044
FXUS02 KWBC 180656
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EDT SUN MAY 18 2025
VALID 12Z WED MAY 21 2025 - 12Z SUN MAY 25 2025
..OVERVIEW
UPPER-LEVEL ENERGY ATOP THE MIDWEST MIDWEEK WILL COMBINE WITH AN
UPPER LOW ANCHORED OVER ONTARIO/QUEBEC TO DEVELOP TROUGHING ACROSS
THE EASTERN U.S. LATER WEEK INTO NEXT WEEKEND, PROMOTING COOLER
TEMPERATURES. THE UPPER ENERGY WILL SUPPORT A SURFACE LOW PRESSURE
SYSTEM TRACKING EAST ACROSS THE OHIO VALLEY TO MID-ATLANTIC
WEDNESDAY AND THEN NORTH NEAR NEW ENGLAND INTO THURSDAY-FRIDAY
BEFORE STALLING OVER ATLANTIC CANADA, WITH RAIN AND THUNDERSTORMS
IN THE VICINITY OF THE LOW. UPSTREAM, AN UPPER RIDGE WILL DEVELOP
OVER THE SOUTHWESTERN U.S. AND PUSH INTO THE CENTRAL U.S. BY LATE
WEEK, LEADING TO ABOVE AVERAGE TEMPERATURES IN THOSE AREAS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE HAS BEEN CONSISTENT WITH VORTICITY/POSSIBLY
TRANSIENT UPPER LOWS ACROSS THE MIDWEST SOUTH OF A CENTRAL CANADA
UPPER HIGH AS THE PERIOD BEGINS WEDNESDAY. THE EXACT PROCESS OF
THIS ENERGY COMBINING WITH THE THE ONTARIO/QUEBEC UPPER LOW IS
SOMEWHAT IN FLUX AND AFFECTS EXACT SURFACE LOW POSITIONS. AT LEAST
THE NEWER 00Z GUIDANCE SHOWS PRETTY GOOD AGREEMENT WITH THE SURFACE
LOW ATOP THE OHIO VALLEY EARLY WEDNESDAY AND CONNECTING WITH
COASTAL LOW DEVELOPMENT LATER WEDNESDAY INTO THURSDAY. HOW FAR
OFFSHORE THE COASTAL LOW MAY BE BY THURSDAY WILL AFFECT THE
PLACEMENT OF RAIN ON ITS BACKSIDE. THE 00Z GFS WAS FARTHER EAST
ENOUGH THAT MOST OF ITS QPF IS OFFSHORE WEDNESDAY, BUT THE BULK OF
NON-NCEP AND AI/ML GUIDANCE SHOWS THE LOW FARTHER WEST, SO THIS
LARGER CLUSTER WAS PREFERRED. A MODEL/ENSEMBLE BLEND WORKED WELL
FOR THE RELOADING UPPER TROUGH IN THE EAST LATE WEEK/WEEKEND.
MODELS SHOW AGREEMENT WITH A SOUTHWEST UPPER RIDGE BUILDING AND
SNEAKING GRADUALLY EAST INTO THE SOUTH-CENTRAL U.S. LATE WEEK.
FARTHER NORTH THERE IS MORE UNCERTAINTY WITH SHORTWAVES IN THE MAIN
FLOW OVER THE NORTHWEST FOR SOME FRONTAL SYSTEM TIMING
DIFFERENCES. THESE SHORTWAVES SHOULD BE GENERALLY WEAK UNTIL ENERGY
IN THE NORTHEAST PACIFIC MAY MOVE INTO THE NORTHWEST FRIDAY OR
SATURDAY. THROUGH THE 12/18Z MODEL CYCLE, GFS RUNS SEEMED LIKE A
WEAKER OUTLIER COMPARED TO OTHER DYNAMICAL AND AI/ML MODELS. THIS
PERSISTS INTO THE NEWER 00Z MODEL CYCLE, BUT WILL NOTE THAT
GENERALLY ALL THE MODELS ARE SLOWER TO BRING IN THE
TROUGHING/POSSIBLE UPPER LOW THAN THE PREVIOUS CYCLE. THE 00Z CMC
IS QUITE DEEP WITH THE UPPER LOW AS WELL. POSSIBLE ENERGY SPLITTING
INTO THE SOUTHERN STREAM INTO CALIFORNIA AND THE GREAT BASIN THIS
WEEKEND DOES NOT SHOW CLEAR CONSENSUS IN THE DETAILS YET.
THE WPC FORECAST WAS BASED ON A BLEND OF MAINLY DETERMINISTIC
MODELS FAVORING THE ECMWF EARLY IN THE FORECAST PERIOD, AND
GRADUALLY INCREASED THE PROPORTION OF ENSEMBLE MEANS TO HALF DAY 6
AND MORE DAY 7 AS SPREAD INCREASED.
