071
FXUS02 KWBC 100700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST TUE DEC 10 2024
VALID 12Z FRI DEC 13 2024 - 12Z TUE DEC 17 2024
...CONTINUING TO MONITOR A POTENTIALLY SIGNIFICANT STORM SYSTEM TO
AFFECT THE PACIFIC NORTHWEST BY THIS WEEKEND...
..OVERVIEW
MULTI-DAY MEANS CONTINUE TO DEPICT NEARLY ZONAL FLOW ACROSS THE
LOWER 48 FROM LATE THIS WEEK INTO THE FIRST HALF OF NEXT WEEK,
CONTAINING A SEQUENCE OF SHORTWAVES WITH VARYING AMPLITUDE AND
TENDING TO BE SPACED A COUPLE DAYS OR SO APART. EACH ASSOCIATED
SURFACE SYSTEM SHOULD PRODUCE EPISODES OF RAIN AND HIGHER
ELEVATION SNOW OVER PORTIONS OF THE WEST AND THEN DEVELOPMENT OF
PRECIPITATION OVER THE SOUTHERN PLAINS AND EXPANDING ACROSS THE
EAST-CENTRAL U.S. THEN CONTINUING INTO PORTIONS OF THE EAST.
NORTHERN AREAS MAY SEE SOME WINTRY WEATHER. WHILE GUIDANCE DEPICTS
THE GENERAL PATTERN WELL, PARTICULAR MODELS/ENSEMBLES HAVE BEEN
VERY STUBBORN WITHIN THEIR RESPECTIVE CLUSTERS WHICH SHOW
SIGNIFICANT DIFFERENCES FOR SYSTEM DETAILS/TIMING. THIS IS
ESPECIALLY THE CASE FOR THE SYSTEM FORECAST TO BE APPROACHING THE
PACIFIC NORTHWEST THIS WEEKEND. EXPECT TEMPERATURES TO BE NEAR OR
ABOVE NORMAL ACROSS MOST OF THE COUNTRY DURING THE PERIOD ASIDE
FROM A CHILLY FRIDAY OVER NORTHERN/EASTERN AREAS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
INITIALLY LOOKING AT THE FORECAST FROM THE BIG-PICTURE PERSPECTIVE,
THE PRIMARY FORECAST PROBLEMS INVOLVE THE EVENTUAL DETAILS OF
FRIDAY'S ROCKIES/PLAINS SHORTWAVE AS IT MOVES INTO THE EAST
FOLLOWED BY THE EVOLUTION/TIMING OF THE FAIRLY STRONG SYSTEM
NEARING THE NORTHWEST BY SATURDAY (AND TROUGH PROGRESSING EASTWARD
THEREAFTER) AS WELL AS THE NEXT SYSTEM FORECAST TO ARRIVE AROUND
MONDAY-TUESDAY. CONTINUING INTO THE NEW 00Z CYCLE, ECMWF/ECENS AND
CMC/CMCENS RUNS REMAIN SIGNIFICANTLY SLOWER WITH THE LATTER TWO
SYSTEMS VERSUS THE GFS/GEFS/UKMET WHILE MACHINE LEARNING (ML)
MODELS STILL FAVOR THE FASTER SIDE OF THE SPREAD--WITH A SMALL
SUBSET EVEN FASTER THAN THE GFS CLUSTER FOR THE NORTHWEST SYSTEM ON
SATURDAY. THE UPDATED FORECAST MAINTAINS RELATIVE CONTINUITY WITH A
TILT MORE TOWARD THE GFS/GEFS GIVEN THE CONSISTENCY OF THE ML
THEMES AS WELL AS THE GENERALLY PROGRESSIVE NATURE OF THE LARGE
SCALE PATTERN.
FOCUSING MORE CLOSELY ON THE SYSTEM DEVELOPING OVER THE EASTERN
PACIFIC FRIDAY INTO THE WEEKEND, EVENTUAL DIFFERENCES IN SMALL-
SCALE BUT VERY IMPORTANT DETAILS BETWEEN THE FASTER/NORTHWARD GFS
AND SLOWER/SOUTHWARD ECMWF CLUSTERS SEEM TO START SHOWING UP
WITHIN THE INITIALLY FLAT NORTH PACIFIC FLOW AROUND LATE WEDNESDAY,
SO THERE IS A CHANCE IT MAY TAKE ANOTHER DAY OR TWO TO ACHIEVE
BETTER CLUSTERING. THE TIMING DIFFERENCES HAVE A SIGNIFICANT IMPACT
ON THE PRECIPITATION TOTALS ALONG THE CENTRAL WEST COAST INTO THE
PACIFIC NORTHWEST (MUCH HEAVIER IN THE SLOWER ECMWF CLUSTER). AS
MENTIONED ABOVE, ML MODELS GENERALLY FAVOR A PROGRESSIVE SOLUTION
WITH THEIR ENVELOPE EVEN EXTENDING BEYOND THE FASTER SIDE OF THE
DYNAMICAL SPREAD. SUCH FAST TIMING MAY BE OVERDONE GIVEN A FAIRLY
GOOD EMERGING CONSENSUS AMONG OPERATIONAL RUNS AND A DECENT NUMBER
OF ENSEMBLE MEMBERS FOR A DEPTH IN THE 970S OR 980S MB.
