701
FXUS21 KWNC 131857
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 13 2023
SYNOPSIS: A MID-LEVEL RIDGE IS EXPECTED TO BRING ABOVE-NORMAL TEMPERATURES TO A
LARGE PART OF THE COUNTRY FROM THE HIGH PLAINS TO THE EAST COAST, BUT EXCESSIVE
HEAT IS NOT ANTICIPATED. RELATIVELY LIGHT RAINFALL IS ANTICIPATED EAST OF THE
FROM THE INTERIOR SOUTHEAST TO NEW ENGLAND, WITH THE COMBINATION OF
ABOVE-NORMAL TEMPERATURES AND MEAGER RAINFALL ANTICIPATED TO KEEP PART OF THE
SOUTHEAST AT RISK FOR RAPID-ONSET DROUGHT. MEANWHILE, WEAK MID-LEVEL
DISTURBANCES ACROSS THE WESTERN CONTIGUOUS U.S. COULD TRIGGER PERIODS OF
MODERATE TO HEAVY RAINFALL OVER THE GREAT PLAINS AND MISSISSIPPI VALLEY, BUT
THE SET-UP DOES NOT SEEM ROBUST ENOUGH TO GENERATE HAZARDOUS RAINFALL TOTALS. A
SERIES OF STORMS OVER THE NORTH PACIFIC OCEAN MAY 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 SATURDAY SEPTEMBER 16 - WEDNESDAY SEPTEMBER 20:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY SEPTEMBER 21 - WEDNESDAY SEPTEMBER 27: MODELS ARE IN DECENT
AGREEMENT REGARDING THE MEAN PATTERN FOR WEEK-2, BUT THE EVOLUTION OF
INDIVIDUAL CIRCULATION FEATURES ARE NOT AS CONSISTENT ACROSS THE RANGE OF
GUIDANCE AS YESTERDAY. IN ADDITION, THE LOCATION OF MID-LEVEL FEATURES IN THE
GEFS ARE SOMEWHAT FARTHER WEST THAN IN THE EUROPEAN AND CANADIAN ENSEMBLE MEANS
OVER MUCH OF THE WESTERN HEMISPHERE.A MID-LEVEL RIDGE AXIS IS EXPECTED TO
EXTEND NORTHWARD THROUGH THE NATION’S MIDSECTION TO A POSITIVE 500-HPA ANOMALY
CENTER LOCATED OVER EASTERN CANADA, DRIFTING SLIGHTLY EASTWARD AS WEEK-2
PROGRESSES. ABOVE-NORMAL TEMPERATURES ARE FAVORED TO BE IN PLACE FROM THE GREAT
PLAINS INTO THE GREAT LAKES WITH MID-LEVEL RIDGING PREDICTED ACROSS THE REGION.
THE WARMTH IS NOT EXPECTED TO REACH THE DANGEROUS, EXCESSIVE LEVELS
PERIODICALLY SEEN THIS PAST SUMMER, SO NO HAZARDS ARE POSTED DIRECTLY
ASSOCIATED WITH THE HEAT.
LATER IN THE WEEK, AS THE MID-LEVEL RIDGE AXIS DRIFTS EASTWARD, TEMPERATURES
ARE EXPECTED TO CLIMB PAST 90 DEG F AT TIMES IN THE CENTRAL GULF COAST REGION.
THIS AREA IS DOWNSTREAM FROM THE RIDGE AXIS, SO RAINFALL AMOUNTS ARE EXPECTED
TO BE LIGHT AT BEST OVER MUCH OF THE LOWER MISSISSIPPI VALLEY AND SOUTHEAST
UNTIL LATE WEEK-2 AT THE EARLIEST. THESE HOT AND DRY CONDITIONS ARE EXPECTED
ACROSS AN AREA WHERE DEPARTURES FROM NORMAL WERE -4 INCHES OR DRIER (TO -12
INCHES LOCALLY) FOR THE PAST 2 TO 3 MONTHS. THE CONFLUENCE OF THESE CONDITIONS
PUTS PARTS OF THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST REGION AT
RISK OF RAPID-ONSET DROUGHT. EXTREME TO EXCESSIVE DROUGHT ALREADY EXISTS JUST
WEST OF THIS AREA AT RISK, FROM CENTRAL TEXAS EASTWARD ACROSS MOST OF LOUISIANA
AND SOUTHWESTERN MISSISSIPPI.
