949
FXUS21 KWNC 152054
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 15 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 DURING THE MIDDLE AND LATTER
PARTS OF WEEK-2. MUCH OF THIS IS EXPECTED TO FALL AS HEAVY SNOW IN THE SIERRA
NEVADA AND SOUTHERN CASCADES. A WEAKENING LOW PRESSURE SYSTEM THAT WAS EJECTED
FROM THE MAIN TROUGH AXIS IS FORECAST TO BRING UNSETTLED WEATHER TO THE
SOUTHWEST AS WEEK-2 STARTS, THEN PROGRESS SLOWLY EASTWARD ACROSS THE SOUTHERN
CONTIGUOUS U.S. (CONUS), BRINGING A RISK FOR HEAVY PRECIPITATION FROM THE
SOUTHERN PLAINS TO THE SOUTH ATLANTIC COAST, WITH INCREASED CHANCES FOR SEVERE
WEATHER IN AND AROUND THE LOWER MISSISSIPPI VALLEY. THIS SYSTEM COULD ALSO
TRIGGER HEAVY SNOW EARLY IN THE PERIOD ACROSS THE CENTRAL ROCKIES AND ADJACENT
HIGH PLAINS.
HAZARDS
MODERATE RISK FOR HEAVY PRECIPITATION FOR MUCH OF WESTERN CALIFORNIA AND
PORTIONS OF THE CENTRAL VALLEYS AND HIGHER ELEVATIONS, MON-FRI, DEC 25-29.
MODERATE RISK FOR HEAVY PRECIPITATION FOR THE LOWER MISSISSIPPI VALLEY AND
ADJACENT AREAS, SAT-MON, DEC 23-25.
MODERATE RISK FOR HEAVY SNOW FOR THE CENTRAL AND NORTHERN SIERRA NEVADA,
SOUTHERN CASCADES, AND NORTHERN COASTAL RANGES, MON-FRI, DEC 25-29.
MODERATE RISK FOR HEAVY SNOW FOR THE HIGHER ELEVATIONS OF COLORADO AND NORTHERN
NEW MEXICO, SAT-SUN, DEC 23-24.
MODERATE RISK FOR HIGH WINDS FOR PORTIONS OF WESTERN CALIFORNIA AND OREGON,
TUE-FRI, DEC 26-29.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PORTIONS OF WESTERN OREGON AND MUCH OF
CALIFORNIA, MON-FRI DEC 25-29.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PORTIONS OF THE SOUTHERN PLAINS, MIDDLE
AND LOWER MISSISSIPPI AND LOWER OHIO VALLEY, AND THE SOUTHEAST, SAT-TUE, DEC
23-26.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR PORTIONS OF ARIZONA, SAT, DEC 23.
SLIGHT RISK FOR HEAVY PRECIPITATION FOR COASTAL PORTIONS OF ALASKA FROM PRINCE
WILLIAMS SOUND SOUTHWARD TO NEAR JUNEAU , SAT-SUN, DEC. 23-24.
SLIGHT RISK FOR HEAVY SNOW FOR THE SIERRA NEVADA, SOUTHERN CASCADES, AND
COASTAL RANGES OF NORTHERN CALIFORNIA, MON-FRI DEC 25-29.
SLIGHT RISK FOR HEAVY SNOW FOR THE CENTRAL ROCKIES AND WASATCH MOUNTAINS,
PORTIONS OF WESTERN AND SOUTHERN WYOMING, AND THE CENTRAL HIGH PLAINS, SAT-SUN,
DEC 23-24.
SLIGHT RISK FOR HIGH WINDS FOR MUCH OF CALIFORNIA AND WESTERN PARTS OF OREGON
AND WASHINGTON, MON-FRI, DEC 25-29.
SLIGHT RISK FOR HIGH WINDS FOR COASTAL PORTIONS OF ALASKA FROM NEAR THE COOK
INLET SOUTH AND EAST THROUGH THE WESTERN HALF OF SOUTHEAST ALASKA, SAT-SUN, DEC
23-24.
