646
FXUS21 KWNC 191907
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 19 2024
SYNOPSIS: ANOMALOUS MID-LEVEL LOW PRESSURE SHIFTING EASTWARD FROM THE
NORTHEASTERN PACIFIC FAVORS AN ELEVATED RISK OF ATMOSPHERIC RIVER (AR) ACTIVITY
TO BRING HEAVY PRECIPITATION, PERIODS OF HIGH WINDS, AND HIGH ELEVATION
SNOWFALL FOR MANY PARTS OF THE WEST COAST AND THE INTERIOR WEST. GIVEN THE
TIMING OF THE AR POTENTIAL, ANY REALIZATION OF THE ASSOCIATED HAZARD RISKS IS
LIKELY TO DISRUPT HOLIDAY TRAVEL NEXT WEEK AND MAY LEAD TO LOCALIZED FLOODING
OVER PARTS OF THE WEST COAST. SURFACE LOW FORMATION IN THE LEE OF THE ROCKIES
MAY LEAD TO HEAVY PRECIPITATION AMOUNTS MAINLY OVER THE MISSISSIPPI, TENNESSEE
AND OHIO VALLEYS. PERSISTENT SURFACE LOW PRESSURE IN THE GULF OF ALASKA IS
EXPECTED TO PROMOTE AN INCREASED RISK OF HEAVY PRECIPITATION AND HIGH WINDS FOR
SOUTHERN MAINLAND ALASKA AND THE SOUTHEAST.
HAZARDS
MODERATE RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, FRI-SAT, DEC 27-28.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, FRI-SUN, DEC 27-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS, FRI-SAT, DEC 27-28.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS FRI-SUN, DEC 27-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR PARTS OF NORTHERN INTERMOUNTAIN
AND NORTHERN ROCKIES, FRI-SAT, DEC 27-28.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR MANY PARTS OF THE NORTHERN
INTERMOUNTAIN, GREAT BASIN, SOUTHWEST, AND THE ROCKIES, FRI-SUN, DEC 27-29.
MODERATE RISK OF HIGH WINDS FOR THE PACIFIC NORTHWEST, NORTHERN HALF OF
CALIFORNIA, NORTHERN AND CENTRAL INTERMOUNTAIN, AND NORTHERN HIGH PLAINS,
FRI-SAT, DEC 27-28.
SLIGHT RISK OF HIGH WINDS FOR THE WEST COAST AND INTERIOR WEST, FRI-SUN, DEC
27-29.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, LOWER AND
MIDDLE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, AND THE SOUTHEAST, FRI-MON,
DEC 27-30.
SLIGHT RISK OF HIGH WINDS FOR SOUTHERN MAINLAND ALASKA AND THE SOUTHEAST,
FRI-MON, DEC 27-30.
POSSIBLE FLOODING FOR PARTS OF CENTRAL AND NORTHERN CALIFORNIA.
DETAILED SUMMARY
FOR SUNDAY DECEMBER 22 - THURSDAY DECEMBER 26:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY DECEMBER 27 - THURSDAY JANUARY 02: BY THE OUTSET OF WEEK-2 (DEC 27),
THERE IS GOOD AGREEMENT IN THE DYNAMICAL MODELS FAVORING AMPLIFIED 500-HPA
TROUGHING OVER THE GULF OF ALASKA, WITH ANOMALOUS RIDGING TO THE SOUTH OVER THE
EASTERN PACIFIC. THE PATTERN REMAINS CONDUCIVE FOR ENHANCED ONSHORE FLOW AND AN
INCREASED RISK FOR ATMOSPHERIC RIVER (AR) ACTIVITY, WHICH LOOKS TO BRING
MULTIPLE WEATHER RELATED HAZARDS FOR MANY PARTS OF THE WEST CONTINUING INTO
LATE NEXT WEEK. CONSISTENT WITH PREVIOUS ENSEMBLE GUIDANCE, BOTH THE GEFS AND
ECMWF FEATURE A DEAMPLICATION OF THESE MID-LEVEL FEATURES OVER THE EASTERN
PACIFIC DURING THE FIRST FEW DAYS OF THE PERIOD. HOWEVER, THERE ARE INDICATIONS
IN THE INTEGRATED VAPOR TRANSPORT (IVT) TOOLS THAT FEATURE A SLIGHTLY MORE
PERSISTENT AR SIGNATURE RELATIVE TO YESTERDAY, WITH THE ECMWF SHOWING THE
STRONGER IVT SIGNALS ON DAY 9 (DEC 28) THAN THE GEFS. THE LINGERING THREAT OF
HEAVY PRECIPITATION IS ALSO SUPPORTED IN THE UNCALIBRATED PRECIPITATION TOOLS
OVER THE WEST COAST, WHERE THE ECWMF MAINTAINS ELEVATED (40%) CHANCES FOR 3-DAY
AMOUNTS EXCEEDING 2 INCHES, WITH HIGHER CHANCES (60-70%) FOCUSED OVER NORTHERN
CALIFORNIA EARLY IN THE PERIOD. BOTH THE GEFS AND ECMWF PROBABILISTIC EXTREMES
TOOLS (PETS) CONTINUE TO DEPICT 30-40% CHANCES FOR 3-DAY PRECIPITATION AMOUNTS
EXCEEDING THE 85TH PERCENTILE, WHICH SUPPORT THE CONTINUATION OF A MODERATE
RISK OF HEAVY PRECIPITATION REMAINS, BUT THIS RISK IS NOW EXTENDED THROUGH DAY
9 (DEC 28) BASED ON THESE TRENDS IN THE RAW AND IVT TOOLS. A SLIGHT RISK OF
HEAVY PRECIPITATION COVERING NEARLY THE ENTIRE WEST COAST REMAINS VALID THROUGH
DAY 10 (DEC 29), BEFORE THE HEIGHT PATTERN DEAMPLIFIES LATER IN WEEK-2.
DUE TO THE 1-DAY EXTENSION OF THE MODERATE RISK OF HEAVY PRECIPITATION RELATIVE
TO THE PREVIOUS OUTLOOK, THE ACCOMPANYING MODERATE RISK FOR HIGH ELEVATION
HEAVY SNOW IS LIKEWISE EXTENDED THROUGH DEC 28, WITH THE BROADER SLIGHT RISK
AREA ALSO VALID THROUGH DEC 29. AS MUCH OF THE ENHANCED PACIFIC MOISTURE WORKS
ITS WAY INTO THE INTERIOR WEST, A MODERATE RISK OF HEAVY SNOW REMAINS POSTED
OVER THE NORTHERN INTERMOUNTAIN AND NORTHERN ROCKIES (DEC 27-28), WITH A
BROADER SLIGHT RISK OF HEAVY SNOW EXTENDING INTO THE FOUR CORNERS (DEC 27-29)
BASED ON THE RAW SNOW TOOLS AND THE SNOW WATER EQUIVALENT (SWE) PET. THE
CONTINUED AR ACTIVITY ALSO SUPPORTS A MODERATE RISK OF HIGH WINDS (DEC 27-28),
THOUGH ITS COVERAGE HAS BEEN REDUCED IN THE UPDATED OUTLOOK OVER THE
NORTHWESTERN CONUS GIVEN THE WEAKER SIGNALS IN BOTH THE GEFS AND ECMWF PETS. A
BROADER SLIGHT RISK OF HIGH WINDS COVERING THE WESTERN THIRD OF THE CONUS
REMAINS POSTED, BUT IS NOW VALID THROUGH DEC 29 WITH THE MID-LEVEL PATTERN
BECOMING MORE ZONAL.
