123
FXUS02 KWBC 280659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 AM 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. A PRIMARY SURFACE LOW IS FORECAST TO
TRACK THROUGH THE OHIO VALLEY TUESDAY AND ACROSS THE NORTHEAST
WEDNESDAY-THURSDAY ACCOMPANIED BY SOME MODEST PRECIPITATION.
UPSTREAM, AN UPPER RIDGE WILL SET UP ATOP THE INTERIOR WEST,
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
MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT WITH THE LARGE
SCALE PATTERN EVOLUTION WITH SOME LINGERING UNCERTAINTIES ON
SMALLER SCALES. MODELS ARE WELL CLUSTERED WITH A SURFACE LOW
POSITION IN THE OHIO VALLEY ON TUESDAY, WITH MORE DIFFERENCES BY
THE TIME THE LOW REACHES THE NORTHEAST IN EMPHASIZING A COASTAL LOW
OR AN INLAND LOW. OVERALL GUIDANCE TRENDED AWAY FROM SHOWING A LOW
AS FAR WEST AS THE LOWER GREAT LAKES, BUT THE 12Z ECMWF AND ONE
AI/ML MODEL (PANGU) WERE STILL FARTHER INLAND THAN THE CONSENSUS
THAT IS OVER NYC OR SO 12Z WEDNESDAY. THE NEW 00Z EC HAS TRENDED
EAST A BIT, WHICH FITS WITH OTHER GUIDANCE BETTER, BUT THERE IS
STILL SOME MINOR TIMING SPREAD. MODELS STILL INDICATE THE LOW
STRENGTHENING AS IT TRACKS NORTH, REACHING OVER OR JUST EAST OF
MAINE BY 12Z THURSDAY.
FARTHER WEST, AFTER A FEW DAYS OF GFS/GEFS RUNS BEING ON THE
STRONG SIDE WITH SHORTWAVE ENERGY REACHING THE WEST AROUND TUESDAY,
RECENT RUNS STARTING WITH THE 12Z GFS HAVE BEEN MODERATED WITH ITS
ENERGY AND MATCH THE OTHER MODELS BETTER. HOWEVER, THE 18Z GFS
BROUGHT IN ENERGY JUST BEHIND THAT SHORTWAVE, WHICH PRODUCED AN
UPPER TROUGH CLOSING OFF A LOW JUST SOUTH/SOUTHWEST OF CALIFORNIA
WEDNESDAY-SATURDAY. THIS SEEMED TO BE AN OUTLIER AS OTHER MODELS
MAINTAINED RIDGING THERE, SO LEANED AGAINST THE 18Z GFS.
AFTER SOME IMPULSES MOVE INTO THE PACIFIC NORTHWEST WEDNESDAY-
THURSDAY, A LARGER SCALE EASTERN PACIFIC UPPER TROUGH IS FORECAST
TO APPROACH FRIDAY AND COME INTO THE NORTHWEST SATURDAY. THE 12Z
ECMWF AND GFS AND THEIR MEANS SHOWED REASONABLE AGREEMENT ON ITS
TIMING, WHILE THE 12Z CMC WAS FASTER THAN PREFERRED GIVEN THE
AMPLIFIED PATTERN AND THE 18Z GFS PERHAPS TOO SLOW. THE 00Z CMC HAS
ENDED UP FLIPPING TO AN EVEN SLOWER SOLUTION THAN CONSENSUS
DESPITE A SHALLOWER TROUGH. THE TIMING OF THIS TROUGH WILL
NATURALLY IMPACT THE PUSHING OF THE UPPER RIDGE EASTWARD TOWARD THE
ROCKIES.
THE WPC FORECAST WAS BASED ON A MULTI-MODEL DETERMINISTIC BLEND
FAVORING THE ECMWF EARLY IN THE FORECAST PERIOD. GRADUALLY
ELIMINATED THE PROPORTIONS OF THE 18Z GFS, 12Z UKMET, AND 12Z CMC
IN TURN, REPLACING THEM WITH THE GEFS AND EC ENSEMBLE MEANS. BY DAY
7 THE MEANS COMPRISED HALF THE MODEL BLEND WITH THE REMAINDER 12Z
GFS/ECMWF.
..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. THEN COLD
CYCLONIC FLOW BEHIND THE STORM SYSTEM WILL LIKELY PRODUCE A PERIOD
OF LAKE EFFECT SNOW INTO THURSDAY AND FRIDAY.
THE PACIFIC NORTHWEST IS FORECAST TO 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 BEFORE A BETTER DEFINED FEATURE
ARRIVES AROUND FRIDAY--ALBEIT WITH SOME TIMING DIFFERENCES. MOST
GUIDANCE AGREES THAT HIGHEST PRECIPITATION WILL OCCUR IN
WASHINGTON/NORTHERN OREGON RATHER THAN CALIFORNIA.
THE EASTERN U.S. CAN EXPECT ONE MORE DAY OF ABOVE AVERAGE
TEMPERATURES (BY 10-15F) ON TUESDAY, BEFORE UPPER TROUGHING PUSHES
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 TO 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
FOR NORTHERN NORTH DAKOTA AND MINNESOTA ON THURSDAY-FRIDAY.
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.
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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