Extended Forecast Discussion NWS Weather Prediction Center College Park MD 147 AM EDT Thu May 6 2021 Valid 12Z Sun May 9 2021 - 12Z Thu May 13 2021 ***Heavy rain and more storms for portions of the South and the Ohio Valley*** ***Unseasonably cool for the western High Plains with some snow for the Rockies*** ...Weather Pattern and Model Guidance... A split flow upper level pattern is expected across the U.S. and southern Canada for the medium range period, with embedded shortwave impulses in the southern stream providing an active weather pattern across the south-central U.S. with multiple rounds of showers and storms. Farther north, a broad trough over the Intermountain West and northern Rockies, and a large Canadian surface high sinking southward across the north-central U.S., will result in below average temperatures from the Rockies to the East Coast. A moderation trend may commence by the middle of next week across the West as an upper level ridge tries to build back in after the trough lifts out of the region. The deterministic model guidance is in decent synoptic scale agreement for the Sunday-Monday time period, with the exception of the 12Z CMC across the northwestern U.S. where it is farther north with the placement of the upper trough, and it is slower with the progression of the surface low across the Ohio Valley. By Monday night, the UKMET becomes stronger than the other guidance with a shortwave tracking towards the Pacific Northwest, and considerably slower and stronger with the trough crossing the Great Lakes region when compared to the ensemble means. Towards the end of the forecast period, the GFS/GEFS is slower with the ejection of the western U.S. trough across the Plains and Midwest, whereas the ECMWF/ECENS are more progressive, which is normally opposite of what is typically seen at day 5-7 time scales. ...Weather Highlights/Hazards... The best rainfall prospects will likely be from the Ohio Valley to the central Appalachians on Mothers Day, associated with a surface low tracking eastward across the region. The potential exists for 1 to 3 inches of rain from southern Indiana to West Virginia. Another zone of heavy rain, along with strong thunderstorms, is also probable across portions of the Deep South 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. Light to moderate rain can also be expected for the western High Plains early next week, and it will be cold enough for accumulating snow for the higher terrain of Colorado and Wyoming. Dry conditions should persist across most of the Intermountain West and the Desert Southwest. 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 even colder across the central and northern Rockies. 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. By the middle of next week, above normal temperatures are likely to make a return to the West Coast region ahead of the next cold front. 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