339
FXUS02 KWBC 060700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST WED DEC 06 2023
VALID 12Z SAT DEC 09 2023 - 12Z WED DEC 13 2023
..PATTERN OVERVIEW
BROAD CYCLONIC MEAN FLOW IS FORECAST TO PERSIST OVER THE EASTERN
TWO-THIRDS OF THE LOWER 48 THROUGH MOST OF THE MEDIUM RANGE
PERIOD. INITIALLY, A DEEP UPPER-LEVEL SHORTWAVE WILL EMERGE FROM
THE WEST LATE THIS WEEK, SPAWNING A SURFACE LOW OVER THE SOUTHERN
PLAINS ON FRIDAY THAT WILL TRACK TO THE NORTHEAST THIS WEEKEND.
SIGNIFICANT PRECIPITATION IS FORECAST FROM THE GULF COAST TO THE
GREAT LAKES AS THIS SYSTEM PROGRESSES, WHICH COULD CAUSE FLOODING
CONCERNS. MEANWHILE, THE PACIFIC NORTHWEST WILL BE DEALING WITH
ANOTHER SURGE OF PACIFIC MOISTURE ON SATURDAY THAT MAY RESULT IN
SCATTERED INSTANCES OF FLOODING. EARLY NEXT WEEK, AN AMPLIFYING
PACIFIC RIDGE WILL HELP TO REINFORCE THE MEAN TROUGH OVER THE
EAST, BUT WITH A POSITIVE TILT THAT WILL LEAD TO MORE BENIGN
WEATHER.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE MOST IMPACTFUL MODEL DIFFERENCES CONTINUE TO CONCERN THE LOW
PRESSURE SYSTEM THAT WILL TRACK FROM THE SOUTHERN PLAINS TO THE
NORTHEAST THIS WEEKEND. MODELS ARE IN FAIRLY GOOD AGREEMENT ON THE
TIMING OF THE LOW ON SATURDAY, BUT NOTABLE DIFFERENCES ARISE AS IT
APPROACHES THE NORTHEAST ON SUNDAY AND SUNDAY NIGHT. THE UKMET AND
THE GFS ARE THE MOST PROGRESSIVE WITH THE SYSTEM WHILE THE ECMWF
IS ON THE SLOWER SIDE, AND THE CMC FALLS IN BETWEEN. THE ENSEMBLE
MEANS FAVOR AN IN BETWEEN SOLUTION, SO A GENERAL DETERMINISTIC
MODEL BLEND WAS USED FOR DAYS 3-4. MODEL GUIDANCE SPREAD IS AS
EXPECTED THROUGH THE REST OF THE PERIOD. THERE IS A GENERAL
AGREEMENT ON THE MEAN TROUGH BECOMING POSITIVELY TILTED WITH SOME
DIFFERENCES IN TIMING OF SHORTWAVES CIRCULATING THROUGH. TO SMOOTH
OUT DIFFERENCES, THE GEFS AND EC ENSEMBLE MEANS WERE ADDED TO THE
BLEND FOR DAYS 5-7.
..WEATHER/HAZARDS HIGHLIGHTS
ANOTHER SURGE OF PACIFIC MOISTURE IS FORECAST TO IMPACT THE
PACIFIC NORTHWEST ON SATURDAY, AND THE REGION WILL BE PARTICULARLY
SENSITIVE TO ANY ADDITIONAL RAINFALL AFTER RECEIVING SIGNIFICANT
RAINFALL EARLIER IN THE WEEK. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK
CONTINUES TO SHOW A MARGINAL RISK OF EXCESSIVE RAINFALL FOR
WESTERN WASHINGTON AND OREGON INTO 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. PRECIPITATION WILL LIGHTEN UP
SIGNIFICANTLY ACROSS THE REGION ON SUNDAY.
THE SURFACE LOW THAT WILL SOAK THE EASTERN U.S. THIS WEEKEND WILL
LIKELY PRODUCE SIGNIFICANT RAINFALL FROM THE GULF COAST TO THE
GREAT LAKES AND INTERIOR NORTHEAST. A MARGINAL RISK AREA WAS
MAINTAINED ON THE DAY 4 EXCESSIVE RAINFALL OUTLOOK FROM THE GULF
COAST TO PARTS OF THE MID-MISSISSIPPI AND LOWER OHIO VALLEYS, AND
ANOTHER MARGINAL RISK AREA WAS INTRODUCED IN THE DAY 5 EXCESSIVE
RAINFALL OUTLOOK FROM THE FLORIDA PANHANDLE THROUGH THE
MID-ATLANTIC AND INTO THE INTERIOR NORTHEAST. THE PROGRESSIVE
NATURE OF THIS SYSTEM WILL HELP TO LIMIT RAINFALL TOTALS; HOWEVER,
ANOMALOUS MOISTURE AND SUFFICIENT INSTABILITY SHOULD STILL SUPPORT
RAIN RATES HIGH ENOUGH TO PRODUCE SOME FLOODING CONCERNS. IN
ADDITION TO POTENTIAL FLOODING, THUNDERSTORMS WILL LIKELY THREATEN
AREAS ALONG THE GULF COAST AND THE LOWER MISSISSIPPI VALLEY ON
SATURDAY. THE STORM PREDICTION CENTER HAS HIGHLIGHTED AN AREA
CENTERED ON EAST TEXAS AND THE ARK-LA-MISS WHERE SEVERE
THUNDERSTORMS ARE EXPECTED. SOME SNOW WILL BE POSSIBLE IN THE
NORTHWESTERN SIDE OF THE PRECIPITATION SHIELD FROM THE CENTRAL
PLAINS/MIDWEST TO THE GREAT LAKES, BUT DETAILS ARE CURRENTLY
UNCERTAIN DUE TO LACK OF GUIDANCE AGREEMENT FOR SURFACE LOW TRACK.
SOME SNOW MAY ALSO FALL AFTER SYSTEM PASSES, MAINLY OVER THE GREAT
LAKES INTO APPALACHIANS. FINALLY, BRISK TO STRONG WINDS ARE
FORECAST TO DEVELOP OVER PORTIONS OF THE ROCKIES AND PLAINS AS THE
PRESSURE GRADIENT TIGHTENS AROUND THE LOW ON SATURDAY.
OVERALL, THE LOWER 48 WILL TREND DRIER THROUGH THE REST OF THE
PERIOD, BUT THERE ARE SOME LOWER PRECIPITATION CHANCES (20-40%)
THAT WILL ACCOMPANY A COUPLE OF RELATIVELY WEAKER FRONTAL SYSTEMS
MOVING ACROSS THE NATION. IN TERMS OF TEMPERATURES, MUCH OF THE
WEST WILL EXPERIENCE BELOW NORMAL TEMPERATURES IN THE WAKE OF THE
UPPER-LEVEL SHORTWAVE ON SATURDAY, AND WELL ABOVE NORMAL
TEMPERATURES ARE EXPECTED IN THE WARM SECTOR AHEAD OF THE
DEVELOPING PLAINS-GREAT LAKES SYSTEM. HIGHS SHOULD BE 10-25
DEGREES ABOVE NORMAL FROM THE LOWER MISSISSIPPI VALLEY TO THE
NORTHEAST THIS WEEKEND. TEMPERATURES WILL RETURN TO NEAR NORMAL
AFTER THE SYSTEM PASSES, AND NEAR NORMAL TEMPERATURES (OR
TEMPERATURES WITHIN A FEW DEGREES OF NORMAL) WILL PERSIST ACROSS
MOST OF THE NATION THROUGH EARLY NEXT WEEK.
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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