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