DETAILED SUMMARY
FOR MONDAY DECEMBER 18 - FRIDAY DECEMBER 22:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY DECEMBER 23 - FRIDAY DECEMBER 29: TODAY, MODELS ARE SIMILAR TO
YESTERDAY IN THE PLACEMENT AND PROGRESSION OF BROAD-SCALE MID-LEVEL FEATURES,
BUT ARE SIGNIFICANTLY MORE AMPLIFIED. THUS, THE NORTH PACIFIC MID-LEVEL TROUGH,
THE 500-HPA RIDGE COVERING CANADA, AND THE SHORTWAVE TRAVERSING THE SOUTHERN
TIER OF THE CONUS ARE ALL STRONGER THAN YESTERDAY, AND THIS MERIDIONAL FLOW
ENHANCES THE LIKELIHOOD FOR A NUMBER OF HAZARDS ACROSS THE NATION
THE GEFS, ECMWF ENSEMBLE MEAN, AND CANADIAN ENSEMBLE MEAN ALL START WEEK-2 WITH
A MID-LEVEL TROUGH IN THE SOUTHWEST THAT TRACKS EASTWARD ACROSS THE SOUTHERN
TIER OF THE CONUS. MODELS INITIALLY PLACE THE SYSTEM SLIGHTLY FARTHER EAST THAN
YESTERDAY, AND KEEP IT STRONGER AS IT TRAVERSES THE SOUTHERN TIER OF THE
COUNTRY DURING WEEK-2. THE POTENTIAL FOR HEAVY PRECIPITATION MOVES WITH IT FROM
WEST TO EAST ACROSS THE SOUTHERN TIER OF THE NATION. THE INITIAL PLACEMENT OF
THE SYSTEM FARTHER EAST REDUCES THE CHANCES FOR PRECIPITATION HAZARDS IN THE
SOUTHWEST AND THE WEST-CENTRAL ROCKIES AROUND THE WASATCH RANGE COMPARED TO
YESTERDAY. MODERATE RISKS FOR HEAVY PRECIPITATION AND HIGH-ELEVATION SNOWFALL
HAVE BEEN REMOVED HERE, WITH THE SLIGHT RISK OF HEAVY PRECIPITATION IN THE
SOUTHWEST LIMITED TO PART OF ARIZONA ON DEC 23.
FARTHER EAST, HOWEVER, THE MID-LEVEL TROUGH WILL BEGIN TO TAP GULF MOISTURE,
SPREADING IT NORTHWARD ACROSS THE SOUTH-CENTRAL AND SOUTHEASTERN CONUS DURING
THE FIRST HALF OF THE PERIOD. MODELS HAVE BEEN TRENDING WETTER WITH THIS
SYSTEM, EXPANDING THE PRECIPITATION SHIELD FARTHER NORTH AND EAST. AS A RESULT,
A SLIGHT RISK FOR HEAVY PRECIPITATION COVERS A LARGE AREA FROM THE SOUTHERN
HALF OF THE GREAT PLAINS EASTWARD ACROSS THE MIDDLE AND LOWER MISSISSIPPI
VALLEY, LOWER OHIO VALLEY, AND SOUTHEASTERN CONUS FOR DECEMBER 23-26. THE ECMWF
AND GEFS PROBABILISTIC EXTREMES TOOLS (PETS) BOTH INDICATE AT LEAST A 20%
PROBABILITY OF 3-DAY PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE AND AT
LEAST ONE INCH IN MOST OF THIS AREA BEFORE THE SURFACE LOW PRESSURE AND
ASSOCIATED FRONTAL BOUNDARY SHIFT EAST OF THE CONUS. THE ECMWF PET TOOL IS MORE
ROBUST WITH PRECIPITATION EARLY IN THE PERIOD ACROSS THE SOUTH-CENTRAL PLAINS,
BUT ALL OF THE PETS LOOK SIMILAR FARTHER TO THE EAST AND NORTHEAST. THE
CONSENSUS OF THE TOOLS INDICATE THAT THE MOST LIKELY AREA TO RECEIVE HEAVY
PRECIPITATION IS IN AND AROUND THE LOWER MISSISSIPPI VALLEY, WITH THE PETS
SHOWING ODDS EXCEEDING 20 PERCENT OF 3-DAY PRECIPITATION TOTALS OVER 1.5
INCHES, WHICH IS MORE THAN DEPICTED YESTERDAY. BASED ON THESE TOOLS AND MODEL
TRENDS, A MODERATE RISK FOR HEAVY PRECIPITATION IS POSTED FOR THE AREA IN AND
AROUND THE LOWER MISSISSIPPI VALLEY FOR DECEMBER 23-25. IN ADDITION TO HEAVY
PRECIPITATION, THE DYNAMIC MID-LEVEL SYSTEM IS ALSO EXPECTED TO INCREASE THE
RISK OF SEVERE WEATHER IN THIS SAME REGION.
