831
FXUS02 KWBC 170657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 AM EST TUE DEC 17 2024
VALID 12Z FRI DEC 20 2024 - 12Z TUE DEC 24 2024
..OVERVIEW
EXPECT THE UPPER PATTERN TO BECOME MORE AMPLIFIED DURING THE LATE
WEEK THROUGH WEEKEND TIME FRAME AS MEAN TROUGHING BECOMES
ESTABLISHED OVER THE EAST AND A STRONG RIDGE BUILDS OVER THE WEST.
A SHORTWAVE AND ITS SURFACE SYSTEM SHOULD BRING SNOW POTENTIAL
ACROSS THE GREAT LAKES AND A FRESH BLAST OF ARCTIC AIR TO THE EAST
FRIDAY INTO THE WEEKEND. TRAILING ENERGY WILL LIKELY REINFORCE THE
TROUGH AGAIN DURING THE WEEKEND. IN CONTRAST, THE WEST WILL BE
QUITE WARM DUE TO THE MEAN RIDGE ALOFT. A SERIES OF SHORTWAVES WILL
PUSH INTO/THROUGH THE MEAN RIDGE, WITH A COUPLE OF ATMOSPHERIC
RIVERS LIKELY BRINGING A PERIOD OF HEAVY RAIN/MOUNTAIN SNOWS TO
PARTS OF THE WEST COAST. 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.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GUIDANCE SHOWS GOOD AGREEMENT ON THE OVERALL PATTERN EVOLUTION
DURING THE MEDIUM RANGE PERIOD BUT PLENTY OF UNCERTAINTY IN THE
TIMING AND DETAILS OF INDIVIDUAL SYSTEMS. THE REINFORCED EASTERN
U.S. TROUGH THROUGH THE WEEKEND SHOWS GOOD ENOUGH AGREEMENT FOR A
GENERAL DETERMINISTIC MODEL BLEND. OUT WEST, GUIDANCE IS IN
AGREEMENT THE FIRST SYSTEM SHOULD PUSH ONSHORE AROUND SATURDAY, BUT
THERE IS PLENTY OF UNCERTAINTY IN THE QPF DETAILS AND AMOUNTS
ASSOCIATED WITH THIS SYSTEM. THIS SYSTEM SHOULD PUSH INLAND ACROSS
THE ROCKIES AND INTO THE NORTHERN PLAINS EARLY NEXT WEEK, BUT
DISAGREEMENT ON THE TIMING AND TRACK OF THIS SYSTEM EXISTS. A
SECOND WEAKER SHORTWAVE WILL BREAK INTO THE RIDGE SUNDAY INTO
MONDAY, WITH A STRONGER MORE AMPLIFIED TROUGH REACHING THE WEST
COAST AROUND TUESDAY, BUT WITH UNCERTAINTY IN THE TIMING OF
IMPACTS. GIVEN THE LATE PERIOD UNCERTAINTY, WPC USED INCREASINGLY
MORE OF THE ENSEMBLE MEANS IN THE BLEND, WITH MODEST AMOUNTS OF THE
ECMWF FOR SOME ADDED SYSTEM DEFINITION.
..WEATHER/HAZARDS HIGHLIGHTS
FOLLOWING A DEPARTING INITIAL SHORTWAVE OUT OF THE EAST, AND
SUBSEQUENT SYSTEMS, EXPECT LAKE EFFECT SNOWS TO CONTINUE IN
FAVORABLE DOWNWIND LOCATIONS OF LAKES MICHIGAN, ERIE, AND ONTARIO.
WITH LIGHT SNOW POSSIBLE ACROSS PORTIONS OF THE INTERIOR NORTHEAST
AS WELL. BY NEXT WEEKEND 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.
OVER THE WEST, UPPER DYNAMICS AND SURFACE SYSTEMS WILL START TO
PUSH FARTHER EASTWARD BY NEXT 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 WARRANT A MARGINAL RISK ON THE DAY 5
EXCESSIVE RAINFALL OUTLOOK ACROSS COASTAL NORTHERN CALIFORNIA. THIS
ENHANCED ACTIVITY WILL LIKELY CONTINUE THROUGH NEXT MONDAY AS
ANOTHER COUPLE OF ARS MOVE THROUGH, WITH THE LAST ONE POSSIBLY
STRONGER THAN THE PREVIOUS TWO WITH HIGHER POTENTIAL FOR IMPACTFUL
RAINFALL. 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 WEST INTO HIGH
PLAINS ABOVE AVERAGE THROUGH THE PERIOD WITH BROAD COVERAGE OF PLUS
10 TO 20 DEGREE ANOMALIES. A FEW DAILY RECORDS (MAINLY FOR DAYTIME
HIGHS) WILL BE POSSIBLE OVER THE WEST LATE THIS WEEK.
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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