Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 PM EST Wed Jan 29 2020 Valid 12Z Sat Feb 01 2020 - 12Z Wed Feb 05 2020 ...Active Weather Pattern over the West... ...Overview and Guidance/Predictability Assessment... Energies digging to the lee of an upper ridge initially across the western U.S. on Sat will reinforce upper level troughing downstream. Phasing of northern/southern stream features should be ongoing Sat with an amplified upper trough crossing the eastern third of the nation this weekend. A lead Mid-Atlantic coastal low will mainly deepen offshore as it tracks northeastward off New England toward the Canadian Maritimes. Forecast spread is now significantly reduced though, bolstering forecast confidence. Out West, models continue to show good agreement in bringing impulse energy tapping an atmospheric river onshore into a wet Pacific Northwest in advance of a vigorous trough set to slam into the Northwest mid-weekend. This will act to break down the leading western ridge. A transient component of the ridge should temporarily shift east to the central/eastern U.S. The vigorous trough will dig into the Great Basin Mon, with some solutions developing a closed upper low. Models show some differences on the amplitude of this wave across the Great Basin and Tue across the Four Corners region, but overall consensus is above average. The dynamic system is expected to support heavy terrain/mountain snows along and in the deformation zone to the north of the track from the Northwest through the north-central Great Basin/Rockies. Downstream, models also show reasonably good consensus that a cutoff upper low over northern Mexico, a component which separated from the previous system, should quickly lift northeastward across the central U.S. Mon-Tue as it encounters anticyclonic flow and weakens. By Tue/Wed as the large western trough reaches the central U.S., a relatively strong cold front and emerging Gulf moisture/precipitation pattern should sweep eastward across the east-central U.S. Models show some timing and intensity differences with a wave of low pressure that develops along the front and moves from the south-central states Tue to the upper Ohio Valley/Northeast next Wed. This does result in some cold front timing differences. Guidance is trending in favor of WPC continuity that offered slower trailing frontal progression and that stll seems reasonable considering flow amplitude. This system also offers some snow threat on the northern periphery of the precipitation shield over the Great Lakes and vicinity. With the better clustering of model and ensemble forecasts days 3-7, the WPC medium range product suite was primarily derived from a composite blend of the GFS/ECMWF/UKMET and GEFS/ECMWF ensemble means along with the National Blend of Models. ...Weather/Hazard Highlights... Heavy precipitation (rain and mountain snow) is expected across the Pacific Northwest, especially the Olympics and Cascades, Sat into early Sun ahead of the incoming Pacific shortwave/frontal system. Precipitation could linger into Sun-Mon across the Northwest as onshore flow persists in the wake of the front, but precip should not be quite as heavy by that time. Snow levels should fall by the weekend into early next week, however, as the trough moves onshore - thus heavier snow amounts may be possible across the Cascades Sat-Sun even as the overall intensity of moisture inflow decreases. As low pressure moves along/off the Eastern Seaboard Sat-Sun, current indications are that the most widespread/heaviest precipitation should remain offshore. Given continued uncertainty on just how close the system will track to the coastline, however, some precipitation is possible, especially close to the coast from the Mid-Atlantic to the Northeast. The air mass across the Northeast should be minimally sufficient to support some degree of winter weather, so winter weather outlooks for Sat/Sun indicated a region of low probabilities for significant snow accumulations across mush of the Northeast. Temperatures are forecast to be well above average across the central and eventually the eastern U.S. High temperatures of 15 to 30 deg F above average are possible across the Northern/Central Plains Sat-Sun. These warm temperatures are forecast to spread eastward by Mon-Wed into the Mississippi/Ohio Valleys and eventually the East Coast. Meanwhile, the trough/cold front expected to reach the western U.S. this weekend will bring an end to above average temperatures, with highs expected to be 5 to 10 deg below average by Sun-Mon. Colder temperatures should spread east across the Rockies and into the central U.S. by next Tue-Wed. Ryan/Schichtel Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sat-Mon, Feb 1-Feb 3and Tue-Wed, Feb 4-Feb 5. - Heavy precipitation across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Sun-Mon, Feb 2-Feb 3. - Heavy precipitation across portions of the Central Rockies, the Central Plains, and the Central Great Basin, Mon-Tue, Feb 3-Feb 4. - Flooding possible across portions of the Pacific Northwest and the Northern Great Basin. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Pacific Northwest. 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