Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM 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 medium range period will begin Thursday with likely an amplified upper pattern across the lower 48, with a ridge atop the West and a deepening trough across central parts of the country. By late week there continues to be a significant amount of model spread with what becomes of this trough, with solutions ranging from a low cutting off in the Southern Plains to a more phased large-scale trough progressing into the East. The WPC forecast has been favoring the latter solution for the past few days, which at the surface yields a 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. But the persistence of the minority scenario reduces confidence and there is still a 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 confidence in the forecast for days 3-5 (Thursday-Saturday) is much lower than normal because of the high degree of model spread. Often differences this large in the extended range are somewhat resolved by the early part of the medium range period. The type of solution that WPC has been leaning toward, a more phased trough with perhaps an upper low inside, has been most persistently forecast by the ECMWF runs with notable support by EC and CMC ensemble members in particular. CMC deterministic runs were also in this camp--until the newer 00Z run. Meanwhile GFS runs have varied quite a bit from run to run, but generally toward an upper low closing off in the southern stream and lingering around the Southern Plains. More specifically, the 12Z GFS seemed to be more phased and in better alignment with the EC/CMC while the 18Z and the 00Z runs indicated a slow closed low again. UKMET runs have also been closer to this closed low solution, while GEFS members show some significant spread with some closer to the operational GFS and some with a more EC/CMC phased type solution. 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 in the 12Z guidance but seems to have shifted to a northern track with the newer 00Z guidance--which could lead to less interaction with a surface low farther south. The WPC forecast for all this leaned toward the ECMWF-led phased trough solution yielding a possibly deep surface low in the Northeast. This cluster was closer to continuity and this was a situation where the model guidance becomes so different that attempting to blend models for an in between solution was not really possible. The surface low track trended slower than the previous forecast. This seems like it might be a good thing considering the new 00Z models came in slower, including the ECMWF, though not as slow with the closed low as the other guidance. Perhaps model guidance will go toward this middle ground solution. The model blend was based on a blend of the 12Z ECMWF, 12Z CMC, 12Z GFS (which was more progressive/phased than the other GFS runs), and the GEFS and EC ensemble means. For QPF, blended in a considerable amount of ECMWF and EC mean since the 01Z NBM favors the GEFS/GFS type solutions and the fronts/pressures blend leaned away from that. 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. So the WPC forecast was based on a blend of the deterministic models and means given average agreement. ...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 northeastern U.S. 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. 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 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