Extended Forecast Discussion NWS Weather Prediction Center College Park MD 329 PM EDT Wed May 04 2022 Valid 12Z Sat May 07 2022 - 12Z Wed May 11 2022 ...North-central Great Basin/Rockies cold heavy snow threat to contrast emerging north-central U.S. convection and record south-central U.S to Midwest heat pattern with Omega-style block development this weekend into next week... ...Lead Mid-Atlantic rainfall threat into Saturday with the closed low to settle over a blocked western Atlantic next week... ...Pattern Overview and Guidance Evaluation... Upper trough in the East will move offshore, close off and linger off the Southeast coast through the medium range. There was plenty of spread in the guidance, particularly with the 00z CMC/CMCE/UKMET 06z GEFS, with respect to the intensity of the closed low in the Atlantic beginning on Sunday and continuing through Wednesday. The 00z EC appears to best capture the pattern across the CONUS. There's notable spread in the 00z EC ensemble members with respect to the upper-trough in the west by next Tuesday and Wednesday. Thus, a blend of majority deterministic 00z EC and 06z GFS was utilized through day 6. A more even combination of deterministic and ensemble guidance was blended for day 7. ...Weather/Threat Highlights... A leading closed upper trough and well organized surface low/frontal system will track from the Ohio Valley through the Appalachians/Mid-Atlantic this weekend. Some strong storms and locally heavy downpours are possible considering slow system track and pooled moisture/instability. The system will then stall and retrograde over the western Atlantic next week as part of a developing Omega-style block. Robust digging of a series of dynamic upper systems will lead to a dramatic change over much of the West later this week into next week as an amplified mean upper trough and wavy frontal systems become established over the region. This will support significant cooling to include spotty record values along with a widespread precipitation focus from the Northwest/Sierra through the north-central Great Basin/Rockies. This will include a substantial and lingering threat for widespread heavy snows as further enhanced in favored terrain. Expect temperatures to range as much as 10-20 degrees below normal. Uncertain upper system ejections channeled between amplified eastern Pacific upper ridging/upper troughing over the West and amplified east-central U.S. upper ridging as part of the aforementioned Omega-style block over the lower 48 will trap a slow moving and wavy frontal zone over the north-central U.S. to support an emerging convection/rainfall pattern. This offers a runoff threat given moist ambient Northern Plains soil conditions. The Storm Prediction Center also highlights a concern for severe weather into the eastern Central Plains for at least Sunday and Monday. The pattern meanwhile favors record high temperatures over the south-central U.S., with pre-frontal warmth gradually building northeastward next week through the MS/TN Valleys/Midwest under the upper ridge, in stark contrast to the wintry conditions over the West and moderated/cooled high pressure dammed airmass to linger over the East on the backside of a difficult to dislodge western Atlantic circulation. Kebede/Schichtel 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sat, May 7. - Heavy precipitation across portions of the Pacific Northwest, Sat-Sun, May 7-May 8. - Heavy snow across portions of the Central Rockies, the Northern Plains, the Northern Rockies, and the Northern Great Basin, Sat-Tue, May 7-May 10. - Severe weather across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Mon, May 9. - Severe weather across portions of the Central Plains, Sat, May 7. - Severe weather across portions of the Central Plains, the Middle Mississippi Valley, and the Upper Mississippi Valley, Sun, May 8. - Flooding possible across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Southern Plains, the Middle Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley and the Northern Plains. - Excessive heat across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Tue, May 7-May 10. - High winds across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Rockies, California, the Southern Plains, and the Southwest, Sun-Mon, May 8-May 9. - High winds across portions of the Mid-Atlantic, Sat-Tue, May 7-May 10. - Enhanced wildfire risk across portions of the Southern Rockies, the Southern Plains, and the Southwest, Sun, May 8. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#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