846
FXUS21 KWNC 201903
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 20 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.
HAZARDS
MODERATE RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, SAT, DEC 28.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, SAT-SUN, DEC 28-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS, SAT, DEC 28.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS SAT-SUN, DEC 28-29.
MODERATE RISK OF HIGH WINDS FOR THE PACIFIC NORTHWEST, NORTHERN HALF OF
CALIFORNIA, NORTHERN AND CENTRAL INTERMOUNTAIN, AND NORTHERN HIGH PLAINS, SAT,
DEC 28.
SLIGHT RISK OF HIGH WINDS FOR THE WEST COAST AND INTERIOR WEST, SAT-SUN, DEC
28-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR PARTS OF NORTHERN INTERMOUNTAIN
AND NORTHERN ROCKIES, SAT, DEC 28.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR MANY PARTS OF THE NORTHERN
INTERMOUNTAIN, GREAT BASIN, SOUTHWEST, AND THE ROCKIES, SAT-MON, DEC 28-30.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, LOWER AND
MIDDLE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, AND THE SOUTHEAST, SAT-TUE,
DEC 28-31.
POSSIBLE FLOODING FOR PARTS OF CENTRAL AND NORTHERN CALIFORNIA.
DETAILED SUMMARY
FOR MONDAY DECEMBER 23 - FRIDAY DECEMBER 27:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY DECEMBER 28 - FRIDAY JANUARY 03: SINCE YESTERDAY, DYNAMICAL MODELS
CONTINUE TO FAVOR THE PERSISTENCE OF 500-HPA TROUGHING OVER THE GULF OF ALASKA
WITH ANOMALOUS RIDGING TO THE SOUTH OVER THE EASTERN PACIFIC LATE NEXT WEEK.
THESE MID-LEVEL FEATURES REMAIN CONSISTENT WITH A CONTINUED RISK OF ATMOSPHERIC
RIVER (AR) ACTIVITY EXTENDING FROM WEEK-1 INTO EARLY WEEK-2, BEFORE MUCH OF THE
ENHANCED ONSHORE FLOW PATTERN BREAKS DOWN HEADING INTO THE FOLLOWING WEEKEND.
BOTH RAW AND CALIBRATED MODEL GUIDANCE HAVE TRACKED THE EVOLUTION AND STRENGTH
OF THIS AR ACTIVITY QUITE WELL DURING THE PAST SEVERAL DAYS, AND THERE ARE
LITTLE CHANGES IN THE UPDATED HAZARD OUTLOOK.
BASED ON THE LATEST INTEGRATED VAPOR TRANSPORT (IVT) TOOLS, THERE IS AN
INCREASED AR RISK ON DAY 8 (DEC 28), FOLLOWED BY A WEAKENING OF THIS POTENTIAL
ON DAY 9 (DEC 29) WHICH IS SUPPORTED IN THE PROBABILISTIC EXTREMES TOOLS (PETS)
WITH LESSER CHANCES (20-30%) FOR 3-DAY PRECIPITATION AMOUNTS EXCEEDING THE 85TH
PERCENTILE EARLY IN WEEK-2, COMPARED TO YESTERDAY. ALTHOUGH THE ECWMF FAVORS A
SLIGHTLY STRONGER AR SIGNAL LINGERING INTO DAY 9 COMPARED TO THE GEFS, THE
DEAMPLIFICATION OF MID-LEVEL TROUGHING FAVORED IN BOTH ENSEMBLES IS REFLECTIVE
OF THE ONGOING AR RISK WINDING DOWN. THEREFORE, THE MODERATE RISK OF HEAVY
PRECIPITATION ISSUED OVER MUCH OF THE WEST COAST REMAINS VALID THROUGH DEC 28,
WITH THE BROADER SLIGHT RISK AREA FOR HEAVY PRECIPITATION VALID THROUGH DEC 29.
AS BEFORE, THE TIMING OF THE ENHANCED ONSHORE FLOW SUPPORTS AN ACCOMPANYING
MODERATE RISK AND SLIGHT RISK FOR HIGH ELEVATION HEAVY SNOW OVER THE WEST
COAST, ALSO VALID THROUGH DEC 28 AND DEC 29, RESPECTIVELY. LASTLY,
STRENGTHENING SURFACE PRESSURE GRADIENTS ASSOCIATED WITH AR ACTIVITY IS
EXPECTED TO BRING AN ELEVATED RISK OF HIGH WINDS THROUGHOUT THE WEST. WHILE
PETS SUGGEST MORE OF A PERSISTENT WIND THREAT COMPARED TO PREVIOUS GUIDANCE
EARLY IN WEEK-2, RAW ENSEMBLES SHOW SURFACE PRESSURE GRADIENTS WEAKENING BY DAY
9, AND A MODERATE RISK OF HIGH WINDS FOR DEC 28, ALONG WITH BROADER SLIGHT RISK
AREA FOR DEC 28-29 REMAINS ISSUED.
