744
FXUS02 KWBC 140807
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
306 AM EST THU DEC 14 2023
VALID 12Z SUN DEC 17 2023 - 12Z THU DEC 21 2023
...A WET PATTERN IS LIKELY FOR THE EASTERN SEABOARD INTO NEXT
WEEK...
..OVERVIEW
A SURFACE LOW IS FORECAST TO TRACK ALONG OR NEAR THE EASTERN
SEABOARD DURING THE FIRST PART OF NEXT WEEK, SPREADING WIDESPREAD
PRECIPITATION TO THE EAST, INCLUDING SOME HEAVY RAINFALL CAUSING
FLOODING CONCERNS. HOWEVER, AMOUNTS AND PLACEMENT OF THE HEAVY
RAIN REMAIN QUITE UNCERTAIN GIVEN MODEL DIFFERENCES IN THE UPPER
LEVELS AND THE SURFACE LOW TRACK. BEHIND THESE FEATURES, DRY UPPER
RIDGING WILL BUILD INTO THE INTERIOR WEST AND SHIFT INTO THE
PLAINS AHEAD OF A MEAN EASTERN PACIFIC TROUGH AXIS THAT SHOULD
PUSH PERIODIC PRECIPITATION INTO THE WEST COAST STATES.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE HAS STRUGGLED FOR DAYS REGARDING THE EVOLUTION AND
TRACK OF THE EAST COAST SURFACE LOW DUE TO DIFFERENCES ALOFT AS
NORTHERN STREAM ENERGY DIVES SOUTHEASTWARD ACROSS THE MIDWEST AND
INTERACTS WITH A SOUTHERN STREAM UPPER LOW. RECENT GUIDANCE STILL
SHOWS CONSIDERABLE SPREAD, LEADING TO LOWER THAN AVERAGE
CONFIDENCE IN THE SENSIBLE WEATHER FORECAST DETAILS LIKE QPF,
ESPECIALLY CONSIDERING THIS IS NOW A DAY 3-5 FORECAST. GFS RUNS
HAVE BEEN PERSISTENTLY FARTHER WEST/INLAND WITH THE LOW TRACK
COMPARED TO THE NON-NCEP GUIDANCE FOR SUNDAY-TUESDAY. STILL DID
NOT FAVOR A SOLUTION AS FAR WEST AS THE GFS, BUT THE OTHER
GUIDANCE HAS CONTINUED ITS TREND IN SHIFTING THE LOW WEST LIKELY
STAYING ONSHORE RATHER THAN IN THE WESTERN ATLANTIC, COMPARED TO
AN OFFSHORE TRACK A DAY AGO. THUS THE WPC FORECAST FOR THE
FRONTS/PRESSURES AND QPF/ERO HAVE SHIFTED INLAND AS WELL. THE
12/18Z ENSEMBLE MEANS SEEMED TO BE LAGGING A BIT IN TERMS OF STILL
SHOWING AN OFFSHORE TRACK OF THE LOW AROUND MONDAY. THERE MAY BE A
COUPLE REASONS FOR THIS...WITH SURFACE HIGH PRESSURE COMING IN
BEHIND THE LOW, MORE LOW PRESSURE WILL BE SEEN FARTHER EAST, AND
ALSO PERHAPS BECAUSE THE LOWS OFFSHORE CAN GAIN STRENGTH MORE
QUICKLY THAN ONSHORE LOWS DUE TO LESS FRICTION, SKEWING THE
AVERAGE EAST/OFFSHORE. CERTAINLY THE ENSEMBLE MEMBERS THEMSELVES
HAVE BEEN SHOWING A LOT OF SPREAD. NOT ONLY IS THERE THIS
WEST-EAST SPREAD AMONG MODELS, THERE IS AMPLE NORTH-SOUTH SPREAD
IN THE TIMING OF THE LOW TOO IN THE DETERMINISTIC MODELS AND THE
ENSEMBLE MEMBERS. THE 12Z CMC SEEMED LIKE AN OUTLIER IN SHOWING A
FAST LOW TRACK NORTHWARD, BUT THE NEWER 00Z CMC SEEMS MORE
REASONABLE. THROUGH SUNDAY-MONDAY THE NEWER 00Z ECMWF/GFS/UKMET
SEEMS REASONABLY SIMILAR TO THEIR PREVIOUS RUNS, BUT BY LATE
MONDAY INTO TUESDAY THERE IS A MARKED TREND TOWARD PIVOTING THE
LOW FARTHER INLAND ACROSS THE LOWER GREAT LAKES OR SO, SO THIS
WILL CONTINUE TO BE MONITORED. ADDITIONAL LOW PRESSURE MAY FORM
AND TRACK ACROSS THE ATLANTIC (LIKELY FARTHER EAST) INTO THE
MIDDLE PART OF NEXT WEEK.
UPPER RIDGING WEST OF THESE FEATURES SEEMS WELL HANDLED BY THE
MODEL GUIDANCE. FARTHER UPSTREAM, AN INITIAL UPPER LOW IN THE
EASTERN PACIFIC WILL SEND SHORTWAVE ENERGY EAST INTO THE WEST
EARLY IN THE WORKWEEK, WHICH ENDS UP SHOWING SOME DIFFERENCES IN
THE AMPLITUDE OF SHALLOW TROUGHING IN THE NORTH-CENTRAL U.S. BY
AROUND MIDWEEK (GFS RUNS WITH MORE TROUGHING). MEANWHILE MODELS
SHOW CONSIDERABLE ENERGY DROPPING SOUTHEAST FROM THE NORTHERN
STREAM AND FORMING A CLOSED LOW IN THE PACIFIC. THERE ARE SOME
TIMING AND PLACEMENT DIFFERENCES WITH THE UPPER LOW BUT NOTHING
TOO EGREGIOUS FOR THE LATE MEDIUM RANGE PERIOD.
