600
FXUS02 KWBC 050658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST TUE DEC 05 2023
VALID 12Z FRI DEC 08 2023 - 12Z TUE DEC 12 2023
..PATTERN OVERVIEW
MEAN UPPER-LEVEL TROUGHING WILL GRADUALLY SWING EAST ACROSS THE
CONTINENTAL U.S. THROUGH THE WEEKEND, ACCOMPANIED BY A DEEPENING
SURFACE LOW THAT WILL TRACK FROM THE SOUTHERN PLAINS TO THE
EASTERN GREAT LAKES. THIS SYSTEM IS FORECAST TO BRING MODERATE TO
LOCALLY HEAVY RAIN TO AREAS EAST OF THE MISSISSIPPI THIS WEEKEND.
HEAVY RAINFALL WILL ALSO BE POSSIBLE IN THE PACIFIC NORTHWEST AS
ANOTHER SURGE OF PACIFIC MOISTURE MOVES INTO THE REGION ON
SATURDAY AHEAD OF AN APPROACHING FRONTAL SYSTEM. THE UPPER-LEVEL
MEAN TROUGH WILL FLATTEN OUT OVER THE EAST EARLY NEXT WEEK WHILE
RIDGING BUILDS OVER THE WEST COAST, WHICH WILL LEAD TO RELATIVELY
BENIGN WEATHER.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE PATTERN IN
THE BEGINNING OF THE PERIOD, SO A FAIRLY EVEN BLEND OF THE
AVAILABLE DETERMINISTIC SOLUTIONS WAS USED FOR THE DAYS 3-4
FORECAST. THERE ARE SOME SIGNIFICANT TIMING DIFFERENCES BETWEEN
THE MODELS WITH THE UPPER LEVEL WAVE THAT WILL SUPPORT THE
DEEPENING SURFACE LOW THIS WEEKEND. THE GFS AND ECMWF FORM LOWS OF
SIMILAR STRENGTHS OVER THE SOUTHERN PLAINS, BUT THEN THE GFS PULLS
THE LOW TO THE NORTHEAST MUCH FASTER THAN THE ECMWF. TO GET A
MIDDLE OF THE ROAD SOLUTION, ENSEMBLE MEANS WERE ADDED INTO THE
BLEND FOR THE DAYS 5-7 FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
AFTER THE ONGOING ATMOSPHERIC RIVER EVENT ENDS IN THE PACIFIC
NORTHWEST, THERE WILL BE A BRIEF BREAK IN HEAVY PRECIPITATION ON
FRIDAY BEFORE ANOTHER SURGE OF MOISTURE REACHES THE COAST ON
SATURDAY. THE REGION WILL BE SENSITIVE TO ADDITIONAL RAINFALL
AFTER THE FIRST EVENT, SO A MARGINAL RISK OF EXCESSIVE RAINFALL
WAS INTRODUCED FOR WESTERN WASHINGTON AND OREGON AND FAR
NORTHWESTERN CALIFORNIA WITH AN EMBEDDED SLIGHT RISK AREA ALONG
THE COAST. IN ADDITION TO HEAVY RAINFALL, WIDESPREAD MODERATE TO
LOCALLY HEAVY SNOW WILL BE POSSIBLE IN THE HIGHER ELEVATIONS OF
THE CASCADES AND NORTHERN ROCKIES AS MOISTURE PUSHES INLAND.
THE SURFACE LOW THAT WILL DEEPEN OVER THE SOUTHERN PLAINS FRIDAY
INTO SATURDAY WILL LIKELY CAUSE WIDESPREAD SHOWERS FROM THE GULF
COAST UP THROUGH THE MID-MISSISSIPPI VALLEY AND INTO THE OHIO
VALLEY AND SOUTHERN GREAT LAKES REGION. CONDITIONS WILL BE
FAVORABLE FOR THUNDERSTORM DEVELOPMENT ON SATURDAY FROM THE GULF
COAST TO THE MID-MISSISSIPPI VALLEY, AND THE STORM PREDICTION
CENTER HAS HIGHLIGHTED THIS AREA FOR A SEVERE THUNDERSTORM THREAT.
AS THE LOW PUSHES NORTHEAST ON SUNDAY, PRECIPITATION WILL SPREAD
TO THE EAST COAST AND UP INTO NEW ENGLAND AND THUNDERSTORM CHANCES
WILL DECREASE. LOCALLY HEAVY RAINFALL WILL BE POSSIBLE FROM THE
MID-SOUTH TO THE OHIO VALLEY ON SATURDAY AND FOR PARTS OF THE
MID-ATLANTIC AND NORTHEAST ON SUNDAY. AT THIS TIME, UNCERTAINTY IN
THE TIMING AND LOCATION IS STILL TOO HIGH TO HIGHLIGHT SPECIFIC
AREAS THAT MAY HAVE A RISK OF FLASH FLOODING. PRECIPITATION WILL
COME TO AN END IN THE EAST ON MONDAY AS THE SYSTEM PUSHES
OFFSHORE, AND HIGH PRESSURE WILL BUILD INTO THE CENTRAL U.S. AND
SOUTHEAST.
TEMPERATURES WILL BE MILD FROM THE SOUTHERN PLAINS TO THE UPPER
MIDWEST AND GREAT LAKES ON FRIDAY, REACHING 10-20 DEGREES ABOVE
AVERAGE FOR THIS TIME OF YEAR. WARMER AIR WILL SHIFT INTO THE
MID-ATLANTIC AND NORTHEAST OVER THE WEEKEND, AND THEN TEMPERATURES
WILL RETURN TO NEAR NORMAL EARLY NEXT WEEK. THOUGH NOT ABNORMAL
FOR THIS TIME OF YEAR, COLDER TEMPERATURES WILL PERSIST ACROSS THE
INTERMOUNTAIN WEST THROUGH THE PERIOD.
DOLAN
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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