AS FOR OTHER SYSTEMS, THE NEXT SHORTWAVE REACHING THE WEST
(SUPPORTING A LESS IMPRESSIVE SURFACE SYSTEM THAN ITS PREDECESSOR)
HAS ADJUSTED A LITTLE FASTER IN THE 00Z ECMWF BY NEXT TUESDAY SO
IT IS NOW CLOSER TO THE MAJORITY CLUSTER. THE NEW 00Z CMC HAS
DRAMATICALLY IMPROVED THE DEPICTION OF THIS SHORTWAVE COMPARED TO
THE OLD 00Z RUN. THE SHORTWAVE PROGRESSING INTO THE EAST BY SUNDAY
IS QUESTIONABLY SLOW/CLOSED IN THE 00Z UKMET AND THEN A LOT OF
LOWER-PREDICTABILITY DETAIL DIFFERENCES ARISE FOR HOW THE SHORTWAVE
ENERGY MAY OR MAY NOT SPLIT AND THE RESULTING SURFACE
REFLECTION/PRECIPITATION. A BLEND APPROACH MINIMIZES EMPHASIS ON
ANY PARTICULAR DETAIL THAT HAS LOW CONFIDENCE.
..WEATHER/HAZARDS HIGHLIGHTS
A SHORTWAVE REACHING THE ROCKIES/PLAINS BY FRIDAY WILL CONTINUE
EASTWARD WITH TIME, WITH SOME DETAILS OF ITS ENERGY BECOMING
INCREASINGLY UNCERTAIN TOWARD THE EAST COAST BY SUNDAY-MONDAY. THIS
FEATURE AND ASSOCIATED SURFACE WAVE/FRONTAL SYSTEM MAY SPREAD
PRECIPITATION ACROSS THE EAST-CENTRAL U.S. BY FRIDAY-SATURDAY AND
THEN ACROSS THE EAST COAST. GENERALLY EXPECT LIGHT TO MODERATE
AMOUNTS WITH THIS SYSTEM. LOCALLY HEAVIER TOTALS CANNOT BE RULED
OUT BUT OVERALL GUIDANCE SIGNALS ARE TOO WEAK TO SUGGEST ANY RISK
AREAS IN THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS. DETAILS WILL
DEPEND ON FINER SCALE SHORTWAVE DETAILS THAT HAVE LOW
PREDICTABILITY SEVERAL DAYS OUT IN TIME. THE NORTHERN PART OF THE
PRECIPITATION SHIELD MAY CONTAIN SOME WINTRY WEATHER AT TIMES,
THOUGH PROBABILITIES FOR 0.25 INCH LIQUID IN THE FORM OF SNOW ARE
CURRENTLY NO HIGHER THAN 10-30 PERCENT.
THE STORM LIKELY NEARING THE PACIFIC NORTHWEST BY SATURDAY WILL
BRING THE POTENTIAL FOR AREAS OF SIGNIFICANT RAIN AND HIGHER
ELEVATION SNOW TO LOCATIONS ALONG THE CENTRAL AND NORTHERN WEST
COAST. BRISK TO STRONG WINDS WILL ALSO BE POSSIBLE. GUIDANCE
CONTINUES TO SHOW SIGNIFICANT DIFFERENCES IN EVOLUTION DETAILS AND
TRACK/TIMING FOR THIS SYSTEM (THOUGH WITH A LITTLE LESS SPREAD FOR
DEPTH). THESE DIFFERENCES YIELD EITHER CONSIDERABLE OR LESS
EXTREME PRECIPITATION OVER PARTS OF WESTERN OREGON/NORTHERN
CALIFORNIA, THOUGH WITH THIS REGION SITTING WITHIN THE BEST OVERLAP
AMONG SOLUTIONS FOR HEAVIEST ACTIVITY. THUS THE DAY 4 ERO
MAINTAINS A MARGINAL RISK CLOSE TO CONTINUITY, WITH THE DAY 5 ERO
INTRODUCING A MARGINAL RISK THAT EXPANDS A LITTLE TO THE SOUTH AND
EAST.
SOME OF THIS WEST COAST SYSTEM'S MOISTURE SHOULD CONTINUE ACROSS
THE WEST DURING THE WEEKEND, WITH THE SHORTWAVE/SURFACE SYSTEM
LIKELY PRODUCING ANOTHER AREA OF PRECIPITATION OVER THE EAST-
CENTRAL STATES BY AROUND THE START OF NEXT WEEK. EXPECT ANOTHER
PACIFIC SYSTEM TO PRODUCE ADDITIONAL RAIN AND HIGHER ELEVATION SNOW
ACROSS THE WEST DURING THE FIRST PART OF NEXT WEEK.
FRIDAY WILL BE ON THE CHILLY SIDE ACROSS THE NORTHERN TIER AND THE
EAST, WITH LOCATIONS OVER THE UPPER MIDWEST NEAR THE CANADIAN
BORDER LIKELY SEEING THE COLDEST ANOMALIES OF 10-20F OR SO BELOW
NORMAL. ASIDE FROM LINGERING MODEST COOL ANOMALIES OVER THE
INTERIOR SOUTHEAST ON SATURDAY, THE REST OF THE LOWER 48 SHOULD
SEE NEAR TO ABOVE NORMAL TEMPERATURES FROM SATURDAY ONWARD. WARMEST
ANOMALIES FOR HIGHS SHOULD BE IN THE PLUS 10-15F RANGE OVER PARTS
OF THE PLAINS ON SUNDAY-MONDAY. EXPECT BROADER COVERAGE OF SOME
AREAS OF PLUS 10-15F ANOMALIES FOR MORNING LOWS.
RAUSCH
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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