975
FXUS02 KWBC 310700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST TUE DEC 31 2024
VALID 12Z FRI JAN 03 2025 - 12Z TUE JAN 07 2025
...BROAD CHANCES FOR WINTRY WEATHER ACROSS THE NORTH-CENTRAL U.S.
TO MID-MISSISSIPPI AND OHIO VALLEYS TO THE EAST COAST OVER THE
WEEKEND INTO EARLY NEXT WEEK...
..OVERVIEW
AS THE MEDIUM RANGE PERIOD BEGINS FRIDAY, BROAD TROUGHING WILL BE
IN PLACE ACROSS THE EAST, ALLOWING FOR ANOTHER COUPLE OF DAYS OF
LAKE EFFECT AND TERRAIN-ENHANCED CENTRAL APPALACHIANS SNOW.
MEANWHILE UPPER RIDGING WILL AMPLIFY OVER THE WEST FOR WARMER
TEMPERATURES WHILE A MODERATE ATMOSPHERIC RIVER WILL PROVIDE SOME
MODEST TO LOCALLY HEAVY PRECIPITATION OVER THE NORTHWEST/NORTHERN
CALIFORNIA CONTINUING INTO FRIDAY AHEAD OF AN EASTERN PACIFIC
TROUGH. THIS TROUGH IS FORECAST TO MOVE INLAND AND TOWARD THE
CENTRAL U.S. OVER THE WEEKEND, WHICH WILL SUPPORT A DEVELOPING LOW
PRESSURE SYSTEM OVER THE PLAINS AND EAST-CENTRAL U.S. EARLY NEXT
WEEK. MOISTURE ACCOMPANYING THIS LOW COULD LEAD TO HEAVY RAINFALL
CURRENTLY EXPECTED IN THE TENNESSEE VALLEY ON SUNDAY, WHILE COLD
TEMPERATURES IN PLACE FARTHER NORTH MAY PRODUCE A BROAD AREA OF
SNOW IN THE NORTH-CENTRAL PLAINS TO MID-MISSISSIPPI VALLEY AND OHIO
VALLEY THIS WEEKEND, WITH ICE LIKELY IN THE TRANSITION ZONE.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS AGREEABLE IN SHOWING A WEST UPPER RIDGE AND
EAST UPPER TROUGH LATE THIS WEEK. THE TIMING OF EASTERN PACIFIC
TROUGHING MOVING INTO THE WEST AND THEN CENTRAL U.S. AND DEVELOPING
SURFACE LOW PRESSURE SHOWS MUCH MORE MODEL VARIABILITY, HOWEVER.
THROUGH THE 12Z RUN, THE CMC HAS BEEN SLOWER THAN CONSENSUS AND
MOST ENSEMBLE MEMBERS WITH THE TROUGH BY LATE WEEK AND BEYOND,
WHICH WPC HAS NOT FAVORED. THE NEW 00Z RUN HAS FORTUNATELY ADJUSTED
FASTER. MEANWHILE, THE 12Z ECMWF WAS ON THE FASTER SIDE IN A
NOTABLE CHANGE FROM ITS PREVIOUS RUNS. EC-BASED AI/ML MODELS AND EC
ENSEMBLE MEMBERS WERE GENERALLY FASTER COMPARED TO THE GEFS AND
CMC SUITES, THOUGH SOME ML MODELS WERE SLOWER THAN THE FAST EC
CONTROL RUN BY MONDAY-TUESDAY WITH THE SURFACE LOW AND UPPER
TROUGH. THE 12/18Z GFS AND 12Z UKMET HAD TIMING THAT WAS IN BETWEEN
THE SLOW 12Z CMC AND FAST 12Z EC. SO THE GFS/UKMET SEEMED TO BE AN
OKAY MIDDLE GROUND THROUGH THE WEEKEND FORECAST, BUT DID NOT FAVOR
THE EVOLUTION OF THE GFS RUNS' LOW TRACKS FROM A SLOW/WEST
POSITION WITH THE SURFACE LOW SUNDAY AND THEN EJECTING EAST WITH
THE 12Z RUN FARTHER NORTH AND 18Z RUN FARTHER SOUTH THAN PREFERRED
BY MONDAY. THE BEST PROXIES FOR AN INTERMEDIATE FORECAST WERE
ACTUALLY THE OLD 00Z (DEC 30) ECMWF, CONTINUITY, AND PERHAPS THE
GEFS MEAN. FORTUNATELY THE NEW 00Z ECMWF HAS SLOWED COMPARED TO ITS
12Z RUN. THERE ARE CERTAINLY STILL MODEL DIFFERENCES IN THE 00Z
MODEL CYCLE WITH THE TIMING AND POSITION OF THE SURFACE LOW,
AFFECTING PRECIPITATION LOCATIONS AND TYPE, BUT AT LEAST LESS SO
THAN THE 12Z CYCLE.
