Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 PM EDT Wed May 05 2021 Valid 12Z Sat May 08 2021 - 12Z Wed May 12 2021 ...Heavy rain possible for portions of the central U.S. this weekend and early next week... ...19Z Update... The 00Z/06Z guidance mostly maintained good continuity from the overnight progs. Uncertainty remains over the West/Rockies early next week with the speed of the upper trough. The 00Z ECMWF/Canadian and their ensembles were displaced a bit farther northeast than the GFS/GEFS, resulting in a bit quicker pace toward the Plains/Midwest of the former compared to the latter. With continued wavering in some GEFS cycles and no clear trend, opted to split the difference with a blended solution. Lead-in setup still suggests a multi-day heavy rainfall pattern east of the Plains though perhaps not in the same place each day as the frontal zone slowly sinks toward the Gulf Coast. The previous discussion from 07Z is below for reference. Fracasso ...Weather Pattern and Model Guidance... The amplified upper level flow pattern during the short range period is expected to become more quasi-zonal by this weekend as the trough over the Northeast lifts out, and the central U.S. ridge flattens out. At the surface, a meandering frontal boundary with multiple waves of low pressure along it will be the main weather maker during the medium range period across the central and southern tier states. Models and ensembles are in relatively good agreement going into the weekend, although there is some variance with the timing and strength of smaller scale systems. The GFS is more progressive with the surface low crossing the Ohio Valley and Mid-Atlantic region through Monday morning, and the CMC is slower than the model consensus. The GFS is also stronger with the upper trough across the western U.S. The WPC fronts/pressures forecast was based on a CMC/ECMWF/GFS blend along with some of the ensemble means through Day 5, and then primarily ECMWF/EC mean/GEFS means by days 6 and 7. ...Weather Highlights/Hazards... The best rainfall prospects will likely be from the Central Plains to the Ohio Valley, where multiple convective complexes are likely in the vicinity of a stationary front and a wave of low pressure. The potential exists for 2 to 4 inches of rain, with locally higher amounts, from eastern Kansas to Kentucky. Another zone of heavy rain is also probable across portions of Arkansas, Mississippi, and Tennessee for next Tuesday and Wednesday with similar rainfall totals possible. Some training of thunderstorms may lead to instances of flooding, and this is something that will continue to be monitored in the days ahead. Widespread moderate to locally heavy rain can also be expected from eastern Colorado to central Montana as the upper trough/closed low builds in overhead combined with moist upslope flow. Dry conditions should persist across most of the Intermountain West and the Desert Southwest, as well as the northern Great Lakes. Temperatures will likely be more reminiscent of March across the central and western High Plains, and adjacent portions of the northern Rockies for the Sunday to Tuesday time period, with highs up to 20 degrees below early May averages. This equates to readings maxing out in the mid 40s to mid 50s for many of those areas, and the higher elevations of Wyoming, Montana, and northern Colorado can expect some late season accumulating snow, perhaps exceeding six inches for some of the higher ranges. Some of these cooler temperatures should also be realized across Great Lakes and Northeast U.S. courtesy of the upper trough and Canadian surface high in place, although not as anomalous. Hamrick 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml