Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Fri Apr 22 2022 Valid 12Z Mon Apr 25 2022 - 12Z Fri Apr 29 2022 ...Overview... As upper/surface lows track into south-central Canada as the medium range period begins Monday, the trailing cold front should sweep across the south-central to eastern U.S. and provide a focus for rain and thunderstorms. Upper troughing over the Midwest/Great Lakes should shift eastward as next week progresses and likely form an upper low over/near the Northeast later in the week, promoting a shift from warmer to cooler than average temperatures and keeping some precipitation over New England. Meanwhile model guidance supports mean troughing from the eastern Pacific into the western U.S., likely with an embedded upper low, leading to potential for rounds of precipitation in the Northwest. Sufficiently strong upper troughing may move into the West by next Friday to enhance precipitation over the northern Rockies/Plains. Expect broad upper ridging between the two troughs, leading to warm temperatures across the Southwest to southern/central High Plains. ...Model Guidance Evaluation... Over recent days the greatest spread and run-to-run variability have been with the overall mean trough over the eastern Pacific and western U.S. Some important details remain unresolved but at least there are common themes emerging. One shortwave reaching the Northwest around Tuesday should continue onward into southern Canada, possibly forming a compact upper low, as it tracks around the northern periphery of the central U.S. mean ridge. The 00Z CMC was farthest south with this energy (adjusting toward consensus in the 12Z run) while otherwise there is still meaningful timing spread with the ensemble means generally on the slow side versus the operational models. Another shortwave dropping into the mean trough should reach near the West Coast by Thursday, with the full trough including the upper low to the north progressing inland through Friday. The majority cluster through the 06Z cycle consisted of the 06Z GFS/00Z ECMWF and CMC along with the ensemble means. The new 12Z GFS has strayed to the faster side while the 00Z GFS tracked its upper low on the south side of the spread late Thursday-early Friday while the 00Z ECMWF could be a tad on the amplified with the trough. There is also suggestion of a weak shortwave reaching the Southwest around Wednesday but it should have at most modest effect and shear out as it heads downstream into the mean ridge. Meanwhile guidance maintains better than average agreement for the idea that energy just west of leading low pressure/upper low over Ontario will form an upper low that should cross New England around late Wednesday-Thursday and may still linger close to the region through Friday. There is still some debate over exactly when the upper low will form (depending in part on what height contour interval is used) and the timing/track into Friday. The 00Z ECMWF is slightly slow around late Wednesday while the 06Z/12Z GFS stray to the northern side of the spread by Friday. The revised forecast incorporated the 00Z ECMWF, 06Z GFS, and 00Z UKMET/CMC (in order of more to less weight) for about the first half of the period and then added some of the 06Z GEFS/00Z ECMWF means by days 6-7 Thursday-Friday. The blend also split GFS input between the 06Z and 00Z runs late in the period due to differing preferences for the New England upper low and western U.S. trough. This approach yielded minimal changes in continuity. ...Sensible Weather/Threats... The cold front trailing from the low moving across Canada early in the workweek will intersect with warm/moist air from the Gulf and lead to widespread rain along the front stretching from the Great Lakes to Ohio/Tennessee Valleys and into the southern Plains Monday, shifting to the Eastern Seaboard Tuesday. While greater potential for excessive rainfall has phased into the short range period, some high rain rates with unstable conditions could extend into Texas and the Lower Mississippi Valley on Monday and cause localized flash flooding. Neutral to wet soils may lower the threshold for runoff issues over the Lower Mississippi Valley while it is worth monitoring the potential for this activity to overcome the drier ground conditions over Texas. Elsewhere, the front should accelerate and lead to lighter overall rainfall amounts over the eastern U.S. Monday-Tuesday. Then as the Northeast upper low develops midweek and surface low pressure pivots over the region, precipitation is forecast to linger especially across New England, with some potential for higher elevations to receive snow. How much precipitation falls will be sensitive to exact system details. Meanwhile in the West, unsettled weather is likely as shortwave energy moves through the mean trough south of a closed upper low positioned off British Columbia through Thursday. Guidance is starting to resolve a better defined episode of rain and high elevation snow in the early-mid part of the week with a leading shortwave (with lower confidence for a second feature) followed by another later in the week with a separate shortwave and then inland progression of the overall trough/upper low by Friday. At that time the combination of upper trough dynamics and developing northerly/easterly low level upslope flow could lead to areas of heavier precipitation over the northern Rockies/Plains. Specifics will take time to resolve. Currently it appears that the higher elevations of the northern Rockies would see some snow with this event while precip type farther east will be sensitive to system details. Meanwhile a frontal boundary draped over the central Plains may help to focus some rainfall as far southeast as the Mid Mississippi Valley late in the week. Warmer than normal conditions for the eastern third of the U.S. Monday-Tuesday will switch to below normal behind the cold frontal passage. The core of that cold air is likely to keep temperatures 15-30F below normal in the Dakotas and Upper Midwest early in the week (with a few record lows possible) and extend 10-20F below normal readings through the Great Lakes/Central Appalachians during Tuesday-Thursday. These temperatures should moderate somewhat late next week. Farther west, upper ridging migrating from the Southwest into the central U.S. will spread warmer than average temperatures from the Southwest (with some record highs possible in southern California Monday) into the Four Corners states and into southern/central parts of the Plains. The High Plains may see temperatures around 15-20F above normal mid-late week. The Northwest will see a gradual cooling trend during the period and slightly below normal highs should extend into California and the Great Basin by late next week. Rausch/Tate 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