176
FXUS02 KWBC 171905
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
205 PM EST SUN NOV 17 2024
VALID 12Z WED NOV 20 2024 - 12Z SUN NOV 24 2024
...STRONG, LONG-DURATION ATMOSPHERIC RIVER WILL TAKE AIM AT
NORTHERN CALIFORNIA/SOUTHWESTERN OREGON FOR MIDWEEK INTO LATE
WEEK...
...PATTERN CHANGE FOR EAST-CENTRAL TO EASTERN U.S. WITH COOLER
TEMPERATURES AND COLD RAIN AND HIGH ELEVATION SNOW...
..OVERVIEW
A LONG DURATION ATMOSPHERIC RIVER IS FORECAST TO AFFECT THE WEST
COAST THIS WEEK AS UPPER AND SURFACE LOWS/FRONTAL SYSTEMS DIRECT A
STRONG MOISTURE PLUME INTO NORTHERN CALIFORNIA/SOUTHWESTERN OREGON
IN PARTICULAR, WHERE SEVERAL INCHES OF RAIN AND MOUNTAIN SNOW ARE
LIKELY. MEANWHILE A DEEP UPPER LOW DEVELOPING ACROSS THE GREAT
LAKES AND NORTHEAST U.S WILL MEANDER SLOWLY IN PLACE LATER THIS
WEEK WITH A BLOCKING HIGH TO ITS NORTH ACROSS CENTRAL CANADA. THIS
WILL RESULT IN COOLER TEMPERATURES, A COLD RAIN FROM THE OHIO
VALLEY TO THE EAST COAST, AND EARLY SEASON ACCUMULATING SNOW FOR
THE CENTRAL APPALACHIANS AND THE HIGHER TERRAIN OF THE INTERIOR
NORTHEAST. BETWEEN THESE TWO REGIONS, AN UPPER RIDGE IS LIKELY TO
BUILD ACROSS THE ROCKIES AND EVENTUALLY ACROSS THE PLAINS BY NEXT
WEEKEND, WARMING UP TEMPERATURES IN THE INTERIOR WEST AND SOUTHERN
PLAINS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE BEGINS THE MEDIUM RANGE PERIOD IN FAIRLY GOOD
AGREEMENT FOR IMPORTANT FEATURES, INCLUDING A DEEP SURFACE LOW
SOMEWHERE IN THE 940S TO 960S IN THE EASTERN PACIFIC OFFSHORE OF
VANCOUVER ISLAND WEDNESDAY, WITH ITS COLD FRONT DIRECTING THE AR
INTO THE WEST COAST. MODELS SHOW THIS LOW GRADUALLY WEAKENING AS IT
MOVES BACK NORTHWEST INTO THURSDAY, GIVING WAY TO ANOTHER LOW WITH
MORE SPREAD IN DEPTH (WEAKER THAN THE FIRST ONE THOUGH) AND
PLACEMENT APPROACHING THE PACIFIC NORTHWEST FRIDAY INTO THE
WEEKEND. THE NEWER 12Z GUIDANCE SUITE SEEMS TO BE COMING IN WITH
BETTER AGREEMENT FOR THIS SECONDARY LOW FORTUNATELY. A MODEL BLEND
SEEMED REASONABLE INITIALLY, BUT THERE WAS INCREASING SPREAD BY
LATE WEEK INTO THE WEEKEND. NAMELY, THE 00Z CMC WAS FARTHER
OFFSHORE WITH AN UPPER LOW AND DIRECTED THE AR FARTHER NORTH INTO
THE PACIFIC NORTHWEST GIVEN ITS MEAN FLOW FROM THE SOUTHWEST RATHER
THAN WEST. MEANWHILE, THE 06Z GFS BROUGHT IN A NOTABLE SHORTWAVE
INTO THE NORTHWEST SATURDAY AND TRACKING INTO THE NORTH-CENTRAL
U.S. BY NEXT SUNDAY. THIS WAS DISMISSED AS AN OUTLIER; HOWEVER, THE
NEWER 12Z GFS HELD ON TO AN EVOLUTION LIKE THIS AND THE 12Z EC IS
NOT TOO OFF EITHER, SO THIS FEATURE WILL CONTINUE TO BE MONITORED
AND MAY NECESSITATE CHANGES IN FUTURE FORECASTS.
FARTHER EAST, MODELS ARE REASONABLY AGREEABLE WITH THE UPPER AND
SURFACE LOWS MOVING ACROSS THE MIDWEST TO EAST. THE 00Z GFS BECAME
THE MAIN OUTLIER BY LATE WEEK AS ITS LOWS WERE DISPLACED TO THE
SOUTHEAST OF CONSENSUS. OTHERWISE A MODEL/MEAN BLEND SEEMED
REASONABLE.
