345
FXUS02 KWBC 010658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST FRI DEC 1 2023
VALID 12Z MON DEC 4 2023 - 12Z FRI DEC 8 2023
...HEAVY RAIN EXPECTED FOR THE PACIFIC NORTHWEST WITH MULTIPLE
ATMOSPHERIC RIVER EVENTS NEXT WEEK...
..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 A SERIES OF
ATMOSPHERIC RIVER EVENTS, 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 WILL LIKELY BE IN PLACE FROM THE
GREAT LAKES TO THE EAST COAST, WITH A DEPARTING STORM SYSTEM ON
MONDAY, FOLLOWED BY A SASKATCHEWAN SCREAMER (A VARIANT OF THE
ALBERTA CLIPPER) 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.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE 00Z MODEL GUIDANCE SUITE IS IN RELATIVELY GOOD MODEL AGREEMENT
TO START THE WEEK, ALTHOUGH THE 12Z RUN OF THE CMC WAS OUT OF
ALIGNMENT WITH THE MODEL CONSENSUS ACROSS THE EASTERN U.S. ON
MONDAY AND THUS IT WAS NOT USED FOR THE DAY 3 FRONTS (00Z RUN NOW
WITH THE CONSENSUS). THE GUIDANCE HAS ALSO SOLIDIFIED ON THE
BUILDING RIDGE OVER THE CENTRAL U.S. THROUGH MIDWEEK, BEFORE
TRANSITIONING MORE TO A ZONAL FLOW TYPE PATTERN BY THURSDAY AND
FRIDAY. MODEL UNCERTAINTIES ARE MORE APPARENT BY THE END OF THE
WEEK, WITH THE GFS MORE PROGRESSIVE WITH THE NEXT SHORTWAVE MOVING
INLAND ACROSS THE WESTERN U.S. AND BECOMING STRONGER WITH THE
TROUGH OVER THE GREAT LAKES REGION BY THE END OF THE WEEK, SO THE
GFS WAS GRADUALLY PHASED OUT BY FRIDAY. ENSEMBLE MEANS WERE
GRADUALLY INCREASED TO ABOUT 50% BY THE END OF THE FORECAST PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
OUT WEST, PRECIPITATION WILL BE ONGOING ON MONDAY, 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. GIVEN A
STRONGER MODEL SIGNAL FOR TERRAIN ENHANCED 3-6 INCH RAINFALL
TOTALS ON MONDAY-MONDAY NIGHT ACROSS THE OLYMPIC PENINSULA, A
SLIGHT RISK IS WARRANTED FOR THE DAY 4 EXCESSIVE RAINFALL OUTLOOK
FOR THIS AREA. WITH THE ATMOSPHERIC RIVER SHIFTING SOUTH ACROSS
OREGON ON TUESDAY, A MARGINAL RISK IS WARRANTED FOR THE DAY 5
OUTLOOK FOR WESTERN OREGON AND THE SOUTHERN PORTIONS OF THE
WASHINGTON CASCADES AND OLYMPICS.
FARTHER INLAND ACROSS NORTHERN PORTIONS OF THE INTERMOUNTAIN WEST
AND ROCKIES, SOME ROUNDS OF MODERATE TO LOCALLY SNOW ARE EXPECTED
INTO NEXT MIDWEEK WITH THE PASSAGE OF THE NEXT ORGANIZED SYSTEM.
SNOW IS ALSO EXPECTED FOR NEW ENGLAND ON MONDAY WITH THE COASTAL
LOW NEARBY, WITH MAINE LIKELY TO HAVE THE GREATEST IMPACTS. 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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