THE ANTICIPATED TIMING OF THE AR RISK NEXT WEEK IS ALSO LIKELY TO DISRUPT
HOLIDAY TRAVEL AND MAY LEAD TO LOCALIZED FLOODING OVER PARTS OF CENTRAL AND
NORTHERN CALIFORNIA, WHERE A POSSIBLE FLOODING HAZARD IS POSTED. SOIL MOISTURE
CONTENT IN THE HIGHLIGHTED REGION IS CURRENTLY REGISTERING IN THE UPPER
PERCENTILES BASED ON WELL ABOVE-NORMAL PRECIPITATION OBSERVED DURING THE PAST
30 DAYS. GIVEN THE INCREASED POTENTIAL FOR HEAVY PRECIPITATION DURING WEEK-1,
PORTIONS OF SOUTHWESTERN OREGON MAY ALSO BE SUSCEPTIBLE TO LOCALIZED FLOODING
DURING WEEK-2.
WHILE THERE IS FAIR AGREEMENT AMONG THE MODELS IN REGARDS TO ENHANCED ONSHORE
FLOW RELAXING OVER THE WEST COAST BY DAY 9 (DEC 28), THE ECMWF AND GEFS
ENSEMBLES DIFFER IN REGARDS TO EVOLUTION OF THE HEIGHT PATTERN, AND THE
STRENGTH OF SHORTWAVE ENERGY PROPAGATING DOWNSTREAM. THE ECWMF FAVORS THE
DEVELOPMENT OF RISING HEIGHTS THROUGHOUT MUCH OF THE INTERIOR WEST, WHEREAS THE
GEFS HAS SHOWN GOOD RUN-TO-RUN CONSISTENCY IN DEPICTING DEEPER SHORTWAVE
TROUGHING WITH A STRONGER SURFACE LOW FORMING IN THE LEE OF THE ROCKIES AND
HIGHER PRECIPITATION AMOUNTS OVER CENTRAL CONUS EARLY IN WEEK-2. DESPITE THESE
DIFFERENCES, GOOD RETURN FLOW FROM THE GULF OF MEXICO REMAINS FAVORED, AND BOTH
THE GEFS AND ECWMF PETS CONTINUE TO SHOW AT LEAST 20% CHANCES FOR 3-DAY AMOUNTS
EXCEEDING THE 85TH PERCENTILE TO SUPPORT THE SLIGHT RISK OF HEAVY PRECIPITATION
MAINLY CENTERED OVER THE MISSISSIPPI VALLEY. DUE TO WELL ABOVE-NORMAL
PRECIPITATION RECEIVED NEAR THE MISSISSIPPI-OHIO RIVER CONFLUENCE IN RECENT
WEEKS, ANY HEAVY PRECIPITATION REALIZED ALONG THE NORTHERN PERIPHERY OF THE
SLIGHT RISK AREA MAY LEAD TO FLASH FLOODING DURING WEEK-2.
ALTHOUGH MUCH OF THE TROUGHING FAVORED OVER THE NORTHEASTERN PACIFIC IS FAVORED
TO DEAMPLIFY DURING WEEK-2, THE DEVELOPMENT OF ANOMALOUS RIDGING OVER
NORTHWESTERN CANADA IS EXPECTED TO SUSTAIN ONSHORE FLOW, RESULTING IN
ABOVE-NORMAL PRECIPITATION AND PERIODS OF HIGH WINDS PREDOMINANTLY FAVORED OVER
SOUTHERN ALASKA. CONTINUED UNSETTLED WEATHER IS REFLECTED IN THE PETS, THOUGH
THERE IS MORE UNCERTAINTY ON WHETHER PRECIPITATION AMOUNTS AND WIND SPEEDS WILL
EXCEED HAZARD THRESHOLDS DURING THE PATTERN TRANSITION, AND THE SLIGHT RISKS
FOR HEAVY PRECIPITATION AND HIGH WINDS ARE REMOVED IN THE UPDATED OUTLOOK.
FORECASTER: NICK NOVELLA
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