Extended Forecast Discussion NWS Weather Prediction Center College Park MD 136 AM EST Tue Feb 23 2021 Valid 12Z Fri Feb 26 2021 - 12Z Tue Mar 02 2021 ...Potential heavy rainfall from the southern Plains to the Tennessee Valley late this week and early next week... ...Overview and Guidance Evaluation... A fast southwesterly flow aloft is expected to evolve over the CONUS during the period with a couple different shortwaves carving out troughing over the Interior West and Southwest U.S. while a downstream ridge remains locked in place over portions of the Gulf and southern Florida. A quasi-split flow is also expected with a parade of shortwaves skirting the northern tier of the U.S., resulting in a wavy baroclinic zone from portions of the Southern Plains to the Tennessee Valley late this week into early next week. This is where the most active sensible weather is forecast over the period with a multi-day heavy rainfall event possible. While the latest guidance largely agrees on the synoptic evolution during the period, there are considerable timing and strength details in both the southern and northern stream. The first West Coast to Southwest U.S. shortwave arrives by day 5 then quickly passes into the Plains as it weakens and becomes absorbed in the large scale flow. Quick on its heels is another shortwave that drops through the West Coast. The GFS remains too fast and a bit flatter with these features as they move through the flow. Conversely, at times the ECMWF is slower than the rest of the guidance. An early look at the 00Z guidance shows better agreement and timing. For the northern stream, the GFS was much more aggressive/deep as the wave drops through the Great Lakes and this seems a bit of an outlier at this point compared to consensus. However, the trend is for a deeper low tracking through the Great Lakes to New England. Finally by day 7 the next system approaching the West Coast and Four Corners show fairly good agreement despite the typical spread and forecast uncertainty. As a result, the WPC blend favored the deterministic ECMWF/GFS/CMC/UKMET day 3-4, then incorporated more of the ECENS and GEFS to handle some of the greater spread east of the Rockies day 5-7. This resulted in a blend that lied close to continuity. ...Weather/Threats Highlights... The Pacific Northwest to Northern Rockies will see a couple days of widespread precipitation late this week, with the higher elevations of the Cascades, Northern Rockies, and Bitterroots expected to receive heavy snow. That weather system then drops to the Southwest U.S. and gradually opens up over the Plains. A favorable setup for a prolonged period of Gulf moisture to interact with the wavy boundary in place is likely to setup a period of unsettled, active weather including heavy rainfall for portions of the Southern Plains to Tennessee Valley and southern Appalachians beginning late this week into early next week. The latest guidance shows the possibility of several inches of rain totaling over the period. 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. Nighttime lows are forecast to be well above normal (10 to 15F) 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