Extended Forecast Discussion NWS Weather Prediction Center College Park MD 132 AM EST Thu Feb 18 2021 Valid 12Z Sun Feb 21 2021 - 12Z Thu Feb 25 2021 ...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. The first trough should amplify somewhat across the central U.S. by Sunday 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 Northwest around the middle of next week, possibly supporting trough amplification over the West and eventually a renewed southward push of the trailing part of the northern tier front. Synoptically, the latest guidance is still in reasonable agreement through at least day 5. Some timing and detail differences remain with the initial central to eastern U.S. trough, but by early Tuesday, clustering is good for a compact coastal system to be just off New England. Bigger questions arise after Tuesday on timing and amplitude of the next upper shortwave pushing into the Northwest. The GFS/GEFS continue to be weaker/faster while the ECMWF/CMC (with support from their respective ensemble means) are more amplified and as a result, slower. Given the typical low predictability of features at the day 6-7 time frame, and somewhat inconsistent run to run continuity in the guidance, prefer an intermediate solution for the time being. Thus, a majority deterministic model blend (ECMWF/GFS/CMC) for the first half of the period transitions to more weighting towards the ensemble means by the end. This also keeps good continuity with the previous WPC 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. The central to eastern states will continue to be below normal on Sunday, but should finally get back to more normal mid-February values by early next week and possibly even slightly above by the end of the period. The West to northern tier should also trend warmer early in the period, with parts of the northern Plains and Southwest most likely to see daytime highs 10-20F above normal. Late period troughing should bring a new push of colder air int the Northwest and Rockies by next Wednesday-Thursday. Santorelli/Rausch 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