Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Apr 12 2025 Valid 12Z Tue Apr 15 2025 - 12Z Sat Apr 19 2025 ...Overview... Initially, an upper level trough will be moving across the eastern U.S. and a closed upper low will be positioned offshore over the eastern Pacific, with ridging in between over the Intermountain West. By the second half of next week, the eastern trough will shift into the Atlantic followed by the upper ridge, making way for an energetic upper low that is expected to dive south across the West. This low is forecast to interact with the low/trough over the eastern Pacific and eject energy northeast towards the Plains. At the surface, a couple of cold fronts will push across the East Tuesday into Wednesday, supporting precipitation chances from the Great Lakes to the Mid-Atlantic and Northeast, then the focus for precipitation will shift to the Western and Central U.S. as a strong frontal system moves southeast across these regions. Portions of the Rockies could see notable snowfall from this system Wednesday through Friday. Then by next weekend, this frontal system is expected to become elongated, stretching from the Southwest to the Northeast, and there may be a risk of heavy rainfall from the Southern Plains to the Ohio Valley. ...Guidance/Predictability Assessment... Model guidance had been split on the evolution of northern stream energy in the West next week, with some solutions bringing energy south and some pushing it to the east. Today, all available deterministic, ensemble, and AI model guidance are depicting the energy taking a southward track into the Western U.S., which has resulted in significant changes to the forecast since yesterday afternoon. The frontal evolution Thursday through Saturday has changed dramatically, which will result in large changes to the sensible weather forecast. Most notably, the probability of snowfall in the Rockies has increased Wednesday through Friday, and a wetter pattern is favored late next week from the Southern Plains to the Ohio Valley. For WPC's afternoon forecast, a blend of the deterministic 06Z GFS and 00Z ECMWF/CMC/UKMET was used for the first two days, then ensemble means from the 06Z GEFS and 00Z ECENS were added in increasing amounts through the rest of the period. The GFS and ECMWF were fairly agreeable with the exception of timing and placement of some features late in the period, and they were also in line with available AI guidance. The CMC and UKMET also showed good agreement, but deviated a bit from the general consensus on how to handle northern stream energy across the northern tier late next week. For this reason, the GFS and ECMWF were given stronger weight in the WPC forecast blend than the CMC and UKMET. ...Weather/Hazards Highlights... At this time, no risk areas were introduced to the Day 4 (Tuesday) and Day 5 (Wednesday) Excessive Rainfall Outlooks. Precipitation amounts are forecast to be modest early next week, with little to no risk of flash flooding. A surface low tracking through southeastern Canada and its trailing cold front will support some precipitation from the Great Lakes to the Mid-Atlantic and Northeast on Tuesday, some of which may linger into Wednesday for the Interior Northeast. Temperatures on the backside of the low could be cold enough for snow at higher elevations of the Adirondacks and Green and White Mountains. Gusty winds will also be possible with this system from the Midwest/Great Lakes to the Northeast and northern/central Appalachians. Conditions will dry out across the East mid-week as high pressure moves in behind the system. A weakening upper low will be positioned over the eastern Pacific on Tuesday with troughing extending into the Southwest, which may support isolated to scattered precipitation for the Four Corners states. More significant precipitation chances will arrive on Wednesday as the energetic upper low dives southward into the West. Precipitation will spread from the Northern Rockies on Wednesday, to the Great Basin and Central Rockies on Thursday, and into the Four Corners region by Friday. The probability of accumulating snow has increased across the Rockies with this forecast, and some locations could receive notable snowfall from this system. As the energetic upper low dives south, a smaller portion of the northern stream energy will continue east, trailing along the northern tier of the nation. This will support precipitation chances from the Northern Plains to the Midwest and Great Lakes Thursday and Friday. By Saturday, an elongated frontal system is expected to stretch from the Southwest to the Northeast with shower and thunderstorm chances spanning the entire length of the boundary. Southerly flow ahead of the boundary should push warm, moist air into the South-Central U.S., which could create a threat of heavy rain from the Southern Plains to the Mid-Mississippi and Ohio Valleys next weekend. Due to increased sensitivity to rainfall across the Mississippi and Ohio Valleys, rainfall next week could cause additional flooding issues for these regions. However, at this time, this wetter solution is still uncertain. The forecast details will likely change over the next few days and should be closely monitored to assess any possible impacts. Initial mean ridging aloft will allow for above normal temperatures in the West, spreading into the Plains by mid-week. Generally, temperatures are forecast to be 10-20 degrees above average, with highs in the 90s in the Desert Southwest Tuesday and Wednesday and in the Central and Southern Plains on Thursday. Cooler than average temperatures are likely in the Great Lakes and Ohio Valley on Tuesday and into the Eastern U.S. on Wednesday and Thursday. Lows should be 5-10 degrees below average and highs 10-15 below average. After that, below average temperatures are expected to develop southward across the West underneath the upper low while above normal temperatures gradually shift towards the East and Southeast. Dolan/Tate 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