Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Mar 16 2024 Valid 12Z Tue Mar 19 2024 - 12Z Sat Mar 23 2024 ...General Overview... An amplified upper level pattern is expected to be in place across the continental U.S. for the beginning of next week, featuring a deep upper trough across the Eastern U.S., and Rex type blocking over the Western U.S. with a closed upper high over the Pacific Northwest and a loitering upper low over the Desert Southwest. The trough over the East slowly lifts out some by midweek with more broad cyclonic flow aloft. The southwest upper low evolves into an open wave and then ejects eastward across the Southern Plains going into Thursday, and reaching the Southeast U.S. by next Saturday, with a potential coastal low development. Upper ridge breakdown over the Northwest will allow for Pacific troughing to move in with more widespread rain and mountain snow by Friday and into Saturday. ...Guidance/Predictability Assessment... Model and ensemble solutions point to similar weather scenarios for Tuesday and Wednesday, with a composite of best clustered guidance from the 06 UTC GFS and 00 UTC ECMWF/Canadian offering a reasonable forecast basis and detail, mainly consistent with the National Blend of Models and WPC continuity. Growing forecast spread contingent mainly upon variances within northern stream flow through Canada, interaction with East Pacific systems and the timing of ejection of Southwest U.S. upper trough energies late week into next weekend prompted preference to a composite of well clustered guidance of the 06 UTC GEFS mean and 00 UTC ECMWF/Canadian ensembles means that seem to best hightlight systems consistent with predictability. The 12 UTC cycle suite continues to suffer from model-model and run-run issues at these time longer frames. ...Weather/Hazards Highlights... A relatively benign weather pattern will be in place over the lower 48 states Tuesday into Wednesday. However, there will be modest organized precipitation potential for the Northwest, the Southwest/Southern Plains and the Great Lakes/Northeast, mainly given upper support. In this pattern, expect some accumulating snows for favored terrain of both the Northwest and Southern Rockies, while sufficient cold flow with secondary frontal passage may also favor portions of the north-central Appalachians and to the lee of Lake Erie and Lake Ontario. For impactful temperatures, frost/freeze concerns are likely Tuesday morning over portions of The South given the settling of post-frontal Canadian high pressure and radiational cooling for areas where spring green-up has started as temperatures dip 10-15 degress below normal that equate to minimum temperatures ranging from well into the 30s to below freezing. Looking ahead to the end of next week, showers and thunderstorms are expected to significantly increase in coverage across The South/Southeast as Gulf moisture advects inland ahead of a developing surface low. While latest model trends show the greatest rainfall just offshore over the northern Gulf, there is a signal for some heavy amounts inland from Texas through the Gulf Coast into Florida. There is an increasing model signal for a potential coastal low to develop offshore the Southeast heading into next Saturday as the Gulf low crosses Florida, and if this comes to pass, heavy rain and gusty winds could develop for at least the coastal Carolinas, with guidance still quite varied on the inland extent and northward coastal low influence next weekend. Meanwhile well upstream, upper ridge breakdown down over the northwestern U.S. should eventually allow Pacific upper troughing to work inland to support a return to a pattern with more widespread rains and inland/mountain snows into Friday and Saturday, but guidance remains varied with eventual flow evolution. Hamrick/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