985
FXUS21 KWNC 191853
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 19 2023
SYNOPSIS: MID-LEVEL HIGH PRESSURE OVER ALASKA AND A MID-LEVEL TROUGH MOVING
ACROSS THE WESTERN CONTIGUOUS U.S. (CONUS) IS PREDICTED TO PUSH COLD CANADIAN
AIR INTO THE CENTRAL CONUS FOR THE FIRST HALF OF WEEK-2 BEFORE MODERATING.
LOCALIZED HEAVY SNOW COULD LINGER FROM THE END OF WEEK-1 INTO THE START OF
WEEK-2, ESPECIALLY ACROSS HIGHER ELEVATIONS OF THE NORTH-CENTRAL CONUS.
MEANWHILE, ENHANCED MOIST LOW-LEVEL FLOW FROM THE GULF OF MEXICO COUPLED WITH
THE PASSAGE OF A SERIES OF FRONTS OVER THE GREAT PLAINS MAY LEAD TO HEAVY
PRECIPITATION ACROSS PARTS OF THE SOUTHERN PLAINS, THE LOWER AND MIDDLE
MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, AND THE ADJACENT SOUTHEAST IN
ADDITION TO HIGH WINDS FOR CENTRAL CONUS.
HAZARDS
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN
AND CENTRAL PLAINS, FRI-SUN, OCT 27-29.
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN
ROCKIES, NORTHERN AND CENTRAL PLAINS, AND THE UPPER MISSISSIPPI VALLEY,
FRI-MON, OCT 27-30.
SLIGHT RISK OF HEAVY SNOW FOR HIGHER ELEVATIONS OF THE NORTHERN AND CENTRAL
ROCKIES, AND THE NORTHERN AND CENTRAL PLAINS, FRI-SAT, OCT 27-28.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHERN PLAINS, THE
LOWER AND MIDDLE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, AND THE ADJACENT
SOUTHEAST, FRI-SAT, OCT 27-28.
SLIGHT RISK FOR PERIODS OF HIGH WINDS FOR PARTS OF THE CENTRAL AND SOUTHERN
PLAINS AND MIDDLE MISSISSIPPI VALLEY, FRI-SUN, OCT 27-29.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE PACIFIC NORTHWEST AND
NORTHERN CALIFORNIA, TUE-THU, OCT 31-NOV 2.
DETAILED SUMMARY
FOR SUNDAY OCTOBER 22 - THURSDAY OCTOBER 26:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY OCTOBER 27 - THURSDAY NOVEMBER 02: A MID-LEVEL TROUGH IS PREDICTED
TO EXTEND FROM THE UPPER MISSISSIPPI VALLEY TO THE INTERIOR WEST AT THE
BEGINNING OF THE PERIOD, WITH THE GEFS ENSEMBLE MEAN HAVING THE GREATEST
AMPLITUDE COMPARED TO THE ECMWF AND CANADIAN COUNTERPARTS. THIS PATTERN
SUPPORTS A COLD SURFACE AIR MASS EXTENDING SOUTHWARD FROM CANADA, BRINGING
ANOMALOUS COLD TO MUCH OF THE CENTRAL CONUS DURING WEEK-2. A SLIGHT RISK OF
MUCH BELOW NORMAL TEMPERATURES IS HIGHLIGHTED ACROSS PORTIONS OF THE NORTHERN
ROCKIES, NORTHERN AND CENTRAL PLAINS, AND THE UPPER MISSISSIPPI VALLEY, OCT
27-30, WHERE THE GEFS, ECMWF, AND CANADIAN PROBABILISTIC EXTREMES TOOLS (PETS)
SHOW AT LEAST A 20% CHANCE OF MINIMUM TEMPERATURES FALLING BELOW THE 10TH
PERCENTILE CLIMATOLOGICALLY AND 20 DEG F. AN EMBEDDED MODERATE RISK IS
DESIGNATED FOR PARTS OF THE NORTHERN AND CENTRAL PLAINS, OCT 27-29, WHERE THE
GEFS PET INDICATES AT LEAST A 30% CHANCE OF MINIMUM TEMPERATURES DIPPING BELOW
THE LOWEST 5TH PERCENTILE (ECMWF PET SIGNAL FOR THIS THRESHOLD IS SOMEWHAT
WEAKER). THESE COLD TEMPERATURES COULD HAVE NEGATIVE IMPACTS TO AGRICULTURE AND
LIVESTOCK.
