263
FXUS02 KWBC 281859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST SAT DEC 28 2024
VALID 12Z TUE DEC 31 2024 - 12Z SAT JAN 04 2025
..OVERVIEW
MEAN UPPER TROUGHING IS LIKELY TO FORM ACROSS THE CENTRAL AND
EASTERN U.S. TUESDAY-WEDNESDAY AND LAST THROUGH AT LEAST THE REST
OF THE WEEK, WITH SOME EMBEDDED SHORTWAVES PRODUCING LOW
PRESSURE/FRONTAL SYSTEMS. GUIDANCE CONTINUES TO SHOW A PRIMARY
SURFACE LOW TRACKING THROUGH THE OHIO VALLEY TUESDAY AND ACROSS
THE NORTHEAST WEDNESDAY-THURSDAY ACCOMPANIED BY PRECIPITATION OF
VARYING INTENSITY DEPENDING ON LOCATION. COLD CYCLONIC FLOW IN THE
SYSTEM'S WAKE SHOULD SUPPORT MULTIPLE DAYS OF LAKE EFFECT SNOW
THROUGH THE LATTER HALF OF THE WEEK. UPSTREAM, AN UPPER RIDGE WILL
BUILD OVER THE WEST COAST AND THEN THE INTERIOR WEST BY THURSDAY-
FRIDAY, LEADING TO TEMPERATURES WARMING ABOVE NORMAL THERE WHILE
THE EAST COOLS UNDER THE TROUGH. EASTERN PACIFIC ENERGY/TROUGHING
THAT MAY MOVE INLAND BY FRIDAY OR SATURDAY WILL LEAD TO INCREASING
PRECIPITATION CHANCES ACROSS THE NORTHWEST FOR THE LATTER HALF OF
THE WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
DYNAMICAL AND MACHINE LEARNING (ML) MODELS CONTINUE TO AGREE UPON
THE LARGE SCALE PATTERN TRANSITIONING TO AN AMPLIFIED WESTERN
RIDGE/EASTERN TROUGH REGIME, WITH LEADING INTENSIFYING LOW
PRESSURE TRACKING FROM THE OHIO VALLEY ON TUESDAY TO NORTHEAST OR
NORTH OF NEW ENGLAND LATER IN THE WEEK, AND A PACIFIC UPPER TROUGH
PUSHING INTO THE WEST BY NEXT SATURDAY. WITHIN THIS CONSENSUS
EVOLUTION, FINER SCALE QUESTION MARKS PERSIST FOR LOW PRESSURE
AFFECTING THE NORTHEAST AROUND MIDWEEK AND FEATURES BRINGING
MOISTURE TO THE PACIFIC NORTHWEST WEDNESDAY-THURSDAY. MORE NOTABLE
DIFFERENCES EXIST WITH THE EASTERN PACIFIC INTO WESTERN U.S. UPPER
TROUGH AND ASSOCIATED SURFACE PATTERN FRIDAY-SATURDAY.
GUIDANCE HAS CONVERGED WELL FOR THE SURFACE LOW FORECAST TO TRACK
OVER THE OHIO VALLEY ON TUESDAY. THEN SPECIFICS CONTINUE TO BE A
BIT AMBIGUOUS AROUND TUESDAY NIGHT AND EARLY WEDNESDAY WITH
RESPECT TO THE RELATIVE EMPHASIS OF NEW YORK/LAKE ONTARIO VERSUS
COASTAL LOW PRESSURE CENTERS BEFORE GUIDANCE CLUSTERING IMPROVES
TOWARD A COASTAL NEW ENGLAND TRACK LATE WEDNESDAY AND CONTINUED
PROGRESS TO JUST EAST/NORTHEAST OF MAINE BY THURSDAY AND FARTHER
NORTH/NORTHEAST THEREAFTER. DYNAMICAL GUIDANCE CURRENTLY TILTS A
LITTLE MORE TOWARD A COASTAL LOW AS OF 12Z WEDNESDAY WHILE THE ML
MODELS ARE STILL MIXED. TIMING AND STRENGTH DIFFERENCES AFTER THAT
TIME ARE GENERALLY TYPICAL FOR THE TIME FRAME INVOLVED, THOUGH
CONSENSUS SUGGESTED THAT THE 00Z CMC WAS A LITTLE TOO SLOW
(ADJUSTING FAVORABLY IN THE NEW 12Z RUN) AND THE 06Z/12Z GFS
POSSIBLY A LITTLE TOO WEAK.
