988
FXUS21 KWNC 191914
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 19 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FAVORED IN THE NORTH PACIFIC, RESULTING IN
THE POTENTIAL FOR HEAVY PRECIPITATION AND HIGH WINDS DOWNSTREAM ALONG THE
WESTERN TIER OF THE CONTIGUOUS U.S. (CONUS) UNTIL AT LEAST THE MIDDLE OF
WEEK-2. MUCH OF THIS IS EXPECTED TO FALL AS HEAVY SNOW IN THE SIERRA NEVADA AND
SOUTHERN CASCADES. MEANWHILE, A LOW PRESSURE SYSTEM OVER THE CENTRAL CONUS IS
EXPECTED TO PULL GULF OF MEXICO MOISTURE NORTHWARD AHEAD OF IT, ENHANCING THE
POTENTIAL FOR HEAVY PRECIPITATION ALONG THE ENTIRE EASTERN SEABOARD AND THE
APPALACHIANS.
HAZARDS
MODERATE RISK FOR HEAVY PRECIPITATION FOR MUCH OF WESTERN CALIFORNIA, THE
CALIFORNIA VALLEYS, AND HIGHER ELEVATIONS, WED-SAT, DEC 27-30.
MODERATE RISK FOR HEAVY PRECIPITATION FROM THE MID-ATLANTIC REGION NORTHWARD
INTO SOUTHERN NEW ENGLAND, WED-FRI, DEC 27-29.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL AND NORTHERN SIERRA NEVADA,
SOUTHERN CASCADES, AND COASTAL RANGES OF NORTHERN CALIFORNIA, WED-SAT, DEC
27-30.
MODERATE RISK FOR HIGH WINDS FOR PORTIONS OF WESTERN CALIFORNIA, THE NORTHERN
CALIFORNIA VALLEYS, WESTERN OREGON, AND THE NORTHERN SIERRA NEVADA WED-SAT, DEC
27-30.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR MUCH OF THE WEST COAST, WED-TUE, DEC
27-JAN 2.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR MUCH OF THE EASTERN CONUS, WED-FRI, DEC
27-29.
SLIGHT RISK FOR HEAVY SNOW FOR THE SIERRA NEVADA, SOUTHERN CASCADES, AND
COASTAL RANGES OF NORTHERN CALIFORNIA, WED-TUE, DEC 27-JAN 2.
SLIGHT RISK FOR HEAVY SNOW IN THE WASHINGTON CASCADES, WED-THU, DEC 27-28.
SLIGHT RISK FOR HIGH WINDS FOR MUCH OF THE WEST COAST, INCLUDING THE CALIFORNIA
VALLEYS, SIERRA NEVADA, AND SOUTHERN CASCADES, WED-SAT, DEC 27-30.
DETAILED SUMMARY
FOR FRIDAY DECEMBER 22 - TUESDAY DECEMBER 26:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY DECEMBER 27 - TUESDAY JANUARY 02: MULTIPLE MODEL ENSEMBLE MEANS
ARE IN GOOD AGREEMENT INDICATING A MID-LEVEL SOUTHERN-STREAM TROUGH SLOWLY
PROGRESSING FROM THE CENTRAL CONUS TO THE SOUTHEAST AND WESTERN NORTH ATLANTIC.
MEANWHILE, A DEEP MID-LEVEL TROUGH OVER THE NORTH PACIFIC WILL WEAKEN AND MOVE
LITTLE DURING WEEK-2. FARTHER NORTH, A BROAD, STRONG MID-LEVEL RIDGE ACROSS
MOST OF CANADA AND THE NORTHEASTERN CONUS IS EXPECTED TO DRIFT NORTHWESTWARD
DURING WEEK-2, ALLOWING 500-HPA HEIGHTS TO DROP IN THE NORTHEASTERN CONUS BUT
MAINTAINING POSITIVE 500-HPA HEIGHT ANOMALIES ACROSS CANADA. OVERALL, THE
MODELS SHOW THE MID-LEVEL TROUGHS IN THE NORTH PACIFIC AND IN THE SOUTHEAST
WEAKENING A LITTLE FASTER THAN YESTERDAY, WITH LESS RE-AMPLIFICATION OF THE
SOUTHEAST TROUGH AS ITS AXIS PUSHES TOWARD THE EAST COAST. WITH A WEAKER
MID-LEVEL NORTH PACIFIC TROUGH LATER WEEK-2, MODELS BRING NEAR- TO ABOVE-NORMAL
500-HPA HEIGHTS INTO THE WESTERN CONUS BY THE END OF THE PERIOD.
