954
FXUS02 KWBC 130700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST WED DEC 13 2023
VALID 12Z SAT DEC 16 2023 - 12Z WED DEC 20 2023
...A WET PATTERN IS LIKELY OVER THE SOUTH/SOUTHEAST AND
POTENTIALLY UP THE EAST COAST THIS WEEKEND INTO NEXT WEEK...
..OVERVIEW
A SOUTHERN STREAM UPPER LOW TRAVERSING THE SOUTH-CENTRAL TO
SOUTHEASTERN U.S. INTERACTING WITH NORTHERN STREAM ENERGY THIS
WEEKEND INTO EARLY NEXT WEEK WILL PROVIDE SUPPORT FOR A SURFACE
LOW TO STRENGTHEN AND SPREAD WIDESPREAD HEAVY RAINFALL ACROSS
FLORIDA ON SATURDAY. HEAVY RAIN MAY SPREAD FARTHER NORTH INTO
EARLY NEXT WEEK, BUT ITS PLACEMENT REMAINS QUITE UNCERTAIN GIVEN
MODEL DIFFERENCES IN THE UPPER LEVELS AND THE SURFACE LOW TRACK.
BEHIND THESE FEATURES, DRY UPPER RIDGING WILL BUILD IN THE
INTERIOR WEST AHEAD OF A MEAN EASTERN PACIFIC TROUGH AXIS THAT
SHOULD PUSH PERIODIC PRECIPITATION INTO THE WEST COAST STATES.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE HAS SHOWN GOOD AGREEMENT REGARDING THE SOUTHERN
STREAM UPPER LOW LOCATED OVER THE CENTRAL U.S. ON SATURDAY, BUT
HAS VARIED QUITE A BIT OVER THE LAST SEVERAL DAYS WITH A NORTHERN
STREAM SHORTWAVE AND HOW IT INTERACTS AND POTENTIALLY COMBINES
WITH THE SOUTHERN STREAM FEATURE AS IT DIVES SOUTHEAST FROM THE
NORTH-CENTRAL CONUS. THE DIFFERENCES ALOFT LEAD TO CONSIDERABLE
SPREAD IN THE RESULTANT SURFACE LOW STRENGTH AND TRACK FROM MODEL
TO MODEL AND RUN TO RUN. AFTER THE SURFACE LOW DEVELOPS IN THE
GULF, IT LOOKS TO MOVE ACROSS FLORIDA THIS WEEKEND WITH AT LEAST
SOME MODEL/ENSEMBLE AGREEMENT. BUT THEN GIVEN THE DIFFERENCES
ALOFT, THE LOW TRACKS AFTER THAT VARY GREATLY IN THE DETERMINISTIC
MODELS WITH ENSEMBLE MEMBERS SHOWING SOME SPREAD AS WELL.
FORTUNATELY THE 12Z/18Z ENSEMBLE MEANS WERE CLUSTERED FAIRLY WELL
IN SHOWING A LOW TRACKING NORTHEASTWARD INTO THE WESTERN ATLANTIC
THROUGH MONDAY/TUESDAY, THOUGH WITH SLOWER MOVEMENT COMPARED TO
THE PREVIOUS FORECAST. THE 12Z ECMWF APPEARED THE CLOSEST TO THE
ENSEMBLE MEAN LOW TRACK THROUGH MONDAY OR SO. CMC RUNS SHOW A
SIMILAR TRACK BUT FASTER MOVING, AND THE 12Z UKMET IS SIMILAR AS
WELL BUT LIKELY TOO STRONG. THE GFS RUNS HAVE BEEN OUTLIERS IN
SHOWING SURFACE LOWS MUCH FARTHER WEST/INLAND, LEADING TO
WIDESPREAD PRECIPITATION FARTHER WEST THAN THE NON-NCEP GUIDANCE
THAT IS BETTER CLUSTERED WITH THE ENSEMBLE MEANS. SO THE GFS
SOLUTIONS WERE NOT FAVORED. THE NEWER 00Z ECMWF SWITCHES BACK TO A
FASTER LOW TRACK (LOOKING SIMILAR TO THE CMC), SO NOT ONLY IS
THERE THIS WEST-EAST SPREAD, NORTH-SOUTH SPREAD WITH TIMING IS AN
ISSUE. BY TUESDAY-WEDNESDAY THERE IS AMPLE UNCERTAINTY IN WHETHER
THE LOW JUST MEANDERS OVER THE WESTERN ATLANTIC OR RUSHES NORTH,
WITH PERHAPS ADDITIONAL LOW PRESSURE FORMING OVER THE ATLANTIC.
THE WPC FORECAST FAVORED A BLEND OF THE 12Z ECMWF WITH THE GEFS
AND EC ENSEMBLE MEANS THROUGH THE FORECAST PERIOD, WITH SMALL
PROPORTIONS OF OTHER GUIDANCE EARLY ON, INCREASING THE PERCENTAGE
OF ENSEMBLE MEANS TO HALF BY DAY 5 AND MORE BY 6-7 GIVEN THE MODEL
SPREAD. THIS BLEND ALSO WORKED FOR EASTERN PACIFIC TO WEST COAST
TROUGHING, THOUGH THE 12Z ECMWF PULLED A CLOSED UPPER LOW FARTHER
WEST OF OTHER GUIDANCE BY TUESDAY-WEDNESDAY. THE 00Z EC SEEMS MORE
REASONABLE, THOUGH IS ACTUALLY STILL ON THE WESTERN SIDE OF THE
GUIDANCE ENVELOPE AS THE 00Z GFS AND CMC TRENDED EAST TOO.
