Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Mar 25 2025 Valid 12Z Fri Mar 28 2025 - 12Z Tue Apr 01 2025 ...Overview... Latest guidance shows a moderately progressive pattern that will bring active weather to portions of the Lower 48. A southern stream shortwave will promote a threat of heavy rain along the western- central Gulf Coast late this week. Behind this feature, eastern Pacific into western U.S. dynamics will spread precipitation across northern parts of the West and then develop one or more waves/fronts from the Plains eastward with associated areas of enhanced rainfall as well as winter weather potential over far northern latitudes. Another Pacific system should spread precipitation into the West from late weekend into early next week. Very warm temperatures will prevail ahead of the overall area of Plains low pressure, while trailing cooler air will bring readings back down to near or slightly below normal. ...Guidance/Predictability Assessment... Primary forecast differences at the surface and aloft involve the evolution of incoming western energy and evolution of low pressure from the Plains northeastward, followed by low pressure near the West Coast by Sunday-Monday and the eastern Pacific into Lower 48 pattern by early next week. Over recent days the guidance has gradually been shifting emphasis of separate shortwaves coming into the West, with trends toward weakening the first shortwave and emphasizing the second. This leads to a more elongated look for low pressure by Sunday, with separate waves near the lower Great Lakes and south-central Plains. The most common theme in the guidance is for a shortwave reaching the east-central U.S. to support a consolidating surface system (with a fair amount of spread for depth) reaching the lower-eastern Great Lakes or Upper Ohio Valley by early Monday and then near the Canadian Maritimes by Tuesday. Machine learning (ML) models have occasionally showed more varied solutions, tempering confidence. Guidance has continued to show a system affecting the West Coast Sunday into Monday but with a very wide spread for strength/track and no clear clustering. Preference incorporates some ideas of ensemble means that have been keeping the overall area of low pressure offshore, but the new 00Z GFS and ECMWF bring it to the Pacific Northwest coast. ML models offer little help, also showing wide variance and generally weaker solutions than dynamical guidance. Toward the end of the period the specifics over the Lower 48 become influenced by developing differences within a large scale blocky Pacific pattern, ultimately yielding model/ensemble spread ranging between a trough and a ridge over some areas between the eastern Pacific and central U.S. A general model/ensemble blend provides an intermediate solution that reflects at least a modest portion of West Coast shortwave energy as of Monday continuing inland through Tuesday, with latest GFS/GEFS runs tilting this way versus the 12Z GFS that had more ridging over the west-central states. Guidance preferences among 12Z/18Z runs led to starting with greater 18Z GFS/12Z ECMWF emphasis relative to the 12Z CMC/UKMET for the first half of the period, followed by steadily increasing 18Z GEFS/12Z ECens input such that day 7 Tuesday consisted of 60 percent means and 40 percent GFS/ECMWF. ...Weather/Hazards Highlights... Latest guidance is consistent with the general theme of heavy rain potential along and near the west-central into central Gulf Coast, as an upper trough interacts with low level Gulf moisture and a warm front. The Day 4/Friday Excessive Rainfall Outlook has introduced a Slight Risk area covering parts of southeastern Texas and southwestern Louisiana, with models/ensembles now clustering better for this region seeing the best focus. By Day 5/Saturday, guidance becomes much more diffuse with the rainfall signal. Recent ECMWF/ECMWF AIFS/UKMET runs show the most agreeable theme of some activity from Friday-Friday night continuing into Saturday, with some instability, before trending lighter. Thus the Day 5 ERO depicts a Marginal Risk area along and north of the central Gulf Coast region. Elsewhere, rain and high elevation snow will cross the northern half of the West late this week, followed by an increasing area of precipitation from the north-central Plains through Northeast and extending southward through the Mississippi Valley and points east during the weekend into early next week as one or more frontal waves evolve and the overall frontal system eventually pushes eastward. It may be cold enough to support wintry precipitation types for a time from the northern Plains through the Upper Great Lakes into northern New England. To the south, some rain could be locally heavy over the north central Plains as well as from the Mississippi Valley eastward. In addition, the Storm Prediction Center is monitoring the potential for severe weather from parts of the eastern Plains into a large part of the east- central U.S. during the weekend. Check latest SPC outlooks for the latest info on severe threats. After a brief break, another area of precipitation should spread across the West from Sunday into early next week. There is still a decent amount of uncertainty for the specifics of supporting eastern Pacific low pressure, but currently expect highest totals to be over the central West Coast/northern California. Very warm temperatures will prevail in the warm sector ahead of the wavy frontal system evolving over the Plains and then pushing eastward. Some areas from the central High Plains into the Midwest may see highs 20-30F above normal on Friday. The warmth will shift south and east over the weekend with plus 20F or greater anomalies for morning lows reaching from the Midwest into the Ohio Valley while plus 10-20F anomalies for highs extend from the southern half of the Plains into the Mid-Atlantic and Southeast. Continued frontal progression will confine above normal readings to the East Coast by Monday. However the Northeast will be on the cool side, to the north of a front settling over or near the northern Mid- Atlantic. Behind the main frontal system, modestly below normal highs will progress from the West late this week into the Great Lakes/Ohio Valley by next Tuesday. Rausch 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