AS WEEK-2 STARTS, STORMY WEATHER IS FORECAST TO BE MOVING INTO THE WEST COAST
DOWNSTREAM FROM THE DEEP MID-LEVEL TROUGH IN THE NORTH PACIFIC, WITH
ATMOSPHERIC RIVER ACTIVITY POSSIBLE. MOST GUIDANCE SHOWS STORMINESS ENCROACHING
ON THE WEST COAST MORE SLOWLY THAN YESTERDAY, WITH HEAVY PRECIPITATION IN
CALIFORNIA COMMENCING DURING DAY 8 (DEC 27), 24 TO 48 HOURS LATER THAN EXPECTED
YESTERDAY. BUT REGARDLESS OF TIMING, THE MID-LEVEL FEATURES IN PLACE ENHANCE
THE CHANCES FOR HEAVY PRECIPITATION, HIGH WINDS, AND HEAVY SNOW IN A LARGE PART
OF THE WESTERN CONUS. THE ENSEMBLES AND THEIR DERIVED TOOLS REMAIN IN ONLY FAIR
AGREEMENT TODAY ON THE LOCATION, STRENGTH, AND TIMING OF THE FEATURES AFFECTING
THE REGION, WHICH CREATES INCONSISTENCIES IN THE GUIDANCE REGARDING THE DETAILS
OF SENSIBLE WEATHER. THE CANADIAN ENSEMBLES FOCUS THE STORMIEST WEATHER FURTHER
NORTH THAN OTHER MODELS, ACROSS CENTRAL AND NORTHERN PORTIONS OF THE WEST
COAST. THE EUROPEAN GUIDANCE SHOWS THIS FEATURE FARTHER SOUTH, BRINGING
ENHANCED CHANCES FOR HEAVY PRECIPITATION AND HIGH WINDS AS FAR SOUTH AS THE
MEXICAN BORDER, AND SHOWING LITTLE POTENTIAL FOR HAZARDS IN WASHINGTON AND
OREGON. THE GEFS SOLUTION LIES BETWEEN THE OTHER TWO, BUT CLOSER TO THE
EUROPEAN ENSEMBLE GUIDANCE, AND IN BETTER AGREEMENT WITH IT THAN YESTERDAY. A
BLEND OF THE GEFS AND EUROPEAN ENSEMBLE MEAN IS FAVORED AT THIS TIME, WHICH
FOCUSES THE HIGHEST RISKS FOR HAZARDOUS CONDITIONS OVER CALIFORNIA, BUT GIVEN
THE POSITIONS OF THE NORTH PACIFIC MID-LEVEL TROUGH AND THE CANADIAN MID-LEVEL
RIDGE, STORMY WEATHER COULD EASILY AFFECT AREAS ACROSS PORTIONS OF THE
NORTHWESTERN CONUS FOR AT LEAST PART OF WEEK-2.
A MODERATE RISK (>40% CHANCE) IS POSTED FOR HEAVY PRECIPITATION OVER MUCH OF
WESTERN CALIFORNIA, THE NORTHERN CALIFORNIA VALLEYS, THE SIERRA NEVADA, AND THE
SOUTHERN CASCADES THROUGH THE MIDDLE OF WEEK-2, WHEN CONDITIONS SHOULD IMPROVE
SOMEWHAT. DUE TO THE UNCERTAINTIES IN TIMING AND LOCATION, A SLIGHT RISK (>20%
CHANCE) FOR HEAVY PRECIPITATION COVERS THE ENTIRE WEST COAST FROM CANADA TO
MEXICO, AND EXTENDS THROUGH ALL OF WEEK-2. FLOODING IS POSSIBLE WHERE THE
PRECIPITATION IS THE HEAVIEST, THOUGH NO SPECIFIC REGION CAN BE IDENTIFIED WITH
AN ENHANCED RISK DUE TO THE UNCERTAINTIES IN THE EVOLUTION OF THIS STORM SYSTEM.
