911
FXUS02 KWBC 190658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM EST TUE NOV 19 2024
VALID 12Z FRI NOV 22 2024 - 12Z TUE NOV 26 2024
***STRONG, LONG-DURATION ATMOSPHERIC RIVER WILL TAKE AIM AT
NORTHERN CALIFORNIA/SOUTHWESTERN OREGON THROUGH LATE WEEK***
..GENERAL 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 AND SOUTHWESTERN OREGON, WHERE SEVERAL INCHES OF RAIN
ARE LIKELY TO CAUSE RIVER RISING AND SOME 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 HAVE COME INTO GOOD AGREEMENT WITH THE LARGE UPPER LOW
AND SURFACE LOW PRESSURE SYSTEM ACROSS THE EASTERN U.S. LATE IN THE
WEEK, AND A GENERAL DETERMINISTIC MODEL COMPROMISE WORKS WELL AS A
STARTING POINT IN THE FORECAST PROCESS. THE GUIDANCE HAS ALSO
IMPROVED WITH THE EXPECTED QUASI-ZONAL FLOW EVOLUTION ACROSS MUCH
OF THE CENTRAL U.S., AND ARE SHOWING MORE SIGNS OF A DEEPENING
TROUGH OVER THE ROCKIES BY EARLY NEXT WEEK, WHICH COULD HERALD THE
ARRIVAL OF AN ARCTIC AIRMASS OVER THE FAR NORTHERN PLAINS. THE
ENSEMBLE MEANS WERE GRADUALLY INCREASED TO ABOUT HALF BY NEXT
TUESDAY AMID GROWING MODEL DIFFERENCES AT THE MESOSCALE LEVEL,
WHILE STILL MAINTAINING SOME OF THE DETERMINISTIC GFS AND ECMWF
SOLUTIONS.
..WEATHER/HAZARDS HIGHLIGHTS
CONFIDENCE IS HIGH FOR A PROLONGED ATMOSPHERIC RIVER EXTENDING INTO
FRIDAY ACROSS PORTIONS OF NORTHWESTERN CALIFORNIA AND INTO
SOUTHWESTERN OREGON. ENHANCED MOISTURE FLUX AHEAD OF A 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 IS BECOMING MORE LIKELY. HOWEVER, THERE IS A MARGINAL RISK
AREA FOR THE LOWER ELEVATIONS OF THE WESTERN SIERRA WHERE UPSLOPE
FLOW MAY RESULT IN SOME RAINFALL TOTALS OF 1-2 INCHES ON DAY 5.
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. COLD AIR ALOFT WILL BE
SUFFICIENT TO SUPPORT SOME 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 EXPECTED. 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.
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 INTO THE
0S AND LOW 10S. EVEN COLDER WEATHER MAY ARRIVE IN TIME FOR
THANKSGIVING FOR THOSE AREAS.
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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