860
FXUS02 KWBC 180640
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
140 AM EST WED DEC 18 2024
VALID 12Z SAT DEC 21 2024 - 12Z WED DEC 25 2024
...HEAVY PRECIPITATION/WINDY PATTERN FOCUS FOR THE PACIFIC
NORTHWEST/NORTHERN CALIFORNIA THIS WEEKEND INTO THE HOLIDAYS...
..OVERVIEW
PATTERN AMPLIFICATION ACROSS THE CONUS DURING THE MEDIUM RANGE
PERIOD IS EXPECTED BY THE WEEKEND AS DEEP TROUGHING BECOMES
ESTABLISHED OVER THE EAST, AND A STRONG RIDGE BUILDS OVER THE WEST-
CENTRAL STATES. A SERIES OF SHORTWAVES ORIGINATING FROM AMPLIFIED
EAST PACIFIC TROUGHING WILL PUSH INTO/THROUGH THE MEAN RIDGE OUT
WEST, BRINGING A COUPLE OF ATMOSPHERIC RIVERS AND HEAVY
RAIN/MOUNTAIN SNOWS TO PARTS OF THE WEST COAST WELL INTO THE
UPCOMING HOLIDAY WEEK. CURRENT FORECASTS SHOW CONTINUED EASTWARD
PROGRESS OF THE LEADING PACIFIC SHORTWAVE INTO THE PLAINS BY EARLY
NEXT WEEK HELPING TO PUSH THE EASTERN U.S. UPPER TROUGH EASTWARD
WITH A RETURN TO A MORE PROGRESSIVE (BUT STILL POTENTIALLY ACTIVE)
OVERALL PATTERN.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS CONTINUE TO SHOW GOOD AGREEMENT ON THE OVERALL PATTERN
EVOLUTION DURING THE MEDIUM RANGE PERIOD, BUT CONTINUED UNCERTAINTY
IN THE DETAILS AND TIMING OF INDIVIDUAL SYSTEMS. THE MODELS HAVE A
GOOD HANDLE EARLY PERIOD ON THE PERIODICALLY REINFORCED EASTERN
TROUGH, ALTHOUGH THERE IS SOME UNCERTAINTY ON THE AMPLITUDE OF A
SHORTWAVE DROPPING DOWN THE WEST SIDE OF THE TROUGH THROUGH THE
MIDWEST AND INTO THE MID-ATLANTIC. BEYOND THIS, THE ATTENTION
SHIFTS OUT WEST AS A SERIES OF SHORTWAVES PROGRESS INLAND ACROSS
THE WEST INTO THE CENTRAL U.S. DURING THE PERIOD. MODELS AGREE ON
THE PRESENCE OF THESE SYSTEMS, BUT SOME TIMING AND STRENGTH
QUESTIONS, WHICH HAVE HUGE IMPLICATIONS ON THE QPF AMOUNTS AND
COVERAGE ALONG ESPECIALLY THE WEST COAST. AS THE INITIAL SHORTWAVE
ON SATURDAY MOVES INLAND, IT MAY EVENTUALLY SPIN UP A BRIEF AND
COMPACT CLOSED LOW OVER/NEAR THE GREAT LAKES EARLY NEXT WEEK, WHILE
ADDITIONAL SOUTHERN STREAM ENERGY INDUCES A WEAK AND UNCERTAIN
SURFACE SYSTEM ACROSS THE SOUTHERN PLAINS. THE STRONGEST OF THE
SHORTWAVES LOOKS TO ENTER THE WEST COAST BY NEXT TUESDAY, WITH SOME
GENERAL AGREEMENT SOUTHERN STREAM ENERGY MAY HELP AMPLIFY THIS
TROUGH OVER THE GREAT BASIN OR SOUTHWEST U.S. BY CHRISTMAS.
THE WPC BLEND FOR TONIGHT WAS ABLE TO USE A NEARLY EQUAL BLEND OF
THE ECMWF, GFS, AND UKMET (THE CMC WAS MORE AMPLIFIED WITH THE
EASTERN U.S. SHORTWAVE THIS WEEKEND) THROUGH DAY 5. AFTER DAY 5,
GRADUALLY INCREASED THE ENSEMBLE MEANS TO HALF THE BLEND PERCENTAGE
TO HELP MITIGATE THE UNCERTAINTY WITH THE WESTERN U.S. SHORTWAVES.
..WEATHER/HAZARDS HIGHLIGHTS
FOLLOWING A DEPARTING INITIAL SHORTWAVE OUT OF THE EAST THIS
WEEKEND, EXPECT LAKE EFFECT SNOWS TO CONTINUE IN FAVORABLE DOWNWIND
LOCATIONS OF LAKES MICHIGAN, ERIE, AND ONTARIO INTO EARLY NEXT
WEEK. THE TRAILING ARCTIC AIRMASS SHOULD BRING DAYTIME HIGHS DOWN
TO AS MUCH AS 15 TO 20 DEGREES BELOW NORMAL FOR ESPECIALLY PARTS OF
THE NORTHEAST AND CLOSE TO THAT IN THE MID- ATLANTIC INTO EARLY
NEXT WEEK, WITH MODERATION EXPECTED THEREAFTER AND JUST IN TIME FOR
THE CHRISTMAS HOLIDAY.
OVER THE WEST, UPPER DYNAMICS AND WINDY SURFACE SYSTEMS WILL START
TO PUSH FARTHER EASTWARD BY THE WEEKEND, WITH THE FIRST OF A
SERIES OF ATMOSPHERIC RIVERS ON SATURDAY AND INCREASING RAINFALL
POTENTIAL FOR PORTIONS OF NORTHERN CALIFORNIA AND MOUNTAIN SNOW IN
THE HIGHER TERRAIN OF THE CASCADES AND CALIFORNIA. THIS SYSTEM
SHOULD BE FAIRLY PROGRESSIVE, BUT MODEST MOISTURE VALUES AND SOME
HIGHER RAIN RATES WERE ENOUGH TO CONTINUE A MARGINAL RISK ON THE
DAY 4 EXCESSIVE RAINFALL OUTLOOK ACROSS COASTAL NORTHERN
CALIFORNIA. THIS ENHANCED ACTIVITY WILL LIKELY CONTINUE INTO NEXT
WEEK AS ANOTHER COUPLE OF ARS MOVE THROUGH, WITH THE LAST ONE
POSSIBLY STRONGER THAN THE PREVIOUS TWO WITH HIGHER POTENTIAL FOR
IMPACTFUL RAINFALL EARLY TO MID NEXT WEEK. AS THE LEADING SHORTWAVE
REACHES THE CENTRAL U.S. BY NEXT MONDAY, PRECIPITATION MAY DEVELOP
OVER PARTS OF THE PLAINS/MIDWEST BUT WITH LOW CONFIDENCE IN
SPECIFICS AT THIS TIME.
MEAN RIDGING ALOFT WILL GENERALLY KEEP TEMPERATURES OVER THE
INTERIOR WEST AND INTO THE PLAINS ABOVE AVERAGE THROUGH THE PERIOD
WITH BROAD COVERAGE OF PLUS 10 TO 20 DEGREE ANOMALIES. A FEW DAILY
RECORDS (MAINLY FOR DAYTIME HIGHS) ARE POSSIBLE FOR THE WEST THIS
WEEKEND.
SANTORELLI
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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