Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EST Mon Feb 22 2021 Valid 12Z Thu Feb 25 2021 - 12Z Mon Mar 01 2021 ...Potential heavy rainfall from the southern Plains to the Tennessee Valley late this week and early next week... ...Overview and Guidance Evaluation... A general synoptic pattern that features upper trough energies lifting across the Southwest together with a persistent upper ridge over the southern Gulf of Mexico will resume during the medium range period. The resulting southwesterly flow in between is likely to lift Gulf moisture across the southern U.S. and into portions of the eastern U.S., especially by late this week as well as early next week. Waves of low pressure developing along a slow-moving front would provide the focus for some areas of heavy rainfall across the interior southern U.S. to the Tennessee Valley. There has been a noticeable trend in the models of ejecting more upper-level energy from the Southwest into the central Plains early in the medium range period. This trend has led to a slower moving front across the Deep South and a better defined axis of potentially heavy rainfall across the southern Plains Thursday to Friday. The extra upper-level energy is then carried downstream to promote upper-level troughing and result in a better defined low pressure system moving across the Great Lakes on Saturday. Meanwhile, the bulk of the moisture in the South is pushed faster toward the east and northeast into the East Coast on Saturday ahead of the developing system with a warm front lifting across the Southeast. By late in the weekend into early next week, model guidance has been quite consistent in setting up an axis of heavy rain across the Mid-South into the Tennessee Valley and towards the Appalachians. Another upper-level trough that gradually digs down into the Southwest this weekend will then send additional upper-level energy into the southern Plains. When combining with broad troughing associated with a cold air mass settling into the northern tier states, the synoptic pattern will appear to help sustain the axis of heavy rainfall across Mid-South into early next week, even though models continue to show some spread regarding the details of the pattern evolution. Meanwhile, the Pacific Northwest should experience a couple of periods of precipitation during the medium-range--first one late this week followed by the next one Sunday into Monday, with relatively consistent model timings. The WPC morning package was derived based on the consensus of the 00Z ECMWF/00Z EC mean, the 06Z GFS/06Z GEFS, and smaller contribution from the 00Z CMC/CMC mean with more of the ensemble means on day 6-7 to account for the forecast uncertainty. ...Weather/Threats Highlights... The Pacific Northwest can expect a period of precipitation late this week into the first part of the weekend, with higher elevations of the Cascades and Northern Rockies/Bitterroots likely to see the most snow. As that system drops to the Four Corners region and then pushes into the Southern Plains, several days of Gulf moisture interacting with frontal boundaries across the South could result in multiple waves of precipitation. Details and totals are less certain but the setup favors potentially heavy rainfall somewhere from the Arklatex, portions of AL/MS to lower TN Valley. Below normal temperatures are forecast predominately across the Rockies during the period and then in the wake of the late weekend and early next week system. Those below normal readings could spill into the Plains. Departures of 10F to as much as 20F are possible. Elsewhere, no significant departures from normal are forecast during the period. Kong/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 Hazards: - Heavy rain across portions of the Southern Plains, the Middle/Lower Mississippi Valley, the Tennessee Valley, the Ohio Valley, and the Central/Southern Appalachians, Thu-Mon, Feb 25-Mar 1. - Heavy snow across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Thu-Fri, Feb 25-Feb 26. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast and the Mid-Atlantic. - Flooding likely across portions of the Southeast. 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