239
FXUS02 KWBC 181838
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
138 PM 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
NOT MUCH CHANGE TO REPORT FROM THE OVERNIGHT FORECAST PACKAGE. UPCOMING
PATTERN AMPLIFICATION ALOFT OVER 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 DYNAMIC 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 AND
ONWARD BY EARLY NEXT WEEK TO HELP PUSH THE EASTERN U.S. UPPER
TROUGH EASTWARD WITH A RETURN TO A MORE PROGRESSIVE (BUT STILL
POTENTIALLY ACTIVE) OVERALL PATTERN.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL FORECAST CLUSTERING AND PREDICTABILITY SEEM MUCH BETTER THAN
AVERAGE FOR BOTH THE LARGER SCALE FLOW EVOLUTION AND MANY EMBEDDED
SYSTEMS DAYS 3-7. A PREFERRED COMPOSITE OF THE 06 UTC GFS AND 00
UTC ECMWF/UKMET/CANADIAN MODELS WAS USED FOR WPC PRODUCT CREATION
FOR DAYS 3-7. THIS GENERALLY SEEMED CONSISTENT WITH WPC AND
GUIDANCE CONTINUITY ALONG WITH THE 13 UTC NATIONAL BLEND OF MODELS
THERE IS ALSO GOOD MULTI-MODEL ENSEMBLE AND MACHINE LEARNING MODEL
SUPPORT. THIS BLEND PROCESS TENDS TO MITIGATE LINGERING SMALLER
SCALE MODEL VARIANCES AS CONSISTENT WITH INDIVIDUAL SYSTEM
UNCERTAINTIES. LATEST INFORMATION FROM THE NEWER 12 UTC GUIDANCE
SUITE SO FAR REMAINS WELL IN LINE WITH THIS OVERALL FORECAST PLAN.
..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 QUITE COLD 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 AND INLAND OVER
THE NORTHWESTERN U.S.. 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/SATURDAY EXCESSIVE
RAINFALL OUTLOOK ACROSS COASTAL NORTHERN CALIFORNIA. THE NEXT
DYNAMIC SHORTWAVE WORKING INLAND DAY 5/SUNDAY LED TO THE
INTRODUCTION OF ANOTHER MARGINAL RISK AREA SPREAD UP THROUGH THE
PACIFIC NORTHWEST. THIS ENHANCED ACTIVITY WILL LIKELY CONTINUE
INTO NEXT WEEK AS ANOTHER COUPLE OF ARS MOVE THROUGH, WITH THE LAST
ONE AND MAIN DEEP LOW WILL LIKLEY BE STRONGER THAN THE PREVIOUS
TWO WITH HIGHER POTENTIAL FOR IMPACTFUL RAINFALL EARLY TO MID NEXT
WEEK. THIS LEADS INTO A WET WEEK 2 PERIOD AS HERALDED BY CPC. 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 10-20 DEGREE ANOMALIES. A FEW DAILY
RECORDS (MAINLY HIGHS) ARE POSSIBLE FOR THE WEST THIS WEEKEND.
SANTORELLI/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 (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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