Extended Forecast Discussion NWS Weather Prediction Center College Park MD 101 AM EST Mon Feb 22 2021 Valid 12Z Thu Feb 25 2021 - 12Z Mon Mar 01 2021 ...Potential Heavy Rainfall For Southern U.S. Late This Weekend and Early Next Week... ...Overview and Guidance Evaluation... The synoptic pattern across the CONUS during the period is expected to feature general troughing over the interior West with a persistent upper ridge over the southern Gulf of Mexico. The resulting southwesterly flow in between is likely to return Gulf moisture over the southern U.S. into portions of the eastern U.S., especially by this weekend into early next week. Waves of low pressure lifting through the flow could provide the focus for some areas of heavy rainfall over the Southern U.S.. In general, the latest model guidance shows fairly good agreement through about day 5. An initial quasi-split flow on day 3 /Thursday/ is expected to become predominately troughing over the Southwest U.S. and downstream ridging anchored over the Gulf of Mexico and southern Florida. By day 6/7, a more vigorous shortwave trough dropping down the West Coast is forecast to reach the Four Corners region. Here, the typical timing/strength biases are seen where the GFS is much faster/progressive ejecting the shortwave and its associated low downstream across the CONUS. Meanwhile, the ECMWF bottles the energy up in the form of a closed low that is slow to push out into Plains. By day 7, the differences are quite notable, ranging from the GFS with its shortwave axis over Oklahoma to the ECMWF over New Mexico. Another Pacific shortwave approaching the West Coast on day 7 shows similar timing issues as well. The CMC for the most part was a compromise deterministic solution though at times its evolution of the Southwest U.S. was off. Given these differences, the WPC blend incorporated mostly the 00Z ECMWF, 18Z GFS, and 00Z CMC on day 3-5 followed by mostly a ECENS/GEFS blend on day 6-7 to account for the forecast uncertainty. Forecast confidence was average to above average days 3-5 then lowered to average to slightly below average day 6-7. ...Weather/Threats Highlights... The Pacific Northwest can expected a few rounds 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. 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