Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EST Tue Nov 22 2022 Valid 12Z Fri Nov 25 2022 - 12Z Tue Nov 29 2022 ...Meaningful precipitation is possible across the southeastern U.S. late this week and potentially into the Northeast early next week... ...Overview... Model guidance has remained in better agreement with the overall pattern across the country late this week into early next week. The WPC forecast indicates that a closed upper low will track across the south-central U.S. along with a developing surface low pressure system for Friday-Saturday. This should lead to widespread rain chances across the southern half of the Plains and Mississippi Valley and farther east into the Southeast, Ohio and Tennessee Valleys, and the East Coast for late week into the weekend. There is some chance of snow on the backside of the closed low in the southern High Plains on Friday. By early next week, the upper pattern is likely to transition to more zonal flow, but with troughing and fronts coming into the Northwest promoting increasing precipitation chances across the West Coast into the Great Basin and Rockies Sunday-Tuesday. ...Guidance and Predictability Assessment... The model guidance has remained in relatively good agreement overnight in terms of the initial overall upper-level pattern given the noted uncertainty the past few days. This is specifically true for the trend of the ECMWF to follow the other deterministic guidance in the development and later progression of a closed upper-level low over the Southern Plains. As a comparison, the GFS has remained consistent in this solution over multiple runs the past day while the ECMWF has further converged towards this solution as of the 00Z run in both timing and in depicting an increasingly deeper low. However, there are a number of more detailed specifics in the phasing and amplitude of waves later into the period that still leads to areas of more significant uncertainty. Given the similar trend across the deterministic guidance for the closed low over the Southern Plains, the WPC forecast initially starts with an even blend of the 00Z ECMWF/UKMET/CMC and 06Z GFS. The guidance starts to diverge in the handling of a developing shortwave upstream over the western U.S. While the timing and amplitude differ some, the 00Z ECMWF/UKMET/CMC all depict a developing wave over the Northwest while the 00Z/06Z runs of the GFS were relative outliers in comparison with only a modest, subtle wave depicted. Thus, to maintain the progression of the low to the east while including a slightly more amplified shortwave in the west, the 00Z ECens mean was progressively increased in the WPC forecast blend as the 06Z GFS was decreased given the relatively similar evolution of the eastern low in the other deterministic guidance. Incorporating more of the 00Z ECens mean also helped to maintain a lower amplitude shortwave given the lingering uncertainty in this solution in both the progression and energy of the wave and subsequent down-scaled effects on the weather pattern. Later in the period, the focus turns towards the West again as a highly amplified, positively tilted upper-level trough develops upstream off the West Coast before digging into the Southwest. Both the 06Z GFS and 00Z CMC are more amplified than the 00Z ECMWF, while the 00Z GEFS and ECens means are also expectedly less amplified compared to the deterministic guidance. The WPC forecast blend in the late period included increased reliance on the means given greater uncertainty but still relied on a contribution of the deterministic guidance to provide a bit more amplification of the trough over the Southwest given how far south the trough digs in the 06Z GFS and 00Z CMC. Use of the ensemble means helps to address an additional lingering area of uncertainty with another subsequent upstream wave developing late in the period. The means include the potential of a shortwave over the Pacific off British Columbia but again allow for a less amplified pattern until more specifics on the timing and progression of these waves can be resolved. The precipitation forecast included an increase in rain amounts from the Southeast up the East Coast in the warm sector of the eastern U.S. system compared to the previous forecast as both the NBM and deterministic guidance had trended upward across this region. In comparison, precipitation amounts came down a bit across the Midwest and Ohio Valley. A general model blend kept things relatively similar during the middle of the period between the eastern U.S. system and increase in precipitation spreading across the West. Late in the period, the trend in the guidance was for higher precipitation amounts in the Pacific Northwest. ...Weather/Hazard Highlights... Ahead of the closed upper low in the Southern Plains and a couple of low pressure/frontal systems, Gulf moisture is likely to stream into central and eastern parts of the lower 48. Rain and perhaps some thunderstorms are expected from Texas/Oklahoma east across the Lower Mississippi Valley and Southeast on Friday. Locally heavy rainfall causing isolated flash flooding is possible. Beneath the upper-level low and along the backside of the subsequent surface low track, there has been an increasing trend in the potential for at least a few inches of snow from eastern New Mexico through the Texas Panhandle and into western Oklahoma. The low is forecast to track northeastward and spread precipitation up the East Coast as well as across the Middle Mississippi Valley and Ohio/Tennessee Valleys into the weekend. Locally heavy rainfall is again possible on Saturday for portions of the southern Appalachians and Piedmont. 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. There could also be some lake effect rain/snow showers early next week behind the upper trough. The Pacific Northwest into the Northern Rockies can expect some light to moderate precipitation late this week in conjunction with a couple of frontal systems. Precipitation is forecast to spread southward across California and 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. Some heavier snow amounts may be possible for the Cascades and the Sierra Nevada. The upper low now forecast atop the south-central U.S. late week will cause some cooler than normal temperatures there, particularly in terms of highs that could be 10-20F below normal mainly in Texas on Friday, which will help contribute to the possibility of snow over the region as the guidance has trended colder. Meanwhile, lows are likely to be warmer than normal for the central/eastern Gulf Coast states and northward into the Tennessee/Ohio Valleys Friday given the Gulf inflow ahead of the upper low, shifting into the East by Sunday. Slightly milder than normal high temperatures for the north-central U.S. Friday-Saturday should moderate to near normal by Sunday. As upper troughing influences the West early next week, expanding coverage of below average temperatures is likely there, while the south-central U.S. flips to slightly above normal ahead of the 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 Hazards: - Heavy rain across portions of the Southeast, the Mid-Atlantic, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Fri-Sat, Nov 25-Nov 26. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Mon, Nov 28. - Heavy snow across portions of the Southern Rockies and the Southern Plains, Fri, Nov 25. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast. 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