623
FXUS21 KWNC 121816
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 12 2023
SYNOPSIS: A MID-LEVEL RIDGE SHOULD KEEP TEMPERATURES ABOVE NORMAL ACROSS THE
PLAINS AND MUCH OF THE NORTHERN TIER OF THE COUNTRY, BUT EXCESSIVE HEAT
CONDITIONS ARE NOT ANTICIPATED. BELOW-NORMAL PRECIPITATION (IF ANY) IS
ANTICIPATED EAST OF THE RIDGE AXIS, AND WHILE HEAT MAY NOT BE EXCESSIVE, IT
SHOULD BE HOT ENOUGH TO KEEP PART OF THE SOUTHEAST AT RISK FOR RAPID-ONSET
DROUGHT. ABOVE-NORMAL TEMPERATURES WILL EXPAND INTO THE EAST AS THE WEEK
PROGRESSES WHILE TEMPERATURES AVERAGE BELOW NORMAL IN THE WEST, BUT NEITHER
SIDE OF THE CONUS SHOULD HAVE TEMPERATURES APPROACHING HAZARDS THRESHOLDS.
MODERATE TO HEAVY PRECIPITATION IS POSSIBLE IN PARTS OF THE PLAINS AND
MISSISSIPPI VALLEY DOWNSTREAM FROM THE WESTERN CONUS TROUGH, BUT WITH
RELATIVELY WEAK CIRCULATION FEATURES, EXTREME AMOUNTS LOOK UNLIKELY. PERSISTENT
SOUTHERLY FLOW FROM THE NORTH PACIFIC IS EXPECTED TO BRING WET AND BREEZY
CONDITIONS TO SOUTH-CENTRAL AND SOUTHEASTERN ALASKA, BUT AS IS THE CASE ACROSS
THE CONUS, CONDITIONS ARE NOT EXPECTED TO REACH HAZARDS THRESHOLDS.
HAZARDS
RAPID ONSET DROUGHT RISK FOR PARTS OF THE LOWER MISSISSIPPI VALLEY AND
SOUTHEAST.
DETAILED SUMMARY
FOR FRIDAY SEPTEMBER 15 - TUESDAY SEPTEMBER 19:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY SEPTEMBER 20 - TUESDAY SEPTEMBER 26: MODELS ARE IN GOOD AGREEMENT
FOR WEEK-2, SHOWING CIRCULATION FEATURES THAT MAY BRING ANOMALOUS TEMPERATURES
OR PRECIPITATION TO PARTS OF THE NATION, BUT WHICH APPEAR TO BE TOO WEAK TO
DRIVE THE EXTREME WEATHER THAT HAS PREVAILED FOR MUCH OF THE SUMMER -
ESPECIALLY THE HEAT. THE PERIOD STARTS WITH A MID-LEVEL RIDGE AXIS IN CENTRAL
NORTH AMERICA REACHING SOUTHWESTWARD FROM A POSITIVE 500-HPA ANOMALY CENTER
NEAR HUDSON BAY. MEANWHILE, WEAK MID-LEVEL TROUGHS EXTEND ALONG OR NEAR BOTH
THE EAST AND WEST COASTS OF THE CONTINENT. DURING WEEK-2, THE POSITIVE
MID-LEVEL HEIGHT ANOMALY CENTER NEAR HUDSON BAY IS FORECAST TO WEAKEN SLIGHTLY
AND REMAIN APPROXIMATELY STATIONARY, KEEPING THE MID-LEVEL FEATURES OVER THE
CONUS QUASI-STATIONARY AS WELL. THERE MAY BE SOME SLIGHT DEEPENING OF THE
WESTERN CONUS TROUGH TOWARD THE END OF WEEK-2, BUT MOST OF NORTH AMERICA IS
EXPECTED TO SEE SOME WEAKENING OF WHAT IS ALREADY A FAIRLY FLAT, ZONAL
MID-LEVEL FLOW. THE GEFS AND CANADIAN ENSEMBLE MEANS HAVE SHIFTED TOWARD
YESTERDAY’S EUROPEAN SOLUTION TOWARD THE END OF WEEK-2, RESULTING IN A STRONGER
TROUGH ALONG THE WESTERN NORTH AMERICAN COAST, AND AN ILL-DEFINED, DISSIPATING
TROUGH IN THE EASTERN CONUS SOUTH OF THE HUDSON BAY RIDGE. THE BOTTOM LINE,
HOWEVER, IS THAT NONE OF THESE FEATURES ARE REMARKABLE, AND IN THOSE AREAS WITH
ENHANCED CHANCES FOR ANOMALOUS WEATHER, CONDITIONS ARE NOT EXPECTED TO BE
EXTREME ENOUGH TO WARRANT POSTING ANY HAZARDS. THE ONLY EXCEPTION IS IN THE
INTERIOR SOUTHEAST, WHERE ABNORMALLY WARM AND DRY CONDITIONS WILL OCCUR WHERE
THERE IS ANTECEDENT DRYNESS, INCREASING THE ODDS FOR RAPID ONSET DROUGHT.
