490
FXUS02 KWBC 241839
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 PM EDT SAT MAY 24 2025
VALID 12Z TUE MAY 27 2025 - 12Z SAT MAY 31 2025
...HEAVY RAIN/FLASH FLOODING THREATS FROM THE SOUTH THROUGH THE
SOUTH-CENTRAL APPALACHIANS/MID-ATLANTIC EARLY-MID NEXT WEEK...
..OVERVIEW
THIS UPCOMING MEDIUM RANGE PERIOD WILL FEATURE AN AMPLIFIED AND
SLOW TO PROGRESS UPPER LEVEL PATTERN WITH A COUPLE CUTOFF LOWS THAT
WILL DRIVE SURFACE FRONTAL SYSTEMS ACROSS THE LOWER 48. ONE LOW
WILL WANDER ACROSS THE NORTH-CENTRAL U.S. AND MIDWEST WHILE THE
OTHERS GRADUALLY APPROACH THE SOUTHWEST. THE EASTERN LOW SHOULD
GAIN MOMENTUM LATE IN THE PERIOD AS IT TENDS TO SYNC WITH A BROAD
UPPER LEVEL TROUGH, BUT SOUTHWEST LOWS WILL MOVE SLOWLY AS RIDGING
BUILDS HEAT OVER THE WEST. THIS PATTERN WILL RESULT IN A LINGERING
PERIOD WITH AREAS OF WET UNSETTLED WEATHER TO FOCUS FROM THE SOUTH
UP THROUGH THE SOUTH-CENTRAL APPALACHIANS/MID- ATLANTIC. A SERIES
OF PACIFIC FRONTAL APPROACHES TOWARD/NORTH OF THE PACIFIC NORTHWEST
MAY OFFER SOME MODEST COASTAL RAINS CHANCES NEXT WEEK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
DESPITE A SOMEWHAT COMPLEX FLOW, THE GUIDANCE CONTINUES TO BE
REASONABLY CLUSTERED FOR AT LEAST THE FIRST HALF OF THE PERIOD,
WITH SOME DISAGREEMENT IN THE DETAILS. A SHORTWAVE IN THE NORTHWEST
ON TUESDAY SHIFTS EAST WITH TIME, WITH GUIDANCE UNSURE HOW OR WHAT
ENERGY MAY GET LEFT OVER THE REGION. SAME ISSUES WITH A SECOND
SHORTWAVE INTO THE REGION MID-WEEK. FARTHER EAST, THE GUIDANCE
AGREES AN UPPER LOW WILL MEANDER SLOWLY EASTWARD, EVENTUALLY
BECOMING ABSORBED BY LARGER SCALE TROUGHING FROM CANADA, FROM THERE
ARE UNCERTAINTIES THAT ARISE LATE WEEK ON THE EVOLUTION OF THAT
MERGER AND AMPLITUDE OF TROUGHING OVER THE EAST NEXT WEEKEND. LOTS
OF LATE WEEK/WEEKEND UNCERTAINTY WITH TROUGHING OVER THE PACIFIC AS
WELL. WPC FORECAST TODAY USED A GENERAL MODEL BLEND FOR DAYS 3-5,
INCREASING ENSEMBLE MEAN CONTRIBUTIONS LATER PERIOD TO ACCOUNT FOR
SMALLER SCALE DIFFERENCES. THIS OVERALL MAINTAINED GOOD CONTINUITY
WITH THE PREVIOUS WPC FORECAST.
..WEATHER/HAZARDS HIGHLIGHTS
MAIN DEVELOPING LOWS ALONG A WAVY FRONTAL SYSTEM OVER THE SOUTH
ARE SLATED TO PROGRESS NORTHEASTWARD TUESDAY AND WEDNESDAY. THIS
WILL EXPAND A THREAT OF LOCALIZED HEAVY RAIN UP THROUGH THE SOUTH-
CENTRAL APPALACHIANS REGION AND MID-ATLANTIC, WHILE MOISTURE POOLS
ALONG A TRAILING FRONT BACK ACROSS THE SOUTH. THIS WILL FOCUS
LOCALLY HEAVY CONVECTIVE DOWNPOURS CHANCES THAT MAY EXTEND WELL
INTO NEXT WEEK WITH ADVENT AND REINFORCEMENT FROM MIDWEST
CYCLO/FRONTOGENESIS FROM NORTHERN STREAM ENERGIES. THE WPC
DAY4/TUESDAY EXCESSIVE RAINFALL OUTLOOK (ERO) DEPICTS AN ELONGATED
MARGINAL RISK THREAT AREA FROM TEXAS INTO THE SOUTH-CENTRAL
APPALACHIANS REGION/MID- ATLANTIC GIVEN DEEP MOISTURE AND UPPER
SUPPORT. A DAY 5/WEDNESDAY WPC ERO MARGINAL RISK SHOWS BEST FOCUS
LINGERING OVER THE SOUTHEAST MID- ATLANTIC WITH LEAD LOW PASSAGE
AND ALSO BACK OVER THE SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY
WITH EJECTING SOUTHERN STREAM IMPULSE INDUCED RETURN FLOW.
IN THE WEST, A BUILDING UPPER RIDGE WILL LEAD TO AN EXPANDING AREA
OF HEAT FROM THE INTERMOUNTAIN WEST INTO THE WEST COAST STATES,
WITH TEMPERATURES RISING UPWARDS TO 10 TO 15+ DEGREES ABOVE NORMAL
DURING THE SECOND HALF OF NEXT WEEK THAT IN PARTICULAR MAY LEAD TO
A HAZARDOUS HEAT FOCUS OVER THE CALIFORNIA CENTRAL VALLEY HEADING
INTO AND THROUGH NEXT WEEKEND.
SANTORELLI/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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