159
FXUS02 KWBC 200659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST FRI DEC 20 2024
VALID 12Z MON DEC 23 2024 - 12Z FRI DEC 27 2024
***MULTIPLE ROUNDS OF RAIN AND SNOW EXPECTED FOR THE WEST COAST
REGION NEXT WEEK WITH AN ENERGETIC PACIFIC STORM TRACK***
..GENERAL OVERVIEW
A QUASI-ZONAL UPPER LEVEL FLOW PATTERN IS EXPECTED TO BE IN PLACE
ACROSS THE CONTINENTAL U.S. FOR THE BEGINNING OF NEXT WEEK ONCE THE
DEEP UPPER TROUGH DEPARTS THE EAST COAST. AN UPPER RIDGE TRIES TO
DEVELOP ACROSS THE INTERMOUNTAIN WEST AND ROCKIES ON TUESDAY BEFORE
A POTENT PACIFIC SHORTWAVE MOVES IN AND PRODUCES AN ATMOSPHERIC
RIVER EVENT FOR THE WEST COAST, AND ANOTHER DISTURBANCE DEVELOPS
OVER THE SOUTHERN PLAINS MIDWEEK AND SPURS A NEW SURFACE LOW WITH
COPIOUS RAINFALL AHEAD OF IT. LOOKING AHEAD TO THE END OF THE WEEK,
AN ENERGETIC STORM TRACK FROM THE PACIFIC LEADS TO AN ADDITIONAL
STORM SYSTEM MOVING INLAND ACROSS THE WESTERN U.S. WITH ANOTHER
ROUND OF HEAVY RAIN AND MOUNTAIN SNOW.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE 00Z MODEL GUIDANCE SUITE IS IN GOOD OVERALL SYNOPTIC SCALE
AGREEMENT FOR EARLY IN THE WEEK, ALTHOUGH THE 00Z CMC IS SLOWER
WITH THE ARRIVAL OF A LEAD SHORTWAVE ACROSS THE PACIFIC NORTHWEST,
AND A LITTLE WEAKER WITH THE DEEPER TROUGH MOVING IN LATE
TUESDAY/EARLY WEDNESDAY, AND THEN A LITTLE SLOWER BY WEDNESDAY
NIGHT ACROSS THE ROCKIES COMPARED TO THE MODEL CONSENSUS. THE GFS
BECOMES STRONGER WITH THE TROUGH DEPARTING THE NORTHEAST U.S.
MIDWEEK, BUT RELATIVELY COMPARABLE ELSEWHERE. BY THE END OF THE
WEEK, THINGS BECOME MORE UNCERTAIN WITH THE NEXT STORM SYSTEM
ACROSS THE SOUTHWESTERN U.S., WITH RECENT ECMWF RUNS SLOWER AND
MORE AMPLIFIED THAN THE ENSEMBLE MEANS SUGGEST. THE MAJORITY OF
THE MACHINE LEARNING GUIDANCE ALSO FAVORS A FASTER SCENARIO THAN
THE ECMWF, SO THIS MODEL WAS WEIGHTED LESS BY LATE THURSDAY INTO
FRIDAY, WHILST INCREASING THE ENSEMBLE MEANS TO ABOUT HALF BY THIS
TIME TO ACCOUNT FOR THE INCREASING UNCERTAINTY.
..WEATHER/HAZARDS HIGHLIGHTS
MODERATE TO HEAVY RAINFALL IS EXPECTED ACROSS PORTIONS OF NORTHERN
CALIFORNIA AND INTO WESTERN OREGON ON MONDAY, WHEN A CORE AXIS OF
THE EASTERN PACIFIC TROUGH SHOULD PUSH EASTWARD WITH AN INCREASE
IN THE MOISTURE AND UPSLOPE FLOW ACROSS THE WINDWARD TERRAIN.
MOISTURE AND INSTABILITY WITH THIS ATMOSPHERIC RIVER CONTINUES TO
LOOK SUFFICIENT ENOUGH FOR A CONTINUED MARGINAL RISK ON THE WPC
EXCESSIVE RAINFALL OUTLOOK FOR DAY 4/MONDAY, AND SUBSEQUENT ROUNDS
OF RAIN AND MOUNTAIN SNOW APPEAR LIKELY THROUGH CHRISTMAS AND INTO
THE END OF THE WEEK. MODEL QPFS CONTINUE TO SUGGEST ENHANCED
RAINFALL POSSIBLE OVER THE PACIFIC NORTHWEST, BUT ESPECIALLY FOR
PARTS OF NORTHWEST CALIFORNIA, WITH ACCUMULATING AND POSSIBLE HEAVY
SNOW FOCUS IN THE HIGHER TERRAIN OF THE CASCADES AND THE SIERRA,
THEN WORKING INLAND ACROSS MAINLY THE NORTHWEST AND THE NORTH-
CENTRAL INTERMOUNTAIN WEST/ROCKIES.
FARTHER EAST, THE LEADING SHORTWAVE AND FRONTAL LOW INTO THE
NORTHERN PLAINS WILL SPREAD SOME MAINLY LIGHT RAIN AND SNOW ACROSS
PORTIONS OF THE UPPER MIDWEST AND THE GREAT LAKES INTO MONDAY. THE
SOUTHERN STREAM TROUGH IS EXPECTED TO DEVELOP FURTHER WITH
POTENTIAL CLOSED LOW DEVELOPMENT, WITH AN OVERALL INCREASE IN
MODEL AND NBM QPF DURING THE TUESDAY-WEDNESDAY TIME PERIOD AS
DEEPER MOISTURE SURGES NORTHWARD FROM THE CENTRAL GULF. GIVEN THE
POTENTIAL FOR HIGHER INSTABILITY AND RAINFALL RATES OF AN INCH PER
HOUR OR GREATER WITH THE CONVECTION THAT DEVELOPS, A MARGINAL RISK
WILL BE VALID ON THE NEW DAY 5/TUESDAY EXCESSIVE RAINFALL OUTLOOK
FROM EASTERN TEXAS TO SOUTHERN MISSOURI. SOME STRONG THUNDERSTORMS
MAY ALSO DEVELOP IN THIS SAME GENERAL AREA.
A DEPARTING SURFACE HIGH OVER THE EAST WILL CONTINUE MUCH BELOW
NORMAL TEMPERATURES THROUGH MONDAY, WITH MODERATION EXPECTED GOING
INTO THE CHRISTMAS HOLIDAY. ABOVE NORMAL TEMPERATURES WILL EXPAND
FROM THE WEST INTO THE PLAINS AND MIDWEST THROUGH THE PERIOD.
DAYTIME HIGHS IN SOME LOCATIONS COULD BE 10 TO 20 DEGREES ABOVE
NORMAL.
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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