Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Wed Nov 22 2023 Valid 12Z Sat Nov 25 2023 - 12Z Wed Nov 29 2023 ...Meaningful snow likely Saturday across parts of the south-central Plains... ...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. But on Saturday, the upper energy initially initially emerging from the West looks to produce an area of snow over the south-central Plains (and lingering over the Rockies) with some rain to the south. 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 coincide with much moisture and should produce light precipitation if any. Lake effect precipitation is likely though through the first half of next week. ...Guidance Evaluation/Predictability Assessment... Guidance continues to show difficulty in resolving some of the details within the more agreeable large scale evolution. Recent guidance continues to show indications of a compact surface low developing off the Southeast coast during the weekend in response to weak southern stream energy and then lifting fairly quickly north/northeast through the western Atlantic as the overall Canada into central U.S. trough approaches. CMC runs (00Z, the new 12Z, and many CMC ensemble members) tend to be farther west with its track, and UKMET runs are farther west as well. The 00Z ECMWF seemed to be a reasonable middle ground given the 00/06Z GFS runs were east. The 12Z GFS/GEFS mean trended a bit west from the 00Z/06Z positions, which puts them closer to the EC, so this track was favored. The surface low looks to track to the north of Maine by early Tuesday and strengthen as it combines with amplifying Canadian trough energy and possibly another surface low. The pattern aloft shows increasing spread as the 00Z and 06Z GFS concentrate a deep upper low crossing northern New England. The 12Z GFS backed off on this, but overall guidance has been divergent and erratic with the placement of shortwaves and thus fronts within the broader trough by the 6-7 day time frame (Tuesday-Wednesday). Changes to frontal placement are likely in future forecasts given the varying guidance. Farther west, models show some differences with mid-upper energy pulling off into the Southwest early next week, with the 00Z UKMET seemingly an outlier with a farther north position of an upper low closing off around Monday where the bulk of other guidance has ridging sneaking in toward the West Coast. Upstream, as a closed low tracks east through the Pacific, the 00Z and 06Z GFS showed a farther west position of this feature allowing for a wider ridge axis compared to the well-clustered ECMWF and CMC. The 12Z GFS and GEFS mean have both trended east and are more agreeable with the low/trough to ridge positions now. The WPC forecast utilized a model blend of the 00Z ECMWF, 06Z GFS, 00Z CMC, and 00Z UKMET (in order of most to least weighting) early in the forecast period. Phased out the UKMET and decreased the proportion of the CMC and GFS first and eventually the ECMWF as lead time and spread increased, in favor of more GEFS and EC ensemble mean guidance that are less prone to the waffling that deterministic models show. ...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, with a slight shift southward in this forecast issuance to include far northern Oklahoma per the model trends. Lighter snow activity could linger over the south-central Rockies. Some rain may be possible over the southern Plains. Then a front may carry remaining moisture eastward into Sunday 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 occurs as well as the precipitation type 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. Potential for organized heavy rainfall remains too low 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. 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 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