Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Fri Mar 29 2024 Valid 12Z Mon Apr 01 2024 - 12Z Fri Apr 05 2024 ...A low pressure/frontal system will spread potentially heavy rain and mountain/northern tier snow across the central to eastern U.S. next week as an active new storm digs into a cooling West... ...Guidance/Predictability Assessment... The guidance continues to show good agreement on the large scale for a very amplified pattern during the medium range period. An elongated trough into the Central U.S. early week may shear with some energy hanging back over northern Mexico and the rest of the energy combining with a northern stream shortwave into the Great Lakes to produce a fairly deep upper low that tracks across the Ohio Valley and Northeast next Wednesday-Thursday. The guidance all agrees on this set-up and the general track but there remains noteable uncertainty on the speed of the upper low. The deterministic and ensemble mean solutions show a fairly progressive system, but some of the EC-based ML/AI models suggest a slower/deeper low through the Ohio Valley/Northeast, and the overall trend has been towards a little bit slower/more snowy solution for the Northeast. Meanwhile, the next amplified shortwave should drop into the Northwest around mid-week and lots of uncertainty on its evolution as well. The GFS remains and outlier with a deeper and farther sound cut-off upper low over the Southwest late period, while ensemble means/ECMWF suggest an amplified shortwave over the Northwest/Great Basin. New 12z CMC came in with a farther offshore solution as well, which does have support from some of the latest AI/ML models as well. The WPC forecast today used a blend of the latest deterministic solutions Days 3-5 (Mon-Wed), increasing incrementally the ensemble means (with the ECMWF) later in the period. Overall, this maintained reasonable continuity with the previous forecast which given the uncertainties both in the Northeast and the West, is the preferred approach for now. ...Pattern Overview and Weather/Hazards Highlights... Closed upper troughing with several embedded lows will work into the Four Corners states and northwest Mexico into early next week. This feature and its surface low pressure system will direct lingering moisture into the Southwest and Rockies into Monday and produce terrain enhanced rain and higher elevation snow. Some gusty wind potential on the backside of the system across the southern high plains as well. Aspects of these upper/surface features will eject downstream to interact with increasingly digging northern stream troughing into the north-central U.S.. This should act to consolidate a main surface low that should deepen and move from the central Plains to the Northeast during the workweek. Expansive precipitation coverage is likely with this large system, with snow and freezing rain possible on its north side, and a heavy rain and strong to severe thunderstorm focus within an unstable warm sector and into the low as moisture pools along the surface low's fronts. Well behind this system, a strongly amplifying upper trough and leading and wavy cold front digging into the West should act to cool temperatures and spread enhanced precipitation to include interior and terrain/mountain snows southeastward across the West into next Wednesday-Friday in increasingly unsettled flow. Depicted areas of precipitation are currently on the modest to moderate side given track/moisture, but ample upper support and favored terrain lift may lead to locally heavier focus and activity to monitor. The WPC Winter Weather Outlook shows the greatest snow probabilities over Northwest to northern Intermountain/Rockies terrain/mountains. Meanwhile, the Ohio Valley will be one area of focus for potentially heavy rainfall. Some rain is likely there on Sunday, but amounts are forecast to increase by Monday. A Marginal Risk remains in place from the Middle Mississippi Valley into the Ohio Valley on Monday for the WPC Day 4 Excessive Rainfall Outlook (ERO). Moisture and instability are anomalously high for this time of year, and showers could train along the west to east oriented warm front nearby. By Tuesday the cold front and low should continue to push east and the heaviest rainfall at that point may focus out from the Tennessee Valley and into the Appalachians and parts of the Mid-Atlantic where a Marginal Risk ERO is in place, with lowered flash flood guidance (FFG) values on the northern end and greater instability over southern portions. Accordingly, the Storm Prediction Center is monitoring severe potential in the warm sector from the Plains to just west of the Appalachians for early next week to monitor. On the northern side of the storm system, snow and freezing rain are potential threats from the Upper Midwest and Great Lakes region Monday and into parts of the Northeast by Tuesday-Wednesday and lingering into Thursday with continued wrap- around flow into the main/windy low. There is still considerable uncertainty in the placement and amounts of snow and ice, but there is increasing potential for a significant late season storm, so continue to monitor forecasts. The Interior Northeast may see the heaviest snow amounts overall because of the duration of the potential snow event there compared to the north- central U.S./Midwest/Great Lakes. Santorelli/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