Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Thu Nov 17 2022 Valid 12Z Sun Nov 20 2022 - 12Z Thu Nov 24 2022 ...Synoptic Overview... A highly amplified upper level flow pattern will be in place across the country for the weekend with a deep trough extending from Hudson Bay to the Northeast. The deep trough is forecast to begin lifting out by Monday as a more zonal flow pattern commences across much of the country. Out West, a couple of Pacific storm systems are expected to move inland as the upper ridge axis decays by Sunday and into Monday, and showers increase in coverage across the southern-tier states by midweek ahead of a developing storm system over the Plains. ...Guidance and Predictability Assessment... The 00Z deterministic guidance continues to have an excellent synoptic scale depiction of the broad upper trough over the eastern U.S. that will lift out starting Monday and thus heralding a pattern change, with more quasi-zonal flow across the nation for early next week. However, models differences become significant going into Wednesday and beyond with the trough developing across the Plains. The 12Z ECMWF was a much more progressive solution with the trough/upper low compared to the GFS/CMC, but the latest 00Z run has shifted west and closer to its 00Z run yesterday. Teleconnections with the positive height anomaly over northern California supported a trough position between the slower CMC/GFS and the slower 00Z ECMWF run from yesterday, and not as fast as the 12Z run. The WPC forecast was primarily based on a multi-deterministic model blend, including the 00Z and 12Z runs of the ECMWF, through Monday, with a gradual increase in use of the EC and GEFS means going into the middle of next week. For the QPF forecast, particularly in the vicinity of the Great Lakes, the NBM 4.0 was blended with some of the ECMWF and GFS to increase overall totals on Sunday. ...Weather/Hazard Highlights... Widespread well below average temperatures for this time of year are expected across the eastern half of the country on Sunday and also across much of southern Texas, where highs could be on the order of 15-25 degrees below average in some cases. The eastern U.S. cold weather is forecast to moderate going into the beginning of the work week with readings returning closer to seasonal averages. However, the Dakotas and northern Minnesota will likely remain slightly below average for much of next week, but not as cold as what is expected during the short range period. In terms of precipitation, lake effect snow is likely to be persistent downwind of the eastern Great Lakes through Sunday night owing to strong cold air advection over the warmer waters, and then abating going into Monday. Heavy precipitation is likely to make a return to western Washington and Oregon with moist onshore flow expected, and the expected rain/snow has trended higher since yesterday. Showers and some thunderstorms are likely to gradually increase in coverage across much of the south-central U.S. going into Wednesday and Thanksgiving Day as return flow from the Gulf of Mexico interacts with a front and a shortwave disturbance aloft. Hamrick Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml