230
FXUS02 KWBC 161857
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
157 PM EST MON DEC 16 2024
VALID 12Z THU DEC 19 2024 - 12Z MON DEC 23 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.
AN INITIAL EAST COAST TROUGH WILL DEPART ON THURSDAY IN FAVOR OF AN
AMPLIFYING SHORTWAVE REACHING INTO THE EAST FRIDAY-FRIDAY NIGHT.
THIS SHORTWAVE AND ITS SURFACE SYSTEM SHOULD BRING SNOW POTENTIAL
ACROSS THE GREAT LAKES AND A FRESH BLAST OF ARCTIC AIR TO THE EAST
BY NEXT WEEKEND. TRAILING ENERGY WILL LIKELY REINFORCE THE TROUGH
DURING THE WEEKEND. IN CONTRAST, THE WEST WILL BE QUITE WARM DUE TO
THE MEAN RIDGE ALOFT. PACIFIC SHORTWAVE ENERGY MAY FINALLY PUSH
INTO/THROUGH THE MEAN RIDGE BY THE WEEKEND, WITH AN ATMOSPHERIC
RIVER POTENTIALLY BRINGING A PERIOD OF HEAVY RAIN/MOUNTAIN SNOWS
TO PARTS OF THE WEST COAST. ADDITIONAL MOISTURE SHOULD REACH THE
WEST COAST EARLY NEXT 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.
..GUIDANCE/PREDICTABILITY ASSESSMENT
OVERALL PREDICTABILITY IS ABOVE AVERAGE DUE TO THE GENERAL
DOMINANCE OF MEDIUM TO LARGER SCALE FEATURES, THOUGH SOME TYPICAL
SMALLER SCALE UNCERTAINTIES EXIST WITHIN THE DEEPENING MEAN TROUGH
OVER THE EAST AS WELL AS WITH WEST COAST DETAILS PLUS THE BETTER
DEFINED SHORTWAVE THAT MAY REACH THE CENTRAL U.S. BY NEXT MONDAY.
WITH THE AVERAGE OF MOST OPERATIONAL RUNS FITTING WITHIN THE
TEMPLATE OF THE ENSEMBLE MEANS, A GENERAL MODEL BLEND REPRESENTED
CONSENSUS WELL FOR MOST OF THE PERIOD, WITH THE 00Z GFS PREFERRED
OVER THE 06Z VERSION BY NEXT MONDAY. THE FORECAST INCLUDED A LIGHT
TOUCH OF THE ENSEMBLE MEANS (06Z GEFS/00Z ECENS) AFTER CONCLUSION
OF THE 00Z UKMET.
OVER THE EAST, TYPICAL DETAIL DIFFERENCES PERSIST FOR THE SURFACE
WAVE TRACKING INTO OR NEAR THE LOWER GREAT LAKES BY EARLY FRIDAY.
THE MACHINE LEARNING (ML) MODEL AVERAGE COMPARES WELL TO THE
DYNAMICAL AVERAGE AT THAT TIME WHILE THE NEW 12Z GFS APPEARS TO BE
A BIT ON THE WEAK/SUPPRESSED SIDE. A TRAILING UPPER SHORTWAVE MAY
PRODUCE SOME WEAK WAVINESS OVER THE PLAINS AROUND SATURDAY. ML
MODELS AND THE NEW 12Z GFS SHOW A BETTER SIGNAL FOR THIS THAN MOST
OTHER DYNAMICAL MODEL RUNS. BY NEXT MONDAY THE GFS/ECMWF HAVE BEEN
INCONSISTENT WITH HOW MUCH TROUGHING MAY LINGER NEAR THE EAST
COAST, WITH ENSEMBLES AND ML MODELS ALSO DISPLAYING A FAIR AMOUNT
OF SPREAD IN THE LATEST CYCLE. MAJORITY CLUSTERING SAYS THE TROUGH
AXIS SHOULD BE OFFSHORE BY THEN.
DETAIL DIFFERENCES ALONG THE WEST COAST HAVE COME INTO BETTER
AGREEMENT ASIDE FROM VERY SMALL-SCALE ASPECTS THAT ARE NOT EASILY
RESOLVED SEVERAL DAYS OUT IN TIME. SOLUTIONS DIVERGE SOME FOR
DETAILS OF THE PACIFIC SHORTWAVE CROSSING THE WEST DURING THE
WEEKEND AND REACHING THE CENTRAL U.S. BY NEXT MONDAY. AS OF MONDAY
THE MOST COMMON THEME IS FOR THE NORTHERN PART OF THE SHORTWAVE TO
BE A LITTLE WEAKER AND/OR SLOWER THAN THE 06Z/12Z GFS, WITH THE 00Z
GFS MORE IN LINE WITH CONSENSUS.
..WEATHER/HAZARDS HIGHLIGHTS
THE AMPLIFYING NORTHERN PLAINS INTO EASTERN U.S. UPPER SHORTWAVE
LATE THIS WEEK AND ASSOCIATED SURFACE SYSTEM TRACKING INTO THE
GREAT LAKES REGION BY EARLY FRIDAY COULD BRING ACCUMULATING SNOW
ACROSS PARTS OF THE UPPER MIDWEST TO GREAT LAKES ON THURSDAY, WITH
LAKE EFFECT SNOW LINGERING INTO NEXT WEEKEND IN THE FAVORABLE
LOCATIONS DOWNWIND OF LAKES MICHIGAN, ERIE, AND ONTARIO. RELATIVELY
HIGHEST TOTALS ARE POSSIBLE OVER PARTS OF THE UPPER MIDWEST AND
NORTHWESTERN GREAT LAKES REGION WHERE PROBABILITIES OF AT LEAST
0.25 INCH LIQUID IN THE FORM OF SNOW ARE 30-50 PERCENT. A BRIEF
PERIOD OF BRISK TO STRONG WINDS BEHIND THE SURFACE WAVE MAY LEAD TO
SOME BLOWING SNOW. 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, EXPECT DRY CONDITIONS TO PREVAIL OVER MOST AREAS
ASIDE FROM THE NORTHWEST WHICH MAY SEE PERIODS OF PRECIPITATION
ASSOCIATED WITH SHORTWAVES/MOISTURE ON THE FAR EASTERN PERIPHERY OF
PACIFIC SYSTEMS. UPPER DYNAMICS AND SURFACE SYSTEMS WILL START TO
PUSH FARTHER EASTWARD BY NEXT WEEKEND, WITH THE CORRESPONDING
ATMOSPHERIC RIVER INCREASING PRECIPITATION TOTALS AND SOUTHWARD
COVERAGE ALONG THE WEST COAST. HEAVY RAIN/MOUNTAIN SNOWS WILL BE
POSSIBLE ACROSS COASTAL RANGES OF THE NORTHWEST AND EXTENDING INTO
NORTHERN CALIFORNIA. THIS ENHANCED ACTIVITY WILL LIKELY CONTINUE
THROUGH NEXT MONDAY. 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.
RAUSCH/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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