759
FXUS02 KWBC 300710
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
210 AM EST MON DEC 30 2024
VALID 12Z THU JAN 02 2025 - 12Z MON JAN 06 2025
...COLDER WEATHER IN THE CENTRAL AND EASTERN U.S. ALLOWS FOR BROADENING WINTRY PRECIPITATION CHANCES OVER THE WEEKEND...
..OVERVIEW
A BROAD UPPER TROUGH WILL BE IN PLACE ACROSS THE CENTRAL AND
EASTERN U.S. LATER THIS WEEK, ALLOWING FOR NEAR TO BELOW AVERAGE
TEMPERATURES AND MULTIPLE DAYS OF LAKE EFFECT SNOW IN FAVORED
AREAS. MEANWHILE UPPER RIDGING WILL AMPLIFY OVER THE WEST FOR
WARMER TEMPERATURES WHILE AN ATMOSPHERIC RIVER PRODUCES
PRECIPITATION IN THE NORTHWEST 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 THE
COOL 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 POSSIBLE ICE IN THE TRANSITION ZONE.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS AGREEABLE WITH THE LARGE SCALE PATTERN OF
THE WESTERN RIDGE AND THE EASTERN TROUGH FOR THE EARLY MEDIUM RANGE
PERIOD. SMALL IMPULSES RIDING THE RIDGE INTO THE NORTHWEST SHOW
SOME MODEL SPREAD THAT AFFECTS QPF, AND THERE IS ALSO SOME SPREAD
WITH THE DETAILS OF A WEAK WAVE PRODUCING SOME LIGHT SNOW FROM THE
MID-MISSISSIPPI VALLEY TO THE OHIO VALLEY AND CENTRAL APPALACHIANS.
THESE SMALLER SCALE DIFFERENCES WILL HAVE TO BE RESOLVED DURING
THE SHORT RANGE PERIOD.
BY FRIDAY INTO SATURDAY, THE EASTERN PACIFIC TROUGH ENTERING THE
WEST SHOWS MORE CONSIDERABLE MODEL VARIABILITY WITH ITS TIMING. THE
12Z AND NEW 00Z CMC RUNS WERE SLOWER/FARTHER WEST WITH THE TROUGH
AXIS THAN ALMOST ALL ENSEMBLE MEMBERS. THE 12Z ECMWF WAS ALSO ON
THE SLOW SIDE THOUGH--DESPITE ALL AI/ML MODELS (BASED ON THE EC
INITIAL CONDITIONS) FASTER LIKE THE GFS RUNS AND THE UKMET. FAVORED
THE FASTER SOLUTIONS, WHICH WAS SIMILAR TO THE PREVIOUS FORECAST
AS WELL, AND FORTUNATELY THE NEW 00Z ECMWF HAS TRENDED FASTER. THE
DEPTH OF THE TROUGH AT LEAST IS MORE AGREEABLE THAN IN PREVIOUS
DAYS, AS MODELS HAVE TRENDED AWAY FROM THE FLATTEST SOLUTIONS.
THE TROUGH SHOULD SET UP ATOP THE ROCKIES TO PLAINS EARLY NEXT
WEEK WITH REASONABLE MODEL AGREEMENT. A SURFACE LOW IS LIKELY TO
FORM IN RESPONSE, LIKELY OVER THE SOUTHERN PLAINS THIS WEEKEND AND
THEN TRACKING EAST. THE 00Z ECMWF AND GFS HAPPEN TO AGREE FAIRLY
WELL ON A 12Z MONDAY SURFACE LOW POSITION OVER LOUISVILLE, KENTUCKY
OR SO, BUT MODELS ARE FAR FROM LOCKED IN ON THIS POSITION.
CONTINUE TO MONITOR FUTURE FORECASTS AS THE LOW TRACK WILL IMPACT
PRECIPITATION POSITION AND TYPE.
THE WPC FORECAST WAS BASED ON A BLEND FAVORING THE 12/18Z GFS RUNS
WITH SOME INCLUSION OF THE 12Z ECMWF, 12Z UKMET, AND GEFS AND EC
ENSEMBLE MEANS EARLY IN THE PERIOD. QUICKLY RAMPED UP THE
PROPORTION OF ENSEMBLE MEANS TO OVER HALF BY DAYS 6-7 AMID
INCREASING MODEL SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
LAKE EFFECT SNOW IS LIKELY LATER THIS WEEK WITH COLD AND BRISK
FLOW ACROSS THE GREAT LAKES, LIKELY PEAKING THURSDAY-FRIDAY WITH
SOME CONTINUING INTO SATURDAY. MEANWHILE SHORTWAVE ENERGY AND A
WEAK FRONTAL WAVE MAY PRODUCE A BAND OF LIGHT SNOW LATE THIS WEEK
SOMEWHERE WITHIN AN AREA ENCOMPASSING THE MIDWEST/OHIO VALLEY TO
CENTRAL APPALACHIANS. FARTHER SOUTH, PARTS OF THE WESTERN/CENTRAL
GULF COAST COULD SEE ONE OR MORE PERIODS OF LIGHT RAIN.
AHEAD OF THE EASTERN PACIFIC UPPER TROUGH, MOISTURE IN THE FORM OF
A MODERATE ATMOSPHERIC RIVER WILL BE DIRECTED INTO THE PACIFIC
NORTHWEST TO NORTHERN CALIFORNIA, LIKELY PEAKING THURSDAY NIGHT
INTO FRIDAY MORNING. THERE ARE REMAINING TIMING AND PLACEMENT
DIFFERENCES FOR THE POSITIONING OF PEAK MOISTURE TRANSPORT AND
HEAVIEST RAINFALL. THUS CONTINUED TO HOLD OFF ON ANY EXCESSIVE
RAINFALL AREAS IN THE DAYS 4-5/THURSDAY-FRIDAY EROS, BUT SOME AREAS
WITHIN THIS REGION MAY BE MORE SENSITIVE THAN NORMAL TO HEAVY
RAINFALL WITH THE WET ANTECEDENT CONDITIONS. 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. THERE MAY BE A TRANSITION ZONE OF ICE IN
BETWEEN THE RAIN AND SNOW, AND SOME LOCATIONS COULD TRANSITION FROM
SNOW TO RAIN OR VICE VERSA. THE CENTRAL PLAINS INTO MIDDLE
MISSISSIPPI VALLEY APPEARS MOST LIKELY FOR TRANSITIONING
PRECIPITATION TYPES AT THIS POINT. HEAVY RAIN AND THUNDERSTORMS ARE
POSSIBLE 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/CENTRAL 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.
MEANWHILE THE MEAN TROUGH IN THE CENTRAL/EASTERN U.S. WILL LEAD TO
COOLER THAN NORMAL TEMPERATURES FROM THE NORTH-CENTRAL U.S. INTO
THE MIDWEST TO EASTERN SEABOARD. THE NORTHERN PLAINS IS FORECAST TO
SEE THE MOST ANOMALOUS TEMPERATURES, 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.
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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