963
FXUS02 KWBC 191841
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 PM EST TUE NOV 19 2024
VALID 12Z FRI NOV 22 2024 - 12Z TUE NOV 26 2024
***STRONG, LONG-DURATION ATMOSPHERIC RIVER TO CONTINUE FOR NORTHERN
CALIFORNIA/SOUTHWESTERN OREGON THROUGH LATE WEEK***
..OVERVIEW
A LONG-DURATION ATMOSPHERIC RIVER WILL AFFECT THE WEST COAST
STARTING IN THE SHORT RANGE (WED) AND CONTINUE INTO THE FIRST PART
OF THE MEDIUM RANGE (FRI) AS UPPER AND SURFACE LOWS/FRONTAL SYSTEMS
DIRECT A STRONG MOISTURE PLUME INTO NORTHERN CALIFORNIA AND
SOUTHWESTERN OREGON. SEVERAL INCHES OF RAIN ARE LIKELY TO CAUSE
RISING RIVERS AND SOME LOCAL FLOODING CONCERNS. 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 BECOMES A BIT MORE ZONAL INTO EARLY NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MODELS REMAIN IN GOOD AGREEMENT OVERALL WITH THE PATTERN,
INCLUDING THE LARGE UPPER/SFC LOW PRESSURE SYSTEM ACROSS THE
EASTERN U.S. LATE IN THE WEEK. A BLEND OF THE LATEST 00Z/06Z
DETERMINISTIC MODELS WORKED WELL AS A STARTING POINT FOR FRI-SUN.
THE GUIDANCE HAS ALSO IMPROVED WITH THE EXPECTED QUASI-ZONAL FLOW
EVOLUTION ACROSS MUCH OF THE CENTRAL U.S., BUT SOME DIFFERENCES
APPEAR BY EARLY NEXT WEEK IN THE EASTERN NORTH PACIFIC INTO THE
WEST AND HIGH PLAINS. THE ECMWF ENSEMBLES WERE SLOWER AND DEEPER
THAN THE OTHER MODELS/ENSEMBLES BY NEXT MON-TUE OFF THE PACNW BUT
THE ECMWF AIFS ENS WAS ABOUT AS QUICK AS THE GEFS/NAEFS WHILE THE
DETERMINISTIC GFS/ECMWF/CANADIAN WERE QUICKER. THUS, OPTED TO MEET
IN THE MIDDLE GIVEN THE SPREAD AND UNCERTAINTY IN UPSTREAM
AMPLITUDE THAT COULD SLOW THE FLOW, BUT COULD ALSO LEAD TO AN
UNDERCUTTING KICKER IN THE MID-LATITUDES TO KEEP THE PATTERN
PROGRESSIVE.
..WEATHER/HAZARDS HIGHLIGHTS
THE PROLONGED ATMOSPHERIC RIVER WILL BE ONGOING INTO THE START OF
THE PERIOD, EXTENDING INTO FRIDAY ACROSS PORTIONS OF NORTHWESTERN
CALIFORNIA AND INTO SOUTHWESTERN OREGON. SUSTAINED MOISTURE FLUX
AHEAD OF THE UPPER LOW AND WAVY/SLOW-MOVING COLD FRONT WILL RESULT
IN ENHANCED OROGRAPHIC LIFT AND LIKELY PRODUCE SOME ADDITIONAL 2 TO
4 INCH RAINFALL TOTALS BY EARLY SATURDAY. A SLIGHT RISK AREA IS IN
PLACE FOR DAY 4 ACROSS THESE AREAS, INCLUDING THE COASTAL
RANGES/NORTHERN SIERRA AND EXTENDING TO THE COAST. 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 WILL BE MORE LIKELY. HOWEVER,
THERE IS A MARGINAL RISK AREA FOR THE LOWER ELEVATIONS/FOOTHILLS OF
THE WESTERN SIERRA WHERE UPSLOPE FLOW MAY RESULT IN SOME RAINFALL
TOTALS OF 1-2 INCHES ON DAY 5 (SAT).
IMPACTFUL WEATHER IS ALSO EXPECTED FOR THE NORTHEAST U.S. TO CLOSE
OUT THE WORK WEEK. THIS WILL BE THE RESULT OF A LARGE UPPER LOW
POSITIONED OVER THE REGION, IN COMBINATION WITH A POTENTIALLY
SUB-990 MB SURFACE LOW OVER NEW ENGLAND. MARGINALLY COLD AIR WILL
BE SUFFICIENT TO SUPPORT SOME ACCUMULATING SNOW FOR THE HIGHER
TERRAIN NEAR THE GREAT LAKES AND INTERIOR NORTHEAST (CATSKILLS,
POCONOS, ADIRONDACKS, INTO NEW ENGLAND ETC.), AND ESPECIALLY FOR
THE CENTRAL APPALACHIANS WHERE SOME TOTALS IN EXCESS OF SIX INCHES
ARE POSSIBLE. THERE WILL ALSO BE SOME LAKE EFFECT RAIN AND SNOW IN
THE WAKE OF THIS STORM SYSTEM. THE STRONG SURFACE LOW WILL RESULT
IN WINDY CONDITIONS AND PERIODS OF HEAVY RAIN FOR MUCH OF NEW
ENGLAND THROUGH FRIDAY AFTERNOON AS THE FRONT CLEARS OUT THE
COASTAL AREAS.
TEMPERATURES WILL TURN NOTICEABLY COLDER ACROSS THE EASTERN U.S.
IN THE WAKE OF THE 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 PROMOTES A WARMER FLOW FROM THE GULF OF MEXICO, WITH
HIGHS REACHING INTO THE 80S FOR THE SOUTHERN HALF OF TEXAS. COLDER
AIR IN CANADA MAY TRY TO START INCHING CLOSER TO THE BORDER BY NEXT
WEEK, BUT THE GUIDANCE STILL SHOWS SOME TIMING/LOCATION/TRAJECTORY
DIFFERENCES. REGARDLESS, TEMPERATURES WILL LIKELY BE BELOW NORMAL
ACROSS PARTS OF THE NORTHERN HIGH PLAINS NEXT WEEK (20S).
FRACASSO/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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