282
FXUS21 KWNC 181918
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 18 2024
SYNOPSIS: STRONGLY 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 OVER PARTS OF THE SOUTH-CENTRAL CONUS.
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, THU-FRI, DEC 26-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, THU-SUN, DEC 26-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS, THU-FRI, DEC 26-27.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS THU-SUN, DEC 26-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR PARTS OF NORTHERN INTERMOUNTAIN
AND NORTHERN ROCKIES, THU-SAT, DEC 26-28.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR MANY PARTS OF THE NORTHERN
INTERMOUNTAIN, GREAT BASIN, SOUTHWEST, AND THE ROCKIES, THU-SUN, DEC 26-29.
MODERATE RISK OF HIGH WINDS FOR THE PACIFIC NORTHWEST, NORTHERN HALF OF
CALIFORNIA, NORTHERN AND CENTRAL INTERMOUNTAIN, AND NORTHERN HIGH PLAINS,
THU-SAT, DEC 26-28.
SLIGHT RISK OF HIGH WINDS FOR THE WEST COAST AND INTERIOR WEST, THU-MON, DEC
26-30.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, LOWER AND
MIDDLE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, AND THE SOUTHEAST, THU-MON,
DEC 26-30.
SLIGHT RISK OF HIGH WINDS FOR SOUTHERN MAINLAND ALASKA AND THE SOUTHEAST,
THU-WED, DEC 26-JAN 1.
SLIGHT RISK OF HEAVY PRECIPITATION FOR SOUTHEASTERN MAINLAND ALASKA AND
PORTIONS OF THE SOUTHEAST, THU-WED, DEC 26-JAN 1.
POSSIBLE FLOODING FOR PARTS OF CENTRAL AND NORTHERN CALIFORNIA TO SOUTHWESTERN
OREGON.
DETAILED SUMMARY
FOR SATURDAY DECEMBER 21 - WEDNESDAY DECEMBER 25:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY DECEMBER 26 - WEDNESDAY JANUARY 01: BASED ON THE LATEST 500-HPA
HEIGHT FORECASTS FROM THE DYNAMICAL MODELS, THERE CONTINUES TO BE GOOD
AGREEMENT IN EVOLUTION, POSITION, AND STRENGTH OF THE ANOMALY PATTERN OVER
NORTH AMERICA DURING THE EARLY PORTION OF WEEK-2. HOWEVER, COMPARED TO PREVIOUS
GUIDANCE, THE MULTI-MODEL MEAN SOLUTIONS HAVE BECOME MORE DIVIDED TOWARDS THE
END OF NEXT WEEK, CONTRIBUTING TO ADDED UNCERTAINTY IN THE WEEK-2 OUTLOOK. BUT
AS IT STILL STANDS, THE STRONG MID-LEVEL MEAN TROUGHING FAVORED OVER THE
NORTHEASTERN PACIFIC EARLY IN THE PERIOD REMAINS CONDUCIVE FOR ENHANCED ONSHORE
FLOW AND POSSIBLE ATMOSPHERIC RIVER (AR) ACTIVITY WHICH COULD BRING MULTIPLE
WEATHER RELATED HAZARDS FOR MANY PARTS OF THE WEST.
FOLLOWING THE PASSAGE OF SHORTWAVE TROUGHING OVER THE INTERIOR WEST LATE IN
WEEK-1, BOTH THE GEFS AND ECWMF ENSEMBLES CONTINUE TO POINT TO ADDITIONAL
SHORTWAVE ENERGY WITH A RAPID DECREASE IN HEIGHTS AND A MARKED INCREASE IN
PRECIPITATION OVER THE PACIFIC NORTHWEST AND INTO PARTS OF CENTRAL AND NORTHERN
CALIFORNIA. GOOD RUN TO RUN CONTINUITY EXISTS IN THE INTEGRATED VAPOR TRANSPORT
(IVT) TOOLS, SIGNALING ADDITIONAL AR ACTIVITY WITH >60% (40%) CHANCES FOR IVT
VALUES EXCEEDING 250 (500) KG/M/S ON DAYS 8 AND 9 (DEC 26-27), WHICH REMAINS
INLINE WITH THE PREVIOUS HAZARDS OUTLOOK. GIVEN THIS TIMING, AND GOOD AGREEMENT
IN THE PROBABILISTIC EXTREMES TOOLS (PETS) DEPICTING 30-40% CHANCES FOR 3-DAY
PRECIPITATION AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 2 INCHES EARLY IN THE
PERIOD, THIS SUPPORTS THE CONTINUATION OF A MODERATE RISK FOR HEAVY
PRECIPITATION FOR DEC 26-27. BY DAYS 10 AND 11 (DEC 28-29), THERE ARE A GROWING
NUMBER OF DIFFERENCES IN THE MID-LEVEL HEIGHT FIELDS AMONG THE MODELS, WHERE
THE ECWMF AND CANADIAN FAVOR THE RETURN OF MORE RIDGING OVER THE WEST COAST.
