762
FXUS02 KWBC 300657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST THU NOV 30 2023
VALID 12Z SUN DEC 03 2023 - 12Z THU DEC 07 2023
...AN ACTIVE AND WET PATTERN ON TAP FOR THE NORTHWEST INCLUDING
HEAVY RAIN AND INLAND HEAVY SNOW THREATS...
..OVERVIEW
AN AMPLIFIED AND ACTIVE WEATHER PATTERN IS EXPECTED TO CONTINUE
INTO THE MEDIUM RANGE PERIOD, ESPECIALLY ACROSS THE WESTERN U.S.
AS A SERIES OF ATMOSPHERIC RIVERS IMPACTS THE REGION RAISING
CONCERNS FOR HEAVY RAINFALL ALONG THE WASHINGTON/OREGON COAST AND
FAVORABLE WESTERN FACING SLOPES OF THE CASCADES WITH HEAVY SNOW
FOR PARTS OF THE NORTHERN ROCKIES. DOWNSTREAM, AN AMPLIFIED
TROUGH, PERIODICALLY REINFORCED BY SHORTWAVE ENERGY, WILL SLIDE
FROM THE CENTRAL U.S. INTO THE EAST SUNDAY-MONDAY, WITH A SECOND
SHORTWAVE ACROSS THE NORTH-CENTRAL U.S. TO THE NORTHEAST MID-NEXT
WEEK. IN BETWEEN THIS TROUGH AND THE NEXT SYSTEM OUT OF THE EAST
PACIFIC, A STRONG RIDGE LOOKS TO BUILD INITIALLY OVER THE WEST ON
TUESDAY, MOVING INTO THE PLAINS WEDNESDAY-THURSDAY BRINGING WELL
ABOVE NORMAL TEMPERATURES, ESPECIALLY ACROSS THE NORTHERN AND
CENTRAL PLAINS.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
ON THE LARGE SCALE, THE GUIDANCE SHOWS PRETTY GOOD AGREEMENT
ACROSS THE BOARD, BUT WITH PLENTY OF VARIABILITY IN THE DETAILS.
OUT WEST, MODELS AGREE ON THE PRESENCE OF MULTIPLE ARS DURING THE
PERIOD (THE FIRST ON SUNDAY, AND THE NEXT MOVING IN LATE
MONDAY/EARLY TUESDAY) BUT STILL WITH SOME UNCERTAINTY IN THE AR
TIMING/STRENGTH/DURATION AND IN THE DETAILS AND EXACT LOCATIONS OF
MAXIMUM QPF. CONTINUING TO FEEL THE NBM IS TOO WET WITH QPF, SO
THE WPC FORECAST TEMPERED THAT DOWN SOME USING DETERMINISTIC
GUIDANCE TO ACCOUNT FOR THE LINGERING UNCERTAINTIES. IN THE
MIDWEST AND THE EAST, RUN TO RUN VARIABILITY AND TIMING OF
INDIVIDUAL SHORTWAVES THIS WEEKEND/EARLY NEXT WEEK STILL
TRANSLATES TO EVEN SOME REGIONAL SENSIBLE WEATHER/PRECIPITATION
UNCERTAINTIES. THROUGH YESTERDAY'S 12Z/18Z RUNS, THE GFS IS
NOTABLY FAST WITH AN INITIAL SHORTWAVE INTO THE EAST ON
SUNDAY/MONDAY, WHILE THE UKMET IS A LITTLE SLOWER THAN CONSENSUS
WITH THE OVERALL TROUGH PROGRESSION. WITH THE NEXT SYSTEM DROPPING
INTO THE NORTH-CENTRAL U.S. TUESDAY-WEDNESDAY, THE CMC GETS TOO
AMPLIFIED/SLOW WHILE THE GFS/ECMWF AND ENSEMBLE MEANS SUGGESTS A
FLATTER WAVE AND FASTER PROGRESSION.
ACCORDINGLY, THE WPC FORECAST BLEND FOR DAYS 3-4 FAVORED THE ECMWF
AND THE CMC WITH SOME ENSEMBLE MEAN GUIDANCE EVEN IN THE EARLY
PART OF THE PERIOD TO HELP SMOOTH OUT DISCONTINUITIES. BY DAY 5
AND BEYOND, REPLACED THE CMC WITH THE GFS AND INCREASED MORE OF
THE ENSEMBLE MEANS TO ACCOUNTS FOR LARGER SCALE DIFFERENCES.
