Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Tue Apr 8 2025 Valid 12Z Fri Apr 11 2025 - 12Z Tue Apr 15 2025 ...General Overview... The forecast shows moderate progression with two significant weather systems of note. The leading one involves digging Canada/Great Lakes energy within an amplified trough, forming a closed low over the central Appalachians and Mid-Atlantic by late this week into the weekend. Associated surface low pressure will likely track just off the East Coast during the weekend, with this system producing organized rainfall and perhaps localized high elevation snow. A trailing upper ridge should progress from the Intermountain West/Rockies to the East Coast during the period. Then an upper trough reaching the Pacific Northwest by Saturday will support the second system that should develop over the Plains during the weekend and track through the upper Great Lakes into east-central Canada early next week. Most of this system's rain and higher elevation snow will likely extend over and east-northeast from the northern Rockies. Most rain with the trailing cold front should be fairly light. Another upper trough with more uncertain specifics may reach the West Coast around next Tuesday. ...Guidance/Predictability Assessment... Models and ensembles from the 00Z/06Z cycles continued to agree fairly well for the general pattern through the period. However meaningful embedded differences persist for individual systems. These include the leading one affecting the East Coast, the upper trough and associated surface system progressing from the Northeast into the Great Lakes/Canada Saturday-Tuesday, and the upper trough reaching near the West Coast by next Tuesday. Guidance comparisons and continuity favored an early-mid period blend consisting of 40 percent 00Z ECMWF and a remaining even split among the 06Z GFS/00Z UKMET/00Z CMC. Clustering ultimately favored a shift toward 60 percent total 06Z GFS and 12Z/07 ECMWF relative to 40 percent 06Z GEFS/00Z ECens means by next Tuesday. For the East Coast system, individual dynamical and machine learning (ML) models show varying detail/position differences without pronounced clustering. At least there is a general theme toward the system having less northward elongation/precipitation extent than depicted by latest GFS runs. The new 12Z UKMET strays northeast with the upper low versus most other solutions by Sunday while the 12Z ECMWF/CMC and GEFS mean maintain an upper low path across the Mid-Atlantic. Meanwhile guidance still shows important detail differences for the evolution of shortwave energy reaching the Northwest by Saturday. GFS/CMC runs have been forming an embedded upper low over or near the northern Rockies by Sunday, with the 12Z ICON also showing this (and with the farthest south track later on). UKMET runs (and the 12Z GEFS mean) have had an intermediate upper low track along the Canadian border while faster ECMWF runs wait until farther eastward near the Canadian border to form the low by Monday. Most ML models favor waiting until after Sunday to form an upper low, and with a track near the Canadian border. The preferred solution leans toward an intermediate evolution--an upper low near the Canadian border, Plains low pressure between the weaker ECMWF and deeper GFS/CMC, and eventually slower timing than the 00Z and new 12Z ECMWF (12Z/07 ECMWF closer to the majority cluster by next Tuesday). Northern tier QPF also reflects an intermediate idea between the GFS/CMC versus ECMWF extremes. By late in the period, individual dynamical/ML models and ensembles develop a lot of spread for the character of troughing that may approach the West Coast. This is also the case for leading energy that may pull off from the southern part of the weekend shortwave. At this point there is no meaningful consensus or continuity regarding how much stream separation may occur within the trough and where this separation may occur. The updated blend valid next Tuesday shows some mid-latitude separation based on preferences to the east but confidence along the West Coast is very low and future cycles are likely to reflect different ideas for specifics. ...Weather/Hazards Highlights... The low pressure system exiting the Ohio Valley and reaching the East Coast to close out the week will be the next system to monitor for meaningful sensible weather. A deepening coastal low near the Mid-Atlantic Coast on Friday and into early Saturday should bring higher rain chances and breezy conditions from the Carolinas to New England. A Marginal Risk area continues for Day 4/Friday from Maryland northward to southern New England with more of an enhanced QPF signal in the deterministic guidance, with perhaps 1-2 inches of rainfall for some areas. New guidance continues to show some spread for specifics but the best clustering and ensemble probabilities recommend no change for the current Marginal Risk area at this time. Higher elevation snowfall is also likely across northern New England Friday night into Saturday. The rainfall becomes lighter in general by Saturday (with additional spread for details) across the Northeast, and the Day 5 ERO still reflects no risk areas. The forecast over and east of the northern Rockies from Saturday onward depends on the specifics of potential upper low formation within the trough supporting Plains low pressure development by Sunday. Current preferences lie between the extremes, represented by the GFS/CMC that produce some areas of heavy rain higher elevation snow--due to earlier and farther south upper low formation--versus the much lighter ECMWF and most ML models which maintain an open upper trough until forming a closed low farther eastward. For the most part, the trailing cold front should not have a lot of moisture to work with so any associated rainfall over the central and eastern U.S. should be on the lighter side. Some light precipitation could reach parts of the West Coast by late Monday or Tuesday, depending on very uncertain specifics of incoming Pacific shortwave energy. Monitor future forecasts as guidance eventually refines the details aloft and associated precipitation. In terms of temperatures, the Western U.S. into northern High Plains will see a warm pattern for the end of the week with both daytime highs and overnight lows about 10-20 degrees above average, with some readings potentially 25 degrees above average across portions of Wyoming and Montana. The warmth will reach the central and northern Plains by the weekend as the upper ridge shifts eastward, with widespread 15-25 degree positive anomalies across the Central Plains and Midwest by Sunday. The amplified upper trough initially over the East will keep highs at some locations up to 10-15 degrees or so below normal Friday-Saturday before a steady warmer trend to moderately above normal readings early next week. Cooler air behind the Northwest U.S. front eventually pushing into the East will bring temperatures down to near or slightly below normal. A trailing upper ridge building into the West early next week should support a rebound to somewhat above normal temperatures over the region. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw