Extended Forecast Discussion NWS Weather Prediction Center College Park MD 418 PM EDT Mon May 04 2020 Valid 12Z Thu May 07 2020 - 12Z Mon May 11 2020 ...Record heat for the Southwest/West and record cold for the East... ...Overview and Guidance/Predictability Assessment... Guidance remains in good agreement showing an amplifying pattern over North America. An omega block will develop aloft with deep/cold troughing over the east-central North America. and the eastern Pacific as much of western North America bakes in summertime heat under an upper ridge. The pattern may start to break down early next week as troughing inches closer to the West Coast, but models and ensembles often weaken amplified/blocky patterns too quickly. A lead/deepening Atlantic system just off New England Thu has trended a bit stronger and closer to the coast than in recent days. As that departs, an upper low over Hudson Bay will sink southward through Ontario and into the Great Lakes, bringing in a much colder airmass to many areas east of the Plains for late in the week through the weekend. A series of impulses and surface lows/fronts will dig to the lee of the western continent ridge and rotate from the Rockies and central U.S. to the East under the cold trough. Forecast clustering seems better than normal through medium range time scales and a composite blend of the latest GFS/ECMWF and GEFS/ECMWF ensembles along with the National Blend of Models and WPC continuity seems to provide a good starting point for creation of the WPC medium range product suite. Opted to apply more blend weight to the GFS/ECMWF than usual to produce increased forecast detail consistent with a pattern with above normal predictability. ...Weather/Hazard Highlights... Record heat will continue in the Southwest through about Thu/Fri before shifting into CA and OR as the upper ridge focuses along the West Coast. Temperatures 10-20F above normal should push readings into record territory for AZ and lower CO River with highest readings near 110F. The core of the heat will shift through CA and into OR/NV into the weekend. Expect mainly dry conditions over the West until perhaps late next Mon over coastal OR/WA when a deep Pacific system may get close enough to spread some rain eastward. Much of the East and especially the Great Lakes/Northeast will in stark contrast see cooler than normal temperatures by about 10-20F through much of the period. Coldest air may be around Fri-Sat with only small moderation Sun-Mon. Temperatures may approach record cold levels for some areas with a late frost/freeze where the growing season has typically started by now. Farther south, upper dynamics/instability and lead frontal/moisture surges may fuel some locally heavy to severe convection/rainfall over the Southern Plains and Lower MS Valley late week, possibly into FL into early next week. Meanwhile, expect some coastal New England rain around the ocean storm Thu. Additional rainfall may expand through the OH Valley and New Eng with surface low approach Fri, where temperatures could support accumulating snow over the highest elevations and more conversational snow at middle elevations. Some lake-effect/enhanced rain/snow is possible over MI Fri/Sat as the upper low swings through with its cold core over still chilly but moderating lakes. Fracasso/Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Thu-Fri, May 7-May 8. - Heavy rain across portions of the Southern Plains, Fri-Sat, May 8-May 9. - Heavy rain across portions of the Southeast, Sun-Mon, May 10-May 11. - Flooding possible across portions of the Central Plains and the Southern Plains. - Flooding occurring or imminent across portions of the Southeast, the Middle Mississippi Valley, the Lower Mississippi Valley, the Great Lakes, and the Northern Plains. - Much above normal temperatures across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Fri-Mon, May 8-May 11. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Great Basin, and the Southwest, Thu-Mon, May 7-May 11. - Much above normal temperatures across portions of the Southern Rockies, the Southern Plains, the Southwest, the Central Great Basin, and the Central Rockies, Thu, May 7. - Much below normal temperatures across portions of the Central Plains, the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Northeast, the Southern Appalachians, the Upper Mississippi Valley, the Southeast, the Great Lakes, and the Ohio Valley, Fri-Mon, May 8-May 11. - Heavy precipitation across portions of mainland Alaska, Thu-Fri, May 7-May 8. - Heavy rain across portions of mainland Alaska and the Aleutians, Fri-Sat, May 8-May 9. - Much above normal temperatures across portions of the Alaska Panhandle and mainland Alaska, Fri-Mon, May 8-May 11. - Much above normal temperatures across portions of mainland Alaska, Thu-Mon, May 7-May 11. 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