IN THE WAKE OF THIS SYSTEM OVER THE CENTRAL ROCKIES AND ADJACENT HIGH PLAINS,
THE SNOW WATER EQUIVALENT (SWE) PET BASED ON THE GEFS SHOWS ENHANCED CHANCES
FOR HEAVY SNOW OVER THE CENTRAL ROCKIES AND ADJACENT HIGH PLAINS, AND A SLIGHT
RISK OF HEAVY SNOW IS POSTED HERE FOR THE FIRST TWO DAYS OF WEEK-2, DEC 23-24 .
THE SWE PET AND MODEL ENSEMBLE MEANS DEPICT THE BEST CHANCES FOR ANOMALOUSLY
HEAVY SNOW OVER AND NEAR THE HIGHER ELEVATIONS OF COLORADO, WHERE A MODERATE
RISK OF HEAVY SNOW IS POSTED FOR THE SAME PERIOD.
CONFIDENCE CONTINUES TO SLOWLY INCREASE IN ENHANCED CHANCES FOR PERIODS OF HIGH
WINDS AND HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST COAST AND MUCH OF
CALIFORNIA DURING THE MIDDLE AND LATTER PART OF THE PERIOD. THE ECMWF REMAINS
MORE BULLISH THAN THE GEFS WITH THE CHANCES FOR HEAVY PRECIPITATION, ALTHOUGH
THE GEFS CONTINUES TO SHOW INCREASED CHANCES FOR STORMY WEATHER, MOVING IN THE
DIRECTION OF THE ECMWF ENSEMBLE MEANS AND DERIVED TOOLS. WITH THE INCREASED
AMPLITUDE OF THE NORTH PACIFIC MID-LEVEL TROUGH SHOWN BY ALL GUIDANCE, THE ODDS
FOR EXTREME WEATHER HAVE DECREASED SLIGHTLY IN SOUTHERNMOST CALIFORNIA, BUT
FARTHER NORTH, HAZARDS FOR HIGH WIND, HEAVY PRECIPITATION, AND HEAVY SNOW HAVE
ALL EXPANDED RELATIVE TO YESTERDAY. THE PETS HAVE THE CLOSEST AGREEMENT ACROSS
A LARGE PORTION OF CALIFORNIA, IN PARTICULAR THE WESTERN TIER OF THE STATE,
PART OF THE NORTHERN CENTRAL VALLEY, AND MANY OF THE HIGHER ELEVATIONS. THIS
HAS PROMPTED AN EXPANSION OF THE MODERATE RISK OF HEAVY PRECIPITATION OVER THIS
REGION, WITH A MODERATE RISK OF HEAVY SNOW COVERING THE HIGHER ELEVATIONS OF
THE SIERRA NEVADA, SOUTHERN CASCADES, AND NORTHERN COASTAL RANGES FOR DEC
25-29. IN ADDITION, A STRONG LOW-LEVEL JET IS EXPECTED TO ACCOMPANY THE STORMY
WEATHER IN THE WEST, BRINGING INCREASED ODDS FOR HIGH WINDS TO THE WEST COAST,
CALIFORNIA VALLEYS, AND MANY OF THE HIGHER ELEVATIONS OF CALIFORNIA AS WELL.
THE AREA OF MODERATE RISK IS EXPANDED RELATIVE TO YESTERDAY, REACHING FROM
SOUTHERN OREGON SOUTHWARD TO NEAR MONTERREY ALONG THE WEST COAST FOR DEC 26-29.
ENSEMBLE SOLUTIONS FROM THE GEFS, ECMWF, AND CMCE ALL INDICATE A SERIES OF LOW
PRESSURE SYSTEMS MOVING FROM THE BERING SEA INTO THE GULF OF ALASKA EARLY
WEEK-2 PERIOD, RESULTING IN ELEVATED CHANCES FOR HIGH WIND AND HEAVY
PRECIPITATION. PETS FROM ALL THREE MODELS ARE SLIGHTLY LESS ROBUST WITH
INCLEMENT WEATHER THAN YESTERDAY, BUT STILL INDICATE AT LEAST A 20% PROBABILITY
OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND AT LEAST 25 MPH ALONG THE
SOUTH-CENTRAL AND SOUTHEASTERN COASTLINE, AND 20% PROBABILITY OF 3-DAY
PRECIPITATION TOTALS EXCEEDING THE 85TH PERCENTILE ROUGHLY FROM PRINCE WILLIAM
SOUND SOUTH AND EAST ALONG THE COAST AND INTO PART OF THE ALASKAN PANHANDLE.
FORECASTER: RICH TINKER
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