Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Mar 20 2025 Valid 12Z Sun Mar 23 2025 - 12Z Thu Mar 27 2025 ...Overview and Guidance/Predictability Assessment... Progressive upper flow into the weekend in two 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 again seem well clustered with the weather pattern evolution through the medium range period, with reasonably good agreement on individual storm systems as well. Forecast confidence remains above average overall with a guidance blend, albeit with more uncertainty in details later period. However, these seem well mitigated consistent with predictability by the blend process and targeted manual adjustments. The WPC forecast was mainly derived from a GFS/ECMWF/CMC/UKMET model composite Sunday into Tuesday before switching to a blend of the better compatible and consistent GEFS/ECMWF/Canadian ensemble means into longer time frames. This forecast is overall mainly in line with the National Blend of Models, machine learning and latest 00 UTC model guidance while also offering good WPC product continuity. ...Weather/Hazards Highlights... West Coast upper ridge riding shortwave troughs shift from the northeast Pacific into BC Sunday with a potent moisture plume to the south that pushes across western WA. A Marginal Risk for excessive rain is maintained only into Day 4/Sunday for coastal Washington as compounding effects of multiple preceding bouts of precipitation may lead to local runoff issues. Amplified upper trough development and deep system genesis early-mid next week should prove slow to approach the West Coast, but should spread orgainzed precipitation back into the Northwest in about a week. The amplifying low over the northern Plains into the Great Lakes Sunday will draw broad scale Gulf moisture up the MS Valley and support a heavy rain and a severe weather threat for parts of Mid- South and lower MS Valley into the TN Valley. The Day 4/Sunday ERO Marginal Risk area is maintained for the Lower Mississippi and Tennessee Valleys. Enhanced rains then spread as a wavy cold front sweeps across the Southeast/Eastern Seaboard Monday into Tuesday. A WPC Day 5/Monday ERO Marginal Risk area was introduced to cover lingering and training cell potential over the eastern Gulf Coast. Meanwhile on the northern periphery of the expanding precipitation shield, reinforcing cold air wrapping around the main deep low appears to support a heavy snow threat for the Great Lakes region Sunday into Monday, spreading over the northern and interior Northeast into Monday and Tuesday with closed upper low/trough approach and with coastal New England triple-point low development. 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