918
FXUS02 KWBC 020700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM EST SAT DEC 2 2023
VALID 12Z TUE DEC 5 2023 - 12Z SAT DEC 9 2023
...HEAVY RAIN EXPECTED FOR THE PACIFIC NORTHWEST WITH AN
ATMOSPHERIC RIVER EVENT TUESDAY AND WEDNESDAY...
..SYNOPTIC OVERVIEW
THE MAIN THEME GOING INTO THE FIRST HALF OF NEXT WEEK WILL BE A
STRONG PACIFIC JET STREAM THAT WILL BE ACCOMPANIED BY AN
ATMOSPHERIC RIVER EVENT, WITH HEAVY RAINFALL EXPECTED FOR THE
PACIFIC NORTHWEST. A BUILDING UPPER RIDGE AXIS IS EXPECTED ACROSS
THE INTERMOUNTAIN WEST AND THEN ACROSS THE PLAINS GOING INTO THE
MIDDLE OF THE WEEK WITH WELL ABOVE AVERAGE TEMPERATURES EXPECTED.
MEANWHILE, A BROAD UPPER TROUGH IS FORECAST TO BE IN PLACE FROM
THE GREAT LAKES TO THE EAST COAST, WITH A DEPARTING STORM SYSTEM
FOLLOWED BY A CLIPPER TYPE SYSTEM CROSSING THE UPPER MIDWEST AND
REACHING THE OHIO VALLEY THROUGH WEDNESDAY WITH SOME SNOW SHOWERS
ACCOMPANYING IT. A MORE ZONAL FLOW PATTERN ALOFT LIKELY RESUMES
AFTER THIS SYSTEM EXITS THE EAST COAST, FOLLOWED BY A POTENTIALLY
ORGANIZED STORM SYSTEM OVER THE WEST-CENTRAL U.S. BY NEXT
SATURDAY.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE 00Z MODEL GUIDANCE SUITE IS IN GOOD OVERALL AGREEMENT ON
TUESDAY, AND A GENERAL MODEL COMPROMISE WORKS WELL AS A STARTING
POINT IN THE FORECAST PROCESS. THIS HOLDS TRUE GOING INTO
WEDNESDAY AS WELL, WITH ONLY SOME MODEST TIMING DIFFERENCES
REGARDING THE ARRIVAL OF FRONTAL BOUNDARIES AND SHORTWAVES ACROSS
THE CENTRAL AND WESTERN STATES. WITH THE UPPER FLOW PATTERN
EVOLVING MORE ZONAL BY THURSDAY, THE GUIDANCE DIFFERS MORE WITH
THE TIMING OF THE WESTERN U.S. TROUGH CROSSING THE ROCKIES, WITH
THE UKMET APPEARING MORE PROGRESSIVE THAN THE MODEL CONSENSUS.
THE GFS IS STRONGER WITH A SHORTWAVE TROUGH OVER THE NORTHEAST
U.S. DURING THIS TIME, BUT CLOSE ENOUGH TO THE CONSENSUS ELSEWHERE
TO REMAIN PART OF THE FORECAST BLEND. BY THE END OF THE FORECAST
PERIOD NEXT SATURDAY, THE MODELS ARE TRENDING STRONGER WITH AN
ORGANIZED STORM SYSTEM OVER THE INTERMOUNTAIN WEST THAT WILL
EVENTUALLY EMERGE OVER THE CENTRAL/SOUTHERN PLAINS, WITH THE
GFS/ECMWF/CMC INDICATING A POTENTIAL SOUTHERN STREAM UPPER TROUGH
DEVELOPING. IT IS INTERESTING TO NOTE THE JMA MODEL IS NOT NEARLY
AS STRONG, SO THERE IS STILL SOME UNCERTAINTY REGARDING HOW THIS
WILL EVOLVE. ENSEMBLE MEANS ACCOUNTED FOR UP TO 50% OF THE
FORECAST BY NEXT SATURDAY.
..WEATHER/HAZARDS HIGHLIGHTS
OUT WEST, PRECIPITATION WILL BE ONGOING ON TUESDAY, WITH A SERIES
OF ATMOSPHERIC RIVERS INTO THE NORTHWEST LASTING INTO THE MIDDLE
OF THE WEEK, WITH RISING CREEKS AND RIVER LEVELS EXPECTED. GIVEN
ENHANCED RAINFALL DURING THE WEEKEND, THE SOILS WILL BE MORE
VULNERABLE AND THUS THE POTENTIAL FOR GREATER RUN-OFF AND
FLOODING. SNOW LEVELS ARE INITIALLY EXPECTED TO BE RATHER HIGH,
AND THEREFORE WESTERN FACING SLOPES OF THE CASCADES MAY BE SUBJECT
TO AN ADDITIONAL RISK OF FLOODING DUE TO SNOW MELT. ACCORDINGLY, A
SLIGHT RISK AREA REMAINS VALID FOR WESTERN SLOPES OF THE
WASHINGTON CASCADES ON TUESDAY, AND ALSO FOR THE OREGON COASTAL
RANGES. WITH THE ATMOSPHERIC RIVER FOCUS MOST LIKELY SHIFTING A
BIT MORE SOUTH INTO OREGON ON TUESDAY, A MARGINAL RISK WAS ALSO
WARRANTED FOR THE DAY 5 OUTLOOK FOR WESTERN OREGON INTO THE
CASCADES AND OLYMPICS. FARTHER INLAND ACROSS NORTHERN PORTIONS OF
THE INTERMOUNTAIN WEST AND ROCKIES, SOME MODERATELY HEAVY SNOW IS
EXPECTED INTO NEXT MIDWEEK WITH THE PASSAGE OF THE NEXT ORGANIZED
SYSTEM. SOME UPSLOPE SNOW SHOWERS ARE ALSO LIKELY FOR THE HIGHER
ELEVATIONS OF THE CENTRAL APPALACHIANS GOING INTO MIDWEEK AS THE
NEXT LOW PRESSURE SYSTEM PASSES BY.
IN TERMS OF TEMPERATURES, MUCH OF THE CENTRAL AND NORTHERN PLAINS
WILL ENJOY LATE SEASON WARMTH WITH HIGHS POSSIBLY 20-30 DEGREES
ABOVE EARLY DECEMBER AVERAGES FROM NORTHEAST COLORADO TO THE
CANADIAN BORDER ON WEDNESDAY AND INTO THURSDAY, WITH SOME RECORD
HIGHS WITHIN THE REALM OF POSSIBILITY. THE SAME IS EXPECTED TO
HOLD TRUE WITH OVERNIGHT LOWS AS WELL ACROSS MUCH OF THE CENTRAL
AND WESTERN U.S., WITH THE GREATEST POSITIVE ANOMALIES ACROSS
MONTANA AND THE DAKOTAS. CONDITIONS SHOULD BE CLOSE TO SEASONAL
AVERAGES FOR THE EASTERN U.S. FOR MUCH OF THE WEEK WITH NO MAJOR
ARCTIC AIR INTRUSIONS EXPECTED.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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