THE MID-LEVEL RIDGE AXIS IN THE CENTRAL CONUS FAVORS ABOVE-NORMAL TEMPERATURES
ACROSS MUCH OF THE NORTHERN TIER OF THE CONUS, THE GREAT PLAINS, THE LOWER
MISSISSIPPI VALLEY, AND SOME ADJACENT AREAS. THIS WILL MAINTAIN ABOVE-NORMAL
TEMPERATURES ACROSS THESE REGIONS, WITH THE LARGEST POSITIVE ANOMALIES EXPECTED
FROM THE GREAT LAKES REGION AND UPPER MISSISSIPPI VALLEY SOUTH AND
SOUTHWESTWARD INTO THE LOWER MISSISSIPPI VALLEY AND SOUTHERN GREAT PLAINS. AS
THE EASTERN TROUGH BECOMES MORE DIFFUSE, WARMER THAN NORMAL WEATHER WILL PUSH
INTO THE EASTERN CONUS AS WELL LATER WEEK-2. THE GEFS ENSEMBLE MEAN AND
RE-FORECAST TOOL ARE CONSIDERABLY WARMER THAN THE EUROPEAN AND CANADIAN
ENSEMBLES, AND EXCEPT FOR THE GEFS EARLY IN THE PERIOD, ALL GUIDANCE KEEPS
TEMPERATURES WITHIN 5 DEG. F OF NORMAL EACH DAY. HIGH TEMPERATURES IN THE 90’S
ARE POSSIBLE ACROSS MOST OF THE PLAINS EARLY IN THE PERIOD, ACROSS THE
SOUTHEAST TOWARD THE END OF WEEK-2, AND ACROSS THE SOUTH-CENTRAL CONUS
THROUGHOUT THE PERIOD. THIS COULD ACCELERATE DEVELOPING DRYNESS IN PART OF THE
SOUTHEAST, BUT TEMPERATURES EVERYWHERE SHOULD BE LOWER THAN DURING THE PERIODS
OF DANGEROUS AND RELENTLESS EXCESSIVE HEAT OBSERVED FOR MUCH OF THE SUMMER.
THIS PRECLUDES POSTING ANY EXCESSIVE HEAT THREATS, BUT CONDITIONS COULD FORCE
RAPID SURFACE MOISTURE LOSS THAT INCREASES THE CHANCES FOR RAPID-ONSET DROUGHT
JUST EAST OF THE CURRENT AREAS OF EXTREME TO EXCESSIVE DROUGHT, WHICH EXTEND
FROM CENTRAL TEXAS EASTWARD THROUGH SOUTHERN MISSISSIPPI.
WITH THE LATEST GUIDANCE FAVORING A SLIGHTLY DEEPER MID-LEVEL TROUGH IN THE
WEST, A PERSISTENT POSITIVE 500-HPA HEIGHT ANOMALY OVER HUDSON BAY WITH A RIDGE
AXIS REACHING INTO THE CENTRAL CONUS, AND A DISSIPATING TROUGH FARTHER EAST,
CYCLONICALLY-CURVED MID-LEVEL FLOW IS FORMED EAST OF THE WESTERN CONUS HEADING
INTO THE MID-LEVEL RIDGE FARTHER EAST. THIS ENHANCES THE ODDS FOR HEAVY
PRECIPITATION OVER PORTIONS OF THE GREAT PLAINS AND MISSISSIPPI VALLEY.
