Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Thu Nov 23 2023 Valid 12Z Sun Nov 26 2023 - 12Z Thu Nov 30 2023 ...Overview... An upper ridge initially extending from the eastern Pacific into northwestern North America as of Sunday will support a fairly deep mean trough over the eastern half of Canada and the lower 48. While ridging over Canada should remain fairly stable through the period, guidance shows more inland progression and gradual weakening of the ridge over the western U.S. followed by the arrival of a modest trough with an opening upper low. Meanwhile, models/ensembles are still having difficulty in resolving the eventual character of weak elongated energy over the southwestern U.S. at the start of next week. This pattern evolution should keep a majority of the lower 48 on the dry side. The main precipitation areas of interest will be over parts of the Gulf Coast states with a wave on Sunday, New England with a brief period of some potentially enhanced activity in association with a compact wave brushing the region around Monday, and the Great Lakes where lake effect snow will be possible over multiple days. Also some light precipitation could reach the West Coast toward midweek with the arrival of the weakening Pacific feature. ...Guidance Evaluation/Predictability Assessment... Based on guidance through the 12Z/18Z cycles, the preferred solution started with a blend of mostly 18Z-12Z GFS/12Z ECMWF with a small contribution of the 12Z CMC for days 3-4 Sunday-Monday and then transitioned toward a day 7 blend consisting of even weight among the 12Z GFS/18Z GEFS mean and 12Z ECMWF/ECens mean. As of the time of forecast preparation, the GFS/ECMWF and their means had generally been the most consistent and in the slight majority for the track and timing of the compact wave forecast to develop offshore the Carolinas on Sunday and track slightly offshore the New England coast as strong upstream troughing approaches. Recent UKMET runs and sporadic CMC runs (including the new 00Z version) have been showing a more coastal track. Latest trends are increasing potential for the western side of the spread as the new 00Z GFS has joined the UKMET/CMC while the 00Z ECMWF has made a subtle westward nudge. Meanwhile another vigorous wave should brush the Upper Great Lakes Sunday-Sunday night and ultimately merge with the coastal wave to the north of Maine. Behind these systems, there is still some ambiguity over surface fronts/waves, reflective of shortwave detail uncertainties. At least the favored blend was able to maintain some degree of continuity with the theme of a general area of low pressure (one or two distinct waves) reaching the Upper Midwest/Great Lakes by day 6 Wednesday. Trailing northern tier details by the end of the period come down to orientation of flow aloft, with the 18Z GFS being most pronounced in bringing upper ridging into the central U.S. in contrast to the 12Z ECMWF's more persistent cyclonic flow. The ensemble means are generally between these extremes and the new 00Z GFS has adjusted somewhat slower with the northern tier part of the upper ridge. Over the West, the UKMET has generally been the least agreeable solution with how its Southwest energy evolves, holding onto an upper low longer than other guidance, though now the new 00Z ECMWF has adjusted that way. The weakening East Pacific trough/upper low has been incrementally trending toward better clustering though some spread remains. The new 00Z CMC becomes a fast extreme with upstream energy by next Thursday. ...Weather/Hazards Highlights... A wave near the Gulf Coast on Sunday may produce areas of rainfall with varying intensity on Sunday, while a separate wave developing off the Carolinas and lifting northward may bring some locally enhanced precipitation to the northern Mid-Atlantic coast and New England Sunday into Monday. Snow will be possible over interior New England, especially in higher elevations. Specifics are still subject to adjustment based on the exact surface low track. With a relative lack of instability, currently expect rainfall over the above areas not to be sufficiently intense to warrant any risk areas in the Days 4-5 Excessive Rainfall Outlooks. Cold flow behind a front crossing the Great Lakes around Monday will likely support areas of lake effect snow that may persist through at least midweek. Some light precipitation, mostly rain, may reach the central-northern parts of the West Coast by midweek with the modest and weakening system approaching the region at that time. The start of the period on Sunday will see mostly below normal temperatures over the lower 48 with decent coverage of highs 10-15F below climatology from the Great Basin into the central-southern Plains. As upper troughing becomes more focused over the eastern U.S. after the weekend, expect the coldest highs versus normal to progress eastward while the western/central parts of the country moderate. Far southern Texas should see the chilly conditions into midweek though. Also, from late weekend into early next week brisk winds will make it feel colder over the southwestern U.S. and vicinity. Toward the East Coast somewhat below normal highs will likely persist into next Thursday. On the other hand, the northern Plains will start out just slightly below normal for highs on Sunday (with above normal lows) and then trend above normal for max/min readings by Tuesday-Thursday. The upper ridge drifting into the Northwest early next week should promote slightly above normal highs near the central/northern West Coast before a trend toward normal as the ridge weakens. 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw