234
FXUS02 KWBC 181912
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
212 PM 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 THROUGH LATE WEEK...
...PATTERN CHANGE FOR EAST-CENTRAL TO EASTERN U.S. WITH COOLER
TEMPERATURES AND COLD RAIN AND HIGHER ELEVATION SNOW...
..OVERVIEW
A LONG DURATION ATMOSPHERIC RIVER IS FORECAST TO AFFECT THE WEST
COAST INTO LATER 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 ARE LIKELY TO CAUSE FLOODING. MEANWHILE A DEEP UPPER LOW
WILL DRIFT SLOWLY FROM THE GREAT LAKES TO THE NORTHEAST U.S.,
LEADING TO COOLER TEMPERATURES AND A COLD RAIN/HIGHER ELEVATION
SNOW FOR THOSE REGIONS. IN BETWEEN, AN UPPER RIDGE IS LIKELY TO
TRAVERSE THE INTERIOR WEST AND CENTRAL U.S. BEFORE THE UPPER
PATTERN FLATTENS OUT INTO EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE BEGINS THE MEDIUM RANGE PERIOD IN FAIRLY GOOD
AGREEMENT FOR IMPORTANT FEATURES, INCLUDING UPPER LOWS OVER THE
NORTHEAST PACIFIC AND THE GREAT LAKES, BUT THE DETAILS ARE MORE
QUESTIONABLE. FOR EXAMPLE EVEN ON THURSDAY, THERE IS MODEL SPREAD
WITH THE POSITIONING OF THE GREAT LAKES REGION SURFACE LOW BEFORE
THE WESTERN ATLANTIC LOW SPINS UP. THE 00Z EC/CMC/UKMET CLUSTERED
WELL WITH A POSITION OVER THE LOWER GREAT LAKES WHILE THE GFS RUNS
AS WELL AS SEVERAL OF THE EC-BASED AI/ML LOWS WERE FARTHER WEST.
THIS HAS IMPACTS ON LOW PLACEMENT INTO LATE WEEK. THE 12Z MODELS
HAVE COME IN WITH BETTER AGREEMENT ON THAT LOW PLACEMENT. THEN FOR
THE WEST, THERE IS GOOD AGREEMENT FOR THE PRIMARY SURFACE LOW TO
DRIFT BACK NORTHWEST WHILE WEAKENING INTO THURSDAY, BUT ANOTHER
SURFACE LOW APPROACHES THE PACIFIC NORTHWEST FROM THE WEST. THERE
REMAINS SOME SPREAD IN DEPTH (WEAKER THAN THE FIRST ONE THOUGH) AND
PLACEMENT, BUT THE NEWER 12Z GUIDANCE SUITE SEEMS TO BE COMING IN
WITH BETTER AGREEMENT FOR THIS SECONDARY LOW FORTUNATELY, OTHER
THAN THE CMC THAT HAS ANOTHER LOW FARTHER SOUTH. THEN INTO THE
WEEKEND, SOME SHORTWAVE ENERGY MAY PUSH ACROSS THE NORTHERN TIER.
THE 00Z GFS WAS PARTICULARLY DEEP IN DEVELOPING THIS TROUGHING,
WITH THE CONSENSUS SHOWING THE EXISTENCE OF THAT FEATURE BUT
SHALLOWER. THIS FEATURE WILL HAVE IMPACTS ON A POSSIBLY DEVELOPING
SURFACE LOW IN THE PLAINS OR MIDWEST AND SENSIBLE WEATHER LIKE QPF
AND TEMPERATURES.
