Extended Forecast Discussion NWS Weather Prediction Center College Park MD 340 PM EST Thu Nov 17 2022 Valid 12Z Sun Nov 20 2022 - 12Z Thu Nov 24 2022 ...Overview... The large scale pattern will be fairly amplified with mean troughing near the West Coast and troughing downstream, with a brief transition of flatter mean flow early next week. This transition represents an important change in the source of flow digging into the central/eastern U.S. mean trough, from a cold Canadian origin Sunday-Monday to a more moderate Pacific origin mid-late week as the West Coast ridge rebuilds. This pattern evolution will favor multiple episodes of rain and high elevation snow over the Northwest, along with rainfall over portions of the Gulf Coast states. By mid-late week this southern tier moisture should expand northward ahead of a surface system supported by an amplifying trough reaching the central U.S. ...Guidance and Predictability Assessment... Models/ensembles agree well into day 4 Monday. Features of note include a leading deep upper trough lifting through the Northeast and a broader upstream trough across southern Canada and the Great Lakes, as well as a weak shortwave emerging from the southern Rockies. After Monday the guidance diverges with various aspects of the forecast, most prominently regarding the timing/amplitude of energy flowing into the northwestern U.S./southwestern Canada (affecting precipitation amounts and southward extent in the Northwest) followed by the details of upper ridging that builds near the West Coast along with corresponding trough amplification farther east. Within the amplifying overall trough, a number of ECMWF/CMC runs have been forming a closed low over or near the southern Rockies by early Thursday while latest GFS runs have depicted such a low farther northeast (Central Plains in the 00Z/06Z runs and Northern Plains in the new 12Z run). Not surprisingly the full ensemble spread encompasses this range of possibilities, including a more open trough. Favoring a blend/mean solution leads to an open trough for the time being while guidance sorts out where an embedded upper low may be more likely to form. Issues also develop with the amplitude and progression of the upper trough over eastern Canada and northeastern U.S., influencing the details of a surface front reaching the Canadian border around Tuesday and possibly sinking southward thereafter. Guidance has varied considerably for the details of this front by late in the period. Based on the array of guidance available through the 00Z/06Z cycles, the updated forecast incorporated an operational composite for about the first half of the period. GFS input incorporated both the 00Z/06Z runs as various details in one or the other compared better to consensus. Thereafter the blend transitioned toward greater emphasis on the ensemble means and some continuity with only modest operational model inclusion by day 7 Thursday given the increasing differences in details at the surface and aloft. This approach helped to maintain as stable of a forecast as possible overall, with the most prominent adjustment being to reflect latest consensus that by midweek shows a more pronounced southward push of the front that reaches near the Canadian border early Tuesday. ...Weather/Hazard Highlights... Widespread well below average temperatures for this time of year are likely across the eastern half of the country and southern Texas on Sunday, with highs up to 15-25F below average. Expect the eastern U.S. cold weather to moderate going into the beginning of the work week with readings returning closer to seasonal averages. However, the Dakotas and 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. The Northeast may see a return to somewhat below normal temperatures by next Thursday, while central U.S. temperatures will depend on the exact evolution of the system that may track over/east of the region late in the period. The West Coast states should trend toward above normal temperatures mid-late week as upper ridging builds over the region. In terms of precipitation, lake effect snow is likely to persist downwind of the eastern Great Lakes through Sunday night owing to strong cold air advection over the warmer waters, and then tapering off going into Monday. Meaningful precipitation is likely to make a return to western Washington and Oregon with moist onshore flow expected and one or more frontal systems approaching. However there is still a fair degree of uncertainty over the southward extent and intensity of rain/mountain snow on a day-to-day basis. Showers and some thunderstorms should gradually increase in coverage across much of the southern tier of the U.S. and eventually northward next week as return flow from the Gulf of Mexico interacts with a front and a shortwave aloft. Details of system evolution will determine if/how much snow may fall over northern tier locations around midweek or so. A period of easterly flow may help to enhance rainfall early next week over the Florida Peninsula. Rausch/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 Hazards: - Heavy rain across portions of the central Gulf Coast region, Thu, Nov 24. - Heavy rain along portions of the east coast of Florida, Mon-Tue, Nov 21-Nov 22. - Heavy snow across portions of the Northeast and the lower Great Lakes, Sun, Nov 20. - Flooding possible across portions of east-central Florida. - Flooding occurring or imminent across portions of east-central Florida. - Much below normal temperatures across the eastern portion of the Great Plains, into the Midwest and mid-Mississippi Valley, as well as the Ohio Valley and much of the Tennessee Valley, Sun, Nov 20. 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