Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EST Sun Nov 20 2022 Valid 12Z Wed Nov 23 2022 - 12Z Sun Nov 27 2022 ...A couple of low pressure systems including a possible nor'easter could cause meaningful precipitation across eastern parts of the U.S. late this week... ...Overview... A quasi-zonal upper pattern with embedded shortwaves expected to be in place across the lower 48 as the medium range period starts early Wednesday should quickly amplify thereafter. This transition will involve an eastern Pacific trough helping to build an upper ridge into the West and digging of downstream energy into a central/eastern U.S. trough by the latter half of the week. However there has been a disconcerting level of guidance spread and run-to-run variability for the ultimate evolution of initial Pacific trough energy, how quickly the western U.S. ridge erodes, and what becomes of the amplifying central/eastern U.S. trough. A fair majority of model guidance continues to advertise a large-scale upper trough tracking east/northeast through the weekend, supporting a central-eastern U.S. frontal system (anchored by low pressure that could be anywhere between Canada and the Great Lakes/Ohio Valley) and a separate East Coast system lifting northward. This pattern would lead to increasing chances for precipitation late this week for the East and a potentially strong storm affecting New England during the weekend. On the other hand, there is a persistent minority scenario that would have an upper low close off over northern Mexico or the Southern Plains, leading to a slower/southern area of significant rainfall and less precipitation/surface development over the Northeast. Meanwhile the differences/adjustments with Pacific trough energy have been leading to significant uncertainty over the coverage, intensity, and duration for Pacific Northwest precipitation as well. ...Guidance and Predictability Assessment... If anything, latest guidance has added more uncertainty to the forecast. There has been a noticeable trend toward more clustering for some degree of separation for initial Pacific trough energy, with the northern stream portion of this flow ultimately serving to flatten the western ridge more quickly by the weekend versus prior consensus--yielding nearly zonal mean flow once again by late in the weekend instead of more persistent ridging over the west-central states. At the same time there is still a lot of spread and variability for how the lower 48 trough will evolve from midweek onward. 00Z/06Z GFS runs continued the theme back to the 12Z/19 run of pulling an upper low down through the Rockies/northern Mexico/Southern Plains region but the 12Z run has reverted to a more progressive trough with an upper low tracking across the mid-latitudes of the East and then the southeast corner of New England (essentially a slower version of the 12Z CMC). Shortwave differences with the GFS in particular actually trace back to the Pacific in the short range. ECMWF/CMC runs that have been part of the majority cluster with the means have trended northward with the surface low anchoring the initial central U.S. front (but the 12Z ECMWF has come back to the south) while maintaining the idea of a leading East Coast development that could ultimately become fairly strong by the time it reaches New England. Ensemble members over the central U.S. by the latter half of the week have actually diverged over the past day. A higher percentage of GEFS members depicted some variation of the 00Z/06Z GFS idea versus 24 hours ago, while the minority of ECens members pulling the upper trough farther west/southwest yesterday has dwindled to only one member (and that one only matching with the UKMET versus the farther southwest 00Z/06Z GFS). Based on the guidance available from the 00Z/06Z cycles, the updated forecast represented various ideas from continuity and significant changes introduced by a majority of guidance. An operational guidance blend early (quickly splitting GFS input with the 06Z GEFS mean) followed by a transition to the GEFS/ECMWF means and 00Z ECMWF/CMC allowed for relative consistency for the potential system along the East Coast, though reflecting the majority cluster that trended northward for the surface low anchoring the initial central U.S. frontal system. GEFS mean inclusion provided a modest nudge toward the possibility of some energy hanging back in the southern stream (which could be from a combination of initial energy near Baja California and any flow from the northern stream). Meanwhile the blend reflects consensus trends toward flatter mean flow by next weekend, with more precipitation reaching the Northwest U.S. ...Weather/Hazard Highlights... Wednesday is likely to be fairly dry across the lower 48, though with perhaps some showers across Florida and the southeastern Atlantic coast as well as parts of the Southern Plains, plus a few rain and snow showers in northern parts of the Rockies and Plains. Then 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 a minority potential for significantly different precipitation coverage and amounts, which could involve more rainfall across the southern tier 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 end of the week and next weekend. Temperatures in the medium range period should be overall more seasonable compared to those of last week. Temperatures are likely to trend above average along the West Coast by midweek 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 by next Sunday given the latest forecast of upper flow becoming more zonal. Conversely, post-frontal cooler temperatures especially in terms of highs should extend from the Mississippi Valley toward the East Coast Friday-Sunday 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. Rausch/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