THE WPC FORECAST WAS BASED ON THE 12/18Z GUIDANCE THAT WAS
AVAILABLE AT FORECAST PRODUCTION TIME. MORE SPECIFICALLY, A BLEND
OF THE 12Z ECMWF/UKMET/GFS/CMC DETERMINISTIC GUIDANCE ALONG WITH A
BIT OF THE GEFS AND EC ENSEMBLE MEANS WAS USED EARLY IN THE
PERIOD. WITH TIME, GRADUALLY TRANSITIONED TOWARD FAVORING THE
ENSEMBLE MEANS BY MORE THAN HALF DAYS 6-7.
..WEATHER/HAZARDS HIGHLIGHTS
THE SURFACE LOW AND DEEP TROUGHING ALOFT WILL SUPPORT HEAVY
RAINFALL THAT COULD BE WIDESPREAD ON SUNDAY AND MONDAY. AS
MENTIONED THERE ARE SIGNIFICANT DIFFERENCES IN THE PLACEMENT AND
AMOUNTS OF THE RAINFALL. THIS FORECAST CONTINUES THE TREND TOWARD
MORE RAINFALL FARTHER WEST IN THE EASTERN U.S. ON SUNDAY. THE
SLIGHT RISK IN THE EXCESSIVE RAINFALL OUTLOOK WAS EXPANDED
WESTWARD AS WELL TO REFLECT THE LATEST MODEL TRENDS, AND NOW
COVERS MUCH OF THE CAROLINAS INTO SOUTHERN VIRGINIA, WHERE MANY
MODELS HAVE MULTIPLE INCHES OF RAIN. INSTABILITY WILL LIKELY BE
MAXIMIZED NEAR THE COAST FOR SOME HIGHER RAIN RATES, BUT THERE IS
ENOUGH DYNAMICAL SUPPORT FOR LIFT THAT HEAVY RAINFALL CAUSING
INSTANCES OF FLOODING IS POSSIBLE INLAND AS WELL. THEN BY MONDAY,
THE LOW LIFTING QUICKLY NORTHWARD SHOULD SPREAD RAIN (AND PERHAPS
SNOW ON THE BACKSIDE, THOUGH THIS IS A FAIRLY WARM SYSTEM OVERALL)
INTO THE NORTHEAST. A MARGINAL RISK IS IN PLACE FOR THE
POTENTIALLY HEAVY RAINFALL. AN UPGRADE TO A SLIGHT RISK MAY BE
NEEDED ESPECIALLY FOR MAINE GIVEN THE WET ANTECEDENT CONDITIONS
AND THUS SENSITIVITY TO HEAVY RAIN THERE, BUT IN COLLABORATION
WITH THE CARIBOU AND GRAY, MAINE NWS OFFICES, HELD OFF FOR NOW.
PRECIPITATION SHOULD PULL AWAY ON TUESDAY BUT LINGERING COASTAL
SHOWERS COULD CONTINUE.
PRECIPITATION CHANCES WILL BE INCREASING NEXT WEEK FOR CALIFORNIA
AS SHORTWAVE ENERGY SUPPORTING A POSSIBLE ATMOSPHERIC RIVER
APPROACHES. RAIN AMOUNTS AND RATES ON SUNDAY AND MONDAY COULD BE
SUFFICIENT TO CAUSE SOME FLOODING CONCERNS, ESPECIALLY GIVEN SOME
INSTABILITY COULD REACH COASTAL AREAS, SO MARGINAL RISKS ARE IN
PLACE IN THE ERO. PRECIPITATION LOOKS TO SHIFT A BIT NORTH MONDAY
COMPARED TO SUNDAY AND REACH THE PACIFIC NORTHWEST INTO
MONDAY-TUESDAY, INCLUDING SOME HIGHER ELEVATION SNOW, THOUGH
CONFIDENCE IN THE DETAILS REMAINS SOMEWHAT LOW. MEANWHILE THE HIGH
PLAINS TO THE MISSISSIPPI VALLEY WILL BE THE DRY SPOT OVER THE
LOWER 48 THROUGH MUCH OF NEXT WEEK.
AREAS FROM THE WEST INTO MUCH OF THE PLAINS WILL SEE WARMER THAN
NORMAL TEMPERATURES NEXT WEEK UNDERNEATH UPPER RIDGING, WITH
TEMPERATURES 10-20F ABOVE AVERAGE PERIODICALLY AFFECTING THE
NORTHERN/CENTRAL HIGH PLAINS INTO THE UPPER MIDWEST. MEANWHILE THE
NORTHEASTERN U.S. SHOULD SEE ABOVE NORMAL TEMPERATURES EARLY IN
THE WEEK BUT MODERATING BACK TOWARD NORMAL BY TUESDAY BEHIND THE
LOW PRESSURE SYSTEM. THE SOUTHEAST INCLUDING FLORIDA LOOKS TO BE
THE MAIN COOLER THAN AVERAGE SPOT NEXT WEEK 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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