Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Mar 18 2025 Valid 12Z Fri Mar 21 2025 - 12Z Tue Mar 25 2025 ...Overview and Guidance/Predictability Assessment... Progressive upper flow with multiple embedded systems this week in two main streams will amplify into next week with the development of a warming/building West Coast ridge and a cooling/deepening and unsettling east-central U.S. trough. Models and ensembles show good agreement on the general evolution of the above-mentioned weather pattern through the medium range period, but with notable disagreement on a couple of features. The first feature of concern is a coastal cyclone that is forecast to form by all of the guidance near coastal New England on Friday. Recent GFS and CMC runs have shown a deeper and slower motion for this cyclone compared with the ECMWF runs. The EC-AIFS has trended toward the faster ECMWF solutions lately. With these in mind, preference has been given to the faster solutions due to ECMWF's better consistency forecasting this system with the support of recent EC-AIFS. The next system of concern will be from this weekend into early next week across the eastern half of the country. Both the GFS and CMC as well as their ensemble means have recently trended farther north regarding the cyclone track in the general direction of the Great Lakes in better agreement with the ECMWF solutions. Therefore, the WPC medium-range forecast package this morning was based on a higher percentages (55%) from the 00Z ECMWF solutions, together with 30% from the 06Z GFS/GEFS, and 15% from the CMC/CMC mean. This blend maintained reasonably good agreement with the National Blend of Models and WPC product continuity. ...Weather/Hazards Highlights... Cyclogenesis near New England coast on Friday would increase the high wind threat for coastal sections. A deeper storm would increase the heavy snow threat for northern/interior Northeast as well. However, only a small high wind area is depicted across the higher elevations of interior New England in favor of the faster and weaker ECMWF solutions. The cyclone is forecast to depart New England as a maritimes hazard heading into the weekend. Well upstream, a series of Pacific systems and moisture feeds will fuel periods of unsettled wet weather to the West/Northwest, with an enhanced moisture/rain focus for the Pacific Northwest and terrain enhanced heavy snows inland to the Rockies. The WPC Day 4/Friday Excessive Rainfall Outlook maintains a Marginal Risk area for coastal northwest Oregon and Washington, with renewed focus to monitor later weekend into early next week. Compounding effects of multiple days of precipitation may lead to local runoff issues. Well downstream, less defined rainy periods over the central to eastern U.S. late this week are expected to become much better organized by Sunday over the mid-Mississippi Valley as a modestly deep cyclone is forecast to track generally across the Great Lakes to the Northeast early next week. Influx of moisture from the Gulf appears to support a heavy rain threat for the Deep South from late Sunday into early Monday. In addition, cold air wrapping around the main low center appears to support a heavy snow threat toward the upper Great Lakes Sunday into Monday, then possibly spreading into interior New England Monday and Tuesday when a triple-point low development is possible near the coast of New England. Kong/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 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