347
FXUS02 KWBC 261908
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
308 PM EDT MON MAY 26 2025
VALID 12Z THU MAY 29 2025 - 12Z MON JUN 02 2025
..OVERVIEW
THIS MEDIUM RANGE PERIOD BEGINS LATER WEEK WITH A SLOW TO PROGRESS
UPPER LEVEL PATTERN OVER THE LOWER 48, AS REINFORCING UPPER
TROUGHING SETS UP OVER THE EAST WITH EMBEDDED UPPER LOWS AT TIMES.
THIS PATTERN WILL RESULT IN A PERIOD WITH AREAS OF WET UNSETTLED
WEATHER TO FOCUS FROM THE SOUTH-CENTRAL U.S. TO THE EAST/SOUTHEAST
U.S. THROUGH THE PERIOD. MEANWHILE AN OFFSHORE SOUTHERN STREAM LOW
WILL ONLY GRADUALLY APPROACH THE SOUTHWEST U.S. AS UPPER RIDGING
BUILDS HEAT OVER THE WEST. A SERIES OF PACIFIC FRONTS APPROACHING
SLUGGISHLY TOWARD THE PACIFIC NORTHWEST MAY OFFER SOME MODEST
COASTAL RAINS CHANCES LATE WEEK INTO NEXT. PRECIPITATION CHANCES
MAY INCREASE OVER THE INTERMOUNTAIN WEST BY SUNDAY AS BROADER
TROUGHING MAY FINALLY COME INTO THE WEST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE SHOWS REASONABLY SIMILAR THEMES ON THE LARGE SCALE
DESCRIBED ABOVE, BUT WITH EMBEDDED SMALLER SCALE DIFFERENCES THAT
CAUSE SENSIBLE WEATHER VARIATIONS. EVEN AT THE START OF THE PERIOD,
MODELS VARY WITH WHETHER THERE WILL BE A CLOSED LOW ATOP THE GREAT
LAKES/OHIO VALLEY VICINITY OR A MORE POSITIVELY TILTED TROUGH WITH
SPLIT ENERGY. GFS RUNS HAVE BEEN INDICATING THE LATTER, AND THE
BULK OF THE NEWER 12Z MODEL GUIDANCE SHOWS A MORE STRUNG OUT TROUGH
TOO. THE DISTRIBUTION OF THIS ENERGY WILL MATTER FOR FRONTAL AND
QPF POSITIONING IN THE EAST--THE UNCERTAINTIES WITH QPF IN THE EAST
COME FROM THESE SHORTWAVES AND POSSIBLY MCV ENERGIES THAT COMBINE
WITH IT. NEWER 12Z MODEL GUIDANCE IS THEN COMING TOGETHER IN
SHOWING ANOTHER SHORTWAVE ROUNDING THE WEST SIDE OF THE TROUGH,
MOVING THROUGH THE NORTH-CENTRAL U.S. SATURDAY INTO THE OZARKS
VICINITY SUNDAY AND INTO THE SOUTHEAST/TN VALLEY ON MONDAY.
BEHIND THE EASTERN TROUGH, MODELS ARE AT LEAST AGREEABLE IN
SHOWING AN UPPER RIDGE DEVELOPING AND RELOADING ACROSS THE INTERIOR
WEST, WHILE AN UPPER LOW IN THE SOUTHERN STREAM MEANDERS JUST WEST
OF NORTHERN MEXICO. WITH THE SOUTHERN STREAM UPPER LOW, GFS RUNS
HAVE BEEN ON THE WESTERN SIDE OF THE SPREAD BUT THE 12Z GFS AT
LEAST TRENDED TOWARD THE NON-NCEP MODELS. NORTHERN STREAM TROUGHING
IN THE EASTERN PACIFIC MAY ENTER THE NORTHWEST THIS WEEKEND OR SO.
THE 00Z ECMWF/CMC PULLED NORTHERN STREAM ENERGY AND TROUGHING
FARTHER WEST/OFFSHORE COMPARED TO THE GFS, ENSEMBLE MEANS, AND MANY
AI/ML MODELS BY SUNDAY-MONDAY, BUT THE NEWER 12Z EC/CMC TRENDED
FAVORABLY EAST, AND NOW MODELS GENERALLY SHOW THE SOUTHERN AND
NORTHERN STREAMS COMBINE TO CREATE FAIRLY DEEP TROUGHING FOR THIS
TIME OF YEAR IN THE WEST BY NEXT MONDAY.
