299
FXUS21 KWNC 221830
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 22 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FORECAST TO BE CENTERED OVER THE INTERIOR
WEST EARLY IN WEEK-2. THIS FEATURE MAY BRING SUBFREEZING TEMPERATURES TO
PORTIONS OF THE ROCKIES AND GREAT BASIN. SURFACE LOW PRESSURE MAY DEVELOP AND
BRING ELEVATED CHANCES FOR HEAVY PRECIPITATION ACROSS PARTS OF THE GREAT PLAINS
AND MISSISSIPPI VALLEY BY THE MIDDLE OF THE PERIOD, ALTHOUGH CONFIDENCE IS LOW
GIVEN THE DISPARITY IN THE MODELS. FURTHER EAST, MID-LEVEL HIGH PRESSURE FAVORS
ABOVE-NORMAL TEMPERATURES AND BELOW-NORMAL PRECIPITATION. THESE CONTINUED DRY
CONDITIONS AND UNSEASONABLY WARM TEMPERATURES RESULT IN AN ENHANCED RISK FOR
RAPID ONSET DROUGHT (ROD) ACROSS PORTIONS OF THE SOUTHEAST AND TENNESSEE VALLEY.
HAZARDS
SLIGHT RISK FOR MUCH BELOW NORMAL TEMPERATURES ACROSS PORTIONS OF THE GREAT
BASIN AND NORTHERN AND CENTRAL ROCKIES, SAT-TUE, SEP 30-OCT 3.
SLIGHT RISK OF HIGH WINDS ACROSS PARTS OF THE NORTHERN AND CENTRAL PLAINS,
SAT-MON, SEP 30-OCT 2.
RAPID ONSET DROUGHT RISK ACROSS MUCH OF ALABAMA AND ADJACENT AREAS OF
MISSISSIPPI, GEORGIA, AND TENNESSEE.
DETAILED SUMMARY
FOR MONDAY SEPTEMBER 25 - FRIDAY SEPTEMBER 29:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY SEPTEMBER 30 - FRIDAY OCTOBER 06: THE ECMWF AND GEFS ENSEMBLE
MEANS ARE IN GOOD AGREEMENT INDICATING MID-LEVEL TROUGHING (RIDGING) OVER THE
WESTERN (EASTERN) CONTIGUOUS U.S. DURING WEEK-2. AMPLIFIED MID-LEVEL TROUGHING
CENTERED OVER THE INTERIOR WEST FROM THE BEGINNING TO MIDDLE OF THE PERIOD
SUPPORTS INCREASED CHANCES FOR EARLY SEASON ANOMALOUS COLD TEMPERATURES ACROSS
THE REGION. A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES IS POSTED ACROSS
PORTIONS OF THE GREAT BASIN AND NORTHERN AND CENTRAL ROCKIES, SEP 30-OCT 30,
WHERE BOTH THE GEFS AND ECMWF PROBABILISTIC EXTREMES TOOLS (PETS) INDICATE AT
LEAST AT LEAST A 20 PERCENT CHANCE OF MINIMUM TEMPERATURES FALLING BELOW THE
15TH PERCENTILE AND NEAR TO BELOW FREEZING. VEGETATION IN THIS AREA IS LIKELY
TO BE VULNERABLE TO ANY FROST OR FREEZES THAT MAY OCCUR DURING THE PERIOD. THIS
POTENTIAL FOR HAZARDOUS COLD DECREASES TOWARDS THE END OF WEEK-2 AS THE
MID-LEVEL TROUGH IS ANTICIPATED TO WEAKEN AND TRANSITION TO MORE ZONAL FLOW.
SNOW MAY ALSO BE POSSIBLE ACROSS THE HIGHER ELEVATIONS OF THESE AREAS. THE GEFS
SNOW WATER EQUIVALENT (SWE) PET INDICATES ELEVATED CHANCES OF 3-DAY ACCUMULATED
SWE EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE ACROSS PARTS OF THE NORTHERN
GREAT BASIN AND NORTHERN AND CENTRAL ROCKIES. HOWEVER, CLIMATOLOGY IS STILL
QUITE LOW AND NO CORRESPONDING RISK OF HEAVY SNOW IS POSTED AT THIS TIME.
TROUGHING OVER THE WEST IS ANTICIPATED TO SUPPORT A SERIES OF SURFACES
DEVELOPING IN THE LEE OF THE ROCKIES AND BRINGING ENHANCED PRECIPITATION AND
POTENTIAL FOR HIGH WINDS INTO PORTIONS OF THE GREAT PLAINS AND MISSISSIPPI
VALLEY TOWARD THE MIDDLE OF WEEK-2. A SLIGHT RISK OF HIGH WINDS IS DESIGNATED
ACROSS PARTS OF THE NORTHERN AND CENTRAL PLAINS WHERE THE PETS SHOW AT LEAST A
20% CHANCE OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND 20 MPH WITH THE
ECMWF AND CANADIAN PETS SHOW AT LEAST A 20% CHANCE OF EXCEEDING 25 MPH IN SOME
AREAS.
