000
FXUS02 KWBC 101859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM 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 THAT 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
EVEN EARLY IN THE FORECAST PERIOD, MODEL GUIDANCE FROM THE 00Z/06Z
CYCLE DIFFERED QUITE A BIT WITH THE EVOLUTION/TIMING OF THE FAIRLY
STRONG SYSTEM NEARING THE NORTHWEST BY SATURDAY AND TROUGH
PROGRESSING EASTWARD THEREAFTER. THERE HAS BEEN A CONSISTENT SIGNAL
FOR ECMWF/ECENS AND CMC/CMCENS RUNS TO BE SIGNIFICANTLY
SLOWER/FARTHER WEST COMPARED TO THE GFS/GEFS/UKMET. BUT, EC-BASED
ML MODELS HAVE UNIFORMLY BEEN ON THE FASTER SIDE OF THE SPREAD,
UNLIKE THE REST OF THE EC SUITE. THIS CREATED LOW CONFIDENCE IN THE
EC/CMC-TYPE SLOWER SOLUTIONS, AND THE MOST RECENT WPC FORECAST
CONTINUED TO FAVOR THE FASTER GUIDANCE CLUSTER. FORTUNATELY, THE
INCOMING 12Z ECMWF AND CMC HAVE SWITCHED TO FASTER TRACKING OF THE
FEATURES TOWARD THE COAST AND INLAND. THUS THE 12Z SUITE IS
YIELDING HIGHER CONFIDENCE IN THIS FASTER PROGRESSION OF THE
UPPER/SURFACE LOWS INTO THE NORTHWEST, THOUGH THE 12Z EC/CMC ARE
STILL SLOWER BY EARLY NEXT WEEK AS THE TROUGH PROGRESSES INTO THE
CENTRAL U.S.
ELSEWHERE, ANOTHER FEATURE TO CONSIDER IS SHORTWAVE TROUGHING
TRAVERSING FROM AROUND THE CENTRAL ROCKIES FRIDAY INTO THE MIDWEST
OVER THE WEEKEND ALONG WITH A GRADUALLY WEAKENING SURFACE LOW. THE
MAIN MODEL OUTLIER WITH THIS WAS THE SLOWER 00Z UKMET, AND
OTHERWISE A MULTI-MODEL BLEND SEEMED REASONABLE DESPITE SOME
DIFFERENCES. THE NEXT SHORTWAVE OR CLOSED LOW APPROACHING THE WEST
FROM THE EASTERN PACIFIC INTO EARLY NEXT WEEK HAS REASONABLE MODEL
SPREAD WITH ITS TIMING.
THE WPC FORECAST FAVORED THE 00Z/06Z GFS, 00Z UKMET, AND GEFS MEAN
EARLY IN THE PERIOD GIVEN THE AFOREMENTIONED EC/CMC DISCREPANCIES.
GRADUALLY INCREASED THE PROPORTION OF THE GEFS MEAN AND A BIT OF
EC MEAN AS THE PERIOD PROGRESSED. HOPEFULLY FUTURE FORECASTS CAN
INCORPORATE MORE EC/CMC GUIDANCE SINCE THEIR 12Z MODELS ARE SHOWING
SOMEWHAT BETTER ALIGNMENT.
..WEATHER/HAZARDS HIGHLIGHTS
THE STORM LIKELY NEARING THE PACIFIC NORTHWEST FRIDAY-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 MAY BE POSSIBLE AT LEAST OFFSHORE.
WHILE THERE ARE STILL SOME MODEL DIFFERENCES, RECENT GUIDANCE IS
SUGGESTING A RELATIVELY FASTER MOVEMENT OF THE ASSOCIATED
ATMOSPHERIC RIVER FOR LESS EXTREME RAINFALL AMOUNTS THAN SOME OLDER
MODELS SUGGESTED. THERE IS STILL CERTAINLY ENOUGH QPF THROUGH
FRIDAY-SATURDAY TO MAINTAIN MARGINAL RISKS FOR FAR SOUTHWEST OREGON
INTO NORTHERN CALIFORNIA, WITH JUST MINOR TWEAKS TO CONTINUITY.
SOME MOISTURE SHOULD CONTINUE ACROSS THE WEST INTO THE WEEKEND AND
HEAVY SNOW IS POSSIBLE IN THE HIGHER ELEVATIONS OF THE CASCADES,
SIERRA NEVADA, AND NORTHERN ROCKIES.
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 IN
THE MID/UPPER MISSISSIPPI VALLEY LATE WEEK INTO THE NORTHEAST OVER
THE WEEKEND, THOUGH PROBABILITIES FOR 0.25 INCH LIQUID IN THE FORM
OF SNOW ARE CURRENTLY NO HIGHER THAN 10-30 PERCENT.
ANOTHER ROUND OF PRECIPITATION IS LIKELY TO DEVELOP ACROSS THE
EAST-CENTRAL U.S. EARLY NEXT WEEK AS THE INITIALLY WESTERN SYSTEM
PUSHES EASTWARD AND PULLS IN MOISTURE. THIS CURRENTLY LOOKS TO BE A
WARMER SYSTEM FOR MOSTLY RAIN, THOUGH WINTRY WEATHER MAY BE
POSSIBLE IN THE FAR NORTHERN TIER. EXPECT ANOTHER PACIFIC SYSTEM TO
PRODUCE ADDITIONAL RAIN AND HIGHER ELEVATION SNOW ACROSS THE WEST
DURING THE FIRST PART OF NEXT WEEK AS WELL.
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.
TATE/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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