831
FXUS02 KWBC 291900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST SUN DEC 29 2024
VALID 12Z WED JAN 01 2025 - 12Z SUN JAN 05 2025
..OVERVIEW
GUIDANCE IS CONSISTENT IN DEPICTING AN AMPLIFIED WESTERN RIDGE AND
EASTERN TROUGH PATTERN BY LATE THIS WEEK, FOLLOWED BY AN EASTERN
PACIFIC TROUGH PASSING THROUGH THE WEST INTO THE CENTRAL U.S. WHILE
RIDGING REBUILDS OFF THE WEST COAST. THIS PATTERN/EVOLUTION WILL
FAVOR COLDER TEMPERATURES FROM THE NORTHERN PLAINS THROUGH MUCH OF
THE EAST AND ABOVE NORMAL READINGS FROM THE GREAT BASIN/SOUTHWEST
INTO FAR SOUTHERN PLAINS. COASTAL LOW PRESSURE WILL BRING RAIN AND
SNOW TO THE NORTHEAST AROUND MIDWEEK, FOLLOWED BY MULTIPLE DAYS OF
LAKE EFFECT SNOW. THE PACIFIC NORTHWEST AND NORTHERN ROCKIES WILL
SEE EPISODES OF RAIN AND MOUNTAIN SNOW MID-LATE WEEK, WITH MOISTURE
EVENTUALLY PUSHING SOMEWHAT SOUTH/EAST AS THE UPPER TROUGH
APPROACHES/CROSSES THE WEST. BY NEXT WEEKEND THE TROUGH MAY SUPPORT
A DEVELOPING LOW PRESSURE SYSTEM OVER THE PLAINS WITH POSSIBLE
NORTH-CENTRAL U.S. SNOW AND SOUTH-CENTRAL U.S. RAIN.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS AGREEABLE THAT UPPER TROUGHING WILL BE IN
PLACE ACROSS THE CENTRAL AND EASTERN U.S. FOR THE LATTER HALF OF
NEXT WEEK. LATEST GUIDANCE CONTINUES TO SHOW GRADUAL CONSOLIDATION
FOR THE SURFACE LOW TRACKING FROM OVER/NEAR NEW ENGLAND AS OF EARLY
WEDNESDAY INTO EASTERN CANADA THEREAFTER. THE CURRENT MAJORITY
SHOWS THE BEST DEFINED LOW PRESSURE INLAND OVER SOUTHERN NEW
ENGLAND AS OF 12Z WEDNESDAY AND THEN REACHING A LITTLE
EAST/NORTHEAST OF MAINE BY EARLY THURSDAY (AS SUPPORTED BY THE
MACHINE LEARNING MODELS). CONSENSUS/CONTINUITY PROVIDE HIGH
CONFIDENCE IN THE FORECAST OF MEAN CYCLONIC FLOW IN THE SYSTEM'S
WAKE. THERE IS STILL THE SIGNAL FOR A WEAK WAVE PROGRESSING FROM
THE PLAINS THROUGH OR NEAR THE TENNESSEE VALLEY AND INTO THE
WESTERN ATLANTIC THURSDAY INTO SATURDAY, THOUGH IF ANYTHING THE
WAVE HAS BECOME A LITTLE LESS DEFINED TODAY. MOST DYNAMICAL
SUPPORT FOR THIS FEATURE COMES FROM SHEARING PACIFIC ENERGY WHICH
LOOKS TO HAVE LOW PREDICTABILITY A FEW DAYS OUT IN TIME, SO
OCCASIONAL ADJUSTMENTS MAY WELL CONTINUE.
