Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 PM EST Fri Nov 16 2018 Valid 12Z Tue Nov 20 2018 - 12Z Sat Nov 24 2018 ...Overview... Upper pattern will be in a state of flux over the lower 48 next week as eastern troughing lifts out and a couple troughs push into/through the west. This will guide a lead and then trailing front into California on Wednesday and then WA/OR/CA on Friday, respectively. ...Guidance/Uncertainty Assessment... Forecast for Tue-Wed was formed from a multi-model deterministic blend of the recent 12Z/18Z guidance, which maintained good continuity to start. The models/ensembles have really struggled with both the northern stream flow across southern Canada and especially over the Pacific (both mid and high latitudes). Rather than being a question of quicker vs slower with a main trough and secondary wave development, the guidance has now shifted toward a lead system on pace with the quicker solutions from 24 hrs ago (ECMWF-led guidance) followed by another system diving out of the higher latitudes as a kicker. The GFS/GEFS have swung much faster than just 24-36 hrs ago with the lead system in the west and was much quicker in the Northeast/southeastern Canada that it was quite suspect. The 12Z ECMWF/Canadian/UKMET were preferred over much of the CONUS from Wed onward trending toward a majority ECMWF ensemble mean weight by next Saturday. Confidence remains no better than average due to the unstable forecasts. This resulted in changes to the ongoing forecast but with the same result in the west -- a much wetter period and cooler temperatures. Adjusted the downstream flow accordingly, which was overall quicker with the lead front moving through the Plains on Friday. ...Weather Highlights/Threats... Thanksgiving summary (Thu 11/22) - Chilly in the Northeast with many areas staying below freezing. Wet along the west coast into the Great Basin and Rockies with near average temperatures and higher elevation snow. Milder and dry for the High Plains with 40s/50s common. Rain chance for the Gulf Coast from Texas eastward through much of Florida to the north of a front in the Florida Straits. Rather quiet to start dominated by NW flow out of central Canada east of 100W. Pesky stationary boundary and inverted trough in the western Gulf will keep a heavy rainfall threat for especially the coast where instability is highest. That will continue through the week with perhaps some enhancement by the incoming southern stream shortwave on Wednesday. The entire west coast will see an increase in first clouds then rain/elevation snow by Wednesday. Focus appears to be around northern California and southwestern Oregon as the upper trough splits N-S. Multi-day rainfall amounts may be in the 3-6 inch range over favored areas and even about and inch to an inch and a half in the Central Valley. Snow levels will fall through the period with several feet of snow likely at the highest elevations. Below average temperatures in the Great Lakes/Northeast will be slow to ease but will trend back toward typical late November values by next weekend. Record cold values are possible on Tuesday and maybe Wednesday. Temperatures in the west will be above average by about 5-10 degrees but will move east as troughing moves inland. The Plains and Upper Midwest will see milder temperatures Wednesday-Saturday, generally above freezing for afternoon highs. Fracasso WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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