Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EST Tue Dec 17 2019 Valid 12Z Fri Dec 20 2019 - 12Z Tue Dec 24 2019 ...Atmospheric River to bring heavy precipitation to the Northwest later this week... ...Parts of the Southeast may see locally heavy rain this weekend... 19Z Update: Models and ensembles are still not in good agreement on the speed and expected evolution of the southern stream system over the Gulf of Mexico this weekend and into next Monday. Both the 00Z and 6Z runs of the GFS were notably faster with the low across the Gulf Coast region compared to the remaining guidance and the GEFS mean. The 12Z GFS has trended slower and a little closer to the model consensus. The UKMET is on the slower side of the model guidance and not favored by the ensemble means. The 12Z ECMWF is now closer to the consensus compared to its 00Z run, which was also farther west. The WPC forecast was based on a blend of the 00Z ECMWF, 00Z CMC, GEFS/EC means, and some previous WPC continuity. The previous forecast discussion is appended below. /Hamrick ...Overview... Upper troughing out of the Gulf of Alaska will slowly ease southeastward into the West Coast by next week, with a several day stretch of modest to locally heavy rain/snow from northern California northward. At about the same time, a somewhat unusual southern stream system is likely to dip into the Gulf of Mexico and cross through Florida with its own locally heavy rain around its circulation. Much of the rest of the CONUS will be dry from the southern Rockies northeastward to the Great Lakes under upper ridging. ...Guidance/Predictability Assessment... Consensus of the recent models/ensembles off the West Coast sufficed as a starting point with better than average predictability despite some lingering timing differences. The 12Z ECMWF was most preferred, followed by the GFS. The 12Z UKMET/Canadian were displaced from the best clustering with other timing issues downstream. As such, for the system into the Gulf, the 12Z Canadian (and 18Z GFS) was likely too quick while the 12Z UKMET was likely too slow with the overall progression. The 12Z ECMWF/GFS were closer to the ensemble means which has been somewhat stable in principle over the last several days (i.e., keeping the system separate from the northern stream). Still would not trust either model verbatim given the unusual track/evolution, but each run offers insight to what could transpire with respect to heavy rain and possibly higher elevation snow in the southern Appalachians. ...Weather Highlights/Threats... Moisture plume off the Pacific should sink southward through coastal OR/CA after Friday as the front moves onshore Saturday. Multiple inches of precipitation are expected over a 2-3 day period with a likely break in between for a given location. Daytime temperatures will be near to above average overall with a trend toward cooler temperatures as the front moves inland this weekend. East of the Rockies, temperatures will be above average with overall dry conditions and upper ridging overhead--some locations 15 to 20 deg F above average. The Northeast will see a moderation after a chilly Friday. Coastal/Gulf system will provide at least modest rainfall for parts of the Southeast, depending on its evolution, with possible locally heavy amounts. Some snow will be possible on its northern fringe in the marginally colder air of the southern Appalachians if the precipitation shield extends that far north. Fracasso 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 California, the Pacific Northwest, and the Northern Great Basin, Fri-Sun, Dec 20-Dec 22. - Heavy rain across portions of the Southeast, Sat-Mon, Dec 21-Dec 23. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Fri, Dec 20. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Fri-Sat, Dec 20-Dec 21. - Flooding possible across portions of the Central Appalachians, the Pacific Northwest, and the Ohio Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Much below normal temperatures across portions of the Lower Great Lakes, the Mid-Atlantic, the Northeast, and the Northern Appalachians, Fri, Dec 20. - High significant wave heights for coastal portions of California and the Pacific Northwest, Fri-Sat, Dec 20-Dec 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