171
FXUS21 KWNC 182056
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 18 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 WEST
COAST AND IN THE HIGHER ELEVATIONS OF CALIFORNIA 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 IN THE MIDDLE OF THE
CONTIGUOUS U.S. (CONUS) IS EXPECTED TO PULL GULF OF MEXICO MOISTURE NORTHWARD
AHEAD OF THE SYSTEM, 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 AND
PORTIONS OF THE CENTRAL VALLEYS AND HIGHER ELEVATIONS, TUE-FRI, DEC 26-29.
MODERATE RISK FOR HEAVY PRECIPITATION IN THE MID-ATLANTIC REGION AND LOWER
NORTHEAST, TUE-FRI, DEC 26-29.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL AND NORTHERN SIERRA NEVADA,
SOUTHERN CASCADES, AND COASTAL RANGES OF NORTHERN CALIFORNIA, TUE-FRI, DEC
26-29.
MODERATE RISK FOR HIGH WINDS FOR PORTIONS OF WESTERN CALIFORNIA AND WESTERN
OREGON, TUE-FRI, DEC 26-29.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR MUCH OF THE WEST COAST, TUE-MON, DEC
26-JAN 1.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR MUCH OF THE EASTERN CONUS, TUE-FRI, DEC
26-29.
SLIGHT RISK FOR HEAVY SNOW FOR THE SIERRA NEVADA,, AND COASTAL RANGES OF
NORTHERN CALIFORNIA, TUE-MON, DEC 26-JAN 1.
SLIGHT RISK FOR HEAVY SNOW IN THE WASHINGTON CASCADES AND SOME ADJACENT AREAS,
TUE-THU, DEC 26-28.
SLIGHT RISK FOR HIGH WINDS FOR MUCH OF THE WEST COAST, INCLUDING THE ,
CALIFORNIA VALLEYS, SIERRA NEVADA, AND SOUTHERN CASCADES, TUE-FRI, DEC 26-29.
DETAILED SUMMARY
FOR THURSDAY DECEMBER 21 - MONDAY DECEMBER 25:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY DECEMBER 26 - MONDAY JANUARY 01: ENSEMBLE MEANS ALL SHOW A SIMILAR
PATTERN FOR WEEK-2. A MID-LEVEL SOUTHERN-STREAM TROUGH IS EXPECTED TO SLOWLY
PROGRESS FROM THE CENTRAL CONUS TO THE SOUTHEAST AND WESTERN NORTH ATLANTIC.
MEANWHILE, A DEEP MID-LEVEL TROUGH IN THE NORTH PACIFIC WILL WEAKEN AND MOVING
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
AND RETROGRADE SLIGHTLY DURING WEEK-2, ALLOWING 500-HPA HEIGHTS TO DROP IN THE
NORTHEASTERN CONUS BUT MAINTAINING POSITIVE 500-HPA HEIGHT ANOMALIES ACROSS
CANADA.
AS WEEK-2 STARTS, STORMY WEATHER IS FORECAST TO COVER MUCH OF THE WEST COAST
DOWNSTREAM FROM THE DEEP MID-LEVEL TROUGH IN THE NORTH PACIFIC, WITH STRONG
ATMOSPHERIC RIVER ACTIVITY POSSIBLE. THIS SET-UP ENHANCES THE CHANCES FOR HEAVY
PRECIPITATION, HIGH WINDS, AND HEAVY SNOW IN A LARGE PART OF THE WESTERN CONUS.
THE ENSEMBLES AND THEIR DERIVED TOOLS ARE ONLY IN FAIR AGREEMENT ON THE
LOCATION, STRENGTH, AND TIMING OF THE FEATURES AFFECTING THE REGION. THE
CANADIAN ENSEMBLES ARE FARTHEST NORTH, FOCUSING THE STORMIEST WEATHER ON
CENTRAL AND NORTHERN PORTIONS OF THE WEST COAST. THE EUROPEAN GUIDANCE IS
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 MODEL GUIDANCE.A BLEND OF THE GEFS AND EUROPEAN ENSEMBLE MEAN
IS FAVORED AT THIS TIME, WHICH FOCUSES THE HIGHEST RISKS FOR HAZARDOUS
CONDITIONS OVER MUCH OF 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 THE PACIFIC NORTHWEST FOR AT LEAST PART OF WEEK-2.
