647
FXUS02 KWBC 180701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST MON DEC 18 2023
VALID 12Z THU DEC 21 2023 - 12Z MON DEC 25 2023
...HEAVY RAIN THREAT FOR SOUTHERN CALIFORNIA/ARIZONA INTO
THURSDAY/FRIDAY...
...EMERGING HEAVY RAINFALL/CONVECTION THREAT FOR THE SOUTH-CENTRAL
U.S. OVER THE EXTENDED HOLIDAY WEEKEND...
..OVERVIEW
A CLOSED UPPER TROUGH/LOW WILL DIG SOUTHEASTWARD OFF CALIFORNIA
INTO LATER THIS WEEK BEFORE WORKING INLAND OVER/JUST SOUTH OF
SOUTHERN CALIFORNIA AND THE SOUTHWEST FRIDAY INTO SATURDAY. THIS
WILL BRING PERIODS OF LEADING ENHANCED PRECIPITATION INTO SOUTHERN
CALIFORNIA AND THE SOUTHWEST. THE EJECTION OF THE DYNAMIC SYSTEM
IS EXPECTED TO SPAWN DOWNSTREAM SOUTH-CENTRAL PLAINS
CYCLOGENESIS/FRONTOGENESIS NEXT WEEKEND WHOSE LEAD RETURN MOISTURE
AND INSTABILITY COULD FUEL AN EMERGING AREA OF ENHANCED
RAINS/CONVECTION.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE WPC MEDIUM RANGE PRODUCT SUITE WAS MAINLY DERIVED FOR THURSDAY
INTO SATURDAY VIA A COMPOSITE OF BEST CLUSTERED GUIDANCE FROM THE
18 UTC GFS AND 12 UTC ECMWF/CANADIAN ALONG WITH COMPATIBLE INPUT
FROM THE 01 UTC NATIONAL BLEND OF MODELS (NBM) AND WPC CONTINUITY
IN AN EFFORT TO PROVIDE SUFFICIENT DETAIL CONSISTENT WITH A
PATTERN WITH SEEMINGLY NEAR NORMAL PREDICTABILITY. THE CLOSER
PROXIMITY OF AN ALTERNATE 12 UTC UKMET CUTOFF COASTAL LOW SOLUTION
HOLDS IN A WETTER PATTERN THAN MOST GUIDANCE FOR EASTERN NEW
ENGLAND IN THIS PERIOD, BUT THE NEWER 00 UTC RUN HAS TRENDED
OFFSHORE.
LATER WEEKEND/MONDAY, A BLEND OF THE ECMWF/CANADIAN AND GEFS/ECMWF
ENSEMBLE MEANS SEEMS TO PROVIDE BEST FEASIBLE DETAIL IN A PERIOD
WITH GROWING FORECAST SPREAD. THE 18 UTC GFS SEEMS A LESS LIKELY
OUTLIER SOLUTION AT THESE LONGER TIME FRAMES IN SHOWING MUCH LESS
UPPER TROUGH DIGGING INTO THE WEST THAN MOST OTHER GUIDANCE. DEEP
SYSTEMS AND ASSOCIATED FLOW AMPLITUDES UPSTREAM OVER THE PACIFIC
SEEM TO BETTER SUPPORT UPPER TROUGH DIGGING INTO THE WEST FOR THIS
PERIOD AND NOW THE LATEST 00 UTC GFS HAS FLIPPED TO THIS PREFERRED
SOLUTION MORE IN LINE WITH OTHER 12/00 UTC CYCLE GUIDANCE.
..WEATHER/HAZARDS HIGHLIGHTS
LOCALLY ENHANCED RAIN CHANCES WILL WORK ACROSS SOUTHERN CALIFORNIA
LATER 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 SEEM SUFFICIENT FOR LOCAL FLOODING
CONCERNS, ESPECIALLY GIVEN SOME INSTABILITY COULD REACH COASTAL
AREAS. THE WPC EXCESSIVE RAINFALL OUTLOOK (ERO) WILL OFFER A
COLLABORATED UPGRADE TO A SLIGHT RISK FOR DAY 4/THURSDAY FOR
SOUTHERN COASTAL CALIFORNIA. ORGANIZED RAINS AND TERRAIN/MOUNTAIN
FOCUSING SNOWS ARE ALSO SET TO WORK INLAND INTO THE SOUTHWEST AND
THE SOUTH-CENTRAL GREAT BASIN/ROCKIES HEADING INTO THE WEEKEND
WITH SYSTEM APPROACH/EJECTION AND A MARGINAL RISK ERO IS PLANNED
FROM THE CALIFORNIA DESERTS TO ARIZONA FOR DAY 5/FRIDAY.
MEANWHILE, SYSTEM/ENERGY DOWNSTREAM PROPAGATION IS SLATED TO
GENERATE AN EMERGING WET PATTERN FROM THE SOUTH-CENTRAL PLAINS
INTO THE LOWER MS VALLEY 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.
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 BY THE WEEKEND.
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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