FARTHER WEST, A MID-LEVEL TROUGH IS EXPECTED TO GRADUALLY AMPLIFY OVER OR NEAR
THE WESTERN CONUS THROUGH MID-WEEK BEFORE STARTING TO WEAKEN TOWARD THE END OF
THE PERIOD. THIS PATTERN MAY SUPPORT ONE OR MORE DISTURBANCES PULLING EASTWARD
FROM THIS TROUGH INTO THE CENTRAL CONUS. THE GEFS BEGINS WEEK-2 WITH A MORE
AMPLIFIED TROUGH THAN THE OTHER TWO ENSEMBLE MEANS, BUT ALL THREE MODELS EVOLVE
TOWARD SIMILAR SOLUTIONS AFTER THE FIRST 24 TO 48 HOURS. AS THESE DISTURBANCES
TRAVERSE THE GREAT PLAINS AND MISSISSIPPI VALLEY, THEY ARE EXPECTED TO INTERACT
WITH A FLOW OF UNUSUALLY WARM AND MOIST AIR FROM THE GULF OF MEXICO TO THE WEST
OF A SURFACE HIGH PRESSURE CENTER. AS A RESULT, SEVERAL PERIODS OF UNUSUALLY
WET WEATHER ARE EXPECTED IN THE CENTRAL CONUS. THIS WILL RESULT IN AN
ABNORMALLY WET WEEK AS A WHOLE, WITH ALL 3 ENSEMBLE MEANS DROPPING BETWEEN 1
AND 2 INCHES OF RAIN ON PORTIONS OF THE PLAINS AND MISSISSIPPI VALLEY. WHILE
THE WET SIGNAL IS MORE ROBUST THAN YESTERDAY, IT STILL DOES NOT LOOK AS THOUGH
RAINFALL ACCUMULATIONS AT ANY GIVEN TIME WILL EXCEED HAZARDS THRESHOLDS.
FARTHER NORTH, AS WEEK-2 GETS UNDERWAY, GUIDANCE GENERALLY AGREES THAT A FLAT
MID-LEVEL RIDGE WITH ANOMALOUSLY HIGH 500-HPA HEIGHTS WILL BE CENTERED SOUTH OF
THE ALEUTIANS IN THE CENTRAL NORTH PACIFIC OCEAN. TO THE NORTH, 500-HPA HEIGHTS
WILL BE CLOSER TO NORMAL, RESULTING IN 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 ONE
OR MORE SURFACE LOW PRESSURE CENTERS TRACKING ACROSS THE GULF OF ALASKA, AND
GENERALLY KEEP 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. THIS SET-UP WILL BE MOST PRONOUNCED DURING
WEEK-1, WITH FLOOD WATCHES COVERING SOUTHEAST ALASKA FOR AT LEAST THE NEXT 48
HOURS. FOR WEEK-2, GUIDANCE IS SLIGHTLY LOWER WITH PRECIPITATION TOTALS
COMPARED TO YESTERDAY. THE EUROPEAN ENSEMBLE MEAN IN PARTICULAR HAS BACKED OFF
THE STRENGTH OF THESE FEATURES TRANSLATING TO DECREASING AMOUNTS OF
PRECIPITATION EXPECTED DURING WEEK-2. A SOLUTION LEANING TOWARD THE MORE ROBUST
CANADIAN ENSEMBLE AND GEFS TOOLS IS PREFERRED, BUT CONFIDENCE IS REDUCED A BIT
FROM YESTERDAY. STILL, STRONG MARITIME WINDS ARE ALSO POSSIBLE NEAR THE COAST
AT TIMES, AND SEVERAL INCHES OF PRECIPITATION ARE EXPECTED OVER SOUTHEASTERN
ALASKA AND ALONG THE SOUTH-CENTRAL COAST. THIS WILL OCCUR FOLLOWING A VERY WET
WEEK-1, POTENTIALLY EXACERBATING IMPACTS FROM PRECIPITATION DURING WEEK-2. AT
THIS TIME, HOWEVER, GIVEN HIGH SEPTEMBER NORMALS OF 8 TO LOCALLY 20 INCHES OF
PRECIPITATION, THE WEEK-2 PRECIPITATION TOTALS ARE ANTICIPATED TO REMAIN BELOW
HAZARDS THRESHOLDS, THUS NO HAZARDS ARE POSTED ANYWHERE IN ALASKA.
FORECASTER: RICH TINKER
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