Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 PM 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 next week and may spread into the north-central U.S. by Tuesday-Wednesday... ...The chance for widespread, potentially heavy rainfall increasing by the middle of next week... ...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 an increasing potential for heavy rainfall ahead of a trailing cold front in the east-central CONUS. ...Guidance and Predictability Assessment... The focus at the start of the forecast period is on the placement and progression of the upper-level closed low over the Southern Plains forecast to lift northeastward across the Midwest over the weekend. In general, the deterministic guidance is relatively similar in terms of the current placement and trend for the closed low to move a bit slower than prior forecasts, resulting in a slower progression of the frontal system at the surface and subsequent downstream effects on precipitation coverage. One notable outlier was the 00 GEFS members that tended to cluster around a more progressive solution. There was more uncertainty with the placement of an upstream shortwave over the western U.S., though the more recent runs of the ECMWF have trended towards a somewhat similar solution to the 06Z GFS and 00Z UKMET/CMC. The phasing slightly differs between the guidance but the forecasts at least tend to agree on a lower amplitude, positively tilted wave over the West. Thus, the initial WPC forecast consisted of a straight blend of the deterministic guidance along with the 00Z ECens mean to smooth out any timing differences and come to a general consensus on the placement of the closed low. The uncertainty increases with the downstream evolution of the closed low, with the run-to-run consistency of the deterministic guidance in disagreement with the progression and lingering intensity of the low as it lifts into the Midwest as an open wave as well as the subsequent upstream shortwave progressing eastward. The ECMWF and GFS have generally trended towards a more progressive solution, which also matches the 00Z UKMET. The 00Z ECens mean is also closer to this solution while the 00Z GEFS mean is even more progressive with the lifting wave. The 00Z CMC differs the most with split northern and southern stream energy, so the forecast blend trended this down with the other guidance up. Attention then turns to the West with the next upstream trough starting to amplify over the Northwest. The 00Z ECMWF and 06Z GFS were relatively similar in phasing and amplitude while the 00Z CMC/UKMET were less amplified. The trend in the WPC forecast blend was to decrease the influence of the 00Z CMC/UKMET with more weight given to the 00Z ECens while introducing 00Z GEFS mean as it was more in line with the other guidance compared to the handling of the closed low earlier in the period. The forecast grows more complex during the latter portion of the period with a bit of a split in the energy between the northern and southern stream over the West. The 06Z GFS trended away from this pattern while the 00Z ECMWF trended towards it, complicating the forecast. Furthermore, there was disagreement in the guidance for the timing and amplitude of the next wave upstream over the Pacific and moving towards the Pacific northwest. There was significant run-to-run uncertainty in both the 00Z ECMWF and 06Z GFS in the handling and progression of these features as the first begins to eject out over the Plains and the second reaches the Pacific Northwest. The individual ensemble members in the 00Z GEFS and ECens showed a good deal of uncertainty in the timing and amplitude of the waves as well. The 00Z CMC was in even less agreement with the overall pattern, developing multiple shortwaves rounding broader larger-scale troughing. Therefore, the latter portion of the forecast included a blend initially edged slightly towards the 00Z ECMWF/06Z GFS deterministic guidance with an increase in the means over the deterministic guidance for the end of the period. This helped to better represent the first trough in the West initially without getting too detailed in a more complex solution given the uncertainty and forecast lead time, with the goal of capturing the general phasing pattern of the waves overall. ...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 and into the Mid-Atlantic and Northeast by Sunday. The trend has been for rainfall to progress slower and be a bit heavier, lingering into Sunday for the Ohio/Tennessee Valleys and Southeast. 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 chance for heavy snow has been increasing across the Cascades and into the Wasatch and Rockies. Heavy snow is also possible in the Sierras, although this has trended downward compared to previous forecasts. The potential for low pressure system development in the central U.S. by mid-week next week is increasing which would bring precipitation chances to portions of the central and eastern U.S. Widespread, potentially heavy rainfall looks possible across the Midwest, Ohio/Tennessee Valleys, and into the Lower Mississippi Valley. The potential for snowfall is also increasing on the backside of the low as it tracks northeastward into the Midwest Tuesday-Wednesday from 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. Putnam/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