THE WPC FORECAST USED A BLEND OF THE 12Z/18Z GFS, 12Z AND OLDER
00Z RUN OF THE ECMWF, AND 12Z UKMET PLUS A BIT OF ENSEMBLE MEANS
FOR THE EARLY PART OF THE PERIOD. ELIMINATED THE FAST 12Z ECMWF
FROM THE BLEND AND GRADUALLY LESSENED THE PROPORTION OF GFS RUNS
WHILE THE TIME LIMITED UKMET PHASED OUT, LEADING TO A MODEL BLEND
OF WELL OVER HALF ENSEMBLE MEANS BY DAYS 6-7 GIVEN THE AMPLE SPREAD
IN THE 12/18Z CYCLE.
..WEATHER/HAZARDS HIGHLIGHTS
LAKE EFFECT SNOW IS LIKELY INTO FRIDAY-SATURDAY WITH COLD AND
BRISK FLOW ACROSS THE GREAT LAKES. FAVORED AREAS COULD SEE HEAVY
SNOW AMOUNTS PILING UP, ESPECIALLY EAST OF LAKE ONTARIO.
ADDITIONALLY, SHORTWAVE ENERGY AND A COLD FRONT MAY PRODUCE LIGHT
SNOW IN THE OHIO VALLEY WITH TERRAIN ENHANCEMENT OF TOTALS IN THE
CENTRAL APPALACHIANS ON FRIDAY. FARTHER SOUTH, PARTS OF THE
WESTERN/CENTRAL GULF COAST COULD SEE PERIODS OF LIGHT RAIN.
AHEAD OF THE EASTERN PACIFIC UPPER TROUGH, MOISTURE IN THE FORM OF
A WEAK TO MODERATE ATMOSPHERIC RIVER WILL BE DIRECTED INTO THE
PACIFIC NORTHWEST TO NORTHERN CALIFORNIA, LIKELY PEAKING THURSDAY
NIGHT INTO FRIDAY MORNING. A MARGINAL RISK HAS BEEN ADDED FOR THE
DAY 4/FRIDAY ERO FROM THE CONTINUATION OF A DAY 3/THURSDAY MARGINAL
ERO. THIS IS LIKELY A LOW END MARGINAL RISK BUT THESE AREAS MAY BE
MORE SENSITIVE THAN NORMAL DUE TO WET ANTECEDENT CONDITIONS. RAIN
RATES COULD BE HALF AN INCH TO AN INCH PER HOUR EARLY FRIDAY.
MODEST PRECIPITATION INCLUDING HIGHER ELEVATION SNOW IS LIKELY INTO
THE INTERMOUNTAIN WEST AND NORTHERN/CENTRAL ROCKIES. ADDITIONAL
ROUNDS OF LIGHT TO MODERATE PRECIPITATION ARE POSSIBLE IN THE
NORTHWEST INTO THE WEEKEND.
