727
FXUS21 KWNC 261927
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 26 2023
SYNOPSIS: MID-LEVEL LOW PRESSURE IS EXPECTED TO TRACK INLAND INTO SOUTHERN
CALIFORNIA AND THE SOUTHWEST ON JANUARY 3 AND 4. LATER IN WEEK-2, THE STRONGEST
ONSHORE FLOW IS FORECAST TO SHIFT NORTHWARD TO NORTHERN CALIFORNIA AND THE
PACIFIC NORTHWEST. MULTIPLE SURFACE LOWS ARE FORECAST TO DEVELOP EITHER ALONG
THE GULF OR EAST COAST FROM JANUARY 3 TO 9. NEAR TO ABOVE-NORMAL TEMPERATURES
ARE FAVORED FOR A MAJORITY OF THE CONTIGUOUS U.S. (CONUS) FROM JANUARY 3 TO 9.
HOWEVER, WITH THE LIKELIHOOD OF A NEGATIVE ARCTIC OSCILLATION DURING EARLY
JANUARY, COLDER TEMPERATURES MAY EVENTUALLY BECOME ESTABLISHED OVER NORTH
AMERICA.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF SOUTHERN CALIFORNIA, WED-THU,
JAN 3-4.
SLIGHT RISK OF HEAVY PRECIPITATION FOR NORTHERN CALIFORNIA AND THE PACIFIC
NORTHWEST, SAT-TUE, JAN 6-9.
SLIGHT RISK OF HEAVY SNOW FOR SIERRA NEVADA MOUNTAINS, KLAMATH MOUNTAINS, AND
CASCADES, SAT-TUE, JAN 6-9.
SLIGHT RISK OF HIGH WINDS FOR COASTAL AREAS OF NORTHERN CALIFORNIA AND THE
PACIFIC NORTHWEST, JAN 6-9.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE GULF COAST, SOUTHEAST, AND
MID-ATLANTIC, WED-TUE, JAN 3-9.
SLIGHT RISK OF HIGH WINDS FOR THE ALEUTIANS, ALASKA PENINSULA, AND COASTAL
SOUTHWESTERN ALASKA, WED-FRI, JAN 3-5.
DETAILED SUMMARY
FOR FRIDAY DECEMBER 29 - TUESDAY JANUARY 02:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY JANUARY 03 - TUESDAY JANUARY 09: THE 0Z GEFS, ECMWF, AND CANADIAN
ENSEMBLE MEANS DEPICT A 500-HPA SHORTWAVE TROUGH TRACKING EASTWARD ACROSS
SOUTHERN CALIFORNIA AND THE SOUTHWEST ON JANUARY 3 AND 4. THIS SHORTWAVE TROUGH
ALONG THE GEFS AND ECMWF PROBABILISTIC EXTREMES TOOLS (PETS) SUPPORT A SLIGHT
RISK OF HEAVY PRECIPITATION FOR SOUTHERN CALIFORNIA EARLY IN WEEK-2. THE DRIER
GEFS MODEL OUTPUT PRECLUDES DESIGNATION OF A MODERATE RISK OF HEAVY
PRECIPITATION. BEYOND THE EARLY PART OF WEEK-2, ENSEMBLE MEAN SOLUTIONS
INDICATE THAT ONSHORE FLOW SHIFTS BACK NORTHWARD. BASED ON THE LONGWAVE PATTERN
AND SUPPORT FROM THE GEFS AND ECMWF PETS (20 TO 40 PERCENT CHANCE OF 3-DAY
AMOUNTS TO EXCEED THE 85TH PERCENTILE AND 2 INCHES, LIQUID EQUIVALENT), A
SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR NORTHERN CALIFORNIA AND THE
PACIFIC NORTHWEST FROM JANUARY 6 TO 9. BASED ON THE AMPLITUDE OF THE LONGWAVE
TROUGH AND 24-HOUR PRECIPITATION AMOUNTS FROM THE ENSEMBLE MEANS, A MODERATE
RISK OF HEAVY PRECIPITATION IS NOT POSTED AT THIS TIME. THE ONSHORE FLOW ALSO
LEADS TO A SLIGHT RISK OF HEAVY SNOW ACROSS THE SIERRA NEVADA MOUNTAINS,
KLAMATH MOUNTAINS, AND CASCADES DURING THIS SAME TIME PERIOD. THE SLIGHT RISK
OF HIGH WINDS IS ONLY POSTED FOR AREAS OF THE PACIFIC NORTHWEST AND NORTHERN
CALIFORNIA, GIVEN THE PREDICTED STRENGTH OF ONSHORE FLOW AND AMPLITUDE OF THE
LONGWAVE TROUGH.
