Extended Forecast Discussion NWS Weather Prediction Center College Park MD 149 PM 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... An unsettled and active weather pattern is forecast to affect much of the central to eastern U.S. later this week while the Pacific Northwest will see a couple weather systems approach the area by late in the weekend and early next week. Ahead of that though, upper ridging over the Rockies will bring a period of warmer/drier weather. ...Model Guidance/Predictability Assessment... The synoptic pattern is fairly well clustered over the CONUS, aside from the 00Z/06Z GFS which is flatter and more progressive with the shortwave trough over the central U.S. on Thursday. Its speed/timing issues remain in place throughout the forecast period and in short, its inclusion in the WPC blend was lowered compared to the other global models. The 00Z ECMWF/CMC/UKMET offered a reasonable blend for day 3-4 with the synoptic system affecting the central/eastern U.S.. All of the models show a fair amount of ridging developing by this weekend over the western U.S. that translates toward the central U.S. by early next week, ahead of the next shortwave trough impacting the Pacific Northwest. Here, the GFS again is too fast and while the ECMWF was on the slower side, a ECENS/GEFS heavy blend for day 6-7 seemed to handle the smaller scale timing differences and also maintains continuity fairly well. ...Weather/Threats Highlights... A surge of warm, moist Gulf of Mexico air will drive more widespread, organized rainfall along a slow moving frontal boundary across parts of the South and Eastern states Thursday into Friday. The heaviest axis of rainfall is expected to fall from the Central Gulf Coast into the Southern Appalachians and Tennessee Valley. On the northern, colder side of the system, a period of light snow from parts of the Great Lakes to interior New England Friday-Saturday is possible. 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 to +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/Taylor 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