Extended Forecast Discussion NWS Weather Prediction Center College Park MD 324 PM EST Mon Dec 16 2019 Valid 12Z Thu Dec 19 2019 - 12Z Mon Dec 23 2019 ...Atmospheric River to Fuel Heavy Precipitation for the Northwest Thursday-Saturday... ...Overview... Closed Gulf of Alaska upper troughing will shift eastward as undercutting waves send protracted periods of heavy rain/mountain snows from the Pacific Northwest to north-central California through the weekend as fueled by an atmospheric river and potent jet. Snow levels will gradually rise, but also work inland to favored terrain of the Northern Great Basin/Rockies. A northern stream will be relatively low amplitude with little precipitation east of the Rockies. Underneath, a somewhat unusual southern stream system may dip into the Gulf of Mexico and Florida this weekend along with an unsettled weather and rainfall risk Sunday/Monday for the Southeast and maritime interests along with modest lead snow/ice potential into the s-central Mid-Atlantic. ...Guidance/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of well clustered guidance from the GFS/ECMWF and GEFS/ECMWF ensemble means days 3-5 (Thu-Sat). While the upcoming heavy precipitation event for the Northwest offers good predictability, forecast confidence is only about average east of the Rockies given uncertainty suggested by the 00/12 UTC UKMET runs that in contrast offer less northern stream amplitude and interaction with a more separated/less progressive southern stream flow. In the southern stream Days 6/7, the 12 UTC GFS is now in much better agreement with recent 00/12 UTC ECMWF and ensemble runs, bolstering confidence with respect to potential FL low development and wrapped precipitation in the Southeast. The 00/06 UTC GFS runs were outlier low amplitude and progressive solutions that did not match WPC continuity. ...Weather Highlights/Threats... Lead system on Thursday should weaken over BC with another right behind it, focused into WA and northern OR with that round of precipitation. As trailing upper troughing sharpens, moisture plume should lift northward Friday as the cold front approaches before sinking southward through coastal OR/CA 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 cooler as the front moves inland this weekend. East of the Rockies, temperatures will be above average with overall dry conditions and upper ridging overhead. Quick cold shot will skirt over the Northeast Thursday (10s/20s for highs and -0s/0s/10s for lows) with a moderation thereafter. Coastal/Gulf system will provide some rainfall for the Southeast, depending on its evolution, with some snow/ice possible on its northern fringe in the marginally colder south-central Mid-Atlantic air at generally higher elevations. Fracasso/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 California, the Pacific Northwest, and the Northern Great Basin, Thu-Sun, Dec 19-Dec 22. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Thu-Fri, Dec 19-Dec 20. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Thu-Sat, Dec 19-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 Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Thu-Fri, Dec 19-Dec 20. - High significant wave heights for coastal portions of California and the Pacific Northwest, Thu-Fri, Dec 19-Dec 20. 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