THE SURFACE LOW OVER THE NORTHERN PLAINS RESPONSIBLE FOR YESTERDAY'S SLIGHT
RISK OF HEAVY SNOW IS ANTICIPATED TO DEPART BY WEEK-2 RESULTING IN THE TIMING
OUT OF A BROADER SNOW RISK AREA. A SMALLER AREA IS POSTED WITH A SLIGHT RISK OF
HEAVY SNOW ACROSS HIGHER ELEVATIONS OF THE NORTHERN AND CENTRAL ROCKIES, AND
THE NORTHERN AND CENTRAL PLAINS OCT 27 TO 28 WITH POTENTIAL UPSLOPE SNOW. THERE
IS HIGH UNCERTAINTY REGARDING THIS HAZARD POTENTIAL AND THE SIGNALS FOR
SUPPORTING HEAVY SNOW ARE WEAKER IN THE PETS COMPARED TO YESTERDAY.
THE AFOREMENTIONED MID-LEVEL TROUGH IS ANTICIPATED TO SUPPORT THE PASSAGE OF A
COLD FRONT ACROSS THE CENTRAL CONUS AT THE BEGINNING OF THE PERIOD. THIS
PATTERN SUPPORTS A SLIGHT RISK OF HEAVY RAINFALL FOR PARTS OF THE SOUTHERN
PLAINS, THE LOWER AND MIDDLE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, AND THE
ADJACENT SOUTHEAST, OCT 27-28. THERE IS GOOD AGREEMENT AMONG THE PETS
INDICATING AT LEAST A 20% CHANCE OF 3-DAY RAINFALL (OCT 27-29) TOTALS EXCEEDING
THE 85TH PERCENTILE AND ONE INCH. DAYS 8 TO 9 HAVE THE GREATEST CHANCES IN
TERMS OF TIMING FOR HEAVY RAINFALL. THIS FRONT MAY ALSO LEAD TO ENHANCED WIND
SPEEDS ACROSS THE CENTRAL CONUS. A SLIGHT RISK OF HIGH WINDS IS DESIGNATED FOR
THE CENTRAL AND SOUTHERN GREAT PLAINS AND MIDDLE MISSISSIPPI VALLEY, OCT 27-29,
PRIMARILY BASED ON WHERE THE ECMWF AND CANADIAN PETS INDICATE AT LEAST A 20%
CHANCE OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND 20 MPH IN ADDITION TO
DETERMINISTIC MODEL GUIDANCE.
THERE IS RELATIVELY GOOD AGREEMENT AMONG MULTIPLE ENSEMBLE MEANS INDICATING A
MID-LEVEL SHORTWAVE TROUGH UNDERCUTTING THE RIDGE OVER ALASKA. THIS PATTERN
WOULD SUPPORT ENHANCED MOISTURE TO PARTS OF THE NORTHWESTERN CONUS, WHICH IS
SEEN IN THE ECMWF AND GEFS ENSEMBLE SHOWING ENHANCED INTEGRATED VAPOR TRANSPORT
(IVT) VALUES EXCEEDING 250 KG/M/S. A SLIGHT RISK OF HEAVY PRECIPITATION IS
DESIGNATED WHERE THE PETS ARE IN GOOD AGREEMENT SHOWING AT LEAST A 20% CHANCE
OF 3-DAY TOTALS EXCEEDING THE 85TH PERCENTILE AND ONE INCH, WITH THE ECMWF
SHOWING MUCH OF THIS AREA EXCEEDING 2 INCHES (GEFS LIMITS THIS POTENTIAL MOSTLY
TO WESTERN OREGON AND WASHINGTON).
ACROSS ALASKA, A STRONG MID-LEVEL RIDGE COVERING THE STATE IS EXPECTED TO
SLOWLY RELAX AS WEEK-2 PROGRESSES. SURFACE LOW FORMATION IS POSSIBLE OVER THE
GULF OF ALASKA THAT COULD SUPPORT ENHANCED PRECIPITATION ACROSS SOME COASTAL
SOUTHERN AREAS INCLUDING THE ALEUTIANS. HOWEVER, CONDITIONS ARE NOT ANTICIPATED
TO REACH HAZARDS CRITERIA AT THIS TIME.
FORECASTER: MELISSA OU
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