Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Nov 22 2023 Valid 12Z Sat Nov 25 2023 - 12Z Wed Nov 29 2023 ...Pattern Overview... Eastern Pacific into northwestern North America upper ridging that drifts at least to the West Coast by next week will promote a deeper/broader mean trough extending from Canada into the lower 48 east of the Rockies. Meanwhile initial western U.S. trough energy should split, with some of it continuing eastward/phasing with an amplifying Canadian shortwave and the rest elongating for a time over the southwestern U.S. with some uncertainty as to its ultimate shape next week. At the surface there is a continued theme of waviness near/south of the Gulf Coast and close to the East Coast, bringing some potential for precipitation along those coasts but with lack of confidence in the specifics. Northern stream fronts/waves will not have a lot of moisture to work and should produce light precipitation if any. The upper energy initially emerging from the West may produce an area of snow over the central Plains (and lingering over the Rockies) on Saturday with some rain to the south. Light precipitation may extend farther east and then the pattern should be favorable for some areas of lake effect snow from Sunday onward. ...Guidance Evaluation/Predictability Assessment... Guidance continues to show difficulty in resolving some of the details within the more agreeable large scale evolution. In latest runs there has been some consensus emerging toward a compact surface wave evolving off the Southeast Coast during the weekend in response to weak southern stream energy and then lifting north/northeast as the overall Canada into central U.S. trough approaches. However the past couple CMC/UKMET runs have depicted a farther west surface low track than the GFS/ECMWF and their means plus the CMCens mean. The amplifying Canadian trough energy should support a well defined wave crossing the Upper Great Lakes or Ontario around Sunday (with trailing front). Then this feature and the western Atlantic wave may interact to produce a stronger system to the northeast of Maine by early Tuesday. The new 00Z GFS is in the minority for its concentrated upper low crossing northern New England though. Behind this evolution, guidance has been divergent and erratic with system details in light of lower predictability shortwave details at the 6-7 day time frame (Tuesday-Wednesday). The most common theme, currently illustrated best by recent ECMWF runs and the GEFS/CMCens means, has generally been toward a wave reaching the northern Plains and then the Great Lakes during that period. 12Z ECMWF-initialized machine learning models also depict this evolution. CMC runs stray faster while the GFS has been inconsistent with the northern tier details from run to run. Spread over the East Pacific/western U.S. after Monday is not as bad as it was 24 hours ago but meaningful differences still exist. In particular, the CMC has been fastest to shift the overall pattern eastward while the GFS is in the minority with how long it holds onto a closed low near Baja California. The latest manual forecast started with a 12Z/18Z operational model composite early in the period and then by around Monday phased out the minority input of the UKMET/CMC due to their lower confidence solutions along the East Coast and with the CMC starting to stray from the majority along the West Coast. Increasing weight of the 18Z GFS/12Z ECens means later in the period led to an even proportion of models/means by day 7 Wednesday, with common aspects of the GFS/ECMWF helping to improve definition but differences over other areas offsetting to keep the forecast close to the means. ...Weather/Hazards Highlights... The upper system emerging from the West may produce a fairly localized area of meaningful snowfall over the central Plains around Saturday. Latest probabilities for at least a quarter inch of liquid in the form of snow show the best potential centered over west-central Kansas. Lighter activity could linger over the south-central Rockies. Some rain may be possible over the southern Plains. A front may carry remaining moisture eastward but likely with fairly light amounts. Low pressure developing off the Southeast Coast during the weekend should lift-northeast thereafter. Coastal North Carolina could see some rain with this system while confidence remains below average for how much precipitation may occur over the Northeast. A passing wave/front around Sunday-Monday and then trailing flow and possibly another system should produce episodes of Great Lakes snow from a combination of lake effect and synoptic influences. The Gulf Coast/Florida may see a rebound in rainfall potential toward midweek but with low confidence in specifics at this time. Currently there is no sufficiently coherent signal for organized heavy rainfall to warrant any risk areas on the Day 4 and Day 5 Excessive Rainfall Outlooks. In a fairly cool pattern over a majority of the lower 48 from the weekend into the middle of next week, the most extreme temperature anomalies should be over the eastern slopes of the central Rockies into central Plains on Saturday with highs 15-25F below normal. Some of this cold air will continue southward into the southern Plains thereafter with highs at least 10-15F below normal Sunday-Monday and perhaps into Tuesday over far southern Texas. Also, during the weekend into early next week brisk winds will make it feel colder over the southwestern U.S. and vicinity. The rest of the central U.S. should moderate close to normal by Tuesday with northern Plains lows possibly trending above normal. Continue to expect slightly above normal temperatures along the central-northern West Coast and perhaps expanding a bit by the middle of next week as East Pacific upper ridging drifts eastward. The East Coast will likely be chilly on Saturday (highs up to 10-15F below normal over the Mid-Atlantic into New England), with most of the East tending to be near to somewhat below normal for the rest of the period. 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