Extended Forecast Discussion NWS Weather Prediction Center College Park MD 331 PM EST Mon Nov 21 2022 Valid 12Z Thu Nov 24 2022 - 12Z Mon Nov 28 2022 ...A couple of low pressure systems including a possible nor'easter could cause meaningful precipitation across central/eastern parts of the U.S. late this week... ...Overview... The start of the extended period will have an amplified upper pattern across contiguous states with a ridge atop the West and a deepening trough across central parts of the country. Although there is a significant amount of model spread with how this trough develops and how fast it tracks to the East, it will have the potential to spread modest to possibly heavy precipitation with it from the Southern Plains possibly to the North East. WPC has been favoring a surface low pressure system moving east across the Great Lakes or nearby with a cold front passing across the central-eastern U.S. and phasing with another low tracking northward near the Atlantic coast, leading to a potentially strong storm affecting New England during the weekend. However, there is the possibility for a slower/southern area of significant rainfall and less precipitation/surface low development over the Northeast. By early next week, the upper pattern is likely to transition to more zonal flow, but with eventual troughing and fronts coming into the Northwest promoting increasing precipitation chances there. ...Guidance and Predictability Assessment... The latest guidance continue to show an upper low develop and close off early in the extend period and will track east/northeast until the end of the forecast early next week. There is a very significant degree of spread with this low feature which yields a low confidence forecast in terms of the details/specifics with each feature. The ECWMF is the most progressive of the global. It closes off the upper low over the central U.S. and keep tracking northeast across the Great Lakes and the Northeast. This places the QPF footprint over the eastern Gulf states and mainly east of the Appalachians, along with the potential for a coast system. The CMC is the first to close off the upper low and favor a slower/deeper progression across the Southern Rockies and into the Southern Plains. The GFS is about 6 hours behind the CMC but favors a similar trend to the CMC. The UKMET depicts a closed off system but does so over the Southern/Central Plains and is a little more progressive than the CMC/GFS solutions. This degree of spread either has little to nor precipitation over Texas/western Gulf with all of the QPF spanning from the eastern Gulf states/Southeast and up the East Coast per the ECWMF. Or, scattered to widespread precipitation over portions of the Southern/Central Plains, Gulf Coast and into the Mississippi/Tennessee Valleys per the CMC/GFS and somewhat per the UKMET. Both types of scenarios can and do happen downstream of ridging so there is low confidence in which to choose. At the surface, there has been some waffling with the placement of lows as well of course, initially with a Great Lakes or southern Canada low that overall seemed to bounce back south. The previous forecast leaned toward the ECMWF-led phased trough solution yielding a possibly deep surface low in the Northeast. However the 00Z run seems to be more of the outlier solution given the the GFS is favoring a similar track as the CMC and the UKMET is closer to those two within the cluster than it is to the ECWMF. The 12Z run of the ECWMF has started to trend toward having a closed low in the vicinity of West Texas and not as progressive of an evolution. So, all in all, this package does break from WPC continuity by utilizing more of the GFS and CMC during the early and middle periods. The model blend was based on a blend of the 00Z ECWMF/CMC,UKMET, 00Z/06Z GFS and the GEFS and EC ensemble means. For QPF, blended in a considerable amount of the CMC, GFS with lighter weighting of ECMWF. After that by early next week, the overall pattern is likely to flatten but with some embedded shortwaves, and more longwave ridging should come into the Northwest around day 7/Monday. ...Weather/Hazard Highlights... As the upper trough amplifies east of the Rockies by Thanksgiving Day and beyond, Gulf moisture should stream into the eastern half of the U.S. ahead of the trough. The southeastern quadrant of the U.S. northward into the Ohio Valley and Great Lakes may see some precipitation around Thanksgiving, while heavier precipitation looks more likely across the Lower Mississippi Valley to the Mid-Atlantic on Friday. Current forecasts continue to indicate that wintry precipitation is a better possibility for the Interior Northeast while the metropolitan areas along the I-95 corridor are more likely to get rain. But stay tuned to future forecasts as these uncertain details like precipitation types in particular areas could change as model guidance eventually comes into better consensus with low tracks and temperatures. Depending on the strength of one or more low pressure centers, winds could become brisk to strong for a time. Also note that there is still potential for significantly different precipitation coverage and amounts, which would involve more rainfall across the southern tier from Texas across the Lower Mississippi Valley and Southeast, and lower totals (with weaker/more offshore low pressure) farther north. Then by next weekend, lake effect snow is once again a possibility behind the upper trough. The Northwest may see a brief period of dry weather but moisture could return into western Washington as soon as Thanksgiving Day, with a series of frontal systems spreading rain and high elevation snow across more of the Northwest and eventually northern California through the weekend, with expanding coverage into the Great Basin/Rockies early next week. Temperatures are likely to trend above average along the West Coast by Thursday with plus 5-15F anomalies expanding into the Rockies and central U.S. later in the week as the upper ridge builds in. Western U.S. temperatures should trend back toward normal and perhaps below average by early next week given the latest forecast of upper flow becoming more zonal and eventually switching to a trough pattern. Farther east, post-frontal cooler temperatures especially in terms of highs should extend from the Mississippi Valley toward the East Coast Friday-Saturday as the upper trough is likely to move through. Anomalies will depend upon the details of the eastern U.S. upper trough and potential East Coast system, which are still quite uncertain. Campbell/Tate/Rausch 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 precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sat-Sun, Nov 26-Nov 27. - Heavy precipitation across portions of the Central Great Basin, California, and the Southwest, Mon, Nov 28. - Heavy rain across portions of the Pacific Northwest, Sat-Sun, Nov 26-Nov 27. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Thu-Fri, Nov 24-Nov 25. - Heavy snow across portions of the Southern Plains, Thu-Fri, Nov 24-Nov 25. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast. - Heavy precipitation across portions of the Alaska Panhandle, Thu-Fri, Nov 24-Nov 25. 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