Extended Forecast Discussion NWS Weather Prediction Center College Park MD 521 PM EDT Wed Apr 26 2023 Valid 12Z Sat Apr 29 2023 - 12Z Wed May 03 2023 ...Multi-day heavy rain threat to focus this weekend into next week over the Northeast and vicinity as a deep upper low settles over and east/northeast of the Great Lakes... ...Amplified pattern causes cooler than normal temperatures in the central/eastern U.S. and warmth in the West... ...Overview and Guidance/Predictability Assessment... Models and ensembles continue to advertise a highly amplified pattern during the period, as a Great Lakes/Northeast U.S./southeast Canada upper low anchors a deep upper trough that becomes established over the East while an eastern Pacific upper low drops down and settles along the California coast. Between these two features, an initial western U.S. ridge will gradually make its way into the High Plains. Along the West Coast the primary issue is exactly how the energy within the overall trough evolves by the days 5-7 Monday-Wednesday time frame, along with the precise position of the likely embedded closed low. Recent GFS/ECMWF runs and their means have been fairly coherent with the upper low, with the ECMWF/ECMWF mean tending to track the upper low farther offshore. The ECMWF mean has been drifting closer to the California coast over the past 24 hours though. GEFS means have been trending southward over the past day or so. The 00Z CMC had a different handling of shortwave energy, leading to a closed low evolution out of sync with the GFS/ECMWF clusters. Recent continuity/trends favored a guidance blend tilted about 60-70 percent toward the 00Z ECMWF/ECMWF mean versus the 06Z GFS/GEFS mean by the latter half of the period, yielding an upper low track along or a fraction offshore the California coast. The new 12Z GFS/GEFS mean/UKMET/CMC have all gravitated toward a similar path, though the 12Z ECMWF has wobbled a little farther offshore. Farther east, there is good agreement for the upper low dropping into the Great Lakes by Sunday-Monday. However timing spread is considerable for the southern stream shortwave emerging from the Plains during the weekend, which combined with uncertainty in details of shortwave energy ahead of the northern stream trough, lead to various possibilities for how the surface pattern will evolve along the East Coast from late weekend into next week. An intermediate solution appears best as a starting point, with the 00Z ECMWF even a little slower than its ensemble mean for the southern stream wave (with the new 12Z run trending much faster) and the GFS appearing to be most eager to consolidate low pressure farther inland by Monday (but now joined by the 12Z ECMWF given its changes with the southern wave). An operational model blend transitioning to the model/mean blend described for the West Coast system provided a reasonable intermediate solution given the current detail uncertainties. ...Weather/Hazards Highlights... The progressive lifting of a leading southern stream upper trough and an associated low/frontal system will increasingly tap Atlantic moisture lifting from the Mid-Atlantic to the Northeast for the weekend with transition to moderate triple point/coastal low development. A surge of wrap-back rains with the lows may produce some local runoff issues. The WPC experimental Day 4 (12Z Saturday-12Z Sunday) Excessive Rainfall Outlook maintains a Marginal Risk area covering locations over and just southwest of southern New England. Meanwhile, a southern stream shortwave emerging from the Plains will support a Gulf Coast wave during the weekend, connecting to the front extending back from the East Coast system. The combination of frontal/wave focus, some instability, and increasing moisture, plus some model/ensemble QPF signals, suggest the potential for sufficiently high rain rates to merit the introduction of a Marginal Risk area in the Day 4 outlook over the Florida Panhandle and some surrounding areas. By the Day 5 (12Z Sunday-12Z Monday) time frame, progression of the southern system pushes the axis of potential heavy rainfall farther east/southeast with a Marginal Risk area maintained from prior issuance. The Day 5 outlook also holds onto a Marginal Risk area over the Northeast as a reinforcing/reforming wave maintains the flow of moisture into the region. However there is increasing detail spread by that time so confidence decreases. Northeast U.S. rainfall should trend lighter after early next week. The amplifying eastern U.S. upper trough/embedded low will support an expanding area of below normal temperatures from the eastern Plains to East Coast. Expect coldest highs of 10-20F below normal over the Great Lakes/Ohio Valley/central Appalachians early next week. Snow will be possible near the upper low track, with best potential over the U.P. of Michigan. Upstream, a strong upper ridge will progress from California and the Interior West into the Rockies and High Plains Saturday-Wednesday. This ridge will support much above average temperatures with some record values possible, especially during the weekend when some highs should reach at least 15-20F above normal. Locations along the West Coast will trend steadily cooler with time as an upper trough/embedded low drop down along the California coast. This system may spread some light precipitation from the Northwest southward into and near California, depending on exact track of the upper low. Rausch/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 rain across portions of the Southeast, Central Gulf Coast, and the Lower Mississippi Valley, Sat-Sun, Apr 29-Apr 30. - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Northeast, and the Lower Great Lakes, Sun, Apr 30. - Heavy rain across portions of the Northeast, Mon-Tue, May 1-May 2. - Flooding possible across portions of the Upper Mississippi Valley, the Middle Mississippi Valley, the Northern Plains, and the Northern Great Basin. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Much above normal temperatures across portions of the Central Great Basin, the Northern Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Sat-Sun, Apr 29-Apr 30. 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