126
FXUS02 KWBC 270659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT SUN APR 27 2025
VALID 12Z WED APR 30 2025 - 12Z SUN MAY 04 2025
...HEAVY RAINFALL THREAT OVER THE SOUTHERN HALF OF THE PLAINS
CONTINUING INTO WEDNESDAY...
..OVERVIEW
THE COMBINATION OF AN EJECTING SOUTHERN ROCKIES SHORTWAVE AND
UPSTREAM ENERGY WILL SUPPORT DEVELOPING PLAINS/MIDWEST LOW PRESSURE
THAT SHOULD REACH SOUTHEASTERN CANADA BY FRIDAY. THIS SYSTEM WILL
LIKELY PRODUCE ITS HEAVIEST RAIN OVER PARTS OF THE SOUTHERN PLAINS
ON WEDNESDAY BEFORE SPREADING A BROADER AREA OF SOMEWHAT LOWER
RAINFALL TOTALS ACROSS THE EAST LATER IN THE WEEK. THE UPPER TROUGH
AND POSSIBLE EMBEDDED UPPER LOW FORECAST TO SETTLE OVER EASTERN
NORTH AMERICA BY NEXT WEEKEND SHOULD BE ONE COMPONENT OF WHAT
GUIDANCE SUGGESTS WILL BECOME A MORE BLOCKY LARGE SCALE PATTERN
ALSO CONSISTING OF AN ARRIVING WEST COAST TROUGH CLOSING OFF ITS
OWN LOW WHILE A RIDGE SETS UP OVER THE CENTRAL U.S. THE WESTERN
SYSTEM WILL SPREAD RAIN AND HIGH ELEVATION SNOW ACROSS THE REGION
LATE WEEK THROUGH THE WEEKEND WHILE SOME AREAS OF RAIN WILL ALSO BE
POSSIBLE OVER THE CENTRAL/SOUTHERN PLAINS IN THAT TIME FRAME.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODELS CONTINUE TO VARY WITH DETAILS OF THE DEVELOPING SYSTEM
FORECAST TO TRACK FROM THE PLAINS/MIDWEST THROUGH SOUTHEASTERN
CANADA DURING MID-LATE WEEK. THIS SPREAD AND VARIABILITY REFLECT
THE SENSITIVITY OF FINER SCALE ASPECTS OF THE SEPARATE PIECES OF
SUPPORTING DYNAMICS. AN INTERMEDIATE DEPTH (BETWEEN THE WEAK 12Z
CMC AND DEEP 12Z ECMWF/UKMET) LOOKS REASONABLE AT THIS TIME, GIVEN
A GRADUALLY DEEPER TREND IN RECENT MACHINE LEARNING (ML) MODELS BUT
NOT TO THE DEGREE OF THE DEEPEST DYNAMICAL SOLUTIONS. ML MODEL
TIMING HAS BEEN LEANING TO THE MIDDLE/FASTER SIDE OF THE ENVELOPE,
CASTING SOME DOUBT ON THE SLOWER 00Z CMC.
AS THE SUPPORTING UPPER TROUGH SETTLES OVER EASTERN PORTIONS OF
CANADA AND THE U.S. BY NEXT WEEKEND, GUIDANCE DIVERGES REGARDING
THE LONGITUDE OF THE TROUGH AND WHETHER THERE WILL BE AN EMBEDDED
UPPER LOW. A NUMBER OF RECENT MODEL RUNS HAVE BEEN SUGGESTING A
MODERATELY SLOW UPPER LOW (THOUGH WITH A TRACK NOT AS FAR SOUTH AS
THE 18Z GFS) WHILE THE ENSEMBLE MEANS HINT AT ANY CLOSED LOW BEING
OVER QUEBEC. THE NEW 00Z ECMWF HAS ADJUSTED NOTICEABLY MORE
PROGRESSIVE THOUGH. A MORE OPEN TROUGH REPRESENTS A SLIGHT MAJORITY
SCENARIO FROM ML MODELS IN THE 12Z/18Z RUNS. A MODEL/ENSEMBLE MEAN
BLEND HELPS TO MAINTAIN A MIDDLE GROUND IN LIGHT OF THE GUIDANCE
SPREAD.
WITH SOME DETAIL DIFFERENCES, MOST GUIDANCE HAS MAINTAINED
AGREEMENT/CONTINUITY WELL FOR THE FAIRLY DEEP UPPER TROUGH EXPECTED
TO REACH THE WEST COAST BY EARLY SATURDAY. IN CONTRAST TO THE
EASTERN TROUGH, A SOLID ML MODEL MAJORITY SIDES WITH MOST
OPERATIONAL GUIDANCE IN DEVELOPING A CLOSED LOW OVER THE WEST BY
EARLY SUNDAY--MOST LIKELY OVER NEVADA/CALIFORNIA.