DESPITE THIS, UNCALIBRATED PRECIPITATION TOTALS DON'T EVINCE MUCH OF A DRYING
TREND OVER THE WEST COAST, AND A SLIGHT RISK OF HEAVY PRECIPITATION REMAINS
POSTED THROUGH DEC 29. THIS SLIGHT RISK AREA IS SLIGHTLY EXPANDED FURTHER
SOUTHWARD INTO SOUTHERN CALIFORNIA WHERE THE GEFS PET SHOWS INCREASED CHANCES
FOR 3-DAY AMOUNTS EXCEEDING 1 INCH EARLY IN THE PERIOD. AN ACCOMPANYING
MODERATE AND SLIGHT RISK HAZARD FOR HIGH ELEVATION HEAVY SNOWFALL ALSO REMAINS
ISSUED FOR THE WEST COAST, VALID FOR THE SAME PERIODS AS THE HEAVY
PRECIPITATION RISKS. 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 26-28), WITH A BROADER SLIGHT
RISK OF HEAVY SNOW EXTENDING INTO THE FOUR CORNERS (DEC 26-29). RELATIVE TO
YESTERDAY, THIS SLIGHT RISK AREA IS EXPANDED FURTHER SOUTH INTO PARTS OF THE
SOUTHWEST AND SOUTHERN ROCKIES BASED ON INCREASED SIGNALS IN THE GEFS SNOW
WATER EQUIVALENT (SWE) PET, DUE TO INCREASED TROUGHING FAVORED IN THE REGION.
THE STRONGEST SIGNALS FOR HAZARDOUS WEATHER ACROSS THE WESTERN CONUS CONTINUE
TO RESIDE IN THE WIND PETS. THE ECWMF FEATURES 70-90% CHANCES FOR WIND SPEEDS
EXCEEDING THE 85TH AND 90TH PERCENTILES TIED TO THE POTENTIAL AR ACTIVITY. THE
GEFS PET CONTINUES TO BE SUBSTANTIALLY WEAKER BY COMPARISON, THOUGH THERE IS
SUFFICIENT SUPPORT BETWEEN THE PETS AND RAW ENSEMBLES FOR THE CONTINUATION OF A
MODERATE RISK OF HIGH WINDS COVERING MUCH OF THE NORTHWESTERN CONUS FOR DEC
26-28. TOWARDS THE MIDDLE OF THE PERIOD, BOTH PETS SHOW A SHARP REDUCTION IN
THE WIND SIGNALS IN PERCENTILE SPACE, BUT SUPPORT THE CONTINUATION OF A BROAD
SLIGHT RISK AREA FOR HIGH WINDS OVER THE WESTERN THIRD OF THE CONUS, VALID
THROUGH DEC 30. THE EXPECTED 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, AND SOUTHWESTERN OREGON, WHERE A POSSIBLE FLOODING
HAZARD IS POSTED. THIS HIGHLIGHTED REGION IS EXPECTED TO RECEIVE POTENTIALLY
HEAVY PRECIPITATION DURING WEEK-1 AND STREAMFLOWS AND SOIL MOISTURE ARE
CURRENTLY REGISTERING IN THE UPPER PERCENTILES BASED ON ABOVE-NORMAL ANTECEDENT
PRECIPITATION DURING THE PAST 30 DAYS.
TIED TO THE SHORTWAVE ENERGY PROPAGATING ACROSS THE INTERIOR WEST, SURFACE LOW
FORMATION CONTINUES TO BE FAVORED IN THE LEE OF THE ROCKIES, WITH GOOD GULF OF
MEXICO RETURN FLOW ESTABLISHED TO INCREASE THE POTENTIAL FOR LOCALLY HEAVY
PRECIPITATION OVER THE SOUTHERN CONUS. RELATIVE TO YESTERDAY, ENSEMBLES NOW
POINT TO A SECONDARY SURFACE LOW FORMING IN THE GREAT PLAINS ON AND AROUND DAY
10 (DEC 28). AS A RESULT, SIGNALS FOR HEAVY PRECIPITATION FOCUSED ACROSS THE
MISSISSIPPI VALLEY IN BOTH THE GEFS AND ECMWF PETS APPEAR MORE PERSISTENT,
LIKELY ASSOCIATED WITH MULTIPLE SURFACE LOWS POSSIBLY IMPACTING THE REGION
THROUGH THE MIDDLE OF WEEK-2. WHILE THE TIMING, LOCATION, AND EVENTUAL TRACK OF
ONE OR MORE SURFACE LOWS REMAINS UNCLEAR, A SLIGHT RISK OF HEAVY PRECIPITATION
IS POSTED AND IS BOTH EXTENDED THROUGH DAY 12, AND SLIGHTLY EXPANDED EASTWARD
INTO THE SOUTHEAST TO ACCOUNT FOR THIS SECONDARY RISK. 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.
MUCH OF THE AMPLIFIED MID-LEVEL TROUGHING FAVORED OVER THE NORTHEASTERN PACIFIC
IS EXPECTED TO BRING AN INCREASED RISK OF HEAVY PRECIPITATION AND HIGH WINDS TO
PORTIONS OF SOUTHERN ALASKA. BASED ON PETS AND ONE OR MORE SURFACE LOW
PASSAGES IN THE GULF OF ALASKA, A SLIGHT RISK OF HIGH WINDS IS POSTED FOR THE
SOUTHCENTRAL MAINLAND THROUGH THE SOUTHEAST, ALONG WITH A SMALLER SLIGHT RISK
OF HEAVY PRECIPITATION ISSUED WITH ITS COVERAGE EXTENDING FROM THE PRINCE
WILLIAM SOUND TO THE WEST OF JUNEAU.
FORECASTER: NICK NOVELLA
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