164
FXUS02 KWBC 200658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM EST WED NOV 20 2024
VALID 12Z SAT NOV 23 2024 - 12Z WED NOV 27 2024
***TURNING MUCH COLDER FOR THE NORTHERN PLAINS, AND SNOWY
CONDITIONS FOR THE SIERRA AND PORTIONS OF THE ROCKIES***
..GENERAL OVERVIEW
A QUIETER OVERALL WEATHER PATTERN CAN BE EXPECTED GOING INTO THE
WEEKEND AFTER THE PROLONGED ATMOSPHERIC RIVER EVENT IN THE SHORT
RANGE PERIOD, AND THE LOW PRESSURE SYSTEM OVER THE NORTHEAST U.S.
WILL BE LIFTING NORTHWARD ACROSS SOUTHEASTERN CANADA. THE UPPER
LEVEL FLOW PATTERN THEN BECOMES MORE QUASI-ZONAL FOR THE SUNDAY TO
MONDAY TIME PERIOD WITH THE WESTERN U.S. TROUGH MOVING QUICKLY
INLAND AND BECOMING LESS AMPLIFIED. HOWEVER, THERE WILL STILL BE
ENOUGH MOISTURE TO PRODUCE MODERATE TO LOCALLY HEAVY SNOW FROM THE
SIERRA TO THE CENTRAL/NORTHERN ROCKIES, AND LINGERING LIGHT TO
MODERATE RAIN FOR THE LOWER ELEVATIONS OF THE WEST COAST. A WAVY
SURFACE FRONT IS EXPECTED TO DEVELOP ACROSS THE PLAINS WITH A MUCH
COLDER AIRMASS MOVING IN ACROSS THE DAKOTAS AND MONTANA EARLY TO
MID NEXT WEEK, AND AN UPPER LEVEL TROUGH LIKELY BUILDS BACK IN
ACROSS THE NORTHWESTERN U.S. BY WEDNESDAY.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE 00Z MODELS REMAIN IN GOOD SYNOPTIC SCALE AGREEMENT OVERALL
WITH THE PATTERN, INCLUDING THE EXITING LARGE UPPER/SURFACE LOW
PRESSURE SYSTEM ACROSS NEW ENGLAND AND INTO SOUTHEASTERN CANADA. A
BLEND OF THE LATEST 12Z/18Z DETERMINISTIC MODELS WORKED WELL AS A
STARTING POINT FOR THE WEEKEND, WITH SLIGHTLY MORE WEIGHTING TO THE
GFS/ECMWF. THE 00Z 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 THEN THE HIGH PLAINS. THE CMC LINGERS THE
TROUGH NEAR THE PACIFIC NORTHWEST MUCH LONGER THAN THE MORE
PROGRESSIVE ECMWF/GFS LOCATIONS BY TUESDAY AND WEDNESDAY, AND THE
CMC IS ALSO MORE AMPLIFIED WITH A TROUGH AND POTENTIAL CLOSED LOW
NEAR THE GREAT LAKES THAT DOES NOT HAVE MUCH ENSEMBLE OR AI
SUPPORT, SO IT WAS NOT USED BEYOND MONDAY. OVERALL MODEL SPREAD
INCREASES SUBSTANTIALLY BY WEDNESDAY WITH THE OVERALL TROUGH
EVOLUTION ACROSS THE WEST AND THE PLAINS, SO FORECAST CONFIDENCE IS
LOW FOR DAY 7. THE ENSEMBLE MEANS ACCOUNTED FOR ABOUT 40-50% OF
THE FORECAST BLEND FOR THE MIDDLE OF NEXT WEEK AMID GROWING MODEL
SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
THERE WILL BE SOME ABATEMENT IN THE ATMOSPHERIC RIVER ACROSS
CALIFORNIA GOING INTO SATURDAY AS THE ATTENDANT COLD FRONT MOVES
INLAND AND THE MOISTURE FLUX ANOMALIES DECREASE SOME. THE BEST
MODEL SIGNAL FOR THE HEAVIEST RAIN IS ACROSS THE WESTERN SLOPES OF
THE SIERRA NEVADA, WHERE THE MARGINAL RISK AREA WILL REMAIN FOR THE
NEW DAY 4 EXCESSIVE RAINFALL OUTLOOK WITH 1 TO LOCALLY 2 INCHES OF
RAIN BY EARLY SUNDAY. NUISANCE LEVEL RAINFALL IS EXPECTED FOR THE
COASTAL RANGES OF CENTRAL AND NORTHERN CALIFORNIA, SO NO RISK AREAS
APPEAR WARRANTED AT THIS TIME FOR THOSE AREAS. SNOW LEVELS WILL BE
DROPPING IN THE WAKE OF THE FRONT, AND THIS IS EXPECTED TO RESULT
IN HEAVY SNOW FOR THE HIGHER ELEVATIONS OF THE SIERRA NEVADA, AND
MODERATE SNOW FOR THE INTERMOUNTAIN WEST RANGES AND EVENTUALLY THE
CENTRAL AND NORTHERN ROCKIES FOR THE SUNDAY TO MONDAY TIME PERIOD.
IMPROVING CONDITIONS ARE ALSO FORECAST FOR THE EASTERN U.S. AS THE
STRONG LOW PRESSURE SYSTEM SLOWLY EXITS THE REGION. THERE WILL
LIKELY BE SOME LAKE EFFECT RAIN/SNOW SHOWERS FOR THE EASTERN GREAT
LAKES AND SNOW FOR THE HIGHER TERRAIN OF NEW ENGLAND ON SATURDAY,
WITH BREEZY TO WINDY CONDITIONS IN NORTHWEST FLOW. MOST AREAS
SHOULD BE DRY BY SUNDAY AS HIGH PRESSURE STARTS BUILDING BACK IN.
DEPENDING ON EVENTUAL SURFACE LOW EVOLUTION ALONG A FRONTAL
BOUNDARY ACROSS THE SOUTH-CENTRAL U.S., SOME HEAVIER SHOWERS AND
PERHAPS SOME THUNDERSTORMS COULD DEVELOP ACROSS PORTIONS OF THE
MID-SOUTH AND INTO THE SOUTHERN OHIO VALLEY BY TUESDAY, AND LIGHTER
RAINFALL EXTENDING EAST ACROSS THE MID-ATLANTIC AND NORTHEAST
TOWARDS MID-WEEK.
TEMPERATURES ARE EXPECTED TO SLOWLY MODERATE TO NEAR AVERAGE
ACROSS THE EASTERN U.S. OVER THE WEEKEND COMPARED TO THE RATHER
CHILLY CONDITIONS LATE THIS WEEK. THERE IS STILL THE POTENTIAL FOR
ARCTIC AIR TO REACH PORTIONS OF MONTANA AND INTO NORTH DAKOTA NEXT
WEEK WITH HIGHS IN THE 10S AND LOW 20S, AND OVERNIGHT LOWS IN THE
0S. THERE IS STILL SOME UNCERTAINTY ON HOW FAR SOUTH THIS ARCTIC
AIRMASS GETS, SO THIS WILL BE REFINED IN FUTURE FORECASTS. WARM
CONDITIONS SHOULD CONTINUE FROM SOUTHERN TEXAS TO FLORIDA, WITH
HIGHS IN THE 70S AND 80S.
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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