DEPICT GREATER THAN A 20 PERCENT CHANCE OF 3-DAY PRECIPITATION AMOUNTS
EXCEEDING THE 85TH PERCENTILE AND NEAR 3 INCHES. DURING THE FINAL WEEK OF
SEPTEMBER, MODEL GUIDANCE INDICATES THAT THE 500-HPA TROUGH SHIFTS SOUTHWARD
WHICH WOULD LEAD TO DECREASING ONSHORE FLOW AND LOWER CHANCES OF HEAVY
PRECIPITATION. A HIGH WIND HAZARD IS NOT WARRANTED SINCE THE CONSENSUS AMONG
THE GEFS AND ECMWF ENSEMBLE MEMBERS IS FOR THESE LOW PRESSURE SYSTEMS TO BE OF
MODERATE STRENGTH.
FORECASTER: BRAD PUGH
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