Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Mar 29 2023 Valid 12Z Sat Apr 01 2023 - 12Z Wed Apr 5 2023 ...Synoptic Overview... One of the weather headlines will be the strong low pressure system crossing the Great Lakes and northern New England on Saturday, and this will then reach southeastern Canada by Sunday. The upper trough associated with this low will be quite progressive and the majority of the bad weather should exit the East Coast by Sunday morning. Meanwhile, energy ahead of the next upper level trough/closed low dropping down from the Gulf of Alaska should reach the Pacific Northwest by Saturday, and then the larger scale trough will likely settle in over the West through early next week. This system will bring another cold front across much of the western U.S. to reinforce the cool weather pattern and support multiple days of rain and potentially heavy mountain snow from the Pacific Northwest and northern California into the northern-central Rockies. This latter western system is expected to produce another surface low developing over the central/southern Plains early next week, with strength and position determined by depth and timing of the ejecting western U.S. shortwave trough. ...Model Guidance/Predictability Assessment... The 00Z model guidance suite has decent synoptic scale agreement for this weekend and a general model compromise works well as a starting point in the forecast process for the storm system exiting the East Coast. The ECMWF is slower with a southern stream shortwave crossing the Desert Southwest and then dampens as it crosses the southern Plains. Behind this East Coast system, guidance continues to portray leading Pacific energy tracking along the U.S.-Canadian border to support a surface system reaching near Lake Superior by next Monday. Most GFS runs and the CMC are on the slower side of the spread while ECMWF runs remain more progressive, with an intermediate/ensemble mean solution working out well. Developing differences with upstream Pacific/Alaska flow late in the period start to influence progression of the upper trough moving into the West and then the Plains early next week. The GFS and GEFS mean remain on the slower side of the guidance with the next major storm system over the central/northern Plains compared to the more progressive ECMWF/ECENS, with the CMC falling between the ECMWF/GFS. The use of the ensemble means was increased to about 50% by next Wednesday to account for these increasing differences. ...Weather/Hazards Highlights... The strong low pressure system tracking from the Great Lakes to southern Quebec will continue to result in impactful weather across much of the East Coast on Saturday ahead of a quick moving cold front. Although the worst of the weather should occur on Friday, there should still be enough forcing and instability across the Southeast and parts of the Mid-Atlantic to fuel a broken line of showers and thunderstorms, and should weaken going through the day on Saturday. However, it will likely be windy for much of the eastern U.S. owing to a strong pressure gradient in place. Windy conditions may also continue into Sunday for New England on the backside of the departing surface low. Snow showers are likely for the Upper Peninsula of Michigan and far northern portions of Maine, and then some snow on the backside of the low across northern New York and into northern Vermont and New Hampshire Saturday night. In terms of excessive rainfall potential, it appears the threat of heavy rainfall and flooding has decreased for the new Day 4 period, with now a smaller Marginal Risk area across portions of southern Georgia, northern Florida, and southeast Alabama with some locally heavier rainfall on the southern end of the front. It is possible this potential could decrease even further with later forecasts. For the Day 5 outlook, a Marginal Risk exists from eastern Texas to southern Mississippi where return flow from the western Gulf increases moisture and instability, and thus the potential for scattered storms with heavy rainfall. Elsewhere across the nation, reloading of upper level troughing over the northwestern U.S. by the weekend and continuing into early next week will support multiple days of rain/mountain snow with terrain enhancement from the Pacific Northwest and northern California into the northern and central Rockies, with the moisture shield pushing gradually farther southeast with time. Heavy snow is likely for the Cascades and the Olympic Mountains where a few feet of snow accumulation is expected at higher elevations. Significant snow may reach the Sierra Nevada by early next week. Showers and thunderstorms are likely to make a return to locations from the central Gulf Coast to the Ohio Valley again Tuesday night and into Wednesday as the next storm system gets better organized across the central Plains. Recent model runs suggest this could also be a significant event with heavy snow northwest of the low track, and strong to severe storms in the warm sector, but uncertainty remains regarding placement at this time. Much of the western U.S. will continue to have cooler than normal temperatures for most of this forecast period as the next upper level trough becomes anchored over the region. The best potential for highs 15 to 25 degrees below normal will be Tuesday and Wednesday from the northern Intermountain West to the Dakotas as the next upper trough settles in overhead. Scattered record cold highs may be possible. Upper level ridging extending from a Gulf of Mexico upper high should maintain warm and humid conditions across the south-central U.S. to spread into the East Coast region on Saturday, with highs generally 5-15F above average and lows around 15-20F warmer than normal. After a brief cooling trend, above normal temperatures should again return to the central U.S. on Sunday into the rest of the East by Monday-Tuesday. A few record warm low temperatures could be tied or set, and next Tuesday may offer the possibility for some record highs over the South. Hamrick 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