Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Apr 13 2025 Valid 12Z Wed Apr 16 2025 - 12Z Sun Apr 20 2025 ...Overview... As the medium range period begins Wednesday, upper troughing will be in place over the East, while an upper ridge sets up over the High Plains just ahead of an energetic trough over western Canada sneaking into the Northwest. This latter trough is forecast to dive south across the Interior West while interacting with southern stream troughing tracking east from the eastern Pacific to form a likely closed low settling over the southwestern U.S. late week. The troughing will bring cooler temperatures and precipitation to the West, including some possibly heavy snow in the northern/central Rockies Wednesday to Friday. Meanwhile some northern stream troughing and a developing low pressure system should lead to widespread rain chances across parts of the Midwest by Thursday and into the Great Lakes and Northeast late week. By Friday into next weekend, the cold front associated with the surface low is forecast to become elongated and settle across the south-central U.S. while moisture streams in ahead of the Southwest upper low. This could cause heavy rainfall in the Southern Plains to Mid/Lower Mississippi Valley, and will continue to monitor this potential. ...Guidance/Predictability Assessment... After a significant pattern change in the last couple of days, models have now stabilized better on the overall pattern described above for mid to late week into next weekend. There remains good model agreement for the first upper trough in the East to shift eastward into the Atlantic later this week and a good signal for ridging behind it. There is relatively more spread in the evolution of the West trough (though certainly much less spread than a day ago). A southern stream upper low is forecast to move from the eastern Pacific Wednesday into California by Thursday, while significant energetic troughing digs south into the Northwest. These should form a closed upper low atop the Great Basin and Southwest late week. On the broad scale these features look combined by Friday or so, but some energy may sneak around the base of the developing upper low and eject into the central U.S. Friday-Saturday and possibly enhance rain. The 12Z CMC/UKMET were a bit northeast with the closed upper low position around Friday compared to other guidance, while GFS runs (including the new 00Z GFS) were farther southwest. A position like the ECMWF seems to be the best middle ground and best aligned with the ensemble means. The Southwest upper low looks to become cut off as northern stream energy tracks eastward without it. There are some position differences in the Southwest upper low by next weekend as it slowly moves eastward. These are mostly minor, as deterministic models and EC-based AI models are in fair alignment, though the 12Z ECMWF became negatively tilted by next Sunday. However, interestingly many of the GFS-based AI models (aside from the Aurora) stayed much farther west in the eastern Pacific with the upper low center, which was not favored. Then regarding the northern tier trough, there are some timing differences with it and the resulting surface low, including the 18Z GFS that was slower than consensus. Newer 00Z models look reasonably agreeable for the late medium range period timeframe. Thus the WPC forecast used a blend of the deterministic models favoring the ECMWF/GFS early in the period, with increasing proportions of the ensemble means to half Day 6 and more Day 7 as model spread increased in the details. There was better continuity with the previous forecast issuance compared to 24 hours ago, so hopefully the forecast will continue to be more stable. ...Weather/Hazards Highlights... Some lingering rain and snow is forecast for the Interior Northeast midweek underneath the upper trough axis. Also on Wednesday into Wednesday night, a shortwave could produce some modest rain across the Middle Mississippi Valley, but without much instability to work with, rain amounts should stay well below any excessive rainfall concerns. Meanwhile, the combining energies in the West will spread significant precipitation chances there. Precipitation is likely in the Northern Rockies Wednesday-Thursday and spreading into the Central Rockies Thursday-Friday. With the cold upper trough, accumulating snow in higher elevations is likely, and snow looks to be heaviest in the higher elevations of Wyoming peaking Thursday. Precipitation amounts have trended up somewhat in the Sierra Nevada across the Great Basin as well, including some higher elevation snow in the Sierra and Wasatch. Some precipitation is forecast to continue in the Four Corners states into next weekend. As the energetic upper low dives south, a smaller portion of the northern stream energy will continue east, trailing along the northern tier of the nation, along with a surface low pressure system. This will support rain and possibly thunderstorm chances in Northern Plains and Midwest on Thursday. Locally heavy rain rates may be possible in thunderstorms, but currently this is not likely to cause any flooding concerns as rain totals should stay modest and storms will be moving quickly. Thus no excessive rainfall area was delineated in Thursday's Day 5 ERO. Rain chances continue into the Great Lakes region Friday and the Northeast/Mid-Atlantic Saturday. Meanwhile the tail end of this frontal system combining with a front ahead of the slow moving Southwest upper low could support a focus for heavy rain in the south-central U.S. as ample moisture streams in. The current forecast shows the heaviest rain across the Southern Plains and toward the Lower/Middle Mississippi Valley, with more moderate rain east into the Ohio Valley. Some areas could overlap with areas of the Ohio and Mississippi Valleys that could still be sensitive from recent flooding, but fortunately the axis of the heaviest amounts through Saturday are to the west. The forecast details could change over the next few days and should be closely monitored to assess any possible impacts. Initial upper ridging across the Interior West to Plains will allow for above normal temperatures there. Temperatures are generally forecast to be 10-20 degrees above average, and even 25F above normal in the Central Plains Wednesday-Thursday as temperatures reach the 90s from Texas to Kansas. Cooler than average temperatures are likely in the East Wednesday and Thursday. Lows should be 5-10 degrees below average and highs 10-20 degrees below average. After that, below normal temperatures are expected to develop southward across the West underneath the upper low while above normal temperatures gradually shift towards the East, becoming more limited to the Southeast next weekend. 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