Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Mar 29 2023 Valid 12Z Sat Apr 01 2023 - 12Z Wed Apr 05 2023 ...Synoptic Overview... Strong low pressure tracking from the Great Lakes through northern New England into southeastern Canada this weekend will provide the first weather headline of the period. The upper trough associated with this low will be quite progressive and the majority of significant weather should exit the East Coast by Sunday morning, though brisk winds could persist a little longer over New England. 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 should support strengthening surface low tracking from near the central Plains northeastward by Tuesday-Wednesday, bringing another episode of cold sector snow, warm sector rain/thunderstorms with some severe potential, and strong winds over some areas. Pattern uncertainty originating from the North Pacific ultimately reaches the eastern Pacific/western U.S. by next Tuesday-Wednesday, with some influence on the timing/evolution of the expected central U.S. storm. ...Model Guidance/Predictability Assessment... A blend of 00Z/06Z models provided a reasonable depiction of significant features for about the first half of the period. Guidance is fairly well clustered for the strong storm tracking northeastward from the Great Lakes on Saturday while a model average provides good continuity for the system tracking along the U.S.-Canadian border. For this latter system, the ECMWF/ECMWF mean continue to lean on the faster side of the spread and the CMC is on the slower side, with other solutions generally in the middle to slower half of the spread. The GFS becomes somewhat faster/weaker than most other guidance with a southern tier shortwave that could play a role in the rainfall forecast along and north of the Gulf Coast. By the early to middle part of next week, initial divergence of the GFS/GEFS mean from most other guidance over and near the North Pacific ultimately leads to significant pattern differences over the eastern Pacific/western U.S. Specifically, the GFS/GEFS mean remain sheared with energy flowing southeastward from Alaska as upper ridging builds into the eastern Pacific while the 00Z ECMWF/CMC and their means (plus the 00Z UKMET through the end of its run) suggest that the Alaskan energy will form an upper low that reaches the northwestern U.S. while the Pacific ridge stays farther westward. Downstream impacts are for the GFS/GEFS to be slower than the majority with the expected central U.S. storm system along with somewhat later deepening. Even with these differences, the ensemble means as a whole have trended noticeably deeper at the surface and aloft to provide increasing confidence in the potential for a significant storm even though details are more of a question mark. Given the split of the guidance, along with continuity that was closer in principle to the majority ECMWF cluster, the updated forecast tilted more in that direction. Minority GFS/GEFS input transitioned to the 00Z run that was a little closer to the other guidance versus the 06Z cycle. Among new 12Z guidance, the UKMET has made the most pronounced change as it has switched to the GFS camp even though some of its upstream details differ some from the GFS. The CMC still falls in line more with the 00Z ECMWF cluster. However, the new 12Z ECMWF has shifted more in the GFS direction, so the most likely scenario is definitely in flux at the moment. ...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 (12Z Saturday-12Z Sunday), with now a smaller Marginal Risk area across portions of the far Southeast with some locally heavier rainfall on the southern end of the front. Progression should keep overall rainfall totals from becoming too high but intense rainfall for a brief period could still pose localized issues over areas that have recently had heavy rain. For the Day 5 outlook (12Z Sunday-12Z Monday), a Marginal Risk exists from eastern Texas into the Lower Mississippi Valley where return flow from the western Gulf increases moisture and instability, with a stationary front beginning to lift north as a warm front to provide some added focus. 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, with probabilities for at least 0.25 inch liquid in the form of snow steadily increasing along an axis from the north-central High Plains northeastward. Strong to severe storms will be possible in the warm sector, with the Storm Prediction Center already monitoring that possibility over the east-central U.S. Meanwhile the best potential for strongest winds with this system extends from the southern Rockies/High Plains into the Upper Midwest, with a much broader area of brisk to windy conditions also possible over the central/eastern U.S. 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, though with some uncertainty over details of trough progression/possible replacement late in the period. The best potential for highs 15 to 25 degrees below normal will be early-mid week from the Great Basin to the Dakotas. 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. and extending 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 and into the rest of the East by Monday-Wednesday. Tuesday-Wednesday will offer the best potential for some daily records, with warm lows tending to be more numerous than record highs (which should be over parts of the South on Tuesday and mainly the Florida Peninsula on Wednesday). Rausch/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