OVERALL, THIS MAINTAINED REASONABLE CONTINUITY WITH THE PREVIOUS
WPC FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
OUT WEST, PRECIPITATION WILL BE ONGOING AS THIS FORECAST PERIOD
BEGINS SUNDAY, WITH A SERIES OF ATMOSPHERIC RIVERS INTO THE
NORTHWEST CONTINUING WELL INTO NEXT WEEK, WITH GROWING TOTALS AND
IMPACTS POSSIBLE. SOME MODEST RAINFALL TOTALS ARE LIKELY ALONG THE
COAST WITH BOTH EVENTS SUNDAY AND AGAIN LATE MONDAY-WEDNESDAY.
WITH HEAVY RAIN ALSO IN THE FORECAST DURING THE SHORT RANGE, THE
SOILS AND RIVERS MAY BE PRIMED INCREASING THE RISK FOR FLOODING
AND FLASH FLOODING. SNOW LEVELS WITH BOTH SYSTEMS WILL BE VERY
HIGH SO ESPECIALLY WESTERN FACING SLOPES OF THE CASCADES MAY BE
SUBJECT TO AN ADDITIONAL RISK OF FLOODING DUE TO SNOW MELT AS ANY
ACCUMULATING SNOW WILL BE CONFINED TO THE VERY HIGHEST PEAKS (OR
FARTHER INLAND). GIVEN THIS, A MARGINAL RISK FOR EXCESSIVE
RAINFALL WAS INCLUDED ON THE DAY 4 AND 5 EROS ACROSS THIS REGION.
IT IS POSSIBLE A SLIGHT RISK WILL BE NEEDED IN SUBSEQUENT UPDATES
OF THE ERO, BUT AT THIS POINT THERE WAS STILL ENOUGH UNCERTAINTY
TO PRECLUDE THAT. FARTHER INLAND ACROSS THE NORTHERN ROCKIES,
HEAVY SNOW WILL BE POSSIBLE INTO SUNDAY, AND POSSIBLY AGAIN MID
NEXT WEEK WITH THE NEXT SYSTEM.
DOWNSTREAM, EJECTING SHORTWAVE ENERGY AND A LEADING/WAVY FRONTAL
SYSTEM MAY CONTINUE TO SUPPORT RAINFALL INTO SUNDAY AND MONDAY
ACROSS THE SOUTHEAST, WITH MUCH OF THE EASTERN U.S. ALSO LOOKING
UNSETTLED THROUGH NEXT WEEK, ALBEIT WITH HIGHER UNCERTAINTY ON
RAINFALL AMOUNTS GIVEN SHORTWAVE DIFFERENCES. HOWEVER, THE
APPROACH AND PASSAGE OF SEVERAL SHORTWAVES AND ADVANCE OF AN
AMPLIFIED MAIN UPPER TROUGH SHOULD SPAWN SEVERAL WAVY FRONTS. SOME
OF THE PRECIPITATION COULD BE IN THE FORM OF SNOW ACROSS PARTS OF
THE GREAT LAKES AND NORTHEAST. THE TERRAIN OF NORTHERN NEW ENGLAND
MAY FOCUS A POSSIBLE SNOWFALL THREAT GIVEN LONGEVITY OF THE
PATTERN AND COLD AIR AVAILABILITY LATER WEEKEND/EARLY NEXT WEEK.
THROUGH SUNDAY, TEMPERATURES ACROSS THE EAST MAY BE MODERATELY
ABOVE NORMAL, WITH SLIGHTLY BELOW NORMAL TEMPERATURES ACROSS THE
INTERIOR SOUTHWEST. THE WESTERN U.S. SHOULD TREND WARMER WITH TIME
AND ESPECIALLY BY NEXT TUESDAY-THURSDAY IN THE NORTHERN HIGH
PLAINS/PLAINS WHERE DAYTIME HIGHS COULD BE 15 TO 25 DEGREES ABOVE
NORMAL. MEANWHILE, THE EASTERN U.S. SHOULD TREND BACK TOWARDS
NORMAL/SLIGHTLY COOLER THAN NORMAL NEXT WEEK AS REINFORCED
TROUGHING AFFECTS THE AREA.
SANTORELLI
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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