Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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 could be in place across the lower 48 as the medium range period begins midweek, but the pattern looks to quickly amplify by Thanksgiving Day with upper trough development in the central U.S. and ridging building across the West behind it. The majority of model guidance continues to advertise the trough tracking eastward through the weekend, supporting a central U.S. low pressure/frontal system crossing the Great Lakes while a western Atlantic low pressure system lifts north (with some interaction possible near New England). This pattern would lead to increasing chances for precipitation late this week for the East. However, there are large scale uncertainties remaining with what becomes of the trough, and smaller scale differences even within the majority cluster of the trough/low tracks that lead to low confidence for precipitation types and intensity across the East. By late week into next weekend, eastern Pacific energy could approach the West Coast, but this aspect of the flow also shows considerable differences with the details and this may play a role in the forecast differences farther east. ...Guidance and Predictability Assessment... Model guidance continues to indicate that energy will dig around the Northern Rockies region to develop a deepening trough progressing into the eastern U.S. by Thursday with a ridge amplifying in the western U.S. behind it. But by Thursday and beyond, spread increases amongst model guidance for the future of the trough. While the majority of model guidance including the ECMWF/CMC and the ensemble means have the trough tracking eastward with time, GFS runs throw a wrench in the overall consensus as the 12/18/00Z runs all show the potential for energy to split off southward and form a closed low somewhere in the south-central U.S. and meandering there as the less energetic northern stream tracks quickly east. GEFS members show quite a bit of spread in this regard with some more like the operational GFS runs and some closer to the majority cluster of other guidance. At this point it seems wisest to consider the GFS runs outliers as the GFS/ECMWF/CMC ensemble means, as well as the deterministic CMC and ECMWF, keep the trough phased and deep across the East. Choosing a non-GFS model blend also maintains WPC continuity better. Note that a GFS-type evolution would greatly shift the surface pattern and emphasis for heaviest precipitation over the central/eastern U.S. The majority cluster still indicates a surface low pressure system tracking into the Great Lakes Thursday-Friday while a separate surface low develops along the East Coast, with possible consolidation of these centers over or near New England by Saturday. There are some certainly some differences between lows from ensemble members and the deterministic models in terms of the exact track and strength of each low, but more in the "typical spread" range once the GFS is excluded. By late week into the weekend the trough seems most likely to continue eastward. Newer 00Z ECMWF and CMC runs show the trough axis shifting a little more quickly through the East along with the surface low exiting faster late week compared to their previous runs. The other major forecast issue arising by Friday-Sunday involves flow within the eastern Pacific trough. There has been some inconsistency in guidance with a couple of rounds of energy and whether flow may separate within the overall trough as well as how energy approaches and enters North America given the overall ridge. GFS runs (particularly the 12Z) have a potent shortwave tracking eastward and pressing the northern side of the ridge south this weekend as the ECMWF deflects it northward. Earlier on there are also some southern stream differences even within the majority cluster for energy around northern Mexico or so, without clear consensus yet as the 12Z ECMWF went without a feature there while the 00Z ECMWF has it while the CMC runs do too. So these features remain uncertain for now. Thus the WPC forecast blend began with a multi-model deterministic blend for day 3/Thursday but with quick decreasing and eventual removal of the 12/18Z GFS components given their flow evolutions appear less likely. The more agreeable GEFS and EC ensemble means were utilized with increasing proportions to half by day 6 and over half day 7 given the increasing model spread even within the majority cluster. This maintained good continuity with previous forecasts while awaiting any potentially significant shifts in the guidance. ...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 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 farther north. Then by next weekend, lake effect snow is once again a possibility behind the trough, and troughing/energy in the eastern Pacific could lead to precipitation coming into the Pacific Northwest and northern California. 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. 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. 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