606
FXUS02 KWBC 171858
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 PM EST SUN DEC 17 2023
VALID 12Z WED DEC 20 2023 - 12Z SUN DEC 24 2023
...EMERGING PRECIPITATION THREAT FOR SOUTHERN CALIFORNIA AND THE
SOUTHWEST...
..OVERVIEW
A CLOSED UPPER TROUGH/LOW WILL DIG SOUTHWARD OFF CALIFORNIA
MID-LATE WEEK BEFORE EJECTING INLAND ACROSS SOUTHERN
CALIFORNIA/NORTHERN MEXICO AND THE SOUTHWEST NEXT WEEKEND. THIS
WILL BRING PERIODS OF ENHANCED PRECIPITATION INTO SOUTHERN
CALIFORNIA, AND THE SOUTHWEST. MEANWHILE, A DEEP UPPER LOW/TROUGH
OVER THE EAST COAST MIDWEEK WILL CUT OFF ANOTHER LOW OFF THE EAST
COAST INTO LATER WEEK, ACTING MAINLY AS A MARITIME THREAT. IN
BETWEEN THE WESTERN AND EASTERN FEATURES, WARMING UPPER RIDING
WILL POSITION OVER THE CENTRAL INTO EAST-CENTRAL U.S..
..GUIDANCE/PREDICTABILITY ASSESSMENT
THERE REMAINS SOME CONSIDERABLE DIFFERENCES IN PLACEMENT OF A CUT
OFF LOW OFF THE EAST COAST DAY 3, THOUGH GUIDANCE HAS GENERALLY
TRENDED SLIGHTLY CLOSER TO THE COAST. THE GFS REMAINS THE FARTHEST
EAST WITH THIS SYSTEM. OUT WEST, GUIDANCE SHOWS BETTER AGREEMENT
TODAY ON THE CLOSED LOW OFF THE CALIFORNIA COAST WHICH SHOULD
SHIFT INLAND AROUND FRIDAY-SATURDAY. THE BIGGER FORECAST QUESTION
IS WITH ADDITIONAL STRONG ENERGY INTO THE PACIFIC NORTHWEST LATE
IN THE WEEK, WHICH SOME GUIDANCE SHOWS CLOSED LOW DEVELOPMENT NEXT
WEEKEND OVER THE INTERIOR WEST. THE GFS IS FASTER WITH THIS
SYSTEM, AND THE ECMWF IS STRONGEST AND DIGS IT FARTHEST SOUTH. THE
MEANS ARE A LOT WEAKER BUT GENERALLY FOLLOW THEIR DETERMINISTIC
COUNTERPARTS.
THE WPC BLEND FOR THE UPDATED FORECAST PACKAGE TODAY USED A
GENERAL MODEL COMPROMISE FOR DAYD 3-4 AMIDST GOOD AGREEMENT. AFTER
THIS, CONTINUED A GENERAL BLEND OF THE GFS, ECMWF, AND CMC, BUT
WITH INCREASING CONTRIBUTIONS FROM THE ENSEMBLE MEANS TO ACCOUNT
FOR THE GROWING SPREAD. THIS APPROACH MAINTAINS GOOD CONTINUITY
WITH THE PREVIOUS WPC SHIFT AS WELL.
..WEATHER/HAZARDS HIGHLIGHTS
LOCALLY ENHANCED RAIN CHANCES WILL SHIFT SOUTHWARD ACROSS
CALIFORNIA THIS WEEK AS A PLUME OF DEEPENED MOISTURE IS CHANNELED
AROUND A CLOSED UPPER TROUGH/SURFACE SYSTEM DUG OVER THE EASTERN
PACIFIC. RAIN AMOUNTS AND RATES MAY BE SUFFICIENT FOR SOME LOCAL
FLOODING CONCERNS, ESPECIALLY GIVEN SOME INSTABILITY COULD REACH
COASTAL AREAS. THE WPC EXCESSIVE RAINFALL OUTLOOK (ERO) OFFERS
MARGINAL RISKS FOR WEDNESDAY INTO THURSDAY FROM CENTRAL TO
SOUTHERN COASTAL CALIFORNIA. ORGANIZED RAINS AND TERRAIN/MOUNTAIN
FOCUSING SNOWS SET TO WORK INLAND THROUGH THE SIERRA AND WITH SOME
THREAT FROM THE SOUTHWEST ACROSS THE SOUTH-CENTRAL GREAT
BASIN/ROCKIES FOR THE HOLIDAY WEEKEND WITH SLOW SYSTEM APPROACH.
MEANWHILE, SYSTEM/ENERGY DOWNSTREAM PROPAGATION MAY ALSO GENERATE
AN EMERGING WET PATTERN FROM THE SOUTH-CENTRAL PLAINS WITH SYSTEM
GENESIS AND WITH MORE UNCERTAINTY TO THE EAST GIVEN RETURN
MOISTURE INFLOW SIGNAL IN GUIDANCE. THIS COULD INCLUDE SOME
NORTHERN TIER WINTRY PRECIPITATION. MEANWHILE UPSTREAM, UPPER
SHORTWAVE TROUGH AND SURFACE SYSTEM PROGRESSION INTO THE NORTHWEST
SHOULD SUPPORT PERIODS OF ENHANCED PRECIPITATION FRIDAY INTO THE
WEEKEND.
TEMPERATURES SHOULD TREND QUITE WARM ACROSS THE CENTRAL U.S.,
ESPECIALLY INTO THE NORTHERN PLAINS AND UPPER MIDWEST NEXT
WEEKEND. DAYTIME HIGHS COULD BE 20 TO 25 DEGREES ABOVE NORMAL IN
THIS REGION. ELSEWHERE, DAYTIME HIGH ANOMALIES RETURN BACK TO
NORMAL ACROSS THE EAST AS UPPER RIDGING BUILDS AND UPPER TROUGHING
INTO THE WEST SHOULD RESULT IN A COOLING TREND LATER THIS WEEK AND
ESPECIALLY NEXT WEEKEND.
SANTORELLI/SCHICHTEL
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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