THE WPC FORECAST BEGAN WITH A BLEND OF MAINLY DETERMINISTIC
MODELS, DECREASING THE PROPORTION OF THE DETERMINISTIC RUNS WITH
TIME AS MODEL SPREAD INCREASED IN FAVOR OF THE GEFS AND EC ENSEMBLE
MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
CONFIDENCE REMAINS HIGH FOR A MAJOR AND PROLONGED ATMOSPHERIC
RIVER EVENT SETTING UP ACROSS PORTIONS OF NORTHERN CALIFORNIA AND
INTO SOUTHWESTERN OREGON INTO THURSDAY-FRIDAY. TWO LOW PRESSURE
AREAS FOCUSING A FRONT JUST OFFSHORE WILL ENHANCE MOISTURE FLUX FOR
SIGNIFICANT INTEGRATED VAPOR TRANSPORT INTO THOSE AREAS, AND THE
AR STALLING WILL LEAD TO A LONG DURATION OF MODERATE TO HEAVY RAIN
AMOUNTS. THROUGH THE EVENT, SOME OF WHICH IS PHASING INTO THE SHORT
RANGE PERIOD, 10 TO 15 INCHES OF QPF ARE FORECAST, AND THE 10 YEAR
AVERAGE RECURRENCE INTERVALS FOR 48 AND 72 HOUR PERIODS ARE LIKELY
TO BE EXCEEDED. NUMEROUS AREAS COULD SEE FLOODING AND BURN SCARS
WILL BE OF PARTICULAR CONCERN. A MODERATE RISK REMAINS IN PLACE FOR
PARTS OF NORTHWESTERN CALIFORNIA WHERE THE HEAVIEST RAINFALL
SHOULD FOCUS FOR DAY 4/THURSDAY. A SLIGHT RISK EXTENDS INLAND EVEN
INTO SOME HIGHER ELEVATION AREAS AS SNOW LEVELS SHOULD BE QUITE
HIGH ON THURSDAY. THE AR MAY START TO MOVE SOMEWHAT BY DAY 5/FRIDAY
FOR LOWER OVERALL RAIN TOTALS, SO MAINTAINED A SLIGHT RISK THEN,
THOUGH A FUTURE UPGRADE MAY BE NEEDED ONCE THE IMPACTS FROM THE
PREVIOUS DAYS' PRECIPITATION ARE MORE CLEAR. THE PATTERN WILL
REMAIN FAVORABLE FOR SOME PRECIPITATION INTO THE WEEKEND. THOUGH
THERE SHOULD BE LESSER AMOUNTS, BUT IN SOME AREAS ANY ADDITIONAL
RAIN MAY CAUSE FLOODING CONCERNS DUE TO THE VERY WET ANTECEDENT
CONDITIONS BY THEN. SNOW LEVELS ARE ALSO FORECAST TO FALL, AND SOME
SNOW MAY BECOME HEAVIER IN THE CASCADES AND SIERRA NEVADA FOR THE
WEEKEND. PRECIPITATION SHOULD EXPAND INLAND, INCLUDING SNOW IN THE
NORTHERN ROCKIES, WITH SOME RAIN AND SNOW IMPACTING THE
INTERMOUNTAIN WEST FARTHER SOUTH INTO EARLY NEXT WEEK. A ROUND OF
PRECIPITATION (POSSIBLY SNOW) MAY OCCUR IN THE NORTHERN PLAINS TO
UPPER MIDWEST, BUT THIS IS UNCERTAIN AT THIS POINT.
IMPACTFUL WEATHER IS ALSO EXPECTED FOR MUCH OF THE EASTERN U.S.
FOR THE LATTER PART OF THE WEEK DUE TO THE UPPER LOW TRAVERSING THE
GREAT LAKES/OHIO VALLEY AND THEN NORTHEAST. 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. PRECIPITATION IS LIKELY WITH THESE
FEATURES AND ON THE BACKSIDE, INCLUDING HIGHER ELEVATION SNOW FOR
THE APPALACHIANS AND INTERIOR NORTHEAST. THE CENTRAL APPALACHIANS
IN PARTICULAR MAY SEE SEVERAL INCHES OF SNOW ESPECIALLY ON
THURSDAY. THE COLD RAIN IN THE NORTHEAST THURSDAY IS NOT FORECAST
TO CAUSE ANY EXCESSIVE RAINFALL CONCERNS SINCE LACK OF INSTABILITY
SHOULD PUT A CAP ON RAIN RATES AND ONGOING DROUGHT WILL MEAN MOST
AREAS ARE NOT PARTICULARLY SENSITIVE TO FLOODING. BREEZY TO WINDY
CONDITIONS ARE FORECAST BROADLY FROM THE MIDWEST TO EAST IN THIS
PATTERN. LAKE EFFECT SNOW AND/OR RAIN ARE LIKELY BEHIND THE LOWS.
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 WORKWEEK. 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 LEADING TO WARMER FLOW FROM THE GULF OF MEXICO, WITH
HIGHS REACHING WELL 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
POSSIBLY DROPPING INTO THE SINGLE DIGITS.
TATE
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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