Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 PM EST Wed Feb 17 2021 Valid 12Z Sat Feb 20 2021 - 12Z Wed Feb 24 2021 ...Much below normal temperatures over the southern Plains and east-central U.S. to moderate and shift eastward this weekend... ...Active weather continues across the Pacific Northwest and northern Rockies through the period... ...Overview and Guidance Evaluation... Progressive mean flow over most of the lower 48 will finally bring an end to the record cold airmass that has plagued the Central U.S. for several days. The pattern will also promote the continuation of an active weather period for the Pacific Northwest into next week with persistent onshore flow and a series of shortwave troughs/impulses. Individual features will include a leading upper trough that will depart from the eastern U.S. this weekend, followed by the next trough which should enter the Northwest by this weekend and amplify somewhat into the Central U.S. before continuing through the East early next week (with associated surface system). Then a strong jet reaching into the Northwest U.S. and western Canada around the start of next week will push a frontal system across the northern tier states. A shortwave dropping through the Northeast Pacific should reach the West around the middle of next week, possibly supporting some waviness and eventually a renewed southward push of the trailing part of the northern tier front. Latest guidance is still in reasonable synoptic agreement through the period. At some valid times GFS runs are a bit sharper and slower than the majority of solutions with the upper trough crossing the lower 48 from the weekend into early next week, and likewise a bit slower for the associated Plains-Great Lakes wave/trailing front. However by early Tuesday the clustering is good for a compact coastal system to be just off New England, with the 00Z ECMWF gravitating toward consensus by trending weaker/eastward from its prior run. Some discrepancies persist for the timing and/or amplitude of upper troughing reaching the West after Monday, involving the strength of a leading shortwave (GFS/GEFS weakest) and specifics of a trailing shortwave dropping into the West from the northeastern Pacific. Small scale of the leading feature and increasing model/ensemble divergence for flow upstream from the second shortwave suggest fairly low predictability as far as resolving which side of the envelope may be more likely, favoring an intermediate solution for the time being. Thus the operational model blend (00Z ECMWF/UKMET/CMC and 06Z GFS) used for the first half of the period transitions to a model/ensemble mean combination later in the forecast. ...Weather/Threats Highlights... The Pacific Northwest into the Rockies will see the heaviest precipitation during the period as a couple of fronts and persistent onshore flow contribute to multiple episodes of coastal rain and heavy mountain snow. Expect the highest totals to be over favored terrain in the Olympics and Washington Cascades (some liquid equivalent totals of 5" or greater possible for the five-day period) with some significant totals reaching into parts of western Oregon. The Plains into Great Lakes/New England low pressure and associated fronts should produce some moderate wintry precipitation on the northern side of the moisture shield, most likely from the Midwest into the Northeast. Areas of light to moderate rain are likely farther south. Some lake effect snow will be possible in the chilly cyclonic flow behind this system. Temperatures will still be up to 10-25F below normal from parts of the southern Plains into the lower Great Lakes and Mid-Atlantic Saturday. Some anomalies exceeding 10F below normal will persist over the East into Sunday while an upstream front will bring moderately below normal readings from the West into the east-central U.S. during the weekend into early next week. The northern tier states should see temperatures rise the farthest above normal during the first half of next week with some locations up to 15-20F above normal. Also during Monday-Wednesday the southwestern states could see highs up to 10-15F above normal. Rausch/Santorelli 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-Mon, Feb 21-Feb 22. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Sat-Wed, Feb 20-Feb 24. - Heavy snow across portions of the Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Sat, Feb 20. - Heavy snow across portions of the Northern Rockies, Sun-Tue, Feb 21-Feb 23. - Flooding possible across portions of the Mid-Atlantic. - Flooding occurring or imminent across portions of the Southeast and 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 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, Sat, Feb 20. - Much below normal temperatures across portions of the Northeast, the Central Appalachians, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, 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