Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 PM EDT Sun Mar 17 2024 Valid 12Z Wed Mar 20 2024 - 12Z Sun Mar 24 2024 ...General Overview... The ongoing blocky upper level pattern with the upper high over the Pacific Northwest and an upper low over the Desert Southwest should break down midweek, while an intial upper trough downstream over the Northeast loses influence. The ejection of the southern stream trough originating from the closed upper low over the Southwest will likely induce an organized surface low over the Gulf that will increase chances for showers and storms from Texas to Florida, and then uncertainly up/off the Southeast/East Coast next weekend as a coastal storm. Meanwhile, multiple and complex energies digging into an upper trough position off the West Coast are expected to increasing work inland heading into next weekend and bring a result to more unsettled and wet/snowy conditions. ...Guidance/Predictability Assessment... The 00/06 UTC model guidance suite is in reasonably good agreement on the synoptic scale Wednesday into Friday overall, albeit with lingerng timing differences with flow embedded shortwaves with development of a more zonal flow pattern. There is better overall agreement in the southern stream shortwave crossing the south- central U.S., but more uncertainty as to where the corresponding surface low develops over the Gulf by late in the week. A model composite seems reasonable in this time frame. The forecast gets tricky going into next weekend for the East Coast regarding the likely evolution of an offshore surface low. Recent runs of the GFS and Canadian tend to offered a more progressive and northward track of the surface low up/off the coast, even compared to the GEFS mean which is much more suppressed and closer to the ECENS guidance, and to a lesser extent an even more surpressed ECMWF. Given upcertainties, opted for the WPC surface progs to gravitate closer to the GEFS/ECMWF ensemble means, but latest 12 UTC model guidance has broadly trended more northward overall. ...Weather/Hazards Highlights... There will likely be an increase in showers and some thunderstorms across portions of Texas and Oklahoma on Wednesday with warm air advection from the western Gulf and lift from frontal boundaries. It does not appear that the convection would be widespread nor heavy enough to warrant any WPC Excessive Rainfall Outlook (ERO) Marginal Risks areas at this time, although trends will continue to be monitored. With the low beginning to develop over the Gulf Thursday into Friday, an axis of greater moisture flux near the coast may result in heavier local downpours from southeast Texas to southern Louisiana, where an ERO Marginal Risk area remains in place. While the greatest rainfall totals will remain offshore over the Gulf, activity will gradually work toward Florida. The eventual evolution and track of the surface low off the Southeast Coast and possibly up off the East Coast next weekend (TBD) remains quite uncertain in guidance, but currently seems to favor enhanced rains up through the coastal Carolinas. Additional activity and enhanced winds up through the coastal Mid-Atlantic and northeast are possible in gradient between the western Atlantic coastal low and ambient high pressure wedged inland over the region. Meanwhile, multiple rounds of mainly light snow are forecast with a heavier snow focus for favored terrain of western Montana and northern Idaho to especially the Canadian Rockies. Enhanced rains are expected to spread from the Pacific Northwest mid-later week down into California by Friday and into Saturday as the next storm system moves into the region, with terrain focusing snows for the central and northern Sierra. Activity should also work inland across the north-central Great Basin and the northern Rockies/Plains, then possibly the Upper Midwest into/through next weekend. Cold Canadian high pressure will ensure winter-like temperatures will be reinforced from the north-central U.S. through the Northeast. 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