Extended Forecast Discussion NWS Weather Prediction Center College Park MD 136 AM EST Wed Feb 24 2021 Valid 12Z Sat Feb 27 2021 - 12Z Wed Mar 03 2021 ...Potential heavy rainfall from the southern Plains to the Tennessee Valley this weekend into early next week... ...Overview and Guidance Evaluation... Quasi-southwesterly flow aloft is expected over the CONUS during the period with a couple Pacific systems dropping into the Four Corners region while a deepening trough quickly moves through the northern stream. Ridging is expected to persist over southern Florida and parts of the Gulf of Mexico and the resulting southwesterly flow will allow Gulf moisture to lift into parts of the CONUS. This moisture and associated boundary will be interacting with high pressure form the north, creating a zone for some episodes of heavy rainfall this weekend into early next week across portions of the Southern Plains to southern Appalachians. Through day 5, the models show fairly good agreement with the Pacific system dropping down to the Four Corners then pushing out into the Plains by early day 6. In the northern stream, the models continue to advertise a deeper solution with the upper level trough, giving way to a stronger low passing through day 3-4-5. In its wake, stronger high pressure is now forecast to drop down into the Plains. This effectively pushes the wavy stationary boundary across the South further south by day 5/6. Meanwhile, there is much uncertainty with the day 6/7 Pacific/West Coast trough. Although many of the models now show a deeper low pinching off well offshore. The GFS appeared to be again too strong with the northern stream energy, though the trend is for a more pronounced trough taking on a negative tilt, so it was not entirely discarded. Elsewhere through day 5, a blend of the available deterministic guidance sufficed. By day 6/7, the forecast confidence was below normal regarding the position of the stationary boundary/front across the South - the models now suggest a stronger high to the north which could push it toward the Gulf Coast (and push heavy rainfall threat to the south as well). Meanwhile, the west coast trough evolution is very uncertain, so a blend heavy on the ensemble means was used. The WPC blend was derived based on the consensus of the 12Z ECMWF/12Z EC mean, the 18Z GFS/18Z GEFS, and smaller contribution from the 12Z CMC, and mostly a blend of the ensemble means on day 6-7 to account for the forecast uncertainty. ...Weather/Threats Highlights... Precipitation spreading into portions of the central to northern Rockies could result in heavy snow for the higher elevations. Meanwhile, upper level troughing over the Four Corners region will allow Gulf moisture to lift northward as waves of low pressure move along a stationary boundary in place. This setup will favor a few rounds of locally heavy rainfall this weekend into early next week across portions of the Arklatex to the TN and OH Valleys. The latest guidance shows the possibility of general rainfall totaling several inches over the period. Farther north, some wintry precipitation can be expected across the central Appalachians Friday night into early Saturday, across the upper Great Lakes early on Saturday, across the Midwest on Sunday and Sunday night, and across northern New England Monday and Monday night. Another round of wintry weather is possible across the Midwest on Monday. Below normal temperatures are expected to remain primarily over the Northern Rockies to start the period with departures of 10F to as much as 20F Saturday while the eastern half of the CONUS experiences slightly above normal daytime highs. Ahead of the stationary boundary expected to be draped across portions of the South, daytime highs as much as 15F above normal are possible Sunday along the Gulf Coast as well as portions of the Northeast and New England. Nighttime lows are forecast to be well above normal (10 to 20F) for much of the central and eastern U.S. Sunday and Monday, ahead of the frontal boundary draped through the MS River Valley to Great Lakes. 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