Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Jan 29 2020 Valid 12Z Sat Feb 01 2020 - 12Z Wed Feb 05 2020 ...Wet Pattern for the Northwest... ...Overview and Guidance/Predictability Assessment... A relatively amplified upper-level pattern looks to persist across the CONUS during the extended forecast period. An upper ridge initially across the western U.S. on Sat (day 3) will promote broad cyclonic flow downstream across the central and eastern U.S. A phasing of northern/southern stream features should be ongoing on Sat with an amplified upper trough crossing the eastern third of the nation Sat into Sun, and an area of low pressure expected to deepen as it tracks northeastward off the Eastern Seaboard. While model spread has reduced over recent days. Differences in the timing during the phasing, and differences in the intensity of the phased system, continue to result in different low tracks along the East Coast - some closer to the coastline and others well offshore. Runs 12Z/18Z runs of the GFS on Tue shifted a bit, with a track farther off the New England coast than the model had shown previously, while the 00Z GFS shifted westward again. Meanwhile, the 12Z ECMWF shifted closer to the coast (west of the 12Z/18Z GFS by Sat night/Sun morning). In contrast to recent days, where the ECMWF was more suppressed, and the GFS was in the center of the ensemble spread - tonight the ECMWF appeared to be much more well-centered within the spread of ensemble members. Thus, the 12Z ECMWF was the preferred solution for the eastern U.S. low pressure system days 3-4. Farther west, models continue to show good agreement on a vigorous positively-tilted shortwave approaching the Northwest Sat night/Sun. This looks sufficient to break down the western ridge, while a somewhat transient/progressive component of the ridge temporarily shifts east to the central/eastern U.S. By Sun night/Mon the shortwave looks likely to dig into the Great Basin, 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. Ahead of the wave, 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 Wed as the large western trough reaches the central U.S., a relatively strong cold front should sweep eastward across the 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 Central Plains on Tue to the Ohio Valley or Great Lakes on Wed, which results in turn results in timing differences for the cold front. Favored the somewhat slower timing shown by the 12Z ECMWF (the 00Z GFS came around to this idea after previously showing a faster frontal wave Tue-Wed). Based on these considerations, the forecast was initially based heavily on the 12Z ECMWF during days 3-4 (Sat-Sun). By days 5-7 (Mon-Wed), a blend of the 12Z ECMWF/18Z GFS (more weight on the ECMWF relative to the GFS) along with increasing ECENS/NAEFS ensemble means was used. ...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 in any precipitation that falls, so winter weather outlooks for Sat/Sun indicated a region of low probabilities for significant snow accumulations across mush of the Northeast - this low probability is primarily accounting for the model spread on the track of the system. 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 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 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