THE PROBABILISTIC EXTREMES TOOL (PET) BASED ON THE EUROPEAN ENSEMBLES SHOWS
VERY HIGH PROBABILITIES FOR HEAVY PRECIPITATION, WITH AT LEAST A 40% CHANCE OF
3-DAY PRECIPITATION TOTALS EXCEEDING THE 15TH PERCENTILE FROM THE
CALIFORNIA/OREGON BORDER SOUTHWARD TO SOUTHEASTERN CALIFORNIA AND EASTWARD INTO
WEST-CENTRAL NEVADA. CHANCES FOR HEAVY PRECIPITATION INCREASE TOWARD
WEST-CENTRAL CALIFORNIA IN THE PET BASED ON THE EUROPEAN ENSEMBLES, REACHING AS
HIGH AS 90 PERCENT ALONG THE IMMEDIATE COAST FROM THE SAN FRANCISCO AREA
SOUTHWARD THROUGH MONTERREY. THE PETS DERIVED FROM THE GEFS AND CANADIAN
ENSEMBLES, HOWEVER, ARE FAR LESS ROBUST, WITH THE ODDS FOR EXCEEDING THIS
THRESHOLD REMAINING BELOW 40 PERCENT THROUGHOUT THE REGION FOR ALL OF WEEK-2.
IN THE HIGHER ELEVATIONS, THE PET TOOL BASED ON THE GEFS SHOWS SLIGHTLY LOWER
CHANCES FOR HEAVY SNOWFALL IN THE HIGHER ELEVATIONS OF CALIFORNIA, BUT THE
CHANGES WERE NOT ENOUGH TO PROMPT ANY ADJUSTMENTS TO THE HEAVY SNOWFALL RISK
AREAS SHOWN YESTERDAY.
THE RAW DYNAMICAL MODELS GENERALLY KEEP ODDS FOR ABOVE-NORMAL PRECIPITATION
DURING WEEK-2 BELOW 60 PERCENT (CHANCES ARE 33 PERCENT CLIMATOLOGICALLY), AND
IN ADDITION, THE GEFS KEEPS WEEK-2 TOTALS WITHIN 2 INCHES OF NORMAL ACROSS THE
WESTERN CONUS. FOR THESE REASONS, IN ADDITION TO THE RELATIVELY LOW ODDS FOR
HEAVY PRECIPITATION SHOWN BY THE GEFS AND CANADIAN ENSEMBLE PETS, NO HIGH RISK
IS DESIGNATED FOR EITHER HEAVY PRECIPITATION OR HEAVY SNOW.
FARTHER NORTH, MODEL GUIDANCE IS WEAKER TODAY IN SUPPORTING A SLIGHT RISK OF
HEAVY SNOW IN THE WASHINGTON CASCADES, BUT GIVEN THE LOCATION OF LARGE-SCALE
MID-LEVEL FEATURES, AND CONSIDERING THE INCONSISTENCIES IN THE FORECAST TOOLS,
THE SLIGHT RISK AREA FOR HEAVY SNOW REMAINS IN PLACE TODAY.
HAZARDOUS WEATHER IS ALSO POSSIBLE ALONG THE EASTERN SEABOARD. DETERMINISTIC
MODELS SHOW A STORM SYSTEM FORMING IN THE CENTRAL CONUS EARLY WEEK-2. SOUTHERLY
WINDS AHEAD OF THE SYSTEM ARE EXPECTED TO TAP GULF OF MEXICO MOISTURE AND
ADVECT IT NORTHWARD THROUGH MOST OF THE EASTERN CONUS. MEANWHILE, A MID-LEVEL
TROUGH WILL BE MOVING SLOWLY ACROSS THE REGION, SETTING THE STAGE FOR
POTENTIALLY HEAVY PRECIPITATION IN MUCH OF THE EAST, INCLUDING AREAS THAT
RECORDED EXCESSIVE RAINS AND FLOODING THIS PAST WEEKEND (DEC 16-18).
