Extended Forecast Discussion NWS Weather Prediction Center College Park MD 437 PM EDT Thu Apr 27 2023 Valid 12Z Sun Apr 30 2023 - 12Z Thu May 04 2023 ...Northeast heavy rain threat this weekend into Monday as a deep upper low settles over/east of the Great Lakes... ...Much cooler than normal temperatures in the east-central U.S. and Intermountain West/Rockies warmth... ...Overview and Guidance/Predictability Assessment... The medium range period kicks off with a pair of upper level waves in the East; one a closed low, the other a shortwave riding around the periphery of the dominant low before getting wound in by early next week. Shortwave energy in the West will amplify over the coast early next week and remain quasi stationary through the week. This morning's guidance signaled a dipole upper low setup in the 500 mb pattern over the CONUS with a slowly decaying ridge propagating across the Rockies and Plains. Deterministic and ensemble guidance agreed more on the evolution of the eastern trough than that of the western one, particularly earlier in the medium range period. A general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS were utilized through day 5. The 06z GEFS and 00z ECE were introduced on day 5 and continued through day 7 to account for growing uncertainties with respect to timing of the western trough. The 00z CMCE was introduced on day 6 and continued through day 7 to help mitigate the deterministic model dispersion in the West. ...Weather/Hazards Highlights... The lifting of a lead southern stream upper trough and frontal system will increasingly advect deep Atlantic moisture into the northern Mid-Atlantic/Northeast this weekend with transition to moderate triple point/coastal low development. A surge of wrap-back rains will likely produce local runoff issues, especially over favored terrain, considering increasing interaction with an amplified closed low set to work slowly out from the Great Lakes/vicinity. The WPC experimental Day 4 (12 UTC Sunday-12 UTC Monday) Excessive Rainfall Outlook (ERO) maintains a "Marginal Risk" threat area. Though, there is a signal for over performance of rain rates occurring somewhere over central New England where 2" ensemble mean exceedance probabilities are over 20-30%. Meanwhile, another southern stream shortwave will bring a trailing cold frontal wave into the Southeast this weekend. The combination of frontal/wave focus, instability and increasing moisture along with some model/ensemble QPF signals, suggest continued potential for sufficiently high rain rates for another "Marginal Risk" area in the Day 4 outlook over north-central Florida and southern Georgia. By the Day 5 (12 UTC Monday-12 UTC Tuesday) time frame, rapid northeastward lifting of this southern system and continued influence from the main northern tier closed low should push the main wrapping axis of potential heavy rainfall to northern New England where a Day 5 ERO "Marginal Risk" area will be initiated prior to the easing of the threat. The amplifying eastern U.S. upper trough/closed 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, with best potential well back 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 generally light precipitation from the Northwest southward into California, depending on exact track of the upper low that is expected to dig sharply southward along/off the West Coast. 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 rain across portions of the Lower Great Lakes, the Mid-Atlantic, and the Northeast, Sun, Apr 30. - Flooding possible across portions of the Middle Mississippi Valley, the Southern Plains, and the Northern Plains. - Flooding occurring or imminent across portions of the Upper/Middle Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley, the Northern Plains, and the Northern Great Basin. - Much above normal temperatures across portions of the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun-Thu, Apr 30-May 4. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Sun-Mon, Apr 30-May 1. 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