088
FXUS02 KWBC 071900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST THU DEC 07 2023
VALID 12Z SUN DEC 10 2023 - 12Z THU DEC 14 2023
..PATTERN OVERVIEW
A PERIOD OF BROAD CYCLONIC MEAN FLOW ALOFT FROM THE SHORT-TERM
TIME FRAME INTO EARLY NEXT WEEK SHOULD TRANSITION BY
WEDNESDAY-THURSDAY TOWARD A RIDGE BUILDING OVER THE NORTHWESTERN
THEN CENTRAL U.S., A CLOSED LOW FORMING OVER OR NEAR THE
SOUTHWEST, AND THE NORTHERN STREAM TROUGH BECOMING MORE CONFINED
TO THE NORTHEASTERN QUADRANT OF THE LOWER 48. A LEADING AMPLIFIED
UPPER SYSTEM/WAVY SURFACE FRONT WILL BRING SIGNIFICANT
PRECIPITATION FROM THE GULF COAST TO THE GREAT LAKES AND NORTHEAST
SUNDAY INTO EARLY MONDAY. MUCH OF THE COUNTRY WILL TREND DRIER
AFTER PASSAGE OF THE EASTERN SYSTEM AND ANOTHER FEATURE INITIALLY
BRINGING PRECIPITATION TO THE NORTHWEST. THE SOUTHERN HIGH PLAINS
AND FLORIDA WILL HAVE THE BEST POTENTIAL FOR ORGANIZED
PRECIPITATION BY MIDWEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GUIDANCE CONTINUES TO BE ON ITS GENERAL PATH TOWARD BETTER
AGREEMENT AND CONTINUITY FOR EVOLUTION OF THE SUNDAY-MONDAY SYSTEM
AFFECTING THE EAST WITH WEAK FRONTAL WAVINESS AS OF EARLY SUNDAY
BECOMING STEADILY STRONGER AS IT CROSSES NEW ENGLAND BY EARLY
MONDAY AND THEN CONTINUES INTO CANADA. THE NEW 12Z CMC LOOKS LIKE
A NOTABLE EXTREME IN BEING SOMEWHAT WEAKER AND SUPPRESSED VERSUS
CONSENSUS FROM EARLY MONDAY ONWARD. PACIFIC SHORTWAVE ENERGY
SUPPORTING THE INITIAL NORTHWEST PRECIPITATION EVENT HAS GENERALLY
BEEN GRAVITATING TOWARD THE SLOWER/STRONGER SIDE OF THE PRIOR
SPREAD. THEN OPERATIONAL MODELS ARE MAINTAINING RECENT SIGNALS FOR
IMMEDIATELY TRAILING ENERGY TO FORM A LOW OVER OR NEAR THE
SOUTHWEST BY MIDWEEK, WITH ENSEMBLE MEAN TRENDS SUPPORTING THE
OPERATIONAL MODEL AVERAGE. HOWEVER BY NEXT THURSDAY THE PAST
COUPLE ECMWF RUNS (00Z AND NEW 12Z VERSIONS) ARE MORE AGGRESSIVE
THAN OTHER GUIDANCE WITH ANOTHER SHORTWAVE REACHING THE WEST COAST
AND HELPING TO EJECT/OPEN UP THE SOUTHWEST UPPER LOW. THUS AT THAT
TIME PREFERENCE LEANS TOWARD THE MAJORITY THAT MAINTAINS A BETTER
DEFINED UPPER LOW.
A FORECAST BLEND STARTING WITH MORE 06Z GFS/00Z ECMWF INPUT
RELATIVE TO THE 00Z CMC/UKMET PROVIDED A GOOD BALANCE OF LATEST
TRENDS AND CONTINUITY FOR THE FIRST HALF OF THE PERIOD. WITH THE
MEANS SUPPORTING THE OPERATIONAL GUIDANCE IN PRINCIPLE LATER IN
THE PERIOD, ASIDE FROM THE AFOREMENTIONED ECMWF QUESTION MARKS
OVER THE WEST, DAYS 6-7 WEDNESDAY-THURSDAY NEEDED TO INCORPORATE
ONLY A MODERATE INPUT OF THE 06Z GEFS/00Z ECENS MEANS. THE BLEND
INCLUDED SOME 12Z/06 ECMWF INPUT LATE TO TEMPER THE MORE RECENT
ECMWF IDEAS OVER THE WEST.
..WEATHER/HAZARDS HIGHLIGHTS
PRECIPITATION OVER THE PACIFIC NORTHWEST SHOULD EXTEND INTO
SUNDAY, WITH RECENT TRENDS FAVORING POTENTIAL FOR SOME MEANINGFUL
TOTALS. THE DAY 4 EXCESSIVE RAINFALL OUTLOOK PERIOD STARTING EARLY
SUNDAY STILL HAS SOME ENHANCED DEEP MOISTURE BUT WHICH DEPARTS
VERY QUICKLY, WHILE MOST GUIDANCE THUS FAR SUGGESTS 1-3 HOUR RAIN
RATES LOW ENOUGH NOT TO BE HAZARDOUS. THEREFORE THE OUTLOOK
MAINTAINS NO RISK AREA. WILL CONTINUE TO MONITOR TRENDS GIVEN THE
WET CONDITIONS LEADING INTO THE OUTLOOK PERIOD AND THE WETTER END
OF THE GUIDANCE SPREAD POSSIBLY MERITING A RISK AREA.
