Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 PM 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. There is also some uncertain potential for additional southern-eastern U.S. rainfall from a second wave up the East Coast late this week. ...Guidance Evaluation/Predictability Assessment... The models and ensembles continue to struggle with resolving important details of the initial trough energy reaching the eastern U.S. midweek and how corresponding coastal low pressure evolves. The 00z UKMET continues to be much slower with southern stream energy through the Southeast, resulting in a slower and more south wave off the East Coast. The GFS is also stronger/slower but usable and not like the UKMET which appears to be an outlier at this point. The upper low that meanders over northern Mexico early in the week should begin to push east as additional troughing enters the West. There are timing differences with this feature and that impacts potential additional coastal low development off the Southeast Coast late in the week and resulting QPF. Troughing into the West around Thanksgiving becomes quite uncertain and related to a building ridge upstream which also affects shortwaves rounding the base of an upper low meandering near Hudson Bay. Despite differences, there was enough agreement between the ECMWF, GFS, and CMC for a purely deterministic model blend for the Wednesday-Thursday forecast which helped bring a good middle ground solution for the quickly moving coastal wave off the Northeast. By Friday though, larger scale differences and plenty of run to run continuity begins to increase fairly quickly and so the WPC forecast at this point trended towards the ensemble means, with some smaller contributions from the GFS and ECMWF for some added forecast details. For the most part, this approach maintained good continuity with the previous WPC forecast amidst a highly uncertain forecast across the board. ...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 modest rainfall over parts of eastern New England. However, given very dry antecedent conditions and a general lack of sufficient instability and anomalous moisture, opted to remove (with support from the affected WFOs) the marginal risk on the Day 4 Excessive Rainfall Outlook. Snow will be possible over parts of interior New England with the Wednesday system as well 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 along a weak and wavy frontal system, with some rainfall possible along parts of the Gulf Coast. 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 followed by a rebound toward normal values. Santorelli/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