360
FXUS02 KWBC 051859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST TUE DEC 05 2023
VALID 12Z FRI DEC 08 2023 - 12Z TUE DEC 12 2023
..PATTERN OVERVIEW
GUIDANCE SHOWS BROADLY CYCLONIC MEAN FLOW SETTING UP OVER THE
LOWER 48 BY THE WEEKEND INTO NEXT WEEK. INITIALLY SETTING UP THIS
PATTERN WILL BE A VIGOROUS SHORTWAVE EMERGING FROM THE WEST LATE
THIS WEEK, SUPPORTING A DEEPENING SURFACE LOW FORECAST TO TRACK
FROM THE SOUTHERN PLAINS THROUGH THE GREAT LAKES. THIS SYSTEM AND
CONNECTING FRONTS WILL LIKELY PRODUCE SIGNIFICANT PRECIPITATION
OVER THE EASTERN THIRD OF THE COUNTRY PLUS A PERIOD OF BRISK TO
STRONG WINDS FROM PARTS OF THE ROCKIES EASTWARD. UPSTREAM PACIFIC
ENERGY FEEDING INTO NORTH AMERICA, FOLLOWED BY AN AMPLIFYING
EASTERN PACIFIC RIDGE, SHOULD HELP TO REINFORCE THE OVERALL MEAN
TROUGH EARLY NEXT WEEK BUT WITH A MORE POSITIVE TILT THAT SHOULD
LEAD TO MORE BENIGN WEATHER. MOISTURE AHEAD OF THE PACIFIC ENERGY
AND ASSOCIATED SURFACE SYSTEM WILL BRING THE POTENTIAL FOR A BRIEF
PERIOD OF HEAVY PRECIPITATION OVER THE PACIFIC NORTHWEST AROUND
SATURDAY.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODELS/ENSEMBLES AGREE IN CONCEPT ON THE IDEA OF THE LEADING
TROUGH EMERGING FROM THE WEST HELPING TO DEEPEN PLAINS THROUGH
GREAT LAKES/CANADA LOW PRESSURE FROM THE END OF THIS WEEK INTO
NEXT MONDAY. HOWEVER THERE IS STILL CONSIDERABLE SPREAD FOR THE
SURFACE LOW TRACK FROM THE MISSISSIPPI VALLEY ONWARD. RECENT GFS
RUNS AND THE 00Z UKMET REPRESENT THE NORTHERN OR NORTHWESTERN SIDE
OF THE SPREAD (12Z UKMET ADJUSTED CLOSER TO THE MIDDLE) WHILE THE
12Z CMC NOW REPRESENTS THE SOUTHEASTERN SIDE OF THE ENVELOPE. SOME
ECMWF RUNS HAD BEEN ON THE SOUTHERN SIDE OF THE SPREAD BUT THE 00Z
VERSION AND THE FAIRLY CONSISTENT ECENS MEAN ARE CLOSER TO THE
MIDDLE OF THE SPREAD. GEFS MEANS LEAN A BIT NORTH IN DEFERENCE TO
THE OPERATIONAL RUNS BUT ARE A BIT LESS EXTREME AT SOME FORECAST
HOURS. GIVEN THE LACK OF PRONOUNCED CLUSTERING SO FAR, PREFER TO
MAINTAIN AN INTERMEDIATE SOLUTION THAT PROVIDES REASONABLE
CONTINUITY. BEHIND THIS SYSTEM THE GUIDANCE HAS MADE MORE
NOTICEABLE TRENDS OVER THE PAST DAY OR SO, TOWARD A MORE AMPLIFIED
EASTERN PACIFIC UPPER RIDGE BY NEXT TUESDAY AND MORE DOWNSTREAM
TROUGHING. ALSO THE LATEST CONSENSUS SHOWS PACIFIC-ORIGIN LOW
PRESSURE REACHING THE VICINITY OF THE GREAT LAKES AT THAT TIME.
THE LATEST GUIDANCE THROUGH THE 00Z/06Z CYCLES FAVORED AN
OPERATIONAL MODEL COMPOSITE FOR THE FIRST HALF OF THE PERIOD AND
THEN A TRANSITION TO INCLUDE SOME 06Z GEFS/00Z ECENS MEANS BY DAYS
6-7 MONDAY-TUESDAY.
