938
FXUS02 KWBC 021901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST SAT DEC 02 2023
VALID 12Z TUE DEC 05 2023 - 12Z SAT DEC 09 2023
...HEAVY RAIN EXPECTED FOR THE PACIFIC NORTHWEST WITH AN
ATMOSPHERIC RIVER EVENT CONTINUING INTO TUESDAY AND WEDNESDAY...
..PATTERN OVERVIEW
A PRIMARY CONCERN INTO NEXT WEEK WILL BE A STRONG PACIFIC JET
STREAM THAT WILL BE ACCOMPANIED BY AN ATMOSPHERIC RIVER EVENT,
WITH HEAVY RAINFALL PATTERN EXPECTED TO CONTINUE 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 SOUTHWEST TO CENTRAL U.S. INTO
NEXT WEEKEND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL AND ENSEMBLE FORECAST SPREAD HAS IMPROVED EARLY-MID NEXT
WEEK IN A PATTERN WITH SEEMINGLY ABOVE NORMAL PREDICTABILITY.
ACCORDINGLY, THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY
DERIVED FROM A COMPOSITE OF THE 06 UTC GFS AND 00 UTC
ECMWF/UKMET/CANADIAN MODELS ALONG WITH THE COMPATIBLE 13 UTC
NATIONAL BLEND OF MODELS (NBM) AND WPC CONTINUITY FOR BEST DETAIL
TUESDAY INTO THURSDAY. NEWER 12 UTC GUIDANCE REMAINS MAINLY IN
LINE AND MULTI-CENTER ENSEMBLE SUPPORT REMAINS VERY GOOD IN THIS
PERIOD.
FORECAST SPREAD INCREASES LATER NEXT WEEK INTO NEXT WEEKEND. A
COMPOSITE BLEND OF BEST CLUSTERED GUIDANCE FROM THE 06 UTC
GFS/GEFS MEAN AND 00 UTC ECMWF/ECMWF ENSEMBLE MEAN WAS PREFERRED
ALONG WITH THE NBM AND WPC CONTINUITY TO ATTEMPT TO MITIGATE
GROWING EMBEDDED SYSTEM DIFFERENCES OVER TIME. THE MOST
SIGNIFICANT GUIDANCE VARIANCE IN THIS BLEND ALOFT WAS THAT THE 06
UTC GFS AND 00 UTC ECMWF MODELS SIMILARLY SHOWED A LESS
PROGRESSIVE AND MORE AMPLIFIED SOUTHERN STREAM UPPER TROUGH OUT
THROUGH THE SOUTHWEST THAN THE GEFS/ECENS MEANS, ALBEIT WITH A
SUBSET OF MODEL MATCHING INDIVIDUAL ENSEMBLE MEMBERS. THE NEWER 12
UTC GFS/ECMWF MODELS HAVE NOW TRENDED MORE PROGRESSIVE/LESS
AMPLIFIED, MORE IN LINE WITH THE ENSEMBLE MEANS.
..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 EARNEST, WITH RISING CREEKS AND RIVER LEVELS
EXPECTED. GIVEN ENHANCED RAINFALL DURING THE WEEKEND AND ON
MONDAY, 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. THEREFORE, A SLIGHT RISK AREA REMAINS VALID FOR
WESTERN SLOPES OF THE WASHINGTON CASCADES ON TUESDAY, AND ALSO FOR
THE OREGON COASTAL RANGES ON TUESDAY (DAY 4). WITH THE
ATMOSPHERIC RIVER FOCUS MOST LIKELY WEAKENING SOME AND SHIFTING A
BIT MORE SOUTH INTO OREGON AND NORTHWEST CALIFORNIA ON WEDNESDAY,
A MARGINAL RISK IS VALID FOR THE DAY 5 OUTLOOK FOR FAR WESTERN
OREGON INTO EXTREME NORTHWEST CALIFORNIA. 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 AND VICINITY 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/SCHICHTEL
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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