423
FXUS02 KWBC 181859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM 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. MEANWHILE SNOW IS LIKELY ON THE BACKSIDE OF THE
SYSTEM FROM THE ROCKIES INTO THE NORTH-CENTRAL PLAINS BY EARLY
NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT MODEL AGREEMENT IS RELATIVELY GOOD WITH THE OVERALL
PATTERN. AS AN ATLANTIC UPPER LOW SHIFTS AWAY FROM THE NORTHEAST
LATER WEEK AND A SHORTWAVE TRACKS EAST ACROSS THE CENTRAL U.S.
WITH JUST SOME TYPICAL TIMING/DEPTH SPREAD, THE MAIN FEATURE OF
CONCERN FOR THE MODEL DIAGNOSTICS IS THE UPPER LOW SHIFTING INTO
CALIFORNIA. THE 00Z UKMET WAS A BIT FARTHER WEST WITH THE CENTER
OF THE LOW COMPARED TO OTHER GUIDANCE--NOT TOO EGREGIOUS, BUT
STILL LEANED AWAY FROM IT. BY FRIDAY-SATURDAY ADDITIONAL ENERGY IS
LIKELY TO DROP INTO THE PACIFIC NORTHWEST AND THEN INTERACT/PHASE
WITH THE SOUTHERN STREAM UPPER LOW AT SOME POINT, THOUGH MODEL
GUIDANCE VARIES ON WHEN AND HOW THIS PHASING SHOULD OCCUR. THE
WESTERN TROUGHING SUPPORTS AN UPPER RIDGE SHIFTING FROM THE
CENTRAL U.S. GRADUALLY INTO THE EASTERN U.S. EARLY NEXT WEEK.
THE UPDATED WPC FORECAST WAS BASED ON A BLEND OF THE 00Z ECMWF/CMC
AND 06Z GFS EARLY IN THE PERIOD, INTRODUCING AND GRADUALLY
INCREASING THE GEFS AND EC ENSEMBLE MEANS INTO THE FORECAST BLEND
TO HALF BY THE LATE PERIOD. THIS MAINTAINED GOOD CONTINUITY WITH
THE PREVIOUS FORECAST FOR THE MOST PART.
..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) CONTINUES TO SHOW
A SLIGHT RISK FOR DAY 4/THURSDAY FOR SOUTHERN COASTAL CALIFORNIA.
ORGANIZED RAINS AND MOUNTAIN 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. A MARGINAL
RISK ERO REMAINS IN PLACE FROM THE CALIFORNIA DESERTS TO ARIZONA
FOR DAY 5/FRIDAY ON THE WARM/RAINY SIDE OF THE SYSTEM. CONSIDERED
A SLIGHT RISK FOR PARTS OF ARIZONA GIVEN THE PRECIPITABLE WATER
ANOMALIES LOOK TO BE WELL OVER THE 90TH IF NOT 95TH PERCENTILE FOR
THIS TIME OF YEAR AND THE AMPLE DIVERGENCE ALOFT WITH THE UPPER
LOW, BUT WITH SOMEWHAT DRY ANTECEDENT CONDITIONS AND A POSSIBLE
LACK OF ANY NOTABLE INSTABILITY, HELD OFF. THEN, SYSTEM/ENERGY
DOWNSTREAM PROPAGATION IS SLATED TO GENERATE AN EMERGING WET
PATTERN WITH POTENTIAL FOR SOME STRONG CONVECTION FOR THE HOLIDAY
WEEKEND FROM THE SOUTH-CENTRAL PLAINS INTO THE LOWER MISSISSIPPI
VALLEY AND CENTRAL GULF COAST STATES TO MONITOR WITH SYSTEM
GENESIS GIVEN RETURN MOISTURE INFLOW SIGNAL IN GUIDANCE AND UPPER
SUPPORT. THE NORTHWARD EXPANSION OF THE BROAD PRECIPITATION SHIELD
WITH FRONTAL WAVE PROPAGATION IS LIKELY TO SPREAD MODERATE
PRECIPITATION AMOUNTS UP THOUGH THE PLAINS/MISSISSIPPI
VALLEY/MIDWEST, WITH A MORE BLOCKY EASTERN U.S. SURFACE RIDGE
HOLDING DOWNSTREAM. THIS COULD INCLUDE SOME NORTHERN TIER WINTRY
PRECIPITATION, AND PROBABILITIES FOR MEANINGFUL SNOW IN THE
NORTH-CENTRAL HIGH PLAINS CAME UP DURING THIS FORECAST FOR EARLY
NEXT WEEK.
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, WHILE LOWS OF 25-35 DEGREES ABOVE AVERAGE ARE
EXPECTED. A FEW RECORD HIGHS ARE FORECAST WHILE RECORD WARM
MINIMUM TEMPERATURES MAY BE WIDESPREAD. 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.
TATE/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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