374
FXUS21 KWNC 311838
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT OCTOBER 31 2023
SYNOPSIS: TOWARD THE END OF WEEK-1 AND INTO THE OUTSET OF WEEK-2, FRONTAL
ACTIVITY AND ASSOCIATED SURFACE LOW PRESSURE IS FORECAST TO BRING ENHANCED
CHANCES FOR HEAVY PRECIPITATION TO THE EASTERN SEABOARD. IN THE WAKE OF THE
PREDICTED SURFACE LOW, LAKE EFFECT SNOW, BELOW NORMAL TEMPERATURES, AND GUSTY
NORTHWESTERLY WINDS ARE POSSIBLE THROUGH THE MIDDLE OF WEEK-2. MID-LEVEL LOW
PRESSURE OVER THE NORTHEASTERN PACIFIC AND RESULTANT ENHANCED ONSHORE FLOW MAY
BRING HEAVY PRECIPITATION AND EPISODES OF HIGH WINDS OVER THE PACIFIC NORTHWEST
AND NORTHERN CALIFORNIA. STRENGTHENING PRESSURE GRADIENTS ASSOCIATED WITH
SURFACE HIGH PRESSURE OVER THE GREAT BASIN MAY LEAD TO EPISODES OF SANTA ANA
WINDS OVER SOUTHERN CALIFORNIA.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE NORTHEAST AND NORTHERN
MID-ATLANTIC, WED, NOV 8.
SLIGHT RISK OF HEAVY SNOW FOR PARTS OF THE GREAT LAKES, OHIO VALLEY,
APPALACHIANS, AND THE INTERIOR NORTHEAST, WED-FRI, NOV 8-10.
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR PARTS OF THE UPPER
MISSISSIPPI VALLEY, GREAT LAKES, AND THE NORTHEAST, WED-SUN, NOV 8-12.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PARTS OF
NORTHERN CALIFORNIA, THU-MON, NOV 9-13.
SLIGHT RISK OF HIGH WINDS OVER PARTS OF THE NORTHEAST, OHIO VALLEY,
APPALACHIANS, AND MID-ATLANTIC, WED-THU, NOV 8-9.
SLIGHT RISK OF HIGH WINDS OVER COASTAL PARTS OF THE PACIFIC NORTHWEST AND
NORTHERN CALIFORNIA, THU-MON, NOV 9-13.
SLIGHT RISK OF HIGH WINDS OVER PARTS OF SOUTHERN CALIFORNIA, WED-SAT, NOV 8-11.
DETAILED SUMMARY
FOR FRIDAY NOVEMBER 03 - TUESDAY NOVEMBER 07:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY NOVEMBER 08 - TUESDAY NOVEMBER 14: THE DYNAMICAL MODELS ARE IN
GOOD AGREEMENT REGARDING THE OVERALL SYNOPTIC PATTERN ACROSS NORTH AMERICA AT
THE START OF WEEK-2. TROUGHS ARE FORECAST ACROSS THE NORTHEAST PACIFIC AND
EASTERN CONUS, WITH RIDGING OVER THE WESTERN CONUS. THE INITIAL TROUGH ACROSS
THE EAST FAVORS THE DEVELOPMENT OF A SURFACE LOW PRESSURE SYSTEM OVER THE
NORTHEAST BRINGING ENHANCED CHANCES FOR HEAVY PRECIPITATION AT THE OUTSET OF
THE PERIOD. THIS EVOLUTION IS FAVORED IN MOST 0Z GEFS AND ECMWF ENSEMBLE
MEMBERS AND THE 0Z DETERMINISTIC ECMWF, ALTHOUGH THE LATEST RUNS OF THE
DETERMINISTIC GFS DEPICT A MORE SHEARED OUT SYSTEM. MODELS ARE ALSO GENERALLY
QUICKER WITH THIS SYSTEM COMPARED TO YESTERDAY AND THERE IS SOME ADDED
UNCERTAINTY INTO HOW MUCH PRECIPITATION WILL OCCUR ON DAY-7 VERSUS DAY-8,
ESPECIALLY FOR AREAS FURTHER SOUTH. BOTH THE ECMWF AND GEFS PROBABILISTIC
EXTREMES TOOLS (PETS) HAVE SIGNIFICANTLY REDUCED HAZARDOUS PRECIPITATION
PROBABILITIES RELATIVE TO YESTERDAY DUE TO THE FASTER PROGRESSION OF THE
SYSTEM. FOR TODAY’S OUTLOOK, A SLIGHT RISK OF HEAVY PRECIPITATION REMAINS
POSTED ACROSS PARTS OF THE NORTHEAST AND NORTHERN MID-ATLANTIC FOR DAY-8 (NOV
8) ONLY.
