Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EST Tue Feb 16 2021 Valid 12Z Fri Feb 19 2021 - 12Z Tue Feb 23 2021 ...Much below normal temperatures over the Great Plains moderate as they shift east this weekend... ...Active weather continues across the Pacific Northwest and northern Rockies through the period... ...Overview and Guidance Evaluation... The very anomalous trough and associated cold airmass currently over the central CONUS will moderate as it shifts east this weekend. The trough axis currently over the west-central CONUS is reinforced/amplified through Thursday night before shifting east; pushing off the East Coast late Friday night. Ridging currently building off the West Coast flattens and shift onshore Thursday, re-amplifying over the eastern CONUS through the weekend. The flattening of the western ridge is due to a potent low lingering over the Gulf of Alaska Thursday and Friday which directs a trough that crosses the CONUS Friday night through early next week. This initial trough passage Friday night resumes an active weather period for the Pacific Northwest which continues well into or through next week in persistent onshore flow with repeating shortwave troughs/impulses. Global deterministic guidance remains in decent synoptic agreement, though timing differences are noted with the two main troughs in the medium range. The 06Z and 12Z GFS made the first wave ejecting east from the central CONUS on Friday more progressive than the 00Z (and now 12Z) ECMWF/CMC/UKMET. Therefore less weight was given to the GFS than recent shifts for the Day 3/4 timeframe. However, the next system crossing the West Coast Friday night and ejecting east of the Rockies on Sunday is now in good agreement for strength and timing between the 12Z GFS and ECMWF which are slower than the more progressive 12Z UKMET and CMC, so a preferred multi-model blend on Days 3/4 becomes more focused on the ECMWF/GFS through Days 4/5 before increasing the contribution to the ECENS/GEFS means through Day 7. ...Weather/Threats Highlights... Lingering lake effect and terrain enhanced snow can be expected Saturday in typical snow belts under the upper trough trailing the exiting low over the Northeast. The next wave pushing onshore Friday night brings an active first half of the weekend to much of the Intermountain West with following onshore flow continuing rainfall along coastal portions of Washington/Oregon with heavy mountain snow in the Cascades and Northern Rockies at least through the middle of next week. Wintry precipitation will cross the central/northern Plains Saturday night/Sunday as a frontal system moves through with rain developing as Gulf moisture is tapped over the mid/lower MS Valley in the Sunday night timeframe. Temperatures will remain dangerously low Friday morning across the battered southern Plains and west Gulf coast with min temps 25 to 30 degrees below normal with dozens more daily records expected to be broken. This airmass shifts east over the weekend, but modifies with min temperatures Saturday generally 20 to 25 degrees below normal from the western Gulf Coast (where a few more record lows can be expected) up through the Midwest. By Sunday morning, temperatures are expected to be 15 to 20 degrees below normal over the upper OH Valley, to the central Appalachians with the anomalous temperatures generally offshore by Monday. Meanwhile, expect a warming trend from west to east under the ridge with above normal temperatures for most of the CONUS by Monday with a warming trend continuing into the week. Jackson 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 precipitation across portions of the Pacific Northwest, Sun-Tue, Feb 21-Feb 23. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Fri-Tue, Feb 19-Feb 23. - Heavy snow across portions of the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Fri-Sat, Feb 19-Feb 20. - Heavy snow across portions of the Northern Rockies, Sun-Mon, Feb 21-Feb 22. - Heavy snow across portions of the Northeast, Fri, Feb 19. - Flooding possible across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, and the Ohio Valley. - Flooding occurring or imminent across portions of the Mid-Atlantic. - Flooding likely across portions of the Southeast and the Mid-Atlantic. - High winds across portions of the Central Rockies, the Central Plains, the Central Great Basin, the Northern Plains, and the Northern Rockies, Sun-Mon, Feb 21-Feb 22. - Much below normal temperatures across portions of the Central Plains, the Lower Mississippi Valley, the Central Appalachians, the Northern Plains, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Southern Appalachians, the Upper Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Fri-Sat, Feb 19-Feb 20. - Much below normal temperatures across portions of the Southern Appalachians, the Great Lakes, the Mid-Atlantic, the Central Appalachians, and the Ohio Valley, Sun, Feb 21. 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