DETERMINISTIC MODELS VARY REGARDING THE AMOUNT AND LOCATION OF THE HEAVIEST
PRECIPITATION, BUT A BLEND OF THE GUIDANCE INDICATES SLIGHTLY LOWER CHANCES FOR
HEAVY PRECIPITATION RELATIVE TO YESTERDAY. SOME MODELS CONTINUE TO PLACE A
SECONDARY LOW PRESSURE SYSTEM CLOSE TO THE EAST COAST, WHICH WOULD ENHANCE
PRECIPITATION TOTALS NEARBY. COMPARED TO YESTERDAY, THE CONSENSUS OF THE
CURRENT DETERMINISTIC MODELS INDICATE THAT THE SECONDARY LOW PRESSURE SYSTEM -
IF IT FORMS - COULD AFFECT AREAS FARTHER NORTH THAN FORECAST YESTERDAY,
INTRODUCING ADDITIONAL UNCERTAINTY INTO THE FORECAST. PET OUTPUT ALSO SHOWS A
LACK OF CONSENSUS AMONG THE ARRAY OF SOLUTIONS IN THE GEFS, EUROPEAN, AND
CANADIAN MODELS BY KEEPING THE ENHANCED CHANCES FOR HEAVY PRECIPITATION
RELATIVELY LOW. NONE OF THE PET SHOW MORE THAN A 40 PERCENT CHANCE FOR
PRECIPITATION EXCEEDING THE 85TH PERCENTILE DURING ANY 3-DAY PERIOD. DESPITE
SLIGHTLY WEAKER SUPPORT IN THE GUIDANCE FOR HEAVY PRECIPITATION COMPARED TO
YESTERDAY, THE HEAVY PRECIPITATION HAZARD IS MAINTAINED FROM YESTERDAY. THE
INCONSISTENCIES AMONG THE TOOLS INCREASES THE ODDS THAT THE DRIER TREND
OBSERVED TODAY IS A TEMPORARY ABERRATION IN THE GUIDANCE AS OPPOSED TO A
DEFINITE TREND, SO THIS TREND TOWARD A DRIER SOLUTION WILL NEED TO CONTINUE FOR
THE NEXT FEW RUNS BEFORE THERE IS ENOUGH CONFIDENCE TO DECREASE OR REMOVE THE
POSTED HEAVY PRECIPITATION HAZARDS. THE ONLY CHANGE MADE WAS TO EXPAND THE
MODERATE RISK AREA INTO PORTIONS OF NEW ENGLAND BASED ON THE POTENTIAL
SECONDARY LOW PRESSURE DEVELOPMENT.
FLOODING IS ONGOING ALONG NUMEROUS RIVERS IN THE EAST DUE TO THE RECENT STORM
SYSTEM, AND WHILE RIVER LEVELS ARE EXPECTED TO SUBSIDE THIS WEEK, RENEWED
FLOODING IS POSSIBLE WITH THE PREDICTED STORM SYSTEM IF THE HEAVY PRECIPITATION
EVENT OCCURS. TEMPERATURES ARE EXPECTED TO BE CONSIDERABLY ABOVE NORMAL AS THE
STORM SYSTEM AFFECTS THE REGION, PRECLUDING THE POSTING OF ANY HEAVY SNOW
HAZARDS AT THIS TIME, THOUGH SOME ACCUMULATION IS POSSIBLE IN WESTERN AND
NORTHERN PARTS OF THE SLIGHT RISK REGION.
MODEL ENSEMBLES AND THE DERIVED PET TOOLS DO NOT SUPPORT ANY HEAVY SNOW, HEAVY
PRECIPITATION, NOR HIGH WIND RISKS ACROSS SOUTHERN ALASKA. THE WEEK-2 POSITION
OF THE MEAN SURFACE LOW PRESSURE IN THE GULF OF ALASKA IS WELL SOUTH OF THE
STATE, KEEPING UNUSUALLY WET AND WINDY WEATHER OFFSHORE.
FORECASTER: RICH TINKER
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