Extended Forecast Discussion NWS Weather Prediction Center College Park MD 233 AM EDT Mon Mar 22 2021 Valid 12Z Thu Mar 25 2021 - 12Z Mon Mar 29 2021 ...Thursday heavy rain threat most likely centered over parts of the South... ...Overview... A couple of vigorous shortwaves embedded within a large scale mean trough over the Central U.S. will support fairly strong surface systems and warm/wet weather for much of the eastern half of the country. This trough should finally begin to shift eastward by the weekend as upper ridging builds across parts of the West bringing a warmer and drier trend to the region. More shortwave energy should also slide into the Pacific Northwest and Northern Rockies early next week. ...Model Guidance/Predictability Assessment... The first shortwave lifts from the Southern Plains into the Midwest and eventually the Northeast Thursday-Saturday. Latest runs of the GFS and the UKMET are faster than the ECMWF/CMC, which also have a little better support from the ensemble means. The next shortwave drops through the Western U.S. and amplifies over the Central U.S. late this week, pushing into the Midwest and East by early next week. For the most part, the guidance is in good agreement with this system, except the GFS does get a little fast with the energy as it lifts through the Great Lakes next Sunday-Monday, likely due to a little bit flatter ridge upstream across the West. However, given the late period timing, run to run continuity for most of the guidance is still poor, so a blend towards the ensemble means served as a good starting point. Another batch of shortwave energy looks to cross into the Pacific Northwest on Sunday, however models continue to show some significant differences both in regards to timing and intensity from one model cycle to another. The ensemble means should again be a good solution at this time. The WPC progs were comprised of a majority blend of the deterministic models days 3-5, with a bit more weighting towards the ECMWF/CMC. By day 6 and 7, uncertainty quickly increases so more of the ensemble means were blended in to help mitigate some of these typical late period differences. ...Weather/Threats Highlights... Moist Gulf of Mexico inflow ahead of mean central U.S. troughing will continue the threat for widespread organized rainfall along a slow moving frontal boundary across parts of the South and Eastern states Thursday into Friday. Models continue to point to the heaviest rainfall axis stretching from the Central Gulf Coast into the Southern Appalachians and Tennessee Valley. Some warm sector convection may be strong/severe and future Storm Prediction Center outlooks will provide more information on this threat as guidance becomes more agreeable for important details. The deepening surface low associated with this system may also bring a period of light snow to the north from parts of the Great Lakes into northern/interior New England Friday and Saturday. The next system dropping through the West will likely bring another round of mountain snows to the Rockies on Thursday, with a drier trend through much of the rest of the period as upper level ridging builds aloft. As this shortwave amplifies and slides eastward next weekend, the models are hinting at another round of moderate to heavy rainfall for parts of the Southeast which should focus along a warm front across the Deep South. The eastern third of the country will stay warm and above normal through much of the period, with the greatest anomalies (+15-20F) across the Northeast Thursday and Friday. Morning lows will be even more so anomalously warm, with several record high minimum temperatures possible late this week stretching from Florida to the Northeast. Out West, temperatures should trend warmer after this week as upper level ridging builds across the region. Santorelli 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml