981
FXUS02 KWBC 110700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST WED DEC 11 2024
VALID 12Z SAT DEC 14 2024 - 12Z WED DEC 18 2024
...CONTINUING TO MONITOR A POTENTIALLY SIGNIFICANT STORM SYSTEM TO
AFFECT THE PACIFIC NORTHWEST BY THIS WEEKEND...
..OVERVIEW
MOST GUIDANCE CONTINUES TO SHOW A SEQUENCE OF SHORTWAVES
APPROXIMATELY 2-3 DAYS APART FROM THE PACIFIC THROUGH THE LOWER 48
FROM THE WEEKEND INTO THE FIRST HALF OF NEXT WEEK. THE CHARACTER OF
THE PATTERN MAY BEGIN TO CHANGE TOWARD THE MIDDLE OF NEXT WEEK AS
THE LAST SHORTWAVE OF THE SERIES REACHES THE WESTERN-CENTRAL U.S.,
WITH LARGER SCALE RIDGING POTENTIALLY BUILDING INTO THE WEST COAST.
EACH SHORTWAVE'S 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. GUIDANCE FINALLY AGREES BETTER FOR THE
SYSTEM APPROACHING THE PACIFIC NORTHWEST BY EARLY SATURDAY BUT
THERE ARE OTHER ASPECTS OF THE FORECAST THAT STILL SHOW A FAIR
AMOUNT OF SPREAD. THE FORECAST PATTERN WILL LEAD TO MOSTLY NEAR OR
ABOVE NORMAL TEMPERATURES DURING THE PERIOD, WITH ONLY LOCALIZED
AREAS OF BELOW NORMAL READINGS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
FOR THE SYSTEM NEARING THE WEST COAST BY EARLY SATURDAY, 12Z
ECMWF/CMC RUNS MADE A SIGNIFICANT DETAIL ADJUSTMENT TO TREND
TOWARD THE GENERAL EVOLUTION OF THE GFS. THIS INVOLVES LEADING
SHORTWAVE ENERGY SUPPORTING A NORTHERN SURFACE LOW THAT TRACKS
TOWARD VANCOUVER ISLAND WHILE TRAILING ENERGY DIGGING UNDERNEATH
PRODUCES A FRONTAL WAVE (ALBEIT WITH SOME LINGERING TIMING SPREAD).
THERE IS STILL SPREAD FOR DEPTH WITHIN THE 970S-980S MB, WITH 12Z
MACHINE LEARNING (ML) MODELS AVERAGING IN THE LOWER HALF OF THE
980S MB. THE ML MODELS HAVE CONSISTENTLY BEEN ADVERTISING A FAIRLY
PROGRESSIVE SOLUTION, AND SOME FROM THE 12Z/10 CYCLE ACTUALLY
SHOWED A LOW TRACK TO THE NORTH OF VANCOUVER ISLAND. A BLEND OF
LATEST OPERATIONAL RUNS PROVIDES GOOD CONTINUITY IN THE FORECAST.
AFTER THIS SYSTEM MOVES INLAND, GUIDANCE CONTINUES TO DIVERGE WITH
RECENT ECMWF/ECENS RUNS LEANING SOMEWHAT ON THE SLOWER SIDE OF THE
SPREAD AS THE UPPER TROUGH CONTINUES ACROSS THE LOWER 48 AS WELL AS
WITH THE NEXT TROUGH LIKELY TO REACH INTO THE WEST BY NEXT TUESDAY.
BY WEDNESDAY THE ML MODELS SUPPORT SUFFICIENT PROGRESSION TO
PRODUCE SOME SURFACE WAVINESS OVER THE PLAINS PER THE GFS/GEFS AND
HINTED AT BY THE 12Z CMCENS. HOWEVER THERE IS LITTLE SUPPORT FOR
SURFACE LOW PRESSURE AS DEEP AS GFS RUNS. ALSO BY WEDNESDAY
SIGNIFICANT DIFFERENCES ARISE OVER EAST-CENTRAL NORTH AMERICA,
WITH 12Z ECMWF/CMC RUNS SHOWING AN UPPER TROUGH OVER THE UPPER
MIDWEST/GREAT LAKES WHILE THE GFS/GEFS SHOW RIDGING. ML MODELS
GENERALLY FAVORED SOMETHING IN THE RANGE OF FLAT WESTERLY OR
SOUTHWESTERLY FLOW TO MODEST RIDGING--FAVORING LEANING AWAY FROM
THE ECMWF/ECENS MEAN.
THE TROUGH/UPPER LOW CROSSING THE EAST CONTINUES TO LOOK TOO SLOW
IN THE UKMET. OTHER MODELS CLUSTER BETTER DURING THE WEEKEND BUT
STILL DIVERGE FARTHER EASTWARD, IN PARTICULAR WITH THE ECMWF
PULLING THE SHORTWAVE ENERGY/UPPER LOW FARTHER SOUTH.
THE EARLY PART OF THE FORECAST STARTED WITH HALF GFS WEIGHT (SPLIT
BETWEEN THE 12Z/18Z RUNS) WITH THE 12Z ECMWF/CMC COMPRISING THE
REST. THEN THE FORECAST SPLIT THE ECMWF COMPONENT BETWEEN IT AND
THE ECENS MEAN TO DAMPEN ECMWF DETAILS OVER THE WESTERN HALF OF THE
COUNTRY. TUESDAY-WEDNESDAY REMOVED THE ECMWF AND INCREASED
CMCENS/GEFS MEANS WHILE DECREASING GFS INPUT SOME. THE END RESULT
YIELDED A PLAINS SURFACE PATTERN COMPATIBLE WITH THE GENERAL THEME
OF THE ML MODELS AND WAS CLOSEST TO THE GEFS MEAN.