..WEATHER/HAZARDS HIGHLIGHTS
AMPLE MOISTURE WITH PRECIPITABLE WATER VALUES LIKELY WELL OVER THE
90TH PERCENTILE WILL BE IN PLACE ACROSS FLORIDA AS THE SOUTHERN
STREAM UPPER LOW ENHANCES LIFT AND DEVELOPS A SURFACE LOW. THESE
FACTORS WILL LEAD TO WIDESPREAD HEAVY RAINFALL OF 2-4 INCHES WITH
LOCALLY HIGHER AMOUNTS LIKELY. A SLIGHT RISK HAS BEEN RAISED FOR
DAY 4/SATURDAY IN THE EXCESSIVE RAINFALL OUTLOOK, AS RAINFALL OF
THIS MAGNITUDE WITH SOME HIGH RAIN RATES COULD CAUSE INSTANCES OF
FLASH FLOODING. HEAVY RAIN AND FLOODING MAY BE A CONCERN FOR THE
LOWER MISSISSIPPI VALLEY AS WELL ON SATURDAY IN THE VICINITY OF
ANOTHER FRONTAL SYSTEM. A MARGINAL RISK CONTINUES TO COVER THIS
POTENTIAL AS RECENT MODELS HAVE TRENDED UP WITH RAINFALL ONSHORE,
THOUGH THERE IS STILL A CHANCE THAT MOST COULD FALL OFFSHORE IN
THE CENTRAL GULF. BY SUNDAY, INCREASING MODEL SPREAD MAKES
RAINFALL TOTALS AND PLACEMENT VERY UNCERTAIN. THE WPC
PRECIPITATION FORECAST IS REFLECTIVE OF THE MODEL PREFERENCE FOR A
LOW MOVING NORTHEASTWARD INTO THE WESTERN ATLANTIC, FOCUSING
RAINFALL FROM GEORGIA INTO THE CAROLINAS INTO THE MID-ATLANTIC. A
MARGINAL RISK IS PLANNED AS A STARTING POINT ACROSS THESE AREAS
FOR DAY 5/SUNDAY. SLIGHT RISK-LEVEL IMPACTS ARE CERTAINLY POSSIBLE
BUT THE UNCERTAINTY IN PLACEMENT OF HEAVY RAIN WAS TOO LARGE TO
DEFINE A HIGHER RISK AREA AT THIS TIME, BUT THE INGREDIENTS ARE
THERE AND AN UPGRADE IS LIKELY AT SOME POINT. HEAVY RAIN COULD
SPREAD INTO THE THE NORTHEAST ON MONDAY DEPENDING ON THE SURFACE
LOW TRACK.
PRECIPITATION CHANCES WILL BE INCREASING OVER THE WEEKEND FOR
CALIFORNIA AS SHORTWAVE ENERGY SUPPORTING A POSSIBLE ATMOSPHERIC
RIVER APPROACHES. RAIN AMOUNTS AND RATES ON SUNDAY COULD BE
SUFFICIENT TO CAUSE SOME FLOODING CONCERNS, ESPECIALLY GIVEN SOME
INSTABILITY COULD REACH COASTAL AREAS, SO A MARGINAL RISK IS IN
PLACE IN THE ERO. PRECIPITATION LOOKS TO PERSIST ACROSS CALIFORNIA
AND REACH THE PACIFIC NORTHWEST INTO MONDAY-TUESDAY, INCLUDING
SOME HIGHER ELEVATION SNOW, THOUGH CONFIDENCE IN THE DETAILS
REMAINS SOMEWHAT LOW. THE INTERIOR WEST TO THE PLAINS WILL BE THE
DRY SPOT OVER THE LOWER 48 THROUGH THE FIRST HALF OF NEXT WEEK.
THE NORTHERN TWO-THIRDS OF THE LOWER 48 SHOULD SEE NEAR TO ABOVE
NORMAL TEMPERATURES THIS WEEKEND INTO EARLY NEXT WEEK, WITH
TEMPERATURES 10-20F ABOVE AVERAGE PERIODICALLY AFFECTING THE
NORTHERN/CENTRAL HIGH PLAINS INTO THE UPPER MIDWEST. THE WEST CAN
ALSO EXPECT GENERALLY ABOVE NORMAL TEMPERATURES UNDERNEATH UPPER
RIDGING. COOLER THAN NORMAL TEMPERATURES WILL BE LIMITED TO THE
SOUTHERN PLAINS (IN TERMS OF HIGHS) EASTWARD INTO THE SOUTHEAST
THIS WEEKEND, MODERATING CLOSER TO NORMAL BY EARLY NEXT WEEK,
THOUGH FLORIDA COULD STILL SEE BELOW NORMAL HIGHS BEHIND MULTIPLE
FRONTAL PASSAGES.
TATE
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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