Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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... Latest models and ensembles continue to advertise a highly amplified pattern during the period, as a Great Lakes/Northeast U.S./southeast Canada closed 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 eastward into the High Plains. Guidance still offer timing/interaction differences with smaller scale embedded systems, but agree with the mid-larger scale development and evolution of an highly amplified upper level flow pattern for North America. The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from the 18 UTC GFS/GEFS mean and the 12 UTC ECMWF/UKMET/Canadian and ECMWF ensemble mean along with the 01 UTC National Blend of Models and WPC continuity. The broad blending process tends to mitigate the smaller scale variances consistent with individual system uncertainty, all within an overall pattern evolution with good predictability. Latest 00 UTC guidance remains in line with this forecast plan. ...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. 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. 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 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