A MEAN 500-HPA TROUGH IS FORECAST OVER THE WEST DURING WEEK-2. THERE ARE LIKELY
TO BE MULTIPLE INDIVIDUAL SHORTWAVE TROUGHS TO EMERGE FROM THIS LONGWAVE TROUGH
AND INCREASE THE CHANCE FOR PERIODS OF MODERATE TO HEAVY PRECIPITATION ACROSS
THE GULF COAST, SOUTHEAST, AND MID-ATLANTIC. BASED ON THE GEFS PET AND
UNCALIBRATED 24-HOUR PRECIPITATION AMOUNTS FROM THE ECMWF ENSEMBLE MEAN, A
SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR PARTS OF THE SOUTHEAST AND
MID-ATLANTIC. DUE TO UNCERTAINTY ON TIMING OF INDIVIDUAL SHORTWAVE TROUGHS,
THIS SLIGHT RISK COVERS THE ENTIRETY OF WEEK-2. THE SPATIAL COVERAGE OF THIS
PRECIPITATION HAZARD IS CONSISTENT WITH THE BACKGROUND EL NIñO. IF A MORE
VIGOROUS SHORTWAVE TROUGH AND ASSOCIATED LOW PRESSURE SYSTEM TRACKS ACROSS THE
EAST, THERE COULD BE AN INCREASED RISK OF HEAVY SNOW FOR THE HIGHER ELEVATIONS
OF THE SOUTHERN TO CENTRAL APPALACHIANS AND PARTS OF THE MID-ATLANTIC. THIS
WILL BE CLOSELY MONITORED IN SUBSEQUENT OUTLOOKS.
IN ALASKA, PRECIPITATION IS FORECAST TO CONTINUE ACROSS MUCH OF SOUTHEAST AND
SOUTH-CENTRAL ALASKA DURING THE WEEK-2 PERIOD. HOWEVER, THERE ARE ONLY
SCATTERED AREAS WHERE 3-DAY PRECIPITATION TOTALS ARE FORECAST TO REACH 2 INCHES
IN THE PETS AND RAW TOOLS. THEREFORE, NO PRECIPITATION HAZARDS ARE POSTED. A
STRONG LOW PRESSURE SYSTEM IS FORECAST TO DEVELOP OVER THE BERING SEA NEXT WEEK
WITH A SOUTHEASTWARD TRACK TO THE GULF OF ALASKA. THE DETERMINISTIC 12Z GFS
ALONG WITH THE DETERMINISTIC 0Z ECMWF AND MANY OF ITS ENSEMBLE MEMBERS DEPICT A
SUB 952-HPA LOW OVER THE BERING SEA AND ALEUTIANS AT THE BEGINNING OF WEEK-2.
BASED ON GOOD MODEL AGREEMENT ON THE STRENGTH OF THIS LOW PRESSURE SYSTEM, A
SLIGHT RISK OF HIGH WINDS IS POSTED FOR THE ALEUTIANS, ALASKA PENINSULA, AND
COASTAL SOUTHWESTERN ALASKA FROM JANUARY 3 TO 5.
FORECASTER: BRAD PUGH
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