THE WPC FORECAST BEGAN WITH A BLEND OF THE 00Z ECMWF/CMC/UKMET AND
06Z GFS. DECREASED THE PROPORTION OF THE DETERMINISTIC RUNS
(ESPECIALLY THOSE OTHER THAN THE ECMWF) WITH TIME AS MODEL SPREAD
INCREASED ESPECIALLY IN THE WEST, IN FAVOR OF THE GEFS AND EC
ENSEMBLE MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
CONFIDENCE REMAINS HIGH FOR A POTENTIALLY MAJOR AND PROLONGED
ATMOSPHERIC RIVER EVENT SETTING UP ACROSS PORTIONS OF NORTHWESTERN
CALIFORNIA AND INTO SOUTHWESTERN OREGON DURING THE WEDNESDAY
THROUGH FRIDAY TIME PERIOD. ENHANCED MOISTURE FLUX AHEAD OF A
MEANDERING FRONT AND AT LEAST TWO LOW PRESSURE AREAS WILL RESULT IN
ENHANCED OROGRAPHIC LIFT AND LIKELY PRODUCE SOME 4 TO LOCALLY 8
INCH RAINFALL TOTALS DURING THIS TIME. SLIGHT RISKS REMAIN IN PLACE
FOR BOTH DAYS 4/WEDNESDAY AND 5/THURSDAY IN THE ERO. BURN SCARS
WILL BE OF PARTICULAR CONCERN FOR FLOODING. BY THURSDAY/DAY 5, SNOW
LEVELS ARE FORECAST TO RISE WITH AN INFLUX OF WARM AIR, SO
EXTENDED THURSDAY'S SLIGHT RISK FARTHER INLAND/INTO HIGHER
ELEVATIONS WHERE RAIN ON SNOW MAY EXACERBATE FLOODING CONCERNS. THE
LONG DURATION OF THE AR STALLING COULD MAKE THIS EVENT BE ON THE
HIGHER END OF THE SLIGHT RISK RANGE, WITH MULTIPLE DAYS OF RAINFALL
COMPOUNDING FLOODING IMPACTS. ADDITIONALLY, AT LEAST MARINE AREAS
SHOULD SEE STRONG WINDS THROUGH WEDNESDAY GIVEN THE DEEP EASTERN
PACIFIC LOW. THE AR SHOULD FINALLY START TO MOVE AND WEAKEN INTO
LATE WEEK AND NEXT WEEKEND, BRINGING PRECIPITATION FARTHER SOUTH
WHILE SNOW LEVELS LOWER INTO THE CASCADES/SIERRA NEVADA, AND
FARTHER EAST WITH SOME CHANCE FOR SNOW TO EXTEND INTO THE NORTHERN
HIGH PLAINS.
IMPACTFUL WEATHER IS ALSO EXPECTED FOR A LARGE PORTION OF THE
EASTERN U.S. FOR THE LATTER HALF OF THE WORK WEEK. THIS WILL BE
THE RESULT OF A LARGE UPPER LOW CLOSING OFF OVER THE MIDWEST AND
SLOWLY MAKING ITS WAY ACROSS THE GREAT LAKES/OHIO VALLEY AND THEN
NORTHEAST AS THE WEEK PROGRESSES. SURFACE LOW PRESSURE WILL TRACK
THROUGH THE GREAT LAKES REGION AND GIVE WAY TO SURFACE CYCLOGENESIS
NEAR THE MID-ATLANTIC AND SOUTHERN NEW ENGLAND IN A MILLER B TYPE
EVOLUTION. SOME STRONG WINDS ARE LIKELY IN THE NORTHERN PLAINS
WEDNESDAY BEHIND THE FIRST SURFACE LOW, WITH BREEZY TO WINDY
CONDITIONS BROADLY FROM THE MIDWEST TO EAST IN THIS PATTERN.
PRECIPITATION IS ALSO LIKELY WITH THESE FEATURES. SOME LOCALLY
HEAVY RAIN IS POSSIBLE IN FLORIDA FOR MIDWEEK WITH MOISTURE
STREAMING IN AHEAD OF THE TAIL END OF ASSOCIATED COLD FRONTS.
COASTAL AREAS OF THE MID-ATLANTIC AND NORTHEAST INCLUDING THE
INTERSTATE 95 CORRIDOR SHOULD ALSO SEE SOME MODEST TO PERHAPS
LOCALLY HEAVY RAIN WEDNESDAY-THURSDAY. THIS DID NOT SEEM TO
NECESSITATE ANY ERO AREAS AS LACK OF INSTABILITY SHOULD PUT A CAP
ON HEAVY TOTALS, AND ONGOING DROUGHT WILL MEAN MOST AREAS ARE NOT
PARTICULARLY SENSITIVE TO FLOODING. COLD AIR ALOFT WILL BE
SUFFICIENT TO SUPPORT ACCUMULATING EARLY SEASON SNOW FOR THE HIGHER
TERRAIN NEAR THE GREAT LAKES AND INTERIOR NORTHEAST, AND
ESPECIALLY FOR THE CENTRAL APPALACHIANS WHERE SOME TOTALS IN EXCESS
OF SIX INCHES ARE A GOOD POSSIBILITY. THERE MAY ALSO BE SOME LAKE
EFFECT SNOW IN THE WAKE OF THIS STORM SYSTEM, ALTHOUGH TEMPERATURES
WILL LIKELY BE MARGINAL AND THIS COULD MIX WITH RAIN AT TIMES.
IN TERMS OF TEMPERATURES, WARM AND SLIGHTLY HUMID CONDITIONS CAN
BE EXPECTED FROM THE MID-ATLANTIC SOUTHWARD TO FLORIDA ON WEDNESDAY
AHEAD OF THE APPROACHING COLD FRONT THAT WILL BRING A RETURN TO
MUCH COOLER CONDITIONS TO CLOSE OUT THE WORK WEEK. HIGHS SHOULD
GENERALLY BE ABOUT 5-10 DEGREES BELOW AVERAGE ACROSS THE INTERIOR
EASTERN U.S. BY FRIDAY, MAINLY BECAUSE OF THE COLD UPPER LOW
OVERHEAD KEEPING WIDESPREAD CLOUD COVER IN PLACE, AND OVERNIGHT
LOWS CLOSER TO AVERAGE. A GRADUAL WARMING TREND IS LIKELY FOR THE
INTERMOUNTAIN WEST AND THE ROCKIES AS THE RIDGE BUILDS IN AFTER
THE COLD CONDITIONS EXPECTED IN THE FIRST HALF OF THE WEEK.
TATE/HAMRICK
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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