..WEATHER/HAZARDS HIGHLIGHTS
RAIN AND THUNDERSTORMS ARE LIKELY ACROSS EASTERN PORTIONS OF THE
OHIO VALLEY INTO THE APPALACHIANS AND MID-ATLANTIC REGION ON
WEDNESDAY WITH THE SURFACE LOW TRACKING ACROSS. THERE WILL BE SOME
MOISTURE AND INSTABILITY BUT CERTAINLY LESS THAN IN AREAS FARTHER
WEST DURING THE SHORT RANGE PERIOD. A MARGINAL RISK REMAINS IN
PLACE FROM THE CENTRAL APPALACHIANS INTO THE NORTHERN MID-ATLANTIC
AND SOUTHERN NEW YORK IN THE DAY 4/WEDNESDAY ERO. RAIN RATES SHOULD
NOT BE TOO EXTREME AND PRECLUDE ANY HIGHER RISK AREAS AT THIS
TIME. THEN AS THE SURFACE LOW PIVOTS NORTHWARD, RAIN AND STORMS
WILL SPREAD ACROSS THE NORTHEAST THURSDAY. A MARGINAL RISK IS
DELINEATED FOR THE NYC AREA INTO PARTS OF NEW ENGLAND ON THE DAY
5/THURSDAY ERO, WITH A GENERAL CONSENSUS OF RAIN TOTALS OF A COUPLE
INCHES. BUT WILL CONTINUE TO WATCH, AS SOME MODEL GUIDANCE HAS THE
SURFACE LOW, AND THUS THE HEAVIEST QPF, FARTHER EAST/OFFSHORE.
COOLER TEMPERATURES WITH THE LOWER HEIGHTS ALOFT COULD PRODUCE SOME
MAY SNOW IN THE HIGHER PEAKS OF THE WHITE MOUNTAINS WITH THIS
SYSTEM. RAIN CHANCES ARE LIKELY TO LINGER IN THE NORTHEAST INTO
LATE WEEK AS THE LOW PULLS AWAY SLOWLY.
SOME ROUNDS OF RAIN ARE POSSIBLE IN MEAN NORTHWEST FLOW BEHIND THE
EASTERN UPPER TROUGH, AFFECTING THE NORTHERN PLAINS TO MIDWEST.
THE NORTHWEST MAY SEE PRECIPITATION WITH SHORTWAVES MOVING THROUGH.
MEANWHILE SOME CONVECTION IS LIKELY EAST OF A SOUTHERN HIGH PLAINS
DRYLINE. BY LATE WEEK INTO THE WEEKEND, SYSTEMS MAY COMBINE AND
PRODUCE BROADER AREAS OF HEAVIER RAIN IN THE CENTRAL U.S., BUT WITH
UNCERTAINTY IN THE DETAILS.
BY WEDNESDAY, THE HEAT ACROSS TEXAS SHOULD ABATE SOMEWHAT AS UPPER
TROUGHING SUPPRESSES THE SUBTROPICAL RIDGE. A FEW RECORD HIGH
MAX/MIN TEMPERATURES ARE STILL POSSIBLE ACROSS THE FLORIDA
PENINSULA WITH HIGHS IN THE MID 90S. THE TROUGH/LOW ALOFT WILL
PROMOTE BELOW NORMAL TEMPERATURES ACROSS THE NORTHERN TIER, WITH
TEMPERATURES OF 15 TO 20 DEGREES BELOW AVERAGE IN THE NORTHERN
PLAINS, ONLY REACHING THE 50S ON WEDNESDAY. BROAD AREAS OF THE
EAST-CENTRAL AND EASTERN U.S. CAN EXPECT BELOW AVERAGE TEMPERATURES
THROUGH THE LATTER HALF OF NEXT WEEK AS THE UPPER TROUGH SETS UP.
MEANWHILE, UPPER RIDGING OVER THE SOUTHWESTERN U.S. WILL RAISE
TEMPERATURES ABOVE NORMAL THERE. THE LOS ANGELES METRO IN
PARTICULAR COULD SEE RECORD HIGHS WITH THE ABOVE AVERAGE CONDITIONS
IN CALIFORNIA WEDNESDAY-THURSDAY. THE WARMTH SHOULD EXPAND EAST
ACROSS THE FOUR CORNERS STATES BY THURSDAY AND INTO THE SOUTHERN
PLAINS LATE IN THE WEEK. HIGHS WILL BE WELL INTO THE 100S IN THE
DESERT SOUTHWEST WITH TEMPERATURES NEARING 100 AGAIN IN PARTS OF
TEXAS EVENTUALLY.
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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