IN THE EAST, A BROAD AREA OF SIGNIFICANT PRECIPITATION WILL
STRETCH FROM THE FLORIDA PANHANDLE THROUGH THE MID-ATLANTIC TO THE
NORTHEAST ON SUNDAY AS A STRONG WAVY FRONT PASSES THROUGH. THE DAY
4 ERO MAINTAINS A MARGINAL RISK OF EXCESSIVE RAINFALL IN THE
UPDATE ISSUANCE. THE PROGRESSIVE NATURE OF THIS SYSTEM WILL TEND
TO KEEP RAINFALL TOTALS FROM BEING TOO EXTREME. HOWEVER, ANOMALOUS
MOISTURE AND SUFFICIENT INSTABILITY (OVER SOUTHERN AREAS) SHOULD
STILL SUPPORT RAIN RATES HIGH ENOUGH TO PRODUCE SOME FLOODING
CONCERNS, AND THERE MAY ALSO BE THUNDERSTORM POTENTIAL IN THE WARM
SECTOR OVER THE SOUTHEAST. WITHIN THE MARGINAL RISK AREA, THERE
MAY BE SOME RELATIVELY GREATER POTENTIAL FOR RUNOFF ISSUES IN THE
NORTHERN MID-ATLANTIC/NORTHEAST URBAN CORRIDOR AND OVER PARTS OF
THE NORTHEAST DUE TO WET GROUND/MELTING SNOW COVER (BEFORE OR
DURING THE EVENT). GUIDANCE SUGGESTING THAT NOT TOO MUCH
INSTABILITY REACHES NORTHERN AREAS, PLUS SOME ONGOING DIFFERENCES
FOR EXACT LOCATION/MAGNITUDE OF HEAVIEST RAINFALL, PRECLUDE ANY
EMBEDDED SLIGHT RISK AREA AT THIS TIME. ENHANCED RAINFALL MAY
EXTEND INTO MAINE EARLY IN THE DAY 5 ERO PERIOD ON MONDAY BUT WITH
SIGNALS NOT YET PRONOUNCED ENOUGH TO MERIT INTRODUCTION OF A
MARGINAL RISK AREA.
REGARDING OTHER ASPECTS OF THE FORECAST, THE SUNDAY-MONDAY EVENT
OVER THE EAST WILL LIKELY CONTAIN SOME WINTRY WEATHER ON THE BACK
SIDE OF THE PRECIPITATION SHIELD. BEST POTENTIAL FOR MEANINGFUL
SNOWFALL (AT LEAST 30 PERCENT PROBABILITY FOR 0.25 INCH LIQUID IN
THE FORM OF SNOW) SHOULD BE OVER THE CENTRAL APPALACHIANS, TO THE
LEE OF THE EASTERN GREAT LAKES, AND FAR NORTHWESTERN NEW ENGLAND.
A PERIOD OF BRISK TO STRONG WINDS WILL ALSO BE POSSIBLE ALONG THE
EAST COAST. THE PRECIPITATION OVER THE NORTHWEST EARLY IN THE
PERIOD WILL INCLUDE SNOW OVER PORTIONS OF THE CASCADES INTO THE
NORTHERN ROCKIES, FOLLOWED BY A DRIER TREND. BY MIDWEEK ONE
PRECIPITATION AREAS OF NOTE SHOULD BE OVER THE SOUTHERN
ROCKIES/HIGH PLAINS AS AN UPPER LOW SETTLES OVER THE SOUTHWEST AND
MOISTURE INTERACTS WITH A FRONT. SOME OF THIS PRECIPITATION MAY BE
SNOW, WITH TYPICAL ELEVATION/COVERAGE UNCERTAINTY. MEANWHILE
EASTERLY FLOW SETTING UP TO THE SOUTH OF STRONG EASTERN U.S. HIGH
PRESSURE SHOULD PROMOTE PERIODS OF RAIN OVER FLORIDA. FINALLY,
LIGHT PRECIPITATION MAY REACH THE PACIFIC NORTHWEST BY NEXT
THURSDAY.
TEMPERATURES NEAR THE EAST COAST WILL BE WELL ABOVE NORMAL ON
SUNDAY, AND CONTINUING OVER NEW ENGLAND INTO MONDAY, AHEAD OF THE
APPROACHING COLD FRONT. THERE WILL BE DECENT COVERAGE OF AT LEAST
PLUS 10-20F ANOMALIES AND A FEW LOCATIONS IN THE NORTHERN
MID-ATLANTIC/SOUTHERN NEW ENGLAND COULD CHALLENGE DAILY RECORD
HIGHS. TEMPERATURES BEHIND THE FRONT WILL TREND TO NEAR OR
SOMEWHAT BELOW NORMAL, WITH THE COOLEST ANOMALIES LIKELY TO
PROGRESS ACROSS THE SOUTHERN TIER SUNDAY-TUESDAY. THE EVOLVING
PATTERN WILL FAVOR A WARMER TREND OVER THE NORTHERN-CENTRAL PARTS
OF THE CENTRAL U.S. NEXT WEEK. ANOMALIES SHOULD BECOME MOST
PRONOUNCED BY NEXT THURSDAY WITH MOST NORTHERN AREAS REACHING
10-20F ABOVE NORMAL.
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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