Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Nov 23 2022 Valid 12Z Sat Nov 26 2022 - 12Z Wed Nov 30 2022 ...Meaningful precipitation is possible across eastern parts of the U.S. this weekend... ...Snow is likely across higher elevations of the West early this week and may spread into the north-central U.S. by Tuesday-Wednesday... ...Overview... The medium range period will begin Saturday with a closed upper low atop the south-central U.S., while a surface low pressure system should lift quickly from the Lower Mississippi Valley toward the Great Lakes region this weekend. This pattern will produce widespread rain chances across the Southeast, where some localized flooding may be possible, and farther north across the Mississippi Valley into the Ohio/Tennessee Valleys, shifting into the Mid-Atlantic and Northeast later Saturday into Sunday. Some precipitation may fall as snow in higher elevations of the interior Northeast. By early next week, the upper pattern looks to have a brief transition toward zonal flow before troughing digs into the western U.S. Monday-Tuesday, increasing precipitation chances there. As the trough shifts eastward, the current forecast shows a consolidating surface low pressure system may shift into the Midwest for Tuesday-Wednesday, with some snow possible north and west of the low for the Northern Plains into Minnesota, and rain ahead of a trailing cold front in the east-central CONUS. ...Guidance and Predictability Assessment... While model guidance agrees in principle for an upper low centered somewhere near the Ark-La-Tex for day 3/Saturday, there are differences in position that may appear relatively minor but considerably affect the placement of the precipitation and the surface low/front positions. In a flip from the past several days, the 12Z ECMWF and EC ensemble mean were among the slowest solutions of the 12/18Z model cycle. The 12Z CMC was fastest while the GFS runs and UKMET seemed like the best in between position in terms of the 12/18Z guidance. The new 00Z CMC has slowed down slightly closer to consensus compared to its previous run, the 00Z GFS and UKMET both slowed a bit as well, and the 00Z ECMWF may be a tad faster than the 12Z run but still on the slow side. So while guidance may be converging somewhat slower there are still some positional differences. The WPC forecast attempted to take a middle ground approach with a blend of the various deterministic guidance for this feature, which meant blending some EC/EC ensemble mean QPF into the NBM (which is designed more like the GEFS mean, so on the fast side) to slow it down. Meanwhile, there are some model differences regarding flow coming into the West. The non-GFS models indicate an initial shortwave coming into California Saturday-Sunday (though the 12Z ECMWF was less aggressive with it than its previous runs) and then generally more troughing influence reaching the northern High Plains by 12Z Monday compared to the ridgier GFS. But at least guidance is reasonably agreeable with more potent energy digging into the West Monday-Tuesday and shifting into the central U.S. Wednesday. GFS and EC runs indicate disparate energy with a separate trough coming in behind by Wednesday, while the CMC is combining those features more, which will likely take some time to be resolved given the energy will track around typically uncertain areas rounding Arctic/Alaska/northern Pacific ridging. Overall the WPC forecast trended toward more utilization of ensemble means to about half by day 6-7, trying to mitigate the individual model spread. But even with some model differences there is the potential for a low pressure system to strengthen in the central High Plains and track toward the Midwest Tuesday-Wednesday. ...Weather/Hazard Highlights... Gulf moisture is forecast to stream into parts of the central and eastern U.S. ahead of the upper and surface lows and produce widespread rain chances this weekend. Some locally heavy rainfall may be possible in the Southeast and into the Southern Appalachians and Piedmont on Saturday, which could cause isolated flash flooding if rain rates are high enough. Precipitation is likely to spread across the Middle Mississippi Valley and Ohio/Tennessee Valleys on Saturday as well, and into the Mid-Atlantic and Northeast late Sunday into Monday. With this low track, most areas are likely to see rain rather than wintry precipitation, but higher elevations of the Interior Northeast could be an exception where snow could fall. The Pacific Northwest into the Northern Rockies can expect some light to moderate precipitation on Saturday in conjunction with a couple of frontal systems. Precipitation is then forecast to spread southward across California and eastward into the Great Basin and Rockies for early next week as the noted upper trough begins to dig into the region. Lower elevation rain and higher elevation snow will be likely. While the details are uncertain, the highest probabilities for heavy snow should initially be across the Cascades and Sierra Nevada, with some snow in the Wasatch/Rockies as well. Then with the potential for low pressure system development in the central U.S., snow is possible on the backside of the low as it tracks northeastward into the Midwest Tuesday-Wednesday. Lower elevation areas that could see snow would include eastern Montana into the Dakotas and Minnesota. This is at the end of the medium range period though so the details of the forecast may need to change with time, so stay tuned to future forecasts. Slightly cooler than normal highs could linger in the south-central U.S. this weekend behind the initial upper low, while milder than normal temperatures particularly in terms of lows are possible in the East. But more widespread cooler than normal temperatures are likely to expand across the western half of the U.S. Monday-Wednesday as deepening troughing takes hold there. Anomalies of 10-20F below average may focus across the northern High Plains. Meanwhile, the south-central and east-central U.S. could see above normal temperatures ahead of the western trough. Tate 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