A CHANGE TO SNOW OR MIXED PRECIPITATION CANNOT BE RULED OUT OVER THE PARTS OF
THE INTERIOR NORTHEAST ON THE BACKSIDE OF THE DEPARTING SURFACE LOW, ALTHOUGH
THE FASTER SPEED OF THE STORM FAVORS MOST OF THE PRECIPITATION TO EXIT BEFORE
COLDER AIR ARRIVES. BEHIND THE SYSTEM, LAKE EFFECT SNOW MAY ALSO BE POSSIBLE
GIVEN ANOMALOUSLY WARM LAKE TEMPERATURES (GREATER THAN 50 DEG F) COUPLED WITH
SUBFREEZING TEMPERATURES ALOFT AND GUSTY NORTHWESTERLY WINDS. THE GEFS-BASED
SNOW WATER EQUIVALENT (SWE) PET ALSO INDICATES A LARGER AREA RELATIVE TO
YESTERDAY WITH GREATER THAN A 20 PERCENT CHANCE SWE EXCEEDS THE 85TH
CLIMATOLOGICAL PERCENTILE DOWNWIND OF LAKES ERIE AND ONTARIO, ALTHOUGH THE LOW
EARLY-SEASON CLIMATOLOGY IS LIKELY PLAYING A ROLE IN THESE HIGHER
PROBABILITIES. IN ADDITION, DAYTIME SURFACE TEMPERATURES MAY REACH OR EXCEED
FREEZING IN SOME AREAS, LIMITING ACCUMULATION POTENTIAL. HOWEVER, GIVEN THE
FAVORABLE SET-UP, A SLIGHT RISK FOR HEAVY SNOW REMAINS POSTED ACROSS THE GREAT
LAKES AND INTERIOR NORTHEAST FOR NOV 8-10. THE TIGHT PRESSURE GRADIENT THAT IS
FORECAST TO DEVELOP ACROSS THE NORTHEAST AND MID-ATLANTIC SUPPORTS A SLIGHT
RISK OF HIGH WINDS OVER THESE AREAS NOV 8-9.
SURFACE HIGH PRESSURE IS FORECAST TO SHIFT SOUTHWARD OUT OF CANADA AND INTO THE
EASTERN CONUS RESULTING IN A PERIOD OF BELOW NORMAL TEMPERATURES ACROSS THE
EAST. BOTH THE GEFS AND ECMWF PETS DEPICT MINIMUM TEMPERATURES FALLING BELOW
THE 10TH CLIMATOLOGICAL PERCENTILE AND 28 DEG F ACROSS PARTS OF THE NORTHEAST
AND GREAT LAKES (20 DEG F ACROSS INTERIOR NEW ENGLAND AND AREAS CLOSER TO THE
U.S./CANADIAN BORDER) WHERE A SLIGHT RISK FOR MUCH BELOW NORMAL TEMPERATURES IS
POSTED NOV 8-12. THE AFOREMENTIONED INCREASED WINDS ALSO FAVOR LOW WIND CHILL
VALUES, WHICH COULD BE HAZARDOUS TO THOSE NOT ACCUSTOMED TO THE COLD GIVEN THAT
IT IS STILL THE EARLY PART OF THE SEASON. THE PETS ALSO SHOW AN INCREASING
POTENTIAL FOR SUBFREEZING MINIMUM TEMPERATURES REACHING AS FAR SOUTH AS THE
TENNESSEE VALLEY AND THE SOUTHEAST. HOWEVER, THE HAZARD IS NOT EXPANDED
SOUTHWARD DUE TO GENERAL UNCERTAINTY AS TO WHETHER SUCH TEMPERATURES WOULD BE
IMPACTFUL ON VEGETATION GIVEN THE COLD AIR OUTBREAK FAVORED OVER THIS PART OF
THE COUNTRY DURING WEEK-1 AND THE LIKELY ENDING OF THE GROWING SEASON.