FORECAST CONSIDERATIONS LED TO STARTING THE FIRST HALF OF THE
FORECAST WITH A 12Z/18Z MODEL COMPOSITE WITH A SLIGHT TILT IN FAVOR
OF THE GFS/ECMWF, FOLLOWED BY ADDING 18Z GEFS/12Z ECENS MEANS SO
THAT SUNDAY'S WEIGHT WAS HALF MODELS AND HALF MEANS.
..WEATHER/HAZARDS HIGHLIGHTS
THE GENERAL SIGNAL FOR HEAVY RAINFALL OVER AND NEAR THE SOUTHERN
PLAINS ON WEDNESDAY HAS BEEN FAIRLY CONSISTENT IN LATEST MODEL
RUNS/FIRST-GUESS FIELDS, AS SURFACE WAVINESS DEVELOPS ALONG A
STALLED FRONT IN RESPONSE TO AN EJECTING ROCKIES SHORTWAVE.
ANOMALOUS MOISTURE AND SUFFICIENT INSTABILITY WILL FURTHER
CONTRIBUTE TO THE HEAVY RAINFALL THREAT. THE DAY 4 ERO REMAINED
FAIRLY CLOSE TO CONTINUITY WITH BOTH THE SLIGHT RISK AREA
ENCOMPASSING PARTS OF TEXAS/OKLAHOMA AND WESTERN
ARKANSAS/SOUTHWESTERN MISSOURI AND THE SURROUNDING MARGINAL RISK
AREA. ACCELERATION OF THE SURFACE LOW AND TRAILING COLD FRONT
DURING DAY 5/THURSDAY SHOULD TEMPER RAINFALL TOTALS SOMEWHAT, BUT
THERE MAY STILL BE POCKETS OF RAIN RATES SUFFICIENT TO CAUSE RUNOFF
ISSUES (ESPECIALLY WHERE THERE ARE WETTER GROUND CONDITIONS
LEADING INTO THE EVENT). THUS THE DAY 5 ERO INTRODUCES A BROAD
MARGINAL RISK AREA FROM PORTIONS OF THE LOWER MISSISSIPPI VALLEY
INTO THE GREAT LAKES. THE STORM PREDICTION CENTER IS ALSO
MONITORING FOR SEVERE THREATS ON WEDNESDAY FOR PARTS OF NORTHERN
TEXAS. CHECK THE SPC OUTLOOKS FOR THE LATEST INFO ON SEVERE
THREATS.
RAIN SHOULD CONTINUE ACROSS THE EAST ON FRIDAY WITH MOSTLY LIGHT
TO MODERATE TOTALS. MOST AREAS SHOULD SEE A DRIER TREND FOR THE
WEEKEND BUT AN UPPER TROUGH ON THE DEEPER/MORE PERSISTENT SIDE OF
THE SPREAD COULD PRODUCE SOME DIURNALLY FAVORED SHOWERS OVER THE
NORTHEAST. MEANWHILE AREAS OF RAIN WITH VARYING INTENSITY SHOULD
DEVELOP OVER THE CENTRAL/SOUTHERN PLAINS LATE WEEK INTO THE
WEEKEND, AND THE SYSTEM PUSHING INTO THE WEST WILL SPREAD RAIN AND
HIGH ELEVATION SNOW INTO THE REGION. LOCATIONS FROM THE PACIFIC
NORTHWEST INTO THE GREAT BASIN SHOULD SEE RELATIVELY HIGHER TOTALS.
BETWEEN THESE PRECIPITATION AREAS, SOME LIGHT ACTIVITY WILL BE
POSSIBLE FROM THE SIERRA NEVADA INTO CENTRAL ROCKIES MID-LATE WEEK
DUE TO WEAK SHORTWAVE ENERGY.
MOST OF THE EAST EXCEPT FOR FAR NORTHERN AREAS SHOULD SEE ABOVE
NORMAL TEMPERATURES MID-LATE WEEK WITH SOME PLUS 10-15F ANOMALIES.
COLD FRONTAL PASSAGE AND ARRIVAL OF UPPER TROUGHING SHOULD BRING
TEMPERATURES DOWN TO NEAR OR BELOW NORMAL DURING THE WEEKEND.
MEANWHILE ABOVE NORMAL TEMPERATURES WILL PROGRESS FROM THE
NORTHWEST INTO THE NORTHERN/CENTRAL PLAINS DURING THE PERIOD,
CORRESPONDING TO MOVEMENT OF AN UPPER RIDGE. SOME PLUS 10-20F
ANOMALIES WILL BE POSSIBLE OVER NORTHERN LOCATIONS MAINLY DURING
THURSDAY-SATURDAY. THEN THE WEST WILL TREND SIGNIFICANTLY COLDER
NEXT WEEKEND WITH THE ARRIVAL OF A DEEP UPPER TROUGH/POSSIBLE LOW,
BRINGING HIGHS DOWN TO 5-15F BELOW NORMAL.
RAUSCH
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page