Extended Forecast Discussion NWS Weather Prediction Center College Park MD 502 PM EDT Thu May 06 2021 Valid 12Z Sun May 09 2021 - 12Z Thu May 13 2021 ...Heavy rain possible for parts of the Ohio Valley, Mid-South, and Southeast next week... ...Chilly temperatures for the western High Plains with some snow for the Rockies... ...Overview... A split-flow and increasingly progressive/zonal upper level pattern is expected across the lower 48 and southern Canada next week, anchored by upper ridging over the northeastern Pacific and across the northwestern Caribbean. Embedded shortwave impulses in the southern stream will provide a focus for multiple rounds of showers and storms over the southeastern 1/3rd of the CONUS. Farther north, a broad trough over the Intermountain West and northern Rockies coupled with a large Canadian surface high sinking southward across the north-central states will result in below normal 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. ...Guidance Evaluation and Preferences... Through the 00Z/06Z cycle, the models and ensembles remained in good agreement overall through the early part of next week. The largest difference remained how quickly (or not) to take the trough over the Rockies Mon-Tue eastward. The GFS/GEFS were generally slower than the ECMWF/ECMWF ensembles, but enough overlap existed between the two ensemble systems that made a blended solution preferable yet again. Overall pattern evolution that will be trending towards more zonal flow suggests that the quicker solutions may be more correct, but if anything the ECMWF ensembles have been trending a bit slower the past couple days. Over the East/Northeast, trend has been for a bit slower pace of the low pressure along a frontal boundary, and a 06Z GFS/00Z ECMWF blend sufficed. ...Weather Highlights/Hazards... The highest rainfall prospects will likely be from the Ohio Valley to the central Appalachians and over the Mid-South on Sunday, associated with an area of low pressure and cold front tracking eastward across the region. The potential exists for 1 to 3 inches of rain from along the Ohio River southward, potentially aided by convection. Another zone of heavy rain is also probable across portions of the lower Mississippi Valley/Deep South next Tuesday and Wednesday. With marginally cold temperatures over the Rockies, accumulating snow is likely for the higher terrain of Colorado and Wyoming while a cold rain may fall in lower elevations. 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/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, though only about 5-15 degrees below normal. By the middle of next week, above normal temperatures are likely to make a return to the West Coast region where 90s to low 100s may again be possible over the lower deserts of AZ/CA/NV. Fracasso/Hamrick Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sun, May 9 and Tue-Wed, May 11-May 12. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Central Appalachians, the Ohio Valley, and the Tennessee Valley, Sun, May 9. - Heavy rain across portions of the Southeast, the Southern Appalachians, and the Tennessee Valley, Mon, May 10. - Flooding occurring or imminent across portions of the Southern Plains. 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