BEHIND THIS SYSTEM, OVER THE PAST DAY OR SO GUIDANCE HAS BEEN
GRADUALLY IMPROVING ITS SIGNAL FOR A WEAK WAVE PROGRESSING FROM THE
PLAINS THROUGH OR NEAR THE TENNESSEE VALLEY AND INTO THE WESTERN
ATLANTIC THURSDAY INTO SATURDAY. MOST DYNAMICAL SUPPORT FOR THIS
FEATURE COMES FROM SHEARING PACIFIC ENERGY WHICH LOOKS TO HAVE LOW
PREDICTABILITY BY 5-7 DAYS OUT IN TIME, SO CHANGES IN SPECIFICS ARE
CERTAINLY POSSIBLE IN FUTURE RUNS.
OVER THE EASTERN PACIFIC/WESTERN U.S., DYNAMICAL AND ML GUIDANCE
CONTINUES TO VARY FOR SURFACE DETAILS NEAR THE PACIFIC NORTHWEST
COAST AROUND MIDWEEK OR SO, AS UPPER DYNAMICS SHEAR OUT AND RISING
HEIGHTS ALOFT EVENTUALLY SUPPORT NORTHEASTWARD PROGRESS OF A MEAN
WARM FRONT. THERE IS NO CLEAR CLUSTERING FOR THIS PART OF THE
FORECAST. BY FRIDAY, THE 00Z ECMWF BECOMES A SOUTHERN EXTREME FOR
THE BETTER DEFINED UPPER TROUGH AND CORRESPONDING EAST PACIFIC
SURFACE LOW, WITH THE BEST CLUSTER OF ML MODELS FAVORING A SURFACE
LOW TRACK NEAR 50N AS OF 12Z FRIDAY PER THE 00Z ECENS MEAN. THEN
GUIDANCE DIVERGES CONSIDERABLY FOR AMPLITUDE OF THE UPPER TROUGH
MOVING INTO THE WEST. LATEST ECMWF RUNS ARE ON THE AMPLIFIED SIDE
OF THE SPREAD (LEADING TO GREATER SOUTHWARD EXTENT OF
PRECIPITATION) WHILE IN CONTRAST THE CMC RUNS ACTUALLY MAINTAIN
RIDGING OVER THE SOUTHERN HALF OF THE WEST. ML MODELS VARY AS WELL
BUT DO NOT SHOW ANY SUPPORT FOR THE CMC. PREFER TO LEAN AWAY FROM
THE ECMWF AS WELL (ESPECIALLY THE 00Z RUN WHICH ALSO STRAYED SLOW
ALOFT AND FOR THE SURFACE LOW), GIVEN THAT PASSAGE OF THE TROUGH
THROUGH A LONGER TERM MEAN RIDGE DOES NOT SEEM TO SUPPORT THE
STRONG SIDE OF THE SPREAD.
A BLEND CONSISTING OF THE 00Z ECMWF AND 00Z/06Z GFS WITH LESSER
WEIGHT OF THE 00Z UKMET/CMC REPRESENTED CONSENSUS OR PREFERRED
INTERMEDIATE SOLUTION, DEPENDING ON THE FEATURE, DURING THE FIRST
HALF OF THE PERIOD. THEN THE FORECAST RAPIDLY TRANSITIONED TOWARD
MORE ENSEMBLE MEAN INPUT (REACHING 60 PERCENT FOR SATURDAY, SPLIT
BETWEEN THE 06Z GEFS/00Z ECENS) WHILE SWITCHING ECMWF INPUT TOWARD
THE 12Z/27 RUN AND PHASING OUT THE 00Z CMC. THIS APPROACH YIELDED
GOOD CONTINUITY WITH MOST ASPECTS OF THE FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
A LOW PRESSURE SYSTEM TRACKING FROM THE OHIO VALLEY INTO THE
NORTHEAST WILL SPREAD PRECIPITATION ACROSS THOSE REGIONS TUESDAY-
WEDNESDAY. PRECIPITATION SHOULD FALL AS RAIN FOR MOST OF THE OHIO
VALLEY TO THE COASTAL AREAS/LOWER ELEVATIONS OF THE NORTHEAST. THE
RAIN RATES AND RAIN AMOUNTS LOOK TO STAY MAINLY UNDER EXCESSIVE
THRESHOLDS. SOME MODERATE TO LOCALLY HEAVY PRECIPITATION MAY FALL
IN THE URBAN CORRIDOR (MOST LIKELY PHILADELPHIA TO NEW YORK CITY)
LATER TUESDAY TO TUESDAY NIGHT THOUGH, AND WILL CONTINUE TO MONITOR
SINCE URBAN LAND USE CAN BE VULNERABLE TO FLASH FLOODING. SNOW IS
LIKELY IN THE HIGHER ELEVATION AREAS OF THE INTERIOR NORTHEAST LIKE
THE ADIRONDACKS AND THE GREEN AND WHITE MOUNTAINS, AND
PROBABILITIES FOR 0.25 INCH OR MORE OF LIQUID EQUIVALENT IN THE
FORM OF SNOW REMAIN 30-70 PERCENT THERE ON WEDNESDAY. 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/CENTRAL
APPALACHIANS.
THE PACIFIC NORTHWEST WILL SEE A RELATIVE BREAK IN PRECIPITATION
ON TUESDAY AFTER A STEADY TRAIN OF ATMOSPHERIC RIVERS INTO THE
REGION. BUT BY TUESDAY NIGHT OR WEDNESDAY MOIST INFLOW MAY GET
RENEWED THERE AND ROUNDS OF PRECIPITATION ARE LIKELY TO CONTINUE
THROUGH LATE WEEK AND AT TIMES FARTHER EAST INTO THE NORTHERN
ROCKIES. PRECIPITATION AMOUNTS FROM DAY TO DAY AND IN TOTAL VARY
CONSIDERABLY IN MODELS AS THERE IS A FAIR AMOUNT OF SPREAD FOR
SYSTEM/FRONTAL SPECIFICS. DURING THE WEDNESDAY-WEDNESDAY NIGHT
PERIOD COVERED BY THE DAY 5 EXCESSIVE RAINFALL OUTLOOK, LATEST
GUIDANCE IS STARTING TO SUGGEST MORE POTENTIAL FOR SOME MOISTURE
FOCUS ALONG THE WEST COAST NEAR THE CALIFORNIA/OREGON BORDER. THE
COMBINATION OF FORECAST TOTALS AND GUIDANCE SPREAD DO NOT SEEM TO
WARRANT A RISK AREA YET. 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. EVEN WITH A BETTER DEFINED FEATURE ARRIVING AROUND
FRIDAY AND PUSHING INTO THE WEST ON SATURDAY, THERE ARE
DIFFERENCES WITH THE SOUTHWARD EXTENT OF MOISTURE FOCUS DUE TO
SPREAD FOR SURFACE LOW TRACK OVER THE PACIFIC ON FRIDAY AND THEN
THE AMPLITUDE OF THE SUPPORTING UPPER TROUGH BY SATURDAY.
THE EASTERN U.S. CAN EXPECT ONE MORE DAY OF ABOVE AVERAGE
TEMPERATURES (BY 10-15F) ON TUESDAY, BEFORE UPPER TROUGHING PUSHES
ALONG A SERIES OF COLD FRONTS THAT GRADUALLY COOL TEMPERATURES TO
NEAR NORMAL ON WEDNESDAY AND GRADUALLY BELOW NORMAL INTO LATE WEEK.
HIGH TEMPERATURES BY SATURDAY ARE FORECAST TO BE AROUND 10-15F
BELOW NORMAL FOR THE OHIO VALLEY TO APPALACHIANS AND MID-ATLANTIC
WHILE LOWS SHOULD BE 5-10F BELOW AVERAGE. COLDER THAN NORMAL
TEMPERATURES WILL ALSO IMPACT THE NORTH-CENTRAL U.S., AND LOWS
COULD REACH 10-15F BELOW ZERO OVER NORTHERN NORTH DAKOTA AND
MINNESOTA BY FRIDAY AND/OR SATURDAY. MEANWHILE, THE AMPLIFYING
UPPER RIDGE OVER THE WEST WILL PROMOTE WARMING, WITH TEMPERATURES
GENERALLY 5-10F ABOVE AVERAGE INCREASING IN COVERAGE BY THE SECOND
HALF OF THE WEEK. LOCALLY HIGHER ANOMALIES ARE LIKELY IN THE
SOUTHWEST AND HIGHS COULD REACH WELL INTO THE 70S. HIGHS OF 5-15F
ABOVE NORMAL MAY REACH INTO THE SOUTHERN HIGH PLAINS BY NEXT
SATURDAY.
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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