THE CONTINUED AR RISK NEXT WEEK IS LIKELY TO DISRUPT HOLIDAY TRAVEL FOR THE
WEST COAST 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.
AS MUCH OF THE ENHANCED PACIFIC MOISTURE WORKS ITS WAY INTO THE INTERIOR WEST,
A MODERATE RISK OF HIGH ELEVATION HEAVY SNOW REMAINS POSTED OVER THE NORTHERN
INTERMOUNTAIN AND NORTHERN ROCKIES (DEC 28), WITH A BROADER SLIGHT RISK OF HIGH
ELEVATION HEAVY SNOW EXTENDING INTO THE FOUR CORNERS. BASED ON THE RAW SNOW
TOOLS AND THE SNOW WATER EQUIVALENT (SWE) PET, THE SLIGHT RISK AREA NOW
EXCLUDES THE SOUTHWEST AND THE SOUTHERN ROCKIES, AND IS EXTENDED THROUGH DEC 30
WHERE ENSEMBLES CONTINUE TO FAVOR SHORTWAVE TROUGHING OVER THE INTERIOR WEST.
THIS SHORTWAVE ENERGY IS EXPECTED TO LEAD TO SURFACE LOW FORMATION IN THE LEE
OF THE ROCKIES, HOWEVER THE ECMWF AND GEFS REMAIN AT ODDS IN REGARDS TO THE
TIMING AND STRENGTH OF SURFACE LOW DEVELOPMENT AND ASSOCIATED TRACKS THROUGH
THE MIDDLE OF THE PERIOD. THE GEFS HAS BEEN BULLISH ON DEVELOPING A SURFACE LOW
BY THE OUTSET OF WEEK-2, WHEREAS THE ECWMF CONTINUES TO FAVOR A STRONGER RIDGE
AXIS THROUGH THE PLAINS, LIMITING ANY SURFACE LOW DEVELOPMENT UNTIL CLOSER TO
DAY 10 (DEC 30). THESE DIFFERENCES ARE REFLECTED IN BOTH THE RAW AND CALIBRATED
PRECIPITATION TOOLS BETWEEN THE GEFS AND ECMWF WITH THE LATTER BEING LESS
SUPPORTIVE OF A HEAVY PRECIPITATION RISK EARLY IN WEEK-2. DESPITE THIS, THE
PREVALENCE OF SHORTWAVE TROUGHING TO INDUCE LEE CYCLOGENESIS AND GOOD RETURN
FLOW FROM THE GULF OF MEXICO WARRANTS THE CONTINUATION OF A SLIGHT RISK OF
HEAVY PRECIPITATION 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.
A NOTABLE CHANGE SINCE YESTERDAY CONCERNS AN INCREASE IN SIGNALS FOR
ACCUMULATING LOWER ELEVATION SNOWFALL OVER THE NORTHERN PLAINS IN THE GEFS SWE
PET AND IN THE UNCALIBRATED ECMWF. A SLIGHT RISK OF HEAVY SNOW WAS CONSIDERED
FOR INCLUSION, HOWEVER DUE TO THE ADDED UNCERTAINTY REGARDING SURFACE LOW
DEVELOPMENT, AS WELL AS STRONG CHANCES FOR ABOVE-NORMAL TEMPERATURES FAVORED
OVER THE MIDWEST WHERE DAYTIME TEMPERATURES MAY STRUGGLE TO FALL BELOW
FREEZING, NO CORRESPONDING SNOW HAZARD IS ISSUED BUT WILL CONTINUE TO MONITOR
THIS POTENTIAL MOVING FORWARD.
MUCH OF THE TROUGHING FAVORED OVER THE NORTHEASTERN PACIFIC IS FAVORED TO
DEAMPLIFY AND RETROGRADE TO THE WEST, WITH ANOMALOUS RIDGING DEVELOPING OVER
NORTHWESTERN CANADA DURING WEEK-2. EVEN WITH THIS TRANSITION, THE PATTERN IS
STILL FAVORABLE FOR CONTINUED ONSHORE FLOW, RESULTING IN ABOVE-NORMAL
PRECIPITATION AND PERIODS OF HIGH WINDS PREDOMINANTLY FAVORED OVER SOUTHERN
ALASKA. THE 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 NO CORRESPONDING
HAZARDS ARE POSTED OVER ALASKA IN THE UPDATED OUTLOOK.
FORECASTER: NICK NOVELLA
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