FOR THESE REASONS, A MODERATE RISK FOR HEAVY PRECIPITATION HAS BEEN POSTED FOR
MUCH OF WESTERN CALIFORNIA, THE NORTHERN CALIFORNIA VALLEYS, THE SIERRA NEVADA,
AND THE SOUTHERN CASCADES. DUE TO UNCERTAINTIES IN TIMING AND LOCATION, A
SLIGHT RISK FOR HEAVY PRECIPITATION COVERS THE ENTIRE WEST COAST FROM CANADA TO
MEXICO. THE MODERATE RISK EXTENDS THROUGH THE FIRST HALF OF WEEK-2, WHEN
MID-LEVEL FEATURES AND ATMOSPHERIC RIVER ACTIVITY SEEM MOST FAVORABLE FOR HEAVY
PRECIPITATION. FLOODING IS POSSIBLE WHERE THE PRECIPITATION IS THE HEAVIEST,
THOUGH NO SPECIFIC REGION CAN BE IDENTIFIED WITH THE ENHANCED RISK DUE TO
UNCERTAINTIES IN THE EVOLUTION OF THIS STORM SYSTEM. DETERMINISTIC MODELS SHOW
ONE OR TWO ADDITIONAL SYSTEMS POTENTIALLY MOVING THROUGH THE WEST, AND ALTHOUGH
THE SET-UP ISN’T AS FAVORABLE FOR EXCESSIVE AMOUNTS LATER IN THE PERIOD, THE
SLIGHT RISK RUNNING THE LENGTH OF THE WEST COAST EXTENDS THROUGH THE ENTIRE
8-14 DAY PERIOD. IN THE HIGHER ELEVATIONS, MUCH OF THE PRECIPITATION COULD FALL
AS SNOW, SO A MODERATE RISK OF HEAVY SNOW IS POSTED FOR THE CENTRAL AND
NORTHERN SIERRA NEVADA, SOUTHERN CASCADES, AND COASTAL RANGES OF NORTHERN
CALIFORNIA FOR THE FIRST HALF OF WEEK-2. A SLIGHT RISK OF HEAVY SNOW COVERS THE
SAME AREAS WITH THE ADDITION OF THE SOUTHERN SIERRA NEVADA, AND EXTENDS THROUGH
THE ENTIRE WEEK-2 PERIOD. THE PROBABILISTIC EXTREMES TOOLS (PETS) BASED ON THE
CANADIAN, EUROPEAN, AND GEFS ENSEMBLES ALL HIGHLIGHT INCREASED ODDS FOR HIGH
WINDS DURING THE FIRST HALF OF WEEK-2, WHEN THE SET-UP IS MOST FAVORABLE FOR
STORMY WEATHER. ALL THREE TOOLS SHOW THE RISK OF HIGH WINDS TO BE DISPLACED
SOMEWHAT NORTHWARD RELATIVE TO THE AREAS MOST AT RISK FOR HEAVY RAIN AND SHOW,
WITH A MODERATE RISK IDENTIFIED ALONG THE COAST FROM NORTHWESTERN OREGON TO
NEAR SALINAS, CA SOUTH OF SAN FRANCISCO. A SLIGHT RISK EXTENDS FARTHER NORTH
(TO THE CANADIAN BORDER) AND SOUTH (TO NEAR LOS ANGELES), AND ALSO INCLUDES THE
CALIFORNIA VALLEYS AND HIGHER ELEVATIONS, WHICH CAN EXPERIENCE HIGH WINDS WHEN
STRONG MID-LEVEL FLOW MIXES DOWN TO GROUND LEVEL DURING STORM SYSTEMS.
HAZARDOUS WEATHER IS ALSO POSSIBLE ALONG THE EASTERN SEABOARD, INCLUDING MUCH
OF THE SOUTHEAST AND APPALACHIANS. DETERMINISTIC MODELS SHOW A STORM SYSTEM
FORMING IN THE CENTRAL CONUS EARLY WEEK-2. SOUTHERLY WINDS AHEAD OF THE SYSTEM
ARE EXPECTED TO TAP ABUNDANT 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 WHILE AMPLIFYING SOMEWHAT, 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 ARE IN FAIR AGREEMENT REGARDING THE EVOLUTION OF THESE
FEATURES, BUT VARY IN THE AMOUNT AND LOCATION OF THE HEAVIEST PRECIPITATION. IN
ADDITION, SOME MODELS SHOW A SECONDARY LOW PRESSURE SYSTEM FORMING ALONG THE
SOUTHEAST COAST AND MOVING NORTHEASTWARD, ENHANCING PRECIPITATION TOTALS,
ESPECIALLY IN THE NORTHEAST. THE UNCERTAINTIES ARE DEMONSTRATED BY THE PET
OUTPUT, NONE OF WHICH SHOW MORE THAN A 40 PERCENT CHANCE FOR PRECIPITATION
EXCEEDING THE 85TH PERCENTILE. BUT A BLEND OF THE ENSEMBLE MEANS PLACES THE
BEST ODDS FOR HEAVY PRECIPITATION OVER THE MID-ATLANTIC REGION AND LOWER
NORTHEAST, WHERE A MODERATE RISK FOR HEAVY PRECIPITATION IS POSTED FOR DAYS
8-11 (DEC 26-29). DUE TO THE UNCERTAINTIES IN THE TIMING, PLACEMENT, AND
MAGNITUDE OF THE HEAVIEST PRECIPITATION, A SLIGHT RISK OF HEAVY PRECIPITATION
IS POSTED ACROSS A LARGE PART OF THE EASTERN CONUS. 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 DURING THE PERIOD. 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 IT IS POSSIBLE IN
WESTERN AND NORTHERN PARTS OF THE SLIGHT RISK REGION.
ENSEMBLE SOLUTIONS FROM THE GEFS, ECMWF, AND CMCE ARE ALL LESS ROBUST WITH A
STORM SYSTEM NEAR THE SOUTHERN PART OF ALASKA THAN THEY WERE LAST WEEK, AND
PLACE THE MEAN POSITIONS OF SURFACE LOW PRESSURE FARTHER WEST, OUTSIDE THE GULF
OF ALASKA. AS A RESULT, ALL HEAVY PRECIPITATION AND HIGH WIND HAZARDS HAVE BEEN
REMOVED ACROSS SOUTHERN ALASKA.
FORECASTER: RICH TINKER
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