274
FXUS02 KWBC 171832
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
132 PM EST TUE DEC 17 2024
VALID 12Z FRI DEC 20 2024 - 12Z TUE DEC 24 2024
...HEAVY PRECIPITATION/WINDY PATTERN FOCUS FOR THE PACIFIC
NORTHWEST/NORTHERN CALIFORNIA THIS WEEKEND INTO THE HOLIDAYS...
..OVERVIEW
IT REMAINS THE CASE THAT 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
MODEL FORECAST SPREAD HAS IMPROVED THROUGH MEDIUM RANGE TIME
SCALES, BOLSTERING FORECAST CONFIDENCE. 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 CONTINUITY AND THE 13 UTC NATIONAL
BLEND OF MODELS. THE BLEND TENDED TO MITIGATE LINGERING SMALLER
SCALE MODEL VARIANCES AS CONSISTENT WITH INDIVIDUAL SYSTEM
UNCERTAINTY. THE NEWER 12 UTC MODELS SUITE ALSO REMAINS IN LINE.
..WEATHER/HAZARDS HIGHLIGHTS
GENERALLY NO CHANGE THAT 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 WINDY 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. THIS LEADS INTO A CLIMATE PREDICTION CENTER FORECAST WET
PERIOD INTO WEEK 2 TO MONITOR. 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) ARE POSSIBLE FOR THE WEST LATE WEEK.
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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