413
FXUS02 KWBC 180658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM EST MON NOV 18 2024
VALID 12Z THU NOV 21 2024 - 12Z MON NOV 25 2024
***STRONG, LONG-DURATION ATMOSPHERIC RIVER WILL TAKE AIM AT
NORTHERN CALIFORNIA/SOUTHWESTERN OREGON FOR THE END OF THE WEEK***
***PATTERN CHANGE FOR EAST-CENTRAL TO EASTERN U.S. WITH COOLER
TEMPERATURES AND COLD RAIN AND HIGHER ELEVATION SNOW***
..GENERAL OVERVIEW
A DEEP UPPER LEVEL LOW DEVELOPING ACROSS THE GREAT LAKES AND
NORTHEAST U.S WILL MAKE WEATHER HEADLINES TO CLOSE OUT THE WORK
WEEK. THIS SYSTEM WILL LIKELY MEANDER SLOWLY IN PLACE FOR A FEW
DAYS WITH A BLOCKING HIGH TO ITS NORTH ACROSS CENTRAL CANADA, AND
THEN SHOULD FINALLY LIFT NORTH ACROSS NORTHEAST CANADA BY SUNDAY.
THIS WILL RESULT IN 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 NEW ENGLAND. OUT WEST, AN
UNSETTLED WEATHER PATTERN ACROSS THE EASTERN PACIFIC IS EXPECTED TO
RESULT IN A PROLONGED ATMOSPHERIC RIVER EVENT FROM NORTHERN
CALIFORNIA TO WESTERN OREGON FOR THE SECOND HALF OF THE WORK WEEK,
WITH SEVERAL INCHES OF RAINFALL LIKELY FOR THE LOWER ELEVATIONS,
AND SNOW MAKES A RETURN TO THE SIERRA NEVADA. BETWEEN THESE TWO
REGIONS, AN UPPER LEVEL RIDGE IS LIKELY TO BUILD ACROSS THE ROCKIES
AND EVENTUALLY ACROSS THE PLAINS BY NEXT WEEKEND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THERE ARE STILL SOME MODEST MODEL DIFFERENCES IN THE EXPECTED
EVOLUTION OF THE LARGE SCALE UPPER TROUGH AND CLOSED LOW ACROSS THE
EASTERN U.S. BY THURSDAY, BUT GENERALLY CLOSE ENOUGH TO SUPPORT A
BLEND OF THE DETERMINISTIC GUIDANCE THROUGH FRIDAY. THIS ALSO HOLDS
TRUE FOR THE SURFACE LOW THAT DEVELOPS NEAR SOUTHERN NEW ENGLAND
AND THEN SLOWLY MOVES NORTH TOWARDS EASTERN QUEBEC. MODEL SPREAD
HAS ALSO IMPROVED WITH THE TWO SEPARATE LOWS OVER THE EASTERN
PACIFIC THAT WILL BE IMPACTFUL TO THE PACIFIC NORTHWEST, BUT MORE
DIFFERENCES APPEAR BY SUNDAY AND INTO MONDAY. AT THE TIME OF THE
FRONTS AND PRESSURES GENERATION, THE 12Z CMC BECAME MORE OUT OF
PHASE WITH THE MODEL CONSENSUS BY THE SECOND HALF OF THE FORECAST
PERIOD, SO IT WAS DROPPED FOR SUNDAY AND INTO MONDAY, AND MORE IN
LINE WITH THE GFS/ECMWF AND THEIR RESPECTIVE MEANS. THE ENSEMBLE
MEANS WERE GRADUALLY INCREASED TO ABOUT 2/3RDS BY NEXT MONDAY AMID
GROWING MODEL DIFFERENCES, WHILE STILL MAINTAINING SOME OF THE
DETERMINISTIC GFS AND ECMWF SOLUTIONS.
..WEATHER/HAZARDS HIGHLIGHTS
CONFIDENCE IS HIGH FOR A PROLONGED ATMOSPHERIC RIVER EVENT SETTING
UP ACROSS PORTIONS OF NORTHWESTERN CALIFORNIA AND INTO
SOUTHWESTERN OREGON DURING THE THURSDAY THROUGH FRIDAY TIME
PERIOD. ENHANCED MOISTURE FLUX AHEAD OF A MEANDERING COLD FRONT AND
TWO SEPARATE LOW PRESSURE AREAS WILL RESULT IN ENHANCED OROGRAPHIC
LIFT AND LIKELY PRODUCE SOME 4 TO LOCALLY 8 INCH RAINFALL TOTALS
BY FRIDAY NIGHT. GIVEN THE DURATION OF THE EVENT, A SLIGHT RISK
AREA IS IN PLACE FOR BOTH DAYS 4 AND 5 ACROSS THESE AREAS,
INCLUDING THE COASTAL RANGES AND EXTENDING TO THE COAST. A DAY 4
(THURSDAY) MODERATE RISK AREA IS NOW PLANNED FOR DEL NORTE AND
HUMBOLDT COUNTIES IN NORTHWESTERN CALIFORNIA, OWING TO HEAVY
RAINFALL AND SATURATED GROUNDS IN THE DAYS LEADING UP TO THIS TIME
PERIOD, AND HIGHLY ANOMALOUS INTEGRATED WATER VAPOR TRANSPORT
ORIENTED NEARLY ORTHOGONAL TO THE TERRAIN. THERE WILL LIKELY BE
SOME ABATEMENT GOING INTO SATURDAY WITH THE FRONT SLOWLY MAKING ITS
WAY INLAND ACROSS THE REGION. SNOW LEVELS WILL INITIALLY BE HIGH,
BUT SHOULD FALL GOING INTO THE WEEKEND, INCLUDING MUCH OF THE
SIERRA RANGE WHERE HEAVY SNOW IS BECOMING MORE LIKELY.
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 AND INDUCING SURFACE
CYCLOGENESIS NEAR THE MID-ATLANTIC AND SOUTHERN NEW ENGLAND. 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. RAINY AND BREEZY TO WINDY CONDITIONS ARE
EXPECTED FOR THE MAJOR CITIES IN SOUTHERN NEW ENGLAND THROUGH
THURSDAY NIGHT.
TEMPERATURES WILL TURN NOTICEABLY COLDER ACROSS THE EASTERN U.S. IN
THE WAKE OF A STRONG COLD FRONT AND THE UPPER LOW MOVING IN
OVERHEAD BY THURSDAY, WITH HIGHS EXPECTED TO BE ABOUT 5 TO 15
DEGREES BELOW AVERAGE TO CLOSE OUT THE WORK WEEK. GIVEN WIDESPREAD
CLOUD COVER, OVERNIGHT LOWS WILL BE CLOSER TO AVERAGE WITH LESS OF
A DIURNAL RANGE FOR MANY AREAS. A WARMING TREND IS FORECAST ACROSS
THE SOUTHERN PLAINS GOING INTO THE WEEKEND AS AN UPPER LEVEL RIDGE
BUILDS IN AND WARMER FLOW FROM THE GULF OF MEXICO, WITH HIGHS
REACHING INTO THE 80S FOR THE SOUTHERN HALF OF TEXAS. SIGNS OF AN
ARCTIC AIRMASS MAY START TO BE NOTICED FOR PARTS OF MONTANA AND
NORTH DAKOTA BY NEXT MONDAY, WITH OVERNIGHT LOWS DROPPING BELOW 10
DEGREES POTENTIALLY.
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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