REFORECAST TOOLS FOR THE EUROPEAN AND CANADIAN ENSEMBLES INDICATE TOTALS MAY
EXCEED AN INCH TOTAL FOR A FEW DAYS AROUND THE MIDDLE OF WEEK-2. HOWEVER, MUCH
OF THIS REGION IS CURRENTLY ENTRENCHED IN SOME DEGREE OF DROUGHT, PARTICULARLY
FROM KANSAS EASTWARD THROUGH MISSOURI. SINCE PRECIPITATION RATES ARE NOT
EXPECTED TO BE EXCESSIVE, INCREASED RAINFALL IN THE REGION MAY DO MORE GOOD
THAN HARM.
TROPICAL EASTERLIES MAY BE SOMEWHAT STRONGER THAN NORMAL ACROSS FLORIDA DURING
PARTS OF THE WEEK-2 PERIOD, AND SOME GUIDANCE - MOST NOTABLY THE EUROPEAN
ENSEMBLE MEANS - BRING ENHANCED CHANCES FOR MODERATE TO HEAVY RAINFALL INTO THE
PENINSULA. SEVERAL INCHES OF RAINFALL ARE POSSIBLE, BUT THIS IS THE WET SEASON
OVER PENINSULAR FLORIDA, AND SEPTEMBER IS ONE OF THE WETTEST MONTHS OF THE
YEAR. NORMAL AMOUNTS RANGE FROM 7.5 INCHES OVER THE NORTHERN PENINSULA TO OVER
10 INCHES FROM MIAMI SOUTHWARD. EVEN IF RAINFALL IS ABOVE-NORMAL IN TOTAL FOR
WEEK-2, THESE CONDITIONS LIKELY WON’T MEET HAZARDS THRESHOLDS.
FARTHER NORTH, AS WEEK-2 GETS UNDERWAY, GUIDANCE GENERALLY AGREES THAT A
MID-LEVEL TROUGH WILL BE ESTABLISHED OVER OR NEAR THE BERING STRAIT, JUST NORTH
OF A FLAT MID-LEVEL RIDGE WITH ANOMALOUSLY HIGH 500-HPA HEIGHTS IN THE CENTRAL
NORTH PACIFIC OCEAN. THESE FEATURES WILL FUEL AN ANOMALOUSLY STRONG MID-LEVEL
JET OVER THE NORTHERN NORTH PACIFIC THAT BECOMES CYCLONICALLY CURVED AND
DIFFLUENT FARTHER EAST OVER SOUTH-CENTRAL AND SOUTHEASTERN ALASKA. THIS WILL
ENHANCE THE IMPACT OF A PERSISTENT SURFACE FLOW OF MOIST PACIFIC AIR INTO THE
STATE, TRIGGERING HEAVY PRECIPITATION ALONG THE SOUTH-CENTRAL ALASKAN COAST AND
IN SOUTHEASTERN PARTS OF THE STATE. STRONG MARITIME WINDS ARE ALSO POSSIBLE
NEAR THE COAST AT TIMES. SEVERAL INCHES OF PRECIPITATION ARE EXPECTED OVER
SOUTHEASTERN ALASKA AND ALONG THE SOUTH-CENTRAL COAST, BUT THESE REGIONS
TYPICALLY RECEIVE 10 TO 20 INCHES OF PRECIPITATION DURING SEPTEMBER. SO, AS IS
THE CASE ACROSS THE CONUS, THE UNREMARKABLE STRENGTH OF THE MID-LEVEL AND
SURFACE FEATURES DOES NOT POSIT EXTREME WEATHER, SO NO HAZARDS ARE POSTED
ANYWHERE IN ALASKA AT THIS TIME.
FORECASTER: RICH TINKER
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