..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 DAY 5 EXCESSIVE RAINFALL OUTLOOK
INTRODUCED A MARGINAL RISK OF EXCESSIVE RAINFALL FOR WESTERN
WASHINGTON AND OREGON INTO FAR NORTHWESTERN CALIFORNIA WITH AN
EMBEDDED SLIGHT RISK AREA ALONG THE COAST. THE UPDATED ERO
ISSUANCE MADE ONLY MODEST ADJUSTMENT TO THESE AREAS. 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 SHOULD QUICKLY
TREND LIGHTER AFTER SATURDAY BUT WITH SOME UNCERTAINTY OVER
SPECIFICS.
THE SURFACE LOW THAT WILL DEEPEN AS IT TRACKS FROM THE SOUTHERN
PLAINS INTO THE GREAT LAKES LATE FRIDAY INTO THE WEEKEND WILL
LIKELY PRODUCE WIDESPREAD RAINFALL 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. MEANWHILE THE DAY 5 ERO HAS
INTRODUCED A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
LOWER OHIO VALLEY SOUTH TO THE GULF COAST. MOISTURE ANOMALIES AND
INSTABILITY ALLOW FOR SOME ENHANCEMENT OF RAINFALL, AND ALTHOUGH
FRONTAL PROGRESSION MAY KEEP TOTALS FROM BECOMING TOO EXTREME,
RAIN RATES COULD STILL BE INTENSE FOR A TIME TO YIELD THE
LOCALIZED TOTALS THAT SOME MODELS ARE PRODUCING. SPECIFICS WITHIN
THIS AREA WILL STILL TAKE TIME TO BE RESOLVED THOUGH. AS THE LOW
PUSHES NORTHEAST ON SUNDAY, PRECIPITATION WILL SPREAD TO THE EAST
COAST AND UP INTO NEW ENGLAND WHILE THUNDERSTORM CHANCES WILL
DECREASE. THE NORTHERN MID-ATLANTIC INTO NORTHEAST WILL SEE THE
BEST POTENTIAL FOR LOCALLY HEAVY RAINFALL ON SUNDAY. SOME SNOW
WILL BE POSSIBLE IN THE NORTHWESTERN PART OF THE PRECIPITATION
SHIELD, BUT DETAILS ARE CURRENTLY UNCERTAIN DUE TO LACK OF
GUIDANCE AGREEMENT FOR SURFACE LOW TRACK. PROBABILITIES FOR A
QUARTER INCH OF LIQUID IN THE FORM OF SNOW ARE LOW ON AN ABSOLUTE
BASIS, BUT WHAT PROBABILITIES DO EXIST COVER PARTS OF THE MIDWEST
INTO THE GREAT LAKES. ALSO SOME SNOW MAY FALL AFTER SYSTEM
PASSAGE, MAINLY OVER THE GREAT LAKES INTO APPALACHIANS. FINALLY,
THE GRADIENT AROUND THIS DEVELOPING STORM WILL PRODUCE A BROAD
AREA OF BRISK TO STRONG WINDS. THE GREATEST WIND THREATS CURRENTLY
APPEAR TO BE BEHIND THE STORM OVER PORTIONS OF THE ROCKIES, BUT
OTHER AREAS AS FAR EAST AS THE EAST COAST (INCLUDING IN THE WARM
SECTOR) SHOULD ALSO MONITOR FORECASTS FOR WIND SPEEDS. THE EASTERN
U.S. WILL TREND DRIER AFTER SUNDAY AS HIGH PRESSURE BUILDS INTO
THE SOUTH AND MID-ATLANTIC.
EXPECT WELL ABOVE NORMAL TEMPERATURES IN THE WARM SECTOR AHEAD OF
THE DEVELOPING PLAINS-GREAT LAKES SYSTEM. FROM FRIDAY INTO THE
WEEKEND, HIGHS SHOULD BE 10-20F ABOVE NORMAL FROM THE PLAINS INTO
THE EAST WITH SOME MORNING LOWS EVEN WARMER VERSUS AVERAGE.
FRONTAL PROGRESSION WILL BRING TEMPERATURES CLOSER TO NORMAL (OR
EVEN A LITTLE BELOW NORMAL OVER THE SOUTHERN PLAINS) OVER THE
CENTRAL U.S. BY THE WEEKEND. ABOVE NORMAL READINGS SHOULD BE
CONFINED TO THE NORTHEAST BY NEXT MONDAY. THE INTERMOUNTAIN WEST
AND CENTRAL-SOUTHERN ROCKIES WILL TEND TO SEE MODERATELY BELOW
NORMAL HIGHS THROUGH THE PERIOD.
RAUSCH/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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