IN THE WEST, THE UPPER RIDGE AXIS SEEMS WELL REPRESENTED IN THE
GUIDANCE WEDNESDAY-FRIDAY BEFORE AN EASTERN PACIFIC TROUGH
APPROACHES. HOWEVER THERE IS CONTINUED SPREAD AND VARIABILITY FOR
ALONG/OFF-SHORE SURFACE DETAILS THAT WILL AFFECT THE PRECIPITATION
FORECAST OVER THE PACIFIC NORTHWEST. THEN THE EASTERN PACIFIC
TROUGH CONTINUES TO SHOW SPREAD IN ITS TIMING AND DEPTH, AFFECTING
THE FORECAST FROM WEST COAST INTO THE PLAINS. AS OF FRIDAY, ECMWF
RUNS CONTINUE TO BE ON THE SOUTHERN SIDE OF THE SPREAD FOR THE CORE
OF THE UPPER TROUGH AND ASSOCIATED SURFACE LOW BUT THE OVERALL
GUIDANCE SPREAD HAS BEEN GETTING TRIMMED MORE ON THE NORTHERN
SIDE--BRINGING THE OVERALL GUIDANCE MEAN SOUTHWARD SOMEWHAT.
ECMWF/ECENS RUNS HAVE BEEN ON THE AMPLIFIED SIDE OF THE SPREAD AS
THE TROUGH PUSHES THROUGH THE WEST NEXT WEEKEND, BUT 06Z/12Z GFS
AND RECENT MACHINE LEARNING MODEL TRENDS SUGGEST A LEANING TOWARD
THE MORE AMPLIFIED HALF OF PRIOR SPREAD AT LEAST (ESPECIALLY
FAVORING DISCOUNTING THE VERY FLAT EARLIER CMC RUNS AND 00Z GFS).
THERE ARE ALSO SIGNIFICANT TIMING DIFFERENCES, WITH THE 12Z CMC AND
NEW 12Z ECMWF ON THE SLOWER SIDE WHILE MOST ML GUIDANCE (MINUS THE
00Z AIFS MEAN) STRAYS FASTER THAN MOST DYNAMICAL GUIDANCE--EVEN
SHOWING A CLOSED UPPER LOW OVER THE MIDWEST AS OF EARLY SUNDAY.
PREFER AN INTERMEDIATE TIMING GIVEN THE SIGNIFICANT SPREAD THAT
CURRENTLY EXISTS.
A 00Z/06Z MODEL COMPOSITE (WITH MORE 00Z ECMWF/06Z GFS EMPHASIS)
REPRESENTED THE BEST IDEAS OF GUIDANCE EARLY IN THE PERIOD. THEN
THE 12Z/28 ECMWF COMPARED A LITTLE BETTER TO OTHER DYNAMICAL
GUIDANCE FROM THE EAST PACIFIC THROUGH THE WEST, WHILE THE 00Z CMC
COMPARED INCREASINGLY POORLY AS WELL. BY LATE IN THE PERIOD A BLEND
AMONG THE 12Z/28 ECMWF AND 06Z GFS PLUS THE 00Z ECENS/06Z GEFS
MEANS PROVIDED A GOOD DYNAMICAL/ML MODEL COMPROMISE ALONG WITH
REASONABLE CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
GRADUALLY DEEPENING LOW PRESSURE TRACKING NEAR THE NEW ENGLAND
COAST WILL BRING PRECIPITATION TO THE NORTHEAST AROUND MIDWEEK.
OVERALL EXPECT COASTAL AND LOWER ELEVATION RAIN, WHILE HIGHER
ELEVATION AREAS OF THE INTERIOR NORTHEAST SUCH AS THE ADIRONDACKS
AND THE GREEN AND WHITE MOUNTAINS WILL SEE ACCUMULATING SNOW.
THERE MAY BE GREATER COVERAGE OF RAIN EARLY IN THE DAY, FOLLOWED BY
EXPANSION OF SNOW AS COLDER AIR FILLS IN RIGHT BEHIND THE LOW. IN
THE WAKE OF THIS SYSTEM, EXPECT MULTIPLE DAYS OF COLD AND BRISK
FLOW ACROSS THE GREAT LAKES AND NORTHEAST, WITH LAKE EFFECT SNOW
PERSISTING THROUGH THE LATTER HALF OF THE WEEK, PERHAPS EXTENDING
INTO SATURDAY OVER SOME LOCATIONS. 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.
