872
FXUS02 KWBC 220655
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
155 AM EST SUN DEC 22 2024
VALID 12Z WED DEC 25 2024 - 12Z SUN DEC 29 2024
***UNSETTLED WEATHER FOR THE WEST COAST WITH PERIODS OF HEAVY RAIN
AND MOUNTAIN SNOW***
..GENERAL OVERVIEW
A PROGRESSIVE AND SPLIT-FLOW UPPER LEVEL PATTERN WILL BE IN PLACE
ACROSS THE NATION WITH MULTIPLE SHORTWAVES CROSSING THE ROCKIES AND
THEN THE PLAINS. THE STORM SYSTEM REACHING THE WEST COAST TUESDAY
IS EXPECTED TO REACH THE WESTERN HIGH PLAINS BY THURSDAY AND
POTENTIALLY FORM A CLOSED UPPER LOW, AND THIS WILL LIKELY BE
FOLLOWED BY ANOTHER WELL DEVELOPED SHORTWAVE TROUGH THAT WILL REACH
THE MIDWEST STATES BY EARLY SUNDAY. MEANWHILE, AN UPPER RIDGE
LIKELY DEVELOPS NEAR THE EAST COAST, AND CONTINUED STRONG ONSHORE
FLOW ACROSS NORTHERN CALIFORNIA TO WESTERN WASHINGTON FROM THE
PACIFIC, WITH MULTIPLE DISTURBANCES MOVING IN FROM AN ENERGETIC
STORM TRACK, WILL KEEP DAILY RAIN AND SNOW CHANCES IN PLACE WITH
SOME ATMOSPHERIC RIVER EVENTS LIKELY.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AT THE TIME OF FRONTS/PRESSURES GENERATION, THE AVAILABLE GUIDANCE
FROM THE EXISTING 12Z/18Z GUIDANCE WAS IN LESS THAN STELLAR
AGREEMENT ACROSS THE DOMAIN FOR CHRISTMAS DAY INTO THE END OF THE
WEEK. THE MAIN DIFFERENCE WAS WITH THE 12/18Z GFS RUNS ACROSS THE
CENTRAL U.S., DEPICTING A MUCH MORE PROGRESSIVE SOUTHERN STREAM
SHORTWAVE NEAR THE GULF COAST ON CHRISTMAS, AND WELL TO THE NORTH
OF THE ECMWF/CMC SOLUTIONS ACROSS THE MIDWEST ON FRIDAY WITH THE
NEXT SYSTEM APPROACHING. UPON EXAMINATION OF THE MACHINE LEARNING
GUIDANCE SUITE, THERE WAS STRONGER SUPPORT FOR A NON-GFS COMPROMISE
FOR THE WEDNESDAY THROUGH FRIDAY TIME PERIOD. THE 00Z GFS IS A
LITTLE CLOSER TO THE CONSENSUS COMPARED TO EARLIER RUNS.
HEADING INTO NEXT WEEKEND, THE OVERALL PATTERN CONSOLIDATES INTO A
MORE AMPLIFIED ONE WITH A DEEPER SYNOPTIC SCALE TROUGH OVER THE
PLAINS AND A RIDGE AXIS OVER THE EAST COAST STATES. THE GFS FALLS
MORE IN LINE WITH THE OVERALL CONSENSUS BY THIS TIME, AND THUS
INCORPORATED BACK INTO THE BLEND FOR DAYS 6 AND 7, WITH A LITTLE
MORE CONFIDENCE COMPARED TO EARLIER IN THE FORECAST PERIOD. THE
ENSEMBLE MEANS WERE GRADUALLY INCREASED TO ABOUT 40-50% BY NEXT
SUNDAY.
..WEATHER/HAZARDS HIGHLIGHTS
IT STILL REMAINS THE CASE THAT MULTIPLE ROUNDS OF MODERATE TO
HEAVY RAINFALL ARE EXPECTED TO PERSIST ACROSS PORTIONS OF NORTHERN
CALIFORNIA AND INTO WESTERN OREGON AND WESTERN WASHINGTON THROUGH
THE END OF THE WEEK, WITH THE HEAVIEST RAIN EXPECTED FOR THE
TERRAIN OF NORTHWESTERN CALIFORNIA AND THE COASTAL RANGE OF OREGON.
SEVERAL INCHES OF RAINFALL IS LIKELY FOR THE COMBINED DAY 4-5 TIME
PERIOD, AND MARGINAL RISK AREAS ARE VALID FOR BOTH OF THESE DAYS.
ADDITIONAL HEAVY RAIN IS EXPECTED GOING INTO NEXT WEEKEND AS WELL.
HEAVY SNOW IS EXPECTED FOR THE HIGHER TERRAIN OF THE CASCADES AND
THE SIERRA NEVADA, THEN WORKING INLAND ACROSS THE NORTHWEST AND
NORTH-CENTRAL INTERMOUNTAIN WEST/ROCKIES.
THE SECOND SOUTHERN STREAM TROUGH IS EXPECTED TO DEVELOP FURTHER
WITH POTENTIAL CLOSED LOW DEVELOPMENT ON THURSDAY ACROSS THE
SOUTHERN PLAINS AS MOISTURE RETURNS NORTHWARD FROM THE GULF OF
MEXICO. GIVEN THE POTENTIAL FOR HIGHER INSTABILITY AND RAINFALL
RATES OF AN INCH PER HOUR OR GREATER WITH THE CONVECTION THAT
DEVELOPS, A MARGINAL RISK IS PLANNED VALID FOR THE WPC DAY
5/THURSDAY EXCESSIVE RAINFALL OUTLOOK FROM THE ARKLATEX REGION TO
SOUTHERN MISSOURI. THERE IS THE POTENTIAL THAT SOME OF THIS
OUTLOOK AREA COULD EVENTUALLY NEED A SLIGHT RISK AS THE EVENT
ENTERS THE SHORT RANGE PERIOD, BUT FOR NOW WILL KEEP IT AS A
MARGINAL RISK. SOME STRONG THUNDERSTORMS MAY ALSO DEVELOP IN THIS
SAME GENERAL AREA.
WIDESPREAD ABOVE NORMAL TEMPERATURES BY LATE DECEMBER STANDARDS ARE
EXPECTED ACROSS BASICALLY ALL OF THE CENTRAL U.S. FOR THE ENTIRE
FORECAST PERIOD, RANGING FROM 5 TO 20 DEGREES ABOVE AVERAGE. THE
GREATEST POSITIVE ANOMALIES ARE LIKELY ACROSS PORTIONS OF THE UPPER
MIDWEST AND EXTENDING INTO THE OHIO VALLEY BY SATURDAY. READINGS
CLOSER TO CLIMATOLOGY ARE EXPECTED NEAR BOTH THE WEST COAST AND THE
EAST COAST, AND NO ARCTIC AIR MASS INCURSIONS ARE EXPECTED OVER THE
NEXT WEEK.
HAMRICK
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC MEDIUM
RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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