THE ECMWF PET IS SUPPORTIVE OF AN INCREASED HEAVY PRECIPITATION RISK ,
DEPICTING AT LEAST A 40 PERCENT CHANCE OF 3-DAY PRECIPITATION EXCEEDING THE
85TH CLIMATOLOGICAL PERCENTILE AND ONE INCH DURING THE OCT 2-4 TIMEFRAME OVER
PARTS OF THESE REGIONS. HOWEVER, THE GEFS PET DOES NOT REACH THESE THRESHOLDS.
ADDITIONALLY, THE UNCALIBRATED ENSEMBLE MEAN TOTALS FROM THE 0Z GEFS, ECMWF,
AND CANADIAN REMAIN FAIRLY LOW (DAILY TOTALS GENERALLY LESS THAN 0.25 INCHES).
DUE TO THE DISCREPANCY IN THE MODELS, NO RELATED HEAVY PRECIPITATION HAZARD IS
POSTED, AND THIS POTENTIAL WILL BE REEVALUATED IN UPCOMING OUTLOOKS . IT IS
NOTEWORTHY THAT SEVERE TO EXCEPTIONAL DROUGHT CONTINUES ACROSS MANY AREAS OF
THE GREAT PLAINS AND MISSISSIPPI VALLEY, SO ANY PRECIPITATION OVER THESE AREAS
MAY END UP BEING MORE OF A WELCOME BENEFIT THAN A HAZARD.
AN AREA OF LOW PRESSURE (NOW DESIGNATED AS POTENTIAL TROPICAL CYCLONE SIXTEEN)
IS FORECAST TO BRING HEAVY PRECIPITATION ACROSS PARTS OF THE EAST COAST OVER
THE NEXT FEW DAYS, WITH THE POSSIBILITY OF ANOTHER DISTURBANCE DEVELOPING OVER
PORTIONS OF THE SOUTHEAST AND MID-ATLANTIC LATER IN WEEK-1. HOWEVER, ALL
INDICATIONS ARE PRECIPITATION CHANCES SHOULD BE DIMINISHING BY THE START OF
WEEK-2 OVER THE EASTERN CONUS AS RIDGING BECOMES ESTABLISHED OVER EASTERN NORTH
AMERICA, WITH MEAN SURFACE HIGH PRESSURE FORECAST OVER THE NORTHEAST DURING
WEEK-2. THUS, THERE IS NO DESIGNATED ASSOCIATED HEAVY PRECIPITATION. RAPID
ONSET DROUGHT (ROD) RISK REMAINS ELEVATED ACROSS MUCH OF ALABAMA AND ADJACENT
AREAS OF MISSISSIPPI, GEORGIA, AND TENNESSEE. THESE AREAS ARE NOT FORECAST TO
BE IMPACTED BY THE AFOREMENTIONED DISTURBANCE OVER THE MID-ATLANTIC IN WEEK-1,
AND ARE ALSO PREDICTED TO BE DOWNSTREAM OF THE POTENTIAL SYSTEM THAT MAY IMPACT
THE GREAT PLAINS AND MISSISSIPPI VALLEY TOWARD THE MIDDLE OF WEEK-2. THEREFORE,
NOT MUCH PRECIPITATION IS FORECAST ACROSS THE HIGHLIGHTED ROD AREA DURING THE
NEXT 2 WEEKS, ALONG WITH ABOVE-NORMAL TEMPERATURES, WHICH WOULD FAVOR DROUGHT
EXPANSION.
A TRANSITION TO MORE TROUGHING IS PREDICTED ACROSS ALASKA WHICH FAVORS
INCREASING CHANCES FOR HEAVY PRECIPITATION ACROSS SOUTHERN AND SOUTHEAST
ALASKA. SEVERAL INCHES OF PRECIPITATION ARE POSSIBLE ACROSS THIS REGION
ACCUMULATED OVER MULTIPLE DAYS. THE ECMWF PET SHOWS AT LEAST A 20% CHANCE OF
3-DAY PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE AND 2 INCHES,
PRIMARILY ACROSS SOUTHEASTERN ALASKA, AND THE CANADIAN PET HAVING MORE LIMITED
SCOPE TO SOUTHERN PORTIONS OF THIS AREA AT THE BEGINNING OF WEEK-2. THE GEFS
PET DOES NOT REACH THESE THRESHOLDS. DUE TO SIGNIFICANT MODEL DIFFERENCES, AN
ASSOCIATED PRECIPITATION HAZARD IS NOT POSTED AT THIS TIME.
FORECASTER: MELISSA OU
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