Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Nov 19 2023 Valid 12Z Wed Nov 22 2023 - 12Z Sun Nov 26 2023 ...Pattern Overview... Behind a system forecast to track over/near New England on Wednesday, an upper ridge over the West will quickly give way to a shortwave trough while an upper ridge setting up over the eastern Pacific/northwestern North America will ultimately tend to favor increasingly large scale troughing that should be aligned from near Hudson Bay southwest into the western U.S. by the end of next weekend. Expect this evolution to bring colder temperatures to the central U.S. in particular by Friday-Sunday, with many other areas trending cooler as well. The New England system will bring some precipitation to the East Coast while the trough reaching the West by Thanksgiving will spread an area of snow across the north-central Rockies. Some of this moisture may continue eastward by next weekend with a wave/frontal system while a potential leading wave could also spread some precipitation closer to the East Coast. There is also a very uncertain potential of some southern eastern U.S. rainfall from a wave behind the leading East Coast system. ...Guidance Evaluation/Predictability Assessment... The models and ensembles continue to have difficulty in resolving important details of the forecast. This includes the forecast of initial trough energy reaching the eastern U.S. by midweek and how corresponding East Coast low pressure evolves, trailing northern Mexico trough energy and its role in the surface/QPF pattern, as well as the combination of the mean upper low over Canada/surrounding flow plus troughing that moves into the West. Given the divergent model solutions, the updated forecast using 12Z/18Z data again relied more than usual on the 18Z GEFS/12Z ECens means, adding in some 12Z ECMWF early in the period and some 18Z GFS mid-late period after its questionable traits over the Northeast (bringing across much lower heights than consensus) exit the forecast domain. Guidance still has no coherent ideas for exactly how trough energy moving into the East will look at the start of the period Wednesday. The GFS/UKMET still dig more northern stream energy into the southern tier U.S. by the start of the period, leading to a farther south track of low pressure near New England (east of Cape Cod) by early Thursday. In contrast, the 00Z CMC is a fast extreme, well into the Canadian Maritimes. The ensemble means provide a reasonable intermediate and more consistent solution for the time being. Over the past day or so there has been a gradually increasing signal for northern Mexico energy to eject northeastward by the latter half of the week, but with very different impacts in terms of the surface reflection and resulting QPF. In general the latest CMC runs have been most pronounced with associated surface waviness and rainfall amounts/coverage. Interestingly, the 12Z cycle of ECMWF-initialized machine learning models show a well-defined southern stream upper low/trough progressing along faster than the current cluster of dynamical guidance, but with no hint of a GFS/UKMET type evolution either. Guidance still shows some spread for the upper trough arriving into the West and then how northern stream flow (plus the Canadian/northern tier trough) may interact. There was some improved clustering as new 00Z runs were coming in but the ECMWF has trended to a more pronounced closed low and greater stream separation than most other guidance by late Friday. With a fair amount of variability, there has been a general signal in models/means toward a wavy frontal system reaching the east-central U.S. by Saturday and more recently a leading wave near the East Coast. These features should continue onward into Sunday. ...Weather/Hazards Highlights... While there is still lower than average confidence for how the system affecting New England on Wednesday (and with lower probability into Thanksgiving) may evolve/track, potential still exists for a period of strong onshore flow over parts of eastern New England. The new Day 4 Excessive Rainfall Outlook that covers the Wednesday-Wednesday night period maintains a Marginal Risk area over eastern New England to reflect the possibility that any enhanced bands of rainfall could lead to localized runoff issues where drainage is impaired due to urbanization/leaves/etc. However initially dry soil conditions should otherwise temper what threats may exist. Elsewhere, snow will be possible over parts of interior New England with the Wednesday system and a brief period of lake effect snow will be possible after system passage. With very uncertain details, a trailing wave may produce some rainfall over parts of the South/East during the latter half of the week. Over the West, a system moving into the region late in the week should bring an area of snow across the north-central Rockies especially around Thanksgiving, with some localized enhancement possible. Some rain/snow may extend east of the Rockies thereafter with a wavy frontal system eventually reaching the East and a separate coastal wave could also spread moisture over parts of the East. Colder air dropping south over the Plains/eastern slopes of the Rockies from Friday into the weekend should should bring highs down to 10-20F below normal over that region. Below normal anomalies will spread elsewhere as well, with the Interior West highs trending to 5-10F below normal late week into the weekend and the Mississippi Valley plus parts of the Ohio Valley at least 10F below normal by Sunday. This will be a significant change from above normal temperatures over the High Plains into the West on Wednesday. The South and East will see a brief period of below normal temperatures and followed by a rebound toward normal values. 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