Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 PM EST Tue Jan 28 2020 Valid 12Z Fri Jan 31 2020 - 12Z Tue Feb 04 2020 ...Wet Pattern for the Northwest... ...Overview and Guidance/Predictability Assessment... Pacific impulses will feed into the Pacific Northwest into the weekend to combine with orographic lift and long fetch deep moisture to offer a heavy precipitation threat. Activity will spread across the n-central Intermountain West/Rockies as energies round a mid-upper level ridge set to build in advance of dynamic mid-upper level trough passage through the region Sun/Mon. This will meanwhile favor cyclonic flow downstream over the central and eastern U.S. that periodically undergos amplification as flow digs to the lee of the ridge. Lead shortwave impulses over the East are expected to favor coastal low developments/rains over the Gulf of Mexico/Southeast Fri to up an Eastern Seaboard frontal zone over the weekend whose enhancing precipitation offers some risk of wintry overrunning receding high pressure/cold air damming from the marginally cold Appalachians/Mid-Atlantic to the colder Northeast. Most guidance offers a modest threat, with deeper coastal storm formation delegated from off New England into the Canadian Maritimes with increasing negative tilt of the supporting upper trough. Flow remains quite uncertain upstream into next week with guidance showing poor run to run consistency. This is especially evident with energy split/focus and surface system reflections over the South that show more modest lead return moisture flow and precipitation in most scenarios in the wake of the lead systems. Given forecast spread and continuity issues through medium range time scales the WPC product suite was primarily derived from a composite blend of more compatable guidance from the latest GEFS/NAEFS/ECMWF ensemble means and the National Blend of Models. ...Weather/Hazard Highlights... Heavy precipitation (rain and mountain snow) is expected across the Pacific Northwest, especially the Olympics and Washington Cascades, Fri-Sat 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 develops in the Gulf of Mexico Fri into Sat, and then moves across the Southeast, a good overrunning setup should aid in the development of a shield of widespread rain from the Gulf Coast to the Southeast and Mid-Atlantic. Models do not show an abundance of cold air in place at that time to support much winter weather, but some possibility of winter weather is indicated by low probabilities on the winter weather outlook Fri for portions of the southern Appalachians and interior Mid-Atlantic in a region of just cold enough low level dammed air. As the low strengthens and moves northeastward, uncertainty in the exact track lends to uncertainty in both precipitation amount and type along the Eastern Seaboard from the Mid-Atlantic to New England. Continued to indicate low to moderate probabilities on the winter weather outlook for Sat/Sat night from the central Appalachians and portions of the Mid-Atlantic region to the Northeast, continuing into Sun across portions of northern New England. 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, Fri-Mon, Jan 31-Feb 3. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Ohio Valley, and the Northern Plains. - Heavy precipitation across portions of the Alaska Panhandle, mainland Alaska, and the Aleutians, Fri-Mon, Jan 31-Feb 3. - High winds across portions of mainland Alaska and the Aleutians, Sat-Sun, Feb 1-Feb 2. - Much below normal temperatures across portions of mainland Alaska, Fri-Mon, Jan 31-Feb 3. 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