Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Sat Nov 17 2018 Valid 12Z Tue Nov 20 2018 - 12Z Sat Nov 24 2018 ...Frigid temperatures expected in time for Thanksgiving for much of the Northeast... ...Overview... The upper-level pattern across North America will be in flux during the extended period, with evolution toward a more chaotic pattern (although not necessarily significantly less amplified). Weakening of the ridge across western North America will open the CONUS up to flow off the Pacific, with a rapid succession of Pacific shortwaves expected to arrive by the middle to end of next week. ...Guidance/Uncertainty Assessment... Consensus has improved during the day 3-5 (Tue-Thu) time period with respect to the flow evolution along the West Coast and strong height falls arriving by mid-week, with the ECMWF slowing a bit and the GFS speeding up slightly. A leading upper low moving onshore in the Southwest early Tue will quickly move east across the southern tier. Confidence is now better that height falls will move onshore along the West Coast late Wed, pushing a surface front into the Great Basin by Thu morning. Upper ridging will quickly erode/shift eastward as this occurs. Farther east, models have shown a nearly universal trend toward greater amplification of a system near the Canadian Maritimes Wed-Thu, with most solutions now showing a closed upper low developing. The ECMWF was the first model to pick up on this, with the GFS coming on board as of the 06Z run. This degree of amplification seems to make sense given blocking flow across the north central Atlantic evident on D+8 anomalies from both the ECENS and GEFS. The implication of this for the CONUS will be even stronger cold advection across the Mid-Atlantic/Northeast just in time for Thanksgiving. The WPC forecast during days 3-5 was based on a blend of the latest ECMWF/GFS/FV3 solutions. During days 6-7 (Fri-Sat), as described above, the flow begins to become more chaotic as smaller scale shortwaves originating over the Pacific play a larger role given the breakdown of the western ridge. Consensus is relatively good that the Pacific trough will reach the central U.S. Thu night Fri, although solutions continue to differ as to how amplified this feature will be. At this point a middle-of-the road approach was preferred here. Another shortwave looks likely to reach the Pacific Northwest on Thu with another on Fri. The GFS brings yet another by Sat but this solutions seems fast in context of the range of model solutions so it is not preferred at this time. Farther east, the leading shortwave which should reach the West Coast on day 3 will be in the vicinity of the Southeast/Gulf Coast by Fri. Models differ on the amplitude of this feature and whether it will trigger development of a wave of low pressure along a surface front in the Gulf of Mexico, with perhaps significant implications in terms of precipitation for portions of South Florida. The ECMWF was most aggressive with developing this wave while the GFS was much less enthusiastic. ECENS members show a fair degree of support for some degree of wave development. Weight in the forecast was shifted toward ECENS and GEFS means during days 6-7, with a bit more weight toward the ECENS mean given somewhat more consistency shown over the past few runs. ...Weather Highlights/Threats... Thanksgiving summary (Thu 11/22) - Cold in the Northeast with high temperatures 20 to near 30 deg F below average, and many areas staying well 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. Ryan/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