Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Mon Mar 18 2024 Valid 12Z Thu Mar 21 2024 - 12Z Mon Mar 25 2024 ...General Overview... A split upper level flow pattern is forecast to be in place across the continental U.S. for the end of the week, with a shortwave trough crossing the Gulf Coast region and broad cyclonic flow across the north-central states. The southern shortwave is expected to spur surface cyclogenesis over the northern Gulf and then the low tracks off the Southeast Coast going into the weekend. Looking ahead to Sunday and next Monday, a larger synoptic scale trough likely develops across the Western U.S. with an organized low pressure system developing over the Plains. ...Guidance/Predictability Assessment... The 00Z model guidance suite is in reasonably good agreement on the synoptic scale on Thursday, although there are some amplitude differences with the Gulf Coast trough and the southward extension of the deep upper low over central Canada. A general model compromise seems reasonable for the end of the week. The forecast gets more challenging 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 offer a more progressive and northward track of the surface low up the coast, even compared to the GEFS mean which is much more suppressed and closer to the ECENS guidance. Given the ongoing uncertainties in the eventual track of that low, the preference is for something closer to the GEFS/ECMWF ensemble means. The WPC QPF was reduced across the Northeast compared to the NBM given the much farther north track of the GFS that is not currently preferred. ...Weather/Hazards Highlights... 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 for the Day 4 period. While the greatest rainfall totals are forecast to remain offshore over the northern Gulf, activity will gradually progress toward the Florida Peninsula on Friday, with potential for some localized heavy rainfall across portions of southern Florida where a Day 5 Marginal Risk area is planned. The eventual evolution and track of the surface low off the Southeast Coast and possibly up off the East Coast Friday into next weekend remains quite uncertain in guidance, but currently seems to favor enhanced rains up through the coastal Carolinas, where a Day 5 Marginal Risk area is also planned. Additional rainfall and enhanced winds near the coastal Mid-Atlantic and Northeast are possible in gradient flow 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, and also the Canadian Rockies. Enhanced rains are expected to spread from the Pacific Northwest down across 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 progress inland across the north-central Great Basin and the northern Rockies/Plains, then probably the Upper Midwest into next weekend. There may be a corridor of more enhanced snow on the northwest side of a developing surface low across the Dakotas and into Minnesota towards the end of the forecast period late Sunday into Monday. There will likely be a stark temperature contrast from the Central Plains/Midwest to the Canadian border late in the week. Winter- like readings will continue across Montana, North Dakota, and into northern Minnesota with a strong arctic high situated over central Canada providing cold northerly flow. Above normal temperatures across the Intermountain West midweek are expected to cool down to below average levels with the upper trough developing late in the forecast period. Even colder temperatures are possible from Montana to the Central Plains by next Monday behind a strong cold front with subfreezing high temperatures for Montana and most of the Dakotas. Hamrick 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