THE WPC FORECAST USED A MULTI-MODEL BLEND WITH MOSTLY
DETERMINISTIC RUNS AND SOME ENSEMBLE MEAN COMPONENT EARLY IN THE
PERIOD, WITH INCREASING PROPORTIONS OF THE MEANS AS THE PERIOD
PROGRESSED AND SPREAD INCREASED, WITH THE EC/NAEFS MEANS REACHING
HALF THE BLEND DAY 6 AND MORE DAY 7.
..WEATHER/HAZARDS HIGHLIGHTS
A MAIN SURFACE LOW SET TO SLOWLY EJECT OUT OF THE GREAT LAKES
LATER THIS WEEK AND FRONTAL AND SECONDARY BOUNDARIES TRAILING DOWN
OVER THE EAST AND BACK ACROSS THE SOUTH WILL CONTINUE TO FOCUS DEEP
MOISTURE WITH POTENTIAL TO FUEL HEAVY RAINS LOCALLY. THREAT
POTENTIAL SHOULD REMAIN THE GREATEST FROM THE SOUTHERN PLAINS AND
GULF COAST STATES TO THE SOUTHEAST/EAST, BUT THERE IS GREATER THAN
AVERAGE UNCERTAINTY IN THE DETAILS AND LOCATIONS OF
IMPACTFUL/HAZARDOUS RAINFALL AS MODEL GUIDANCE IS QUITE VARIABLE.
THIS ISSUANCE OF THE DAY 4/THURSDAY ERO WILL EXPAND THE MARGINAL
RISK IN PLACE FOR MCS AND DRYLINE CONVECTION IN TEXAS EASTWARD INTO
THE SOUTHEAST FOR MOISTURE AND INSTABILITY POOLING ALONG THE
FRONT, WITH MODEL TRENDS TOWARD HEAVIER RAINFALL TO THE EAST. THEN
THE FRONT LINGERING WILL ALLOW FOR ADDITIONAL CONVECTION FROM THE
CENTRAL GULF COAST THROUGH THE SOUTHEAST ON FRIDAY. PER MODEL
TRENDS, EXPANDED THE EXISTING MARGINAL NORTH INTO THE CAROLINAS AS
MODELS LIKE THE 12Z ECMWF, EC-AIFS, AND OTHER NEW MODELS SHOW
FOCUSED HEAVY RAINFALL NORTH COMPARED TO PREVIOUS CYCLES. THE
SMALLER SCALE FEATURES LIKE THE UPPER SHORTWAVES AND THE FRONTAL
POSITIONS WILL GREATLY AFFECT THE POSITION OF THE HEAVY RAIN AND
THE AREAS WILL NEED TO BE REFINED WITH TIME. SOME AREAS MAY
EXPERIENCE PERIODS OF ENHANCED SHORT-RANGE RAINFALL, WHICH COULD
COMPOUND FLOODING THREATS.
THIS ACTIVITY SHOULD PERIODICALLY PERSIST INTO THE WEEKEND AS
BOUNDARIES LINGER AND WEAKEN AND AS UPPER TROUGHING BECOMES MORE
ESTABLISHED OVER THE EAST. RAINS MAY LINGER LONGER OVER PARTS OF
THE SOUTHERN HIGH PLAINS ASSOCIATED WITH RETURN FLOW AHEAD OF A
BUILDING UPPER RIDGE UP OVER THE SOUTHWEST. FARTHER NORTH, A
SECONDARY COLD FRONT AND LOW WILL SPREAD RAIN OVER THE NORTHEAST
PORTION OF THE NATION LATE WEEK. THEN BY EARLY NEXT WEEK, MOIST
INFLOW AHEAD OF A WEST TROUGH COULD SPREAD PRECIPITATION ACROSS THE
INTERMOUNTAIN WEST.
IN THE WEST, A BUILDING UPPER RIDGE WILL LEAD TO AN EXPANDING AREA
OF HEAT FROM THE INTERMOUNTAIN WEST INTO THE WEST COAST STATES
THIS WEEK INTO THE WEEKEND, WITH TEMPERATURES RISING UPWARDS TO 10
TO 20 DEGREES ABOVE NORMAL. THIS IN PARTICULAR MAY LEAD TO A
HAZARDOUS HEAT FOCUS FOR THE CALIFORNIA CENTRAL VALLEY INTO DESERT
SOUTHWEST LATE WEEK/WEEKEND. ABOVE NORMAL TEMPERATURES CLOSER TO
10-15 DEGREES ABOVE NORMAL SHOULD EXPAND TO THE NORTH-CENTRAL U.S.
AS THE PERIOD PROGRESSES.
TATE/SCHICHTEL
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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