AS THE UPPER TROUGH MOVES INTO THE CENTRAL U.S., SURFACE LOW
PRESSURE IS FORECAST TO DEVELOP OVER THE SOUTHERN PLAINS OVER THE
WEEKEND. WHILE THE DETAILS REMAIN NEBULOUS, PRECIPITATION CHANCES
LOOK TO INCREASE GENERALLY ACROSS THE PLAINS AND MISSISSIPPI VALLEY
SATURDAY AND STRETCHING FARTHER INTO THE EAST-CENTRAL U.S. SUNDAY-
MONDAY AS THE LOW MOVES EAST. THIS PRECIPITATION WOULD BE SNOW IN
THE NORTHERN HALF OR SO OF THE U.S. GIVEN THE COLD TEMPERATURES,
WITH RAIN IN THE SOUTH. MODEL GUIDANCE CONTINUES TO SHOW A
RELATIVELY BROAD REGION WHERE THERE MAY BE A TRANSITION ZONE OF
SLEET AND/OR FREEZING RAIN IN BETWEEN THE RAIN AND SNOW, AND SOME
LOCATIONS COULD TRANSITION BETWEEN MULTIPLE PRECIPITATION TYPES.
THE CENTRAL PLAINS INTO MIDDLE MISSISSIPPI VALLEY TO PERHAPS THE
TENNESSEE AND/OR LOWER OHIO VALLEY APPEARS MOST LIKELY FOR
TRANSITIONING PRECIPITATION TYPES AT THIS POINT. THE WINTER WEATHER
OUTLOOK DEPICTS A LARGE AREA OF 10-50% PROBABILITIES OF "PLOWABLE"
SNOW/SLEET FROM THE NORTH-CENTRAL PLAINS EASTWARD ACROSS THE
MIDDLE MISSISSIPPI VALLEY AND OHIO VALLEY THIS WEEKEND. SOME WINTRY
WEATHER MAY PUSH INTO THE MID-ATLANTIC INTO PARTS OF THE NORTHEAST
BY MONDAY, BUT WITH LOW CONFIDENCE. HEAVY RAIN AND THUNDERSTORMS
IN THE WARM SECTOR COULD ALSO BE HAZARDOUS WITH THIS SYSTEM,
CURRENTLY FORECAST TO PEAK IN THE TENNESSEE VALLEY ON SUNDAY. KEEP
AN EYE ON FUTURE FORECASTS AS DETAILS ARE LIKELY TO CHANGE.
GENERALLY, THE MEAN UPPER RIDGE OVER THE WEST WILL PROMOTE ABOVE
NORMAL TEMPERATURES FROM THE GREAT BASIN AND SOUTHWEST TO THE
SOUTHERN PLAINS LATE WEEK INTO THE WEEKEND. HIGHS WILL REACH WELL
INTO THE 70S IN THE DESERT SOUTHWEST, AND TEMPERATURES COULD WARM
INTO THE LOW 80S IN SOUTHERN TEXAS BY NEXT WEEKEND. THESE AREAS ARE
FORECAST TO MODERATE CLOSER TO NORMAL EARLY NEXT WEEK, PERHAPS
REACHING BELOW AVERAGE IN TEXAS. MEANWHILE THE MEAN TROUGH IN THE
CENTRAL/EASTERN U.S. WILL LEAD TO COOLER THAN NORMAL TEMPERATURES
FROM THE CENTRAL U.S. INTO THE MIDWEST TO EASTERN SEABOARD, WITH
MULTIPLE RESURGENCES OF COLD AIR. THE NORTHERN PLAINS ARE FORECAST
TO SEE THE MOST ANOMALOUS TEMPERATURES THROUGH THE WEEKEND, AROUND
10-20F BELOW NORMAL FOR HIGHS AND AROUND 5-15F BELOW NORMAL FOR
LOWS. THIS TRANSLATES TO HIGHS NOT REACHING OR BARELY REACHING 0F
AND LOWS BETWEEN -10F AND -20F IN SOME AREAS. THIS SHOULD NOT BE
RECORD BREAKING COLD, AS DAILY RECORDS THERE FOR THIS TIME OF YEAR
ARE GENERALLY IN THE -30S AND COLDER (LOW MINS) AND AROUND -20F FOR
LOW MAXES. HOWEVER, THE COLD COULD STILL BE HAZARDOUS AS WIND
CHILL VALUES MAY DIP BELOW -20F TO -30F.
TATE
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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