MOIST INFLOW INTO THE WEST SHOULD GET RENEWED BY WEDNESDAY AND
CONTINUE THROUGH LATE WEEK, PRODUCING ADDITIONAL ROUNDS OF
PRECIPITATION IN THE PACIFIC NORTHWEST AND AT TIMES FARTHER EAST
INTO THE NORTHERN ROCKIES. PRECIPITATION AMOUNTS FROM DAY TO DAY
AND IN TOTAL FOR ANY SPECIFIC LOCATION VARY CONSIDERABLY IN THE
MODELS AS THERE IS A FAIR AMOUNT OF SPREAD FOR SYSTEM/FRONTAL
SPECIFICS. THERE IS STILL A SUBSET OF MODEL GUIDANCE SUGGESTING
POTENTIAL FOR A PERIOD OF RELATIVELY HEAVIER PRECIPITATION ALONG
THE WEST COAST NEAR THE CALIFORNIA/OREGON BORDER ON WEDNESDAY.
HOWEVER OTHER SOLUTIONS VARY IN PLACEMENT/COVERAGE AND MAGNITUDE.
THE COMBINATION OF FORECAST TOTALS AND GUIDANCE SPREAD CONTINUE TO
FAVOR NOT INTRODUCING ANY RISK AREAS IN THE DAY 4 ERO. HOWEVER
THIS REGION WILL BE SENSITIVE DUE TO RECENT/ONGOING RAINFALL AND
WILL MONITOR GUIDANCE IN FUTURE CYCLES FOR ANY TRENDS TOWARD
CLUSTERING IN THE WETTER END OF THE ENVELOPE. BY DAY 5 (THURSDAY
THROUGH THURSDAY NIGHT), PACIFIC NORTHWEST PRECIPITATION SHOULD
SHIFT OR EXPAND NORTHWARD AHEAD OF A BETTER DEFINED SYSTEM
APPROACHING BY VERY EARLY FRIDAY, WITH DETAILS AND MAGNITUDE AGAIN
NOT FAVORING ANY ERO RISK AREA AT THIS TIME.
THE UPPER TROUGH NEARING THE WEST COAST AS OF EARLY FRIDAY WILL
CONTINUE ONWARD THEREAFTER, MOST LIKELY REACHING THE CENTRAL U.S.
BY NEXT SUNDAY. RAIN AND HIGHER ELEVATION SNOW SHOULD SPREAD FROM
THE NORTHWEST AND NORTHERN CALIFORNIA INTO THE NORTHERN-CENTRAL
ROCKIES DURING FRIDAY-SATURDAY, WITH CONTINUED UNCERTAINTY FOR
SOUTHWARD EXTENT AND DURATION IN LIGHT OF GUIDANCE SPREAD FOR UPPER
TROUGH AMPLITUDE AND TIMING. SURFACE LOW PRESSURE SUPPORTED BY THE
TROUGH WILL DEVELOP OVER THE PLAINS NEXT WEEKEND. WHILE THE
DETAILS REMAIN NEBULOUS, PRECIPITATION CHANCES LOOK TO INCREASE
GENERALLY ACROSS THE PLAINS AND MISSISSIPPI VALLEY FROM SATURDAY
INTO SUNDAY. THIS PRECIPITATION WOULD BE SNOW IN THE NORTH-
CENTRAL 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.
KEEP AN EYE ON FUTURE FORECASTS AS DETAILS ARE LIKELY TO CHANGE.
BEHIND THIS SYSTEM, ANOTHER FRONT COULD BRING MORE PRECIPITATION TO
THE NORTHWEST NEXT WEEKEND BUT WITH LOW CONFIDENCE IN SPECIFICS
FOR NOW.
GENERALLY, THE MEAN UPPER RIDGE OVER THE WEST WILL PROMOTE ABOVE
NORMAL TEMPERATURES FROM THE GREAT BASIN AND SOUTHWEST TO THE
SOUTHERN/CENTRAL PLAINS THROUGH THE PERIOD, AND 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 (AFTER ONE MORE WARMER THAN AVERAGE DAY IN THE
NORTHEAST ON WEDNESDAY). 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
BARELY REACHING 0F AND LOWS BETWEEN -10F AND -15F OR SO IN SOME
AREAS. MEANWHILE 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.
RAUSCH/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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