..WEATHER/HAZARDS HIGHLIGHTS
THE STORM LIKELY NEARING THE PACIFIC NORTHWEST BY EARLY 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.
BETTER CLUSTERING OF GUIDANCE FOR HOW THIS SYSTEM WILL EVOLVE HAS
CONTINUED INTO THE 00Z MODEL CYCLE, FAVORING SUFFICIENTLY FAST
MOVEMENT OF THE ASSOCIATED ATMOSPHERIC RIVER TO KEEP RAINFALL
TOTALS FROM BEING TOO EXTREME. HOWEVER THERE IS STILL ENOUGH
RAINFALL POTENTIAL FOR THE DAY 4 EXCESSIVE RAINFALL OUTLOOK VALID
ON SATURDAY TO MAINTAIN A MARGINAL RISK AREA OVER PARTS OF
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON. FOR NOW THIS AREA IS
UNCHANGED FROM CONTINUITY AS MOST ENSEMBLE GUIDANCE AND SOME MODEL
RUNS MAINTAIN SUPPORT, BUT OTHER MODELS OFFER POTENTIAL FOR FUTURE
ADJUSTMENTS ONCE CLUSTERING IMPROVES AGAIN. 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 WITH AN EMBEDDED UPPER LOW SHOULD TRACK THROUGH THE
MIDWEST AND GREAT LAKES/OHIO VALLEY DURING THE WEEKEND. THIS
FEATURE AND ASSOCIATED SURFACE WAVE/FRONTAL SYSTEM SHOULD SPREAD
PRECIPITATION FROM THE MISSISSIPPI VALLEY EASTWARD, POSSIBLY
INCLUDING SOME AREAS BACK INTO EASTERN TEXAS AS WELL. MULTI-DAY
TRENDS HAVE BEEN FAVORING SOMEWHAT HIGHER RAINFALL TOTALS WITHIN AN
AXIS FROM EASTERN TEXAS INTO THE OHIO VALLEY ON SATURDAY, BUT
CONSENSUS OF GUIDANCE ASIDE FROM THE UKMET (SUSPICIOUSLY SLOW WITH
THE UPPER LOW) STILL INDICATES THAT SYSTEM PROGRESSION AND RELATIVE
LACK OF INSTABILITY SHOULD KEEP RAIN RATES BELOW EXCESSIVE
THRESHOLDS. MOST OF THIS REGION HAS NEUTRAL TO DRY GROUND
CONDITIONS AS WELL. PARTS OF THE UPPER MIDWEST INTO NORTHEAST COULD
SEE SOME WINTRY PRECIPITATION AT TIMES AS THE MOISTURE SHIELD
PROGRESSES EASTWARD. PROBABILITIES FOR 0.25 INCH LIQUID IN THE
FORM OF SNOW REMAIN BELOW 30 PERCENT. MEANWHILE THE MID-ATLANTIC
MAY SEE A FAIRLY WET SUNDAY.
ANOTHER ROUND OF PRECIPITATION IS LIKELY TO DEVELOP ACROSS THE
EAST-CENTRAL U.S. EARLY NEXT WEEK AS THE SYSTEM NEAR THE WEST COAST
ON SATURDAY PUSHES EASTWARD AND PULLS IN MOISTURE. THIS CURRENTLY
LOOKS TO BE A WARMER SYSTEM PRODUCING MOSTLY RAIN, THOUGH WINTRY
WEATHER MAY BE POSSIBLE IN THE FAR NORTHERN TIER. HIGHEST RAINFALL
TOTALS MAY BE NEAR THE WESTERN GULF COAST AND LOWER MISSISSIPPI
VALLEY, BUT SOME IMPORTANT DETAILS ARE YET TO BE RESOLVED. EXPECT
ANOTHER PACIFIC SYSTEM TO PRODUCE ADDITIONAL RAIN AND HIGHER
ELEVATION SNOW ACROSS THE WEST DURING THE FIRST PART OF NEXT WEEK
AS WELL, AND THEN PRODUCE ANOTHER AREA OF PRECIPITATION OVER THE
CENTRAL U.S. TOWARD WEDNESDAY.
MUCH OF THE LOWER 48 SHOULD SEE NEAR TO ABOVE NORMAL TEMPERATURES
DURING THE PERIOD, WITH THE WARMEST ANOMALIES FOR HIGHS (10-15F
ABOVE NORMAL) FORECAST TO BE OVER THE CENTRAL/EAST-CENTRAL U.S.
SUNDAY-MONDAY AND NEAR THE EAST COAST BY TUESDAY AHEAD OF A COUPLE
COLD FRONTS. SOME DAYS COULD FEATURE SOME POCKETS OF MORNING LOWS
UP TO 15-20F ABOVE NORMAL. LOCALIZED AREAS WITH SLIGHTLY BELOW
NORMAL HIGHS SHOULD INCLUDE SOME AREAS NEAR THE EAST COAST ON
SATURDAY, THE CENTRAL WEST COAST FROM THE WEEKEND INTO MONDAY, AND
THE FAR NORTHERN TIER TUESDAY-WEDNESDAY.
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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