Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EDT Mon Mar 18 2024 Valid 12Z Thu Mar 21 2024 - 12Z Mon Mar 25 2024 ...Late season winter storm possible across portions of the Northern Plains/Upper Midwest Sunday-Monday... ...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 bringing a potential late season winter storm to parts of the Northern Plains. ...Guidance/Predictability Assessment... The latest model guidance continues to show reasonably good agreement on the large scale, but definitely some noticeable differences still in the details, timing, and amplitude of individual systems. There continue to be some amplitude differences with the Southeast/Gulf Coast low, with the GFS on the stronger side of the guidance, bringing the low farther up the East Coast. Ensemble means and other operational models suggest a cutoff upper low off the Southeast develops with a weaker surface low reflection and more easterly/offshore track. By Saturday, the GFS (including the new 12z run today) is notably slower than the rest of the guidance with the upper low/shortwave into the Northwest. The CMC is also faster with it by Sunday-Monday as it amplifies over the Four Corners region. The WPC forecast blend for today used a blend of the operational guidance for Days 3 and 4. After 5, removed the GFS from the blend due to its issues noted above and blended the ensemble means with the ECMWF through day 7. Overall, this maintained good continuity with the previous WPC forecast. ...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 highlighted. 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 second Day 5 Marginal Risk area was placed. 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 heavy 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. Guidance continues to show potential for a late season winter storm 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. Santorelli/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