RIDGING INITIALLY OVER THE WESTERN CONUS IS FORECAST TO QUICKLY SHIFT INLAND
TOWARD THE GREAT PLAINS ALLOWING TROUGHING OVER THE NORTHEAST PACIFIC TO MOVE
CLOSER TO THE WEST COAST. FOLLOWING A PREDICTED LULL IN ATMOSPHERIC RIVER
ACTIVITY ON DAY-8, PRECIPITATION CHANCES ARE FORECAST TO AGAIN INCREASE ON
DAY-9 WITH BOTH THE ECMWF AND GEFS ENSEMBLES DEPICTING AT LEAST A 50 PERCENT
CHANCE INTEGRATED VAPOR TRANSPORT (IVT) EXCEEDS 250 KG/M/S. GIVEN GOOD SUPPORT
IN THE PETS INDICATING AT LEAST A 20 PERCENT CHANCE FOR 3-DAY AMOUNTS EXCEEDING
THE 85TH PERCENTILE WITH SIMILAR CHANCES FOR 1 TO 1.5 INCHES OVER PARTS OF THE
PACIFIC NORTHWEST AND NORTHERN CALIFORNIA, A SLIGHT RISK OF HEAVY PRECIPITATION
IS POSTED FOR NOV 9-13 FOR THESE AREAS. A SLIGHT RISK OF HIGH WINDS ALSO
REMAINS POSTED FOR THIS TIME FRAME FOR COASTAL AREAS BASED ON PETS ALSO
INDICATING INCREASED CHANCES FOR WIND SPEEDS EXCEEDING THE 85TH PERCENTILE. THE
EXCESS IN MOISTURE IS LIKELY TO PRODUCE ACCUMULATING SNOWFALL IN THE HIGHER
ELEVATIONS OF THE CASCADES AND PORTIONS OF THE NORTHERN ROCKIES, THOUGH TOTALS
APPEAR UNLIKELY TO REACH HAZARD THRESHOLDS.
TIED TO THE ANOMALOUS RIDGING FAVORED FURTHER SOUTH, CONDITIONS ARE EXPECTED TO
REMAIN ANOMALOUSLY HOT AND DRY OVER SOUTHERN CALIFORNIA, THE SOUTHWEST, AND
LOWER FOUR CORNERS. WITH MODELS FAVORING MORE MEAN SURFACE HIGH PRESSURE OVER
THE WESTERN GREAT BASIN, THERE IS AN INCREASED POTENTIAL FOR EPISODES OF SANTA
ANA WINDS OVER SOUTHERN CALIFORNIA AND A SLIGHT RISK OF HIGH WINDS IS POSTED
FOR NOV 8-11.
TROUGHING FORECAST OVER THE NORTHEAST PACIFIC TO THE SOUTH OF MAINLAND ALASKA
GENERALLY FAVORS INCREASED ONSHORE FLOW AND SUBSEQUENT ROUNDS OF HEAVY
PRECIPITATION AND HIGH WINDS ACROSS THE SOUTHERN COAST OF THE MAINLAND AND
SOUTHEAST ALASKA. HOWEVER, PRECIPITATION AMOUNTS AND WIND SPEEDS ARE NOT
FORECAST TO REACH HAZARDS THRESHOLDS. TEMPERATURES ARE GENERALLY FORECAST TO BE
ABOVE-NORMAL OVER MOST AREAS, WITH CLOSER TO NORMAL TEMPERATURES FAVORED FOR
THE FAR WESTERN MAINLAND.
FORECASTER: THOMAS COLLOW
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