Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Apr 28 2024 Valid 12Z Wed May 01 2024 - 12Z Sun May 05 2024 ...Rounds of convection are likely in the central U.S. for much of the week... ...Overview... Into midweek, mean troughing will amplify over the Intermountain West and provide support for widespread convection over much of the central U.S., with heavy rain and flash flooding likely Wednesday and Thursday across much of the Plains and Mississippi Valley. Upper troughing and a surface cold front pressing east into late week should push rain chances into the east-central and eastern U.S., though some may remain over the southern Plains near a lingering frontal boundary. Additionally, precipitation including some higher elevation snow is possible at times across the Northwest into the northern/central Rockies, but confidence in the details is still lower than desired. There is also considerable spread for specifics of an amplifying eastern Pacific upper trough that may affect parts of the West by next weekend. ...Guidance/Predictability Assessment... Guidance is showing similarities for some aspects of the general pattern but there is still considerable spread for important features. The most prominent differences involve the upper trough initially amplifying over and ejecting from the West to produce Plains through Great Lakes low pressure, a compact upper low reaching western North America late this week, and eastern Pacific upper troughing that should affect the West by late week/next weekend. The forecast remains somewhat complex for the western-central U.S. during the mid-late week time frame, with dynamical/machine learning (ML) models and ensembles showing various ideas for the combination of the upper trough ejecting from the West and upper low circulation centered over the northern Plains/southern Canada. The 00Z ECMWF-initialized MLs at least generally favor an intermediate solution for the western U.S./Plains feature, between the faster ECMWF and slow/deep/closed 06Z GFS--while showing perhaps a little deeper closed low along the Canadian border than the average of dynamical guidance. Meanwhile as of early Friday the MLs favor associated surface low pressure over the western Great Lakes. There is not yet anything close to a resolution for the track of the compact upper low reaching British Columbia or the Pacific Northwest by Thursday, with continued significant spread and model variability. ECMWF and GFS runs have swapped places over the past day with the GFS now having it track into British Columbia and ECMWF (plus the CMC) farther south. Ensemble means and the MLs favor British Columbia, so preference remains along that track. As was the case yesterday there is better convergence over the far northern Rockies/Canadian border by Friday. Over the eastern Pacific/West Coast by late week and next weekend, the average of MLs recommend an upper trough that could be deeper than some of the dynamical guidance offshore the Northwest as of early Saturday and then closest in principle to latest CMC runs/00Z ECMWF and the ensemble means by Sunday with a broad open trough aligned near the West Coast. There is a minority ML solution has a closed low near the Washington coast, which is one day slower than the new 12Z UKMET. Recent GFS runs have been persistent in pulling an upper low farther south, closer to California. Some GEFS members offer this scenario but otherwise there is minimal support for the GFS evolution. The new 12Z ECMWF introduces another possibility, continued inland progression of an upper low through the Northwest during the weekend. The early-mid part of the updated forecast incorporated a multi- model/multi-run blend of the GFS/ECMWF/CMC to achieve the desired intermediate solution for the western-central U.S. evolution and favored track for the compact upper low tracking into western North America. Then the blend phases out the 06Z GFS and then the 00Z GFS, along with the 12Z/27 ECMWF, due to issues along the West Coast while steadily increasing 06Z GEFS/00Z ECens input to 60 percent by Sunday. ...Weather/Hazards Highlights... On Wednesday, ingredients for widespread rain and thunderstorms potentially causing flash flooding will be in place across central parts of the country. Lowering heights as upper troughing digs in the Intermountain West will provide lift and combine with ample moisture and instability. On Wednesday model guidance already shows heavy rain totals of 3-5 inches in some areas and high rain rates are likely as well. A large Slight Risk remains in place for the Day 4/Wednesday ERO for the central/southern Plains and stretching into the Mid-Mississippi Valley. Within this risk area, there may be somewhat greater potential from eastern Kansas/southeast Nebraska into northwest Missouri and much of Iowa given proximity to the stalling surface front and central Plains wave development by Wednesday night. There remains a little more spread farther south. Then for Thursday, another large Slight Risk serves as a starting point for the Day 5 ERO just shifted a bit eastward from Wednesday as the surface front begins to press eastward. Many locations within these areas will be sensitive to additional rain due to wet antecedent conditions, and potential for embedded upgrades is possible depending on details of prior convection and how guidance consolidates for amounts/location of heaviest rainfall. One region of interest in that regard may extend from eastern Oklahoma/northeastern Texas into western Arkansas, as guidance suggests somewhat slower frontal motion than to the north and there may be relatively greater sensitivity due to wet ground. By Friday, the cold front progressing east in the northern two- thirds of the country or so will transition rain chances more into the Great Lakes and Ohio/Tennessee Valleys, and toward the Appalachians to Eastern Seaboard on Saturday. However, the frontal boundary looks to linger longer across the the southern Plains (possibly beginning to lift north as a warm front by next Sunday) and produce additional rounds of rain and storms through the weekend. Meanwhile, areas from the Northwest into the northern and central Rockies may see rounds of precipitation including snow in higher elevations through the latter half of the week into the weekend. On Wednesday-Thursday, the cooling associated with the upper trough could bring some early May snow into the northern and central Rockies. There is some chance that snow may make it into the Front Range of Colorado on Thursday. There is still quite a bit of model spread into late week, which affects the precipitation forecast. Currently the best guidance cluster shows another round of precipitation in the northern Rockies Friday with a compact upper system crossing the area, while an upstream Pacific system renews precipitation of varying intensity across the Northwest from late Friday or Saturday through Sunday. Temperatures across the south-central U.S. all the way to the Eastern Seaboard will be warmer than average by generally 10-15F on Wednesday, while cooler than normal temperatures (with more localized coverage of minus 10-15F anomalies) are likely for the northwestern quadrant of the lower 48. Then upper troughing and a cold front pushing from the Northwest should cool temperatures to near or below normal across the central U.S. on Thursday or Friday and reaching the east-central U.S. Friday and Saturday. Temperatures over the West may rebound to near normal or moderately above normal levels by late week into next weekend. Confidence in this aspect of the forecast is still not great due to guidance spread for upper flow details, but a decent range of solutions would at least support some warmup for the southern half of the West. Rausch/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