855
FXUS21 KWNC 121822
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 12 2024
SYNOPSIS: ONE OR TWO LOW PRESSURE SYSTEMS MAY DEVELOP NEAR THE SOUTHEAST OR
MID-ATLANTIC COAST DURING THE NEXT WEEK TO TEN DAYS. MID-LEVEL LOW PRESSURE
OVER THE WEST IS FORECAST TO RESULT IN SURFACE LOW DEVELOPMENT ACROSS THE
CENTRAL HIGH PLAINS BY THE BEGINNING OF WEEK-2. THIS LOW PRESSURE SYSTEM IS
THEN EXPECTED TO TRACK NORTHEASTWARD TO THE UPPER MISSISSIPPI VALLEY. ON
SEPTEMBER 20 AND 21, THERE IS AN INCREASED CHANCE OF A FROST OR LIGHT FREEZE
FOR PORTIONS OF THE GREAT BASIN. MID-LEVEL LOW PRESSURE IS EXPECTED TO PERSIST
OVER ALASKA WITH ENHANCED, ONSHORE FLOW AFFECTING SOUTHEASTERN ALASKA.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE MID-ATLANTIC AND
SOUTHEAST, FRI-TUE, SEP 20-24.
SLIGHT RISK OF HIGH WINDS FOR THE COASTAL MID-ATLANTIC AND SOUTHEAST, FRI-SUN,
SEP 20-22.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE GREAT PLAINS AND
MISSISSIPPI VALLEY, FRI-SAT, SEP 20-21.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE ROCKIES AND GREAT PLAINS, FRI-SAT,
SEP 20-21.
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE GREAT BASIN AND
SOUTHWEST, FRI-SAT, SEP 20-21.
SLIGHT RISK OF HEAVY PRECIPITATION FOR SOUTHEASTERN ALASKA, FRI-TUE, SEP 20-24.
DETAILED SUMMARY
FOR SUNDAY SEPTEMBER 15 - THURSDAY SEPTEMBER 19:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY SEPTEMBER 20 - THURSDAY SEPTEMBER 26: A MID-LEVEL TROUGH MAY RESULT
IN THE DEVELOPMENT OF MORE THAN ONE LOW PRESSURE SYSTEM NEAR THE SOUTHEAST
COAST DURING MID TO LATE SEPTEMBER. AS OF 2PM EDT ON SEPTEMBER 12, THE
NATIONAL HURRICANE CENTER STATES THAT THERE IS A 30 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 WELL
BEFORE THE START OF WEEK-2. HOWEVER, FOR THE SECOND CONSECUTIVE DAY, AN
INCREASING NUMBER OF GFS AND ECMWF ENSEMBLE MEMBERS FAVOR 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. SINCE THIS LOW PRESSURE SYSTEM WOULD MOVE VERY SLOWLY, THE SLIGHT
RISK OF HEAVY PRECIPITATION IS VALID FROM SEPTEMBER 20 TO 24. REGARDLESS OF ANY
SURFACE LOW DEVELOPMENT, ENHANCED ONSHORE FLOW LEADS TO A SLIGHT RISK OF HIGH
WINDS (SEPTEMBER 22-24) FROM COASTAL NORTHEASTERN FLORIDA TO THE CHESAPEAKE
BAY. BEYOND SEPTEMBER 24, DAILY ENSEMBLE FORECASTS INDICATE THAT THE ONSHORE
FLOW SHOULD EASE.
THE GEFS AND ECMWF ENSEMBLE MEANS ARE IN GOOD AGREEMENT AND REMAIN CONSISTENT
INDICATING AN AMPLIFIED 500-HPA TROUGH PROGRESSING EASTWARD FROM THE WEST. THIS
LONGWAVE TROUGH ALONG WITH LEESIDE SURFACE CYCLOGENESIS ACROSS THE CENTRAL HIGH
PLAINS SUPPORTS A BROAD AREA HIGHLIGHTED WITH A SLIGHT RISK OF HIGH WINDS FOR
MUCH OF THE ROCKIES AND GREAT PLAINS THROUGH SEPTEMBER 21. BASED ON THE ECMWF
PET AND ITS UNCALIBRATED MODEL OUTPUT, A SLIGHT RISK OF HEAVY PRECIPITATION
(LOCALLY MORE THAN 1 INCH) IS POSTED FOR PARTS OF THE GREAT PLAINS AND
MISSISSIPPI VALLEY FOR THE SAME PERIOD. GIVEN THE ENHANCED LOW-LEVEL MOISTURE
AND INSTABILITY, THE COLD FRONT COULD INITIATE THUNDERSTORMS WITHIN THE
SPECIFIED HEAVY PRECIPITATION HAZARD.
THE AMPLIFIED 500-HPA TROUGH IS ALSO FORECAST TO RESULT IN COOLER-THAN-NORMAL
TEMPERATURES ACROSS THE INTERIOR WEST EARLY IN WEEK-2. AS SURFACE HIGH PRESSURE
BECOMES CENTERED ACROSS THE GREAT BASIN AND SOUTHWEST, THERE IS AN ELEVATED
CHANCE OF A FROST OR FREEZE. A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES IS
POSTED FOR PARTS OF THE GREAT BASIN AND SOUTHWEST WHERE THE GEFS AND ECMWF PETS
SHOW AT LEAST A 20 PERCENT CHANCE OF MINIMUM TEMPERATURES FALLING BELOW THE
15TH PERCENTILE AND 32F. THE PASSAGE OF THE MID-LEVEL TROUGH COULD LEAD TO
ACCUMULATING SNOW IN THE HIGHEST ELEVATIONS OF THE NORTHERN TO CENTRAL ROCKIES,
BUT IS NOT EXPECTED TO REACH HAZARDOUS CRITERIA.
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 MUCH OF WEEK-2. A SLIGHT RISK OF HEAVY PRECIPITATION (VALID SEPTEMBER
20-24) 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. A HIGH WIND HAZARD IS NOT POSTED AT THIS
TIME 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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