Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Oct 11 2021 Valid 12Z Thu Oct 14 2021 - 12Z Mon Oct 18 2021 ...Heavy rain threat over parts of the southern Plains may extend into Thursday... ...Overview... Expect an amplified western trough/eastern ridge upper pattern at the start of the period on Thursday, supporting unseasonably chilly temperatures over the West and very warm conditions over the East. A front extending from low pressure tracking into Canada will separate the two air masses, with moisture from eastern Pacific Tropical Storm Pamela (forecast to pass just to the southeast of the southern tip of Baja California as a major hurricane around Tuesday night/early Wednesday) likely enhancing rainfall along the front. Highest totals are likely over the southern Plains but some heavy rain is possible farther northeast. Then during the weekend into early next week the mean flow will become less amplified and more progressive. This will support a drying trend over the central/eastern U.S. and a moderation of temperatures closer to normal over much of the lower 48. ...Guidance Evaluation/Preferences... The combination of exact timing of the western upper trough as it emerges into the central U.S. along with timing of Pamela continue to present forecast challenges for resolving the details of one or more frontal waves that should track northeastward from the southern Plains. Recent GFS runs have tended to be faster than the ECMWF with the western trough and slower with Pamela. CMC runs have been slowest with the western trough and fastest with Pamela. A steady faster trend for Pamela in the GFS over the past 12 hours is narrowing that spread though. After early Friday UKMET runs have been straying a lot flatter than consensus over western North America where consensus has been building a ridge--leading to questionably fast progression of the trough heading into the East. By the latter half of the period some Pacific/western North America details have changed over the past day or so. Latest trends are for a more pronounced upper trough to develop south of Alaska around early next week, ultimately leading to greater inland progression of the shortwave currently forecast to be nearing the Pacific Northwest/British Columbia by very early Sunday. On the positive side, by day 7 Monday the 12Z/18Z models and their ensemble means were quite agreeable for the system tracking across southern Canada and bringing a frontal system into the northern Plains. After early Sunday the new 00Z CMC goes wildly astray from its continuity or other guidance, digging a deep trough into the West by Monday. Teleconnections relative to the upstream Pacific trough greatly favor the other models and ensemble means. The updated forecast started with a blend of 12Z/18Z operational models from Thursday into early Friday but then removed the UKMET and minimized CMC influence since those models were on opposite extremes of the envelope over the central/eastern U.S. and the UKMET compared increasingly poorly to consensus over the West. The blend eventually went to a model/mean mix consisting mostly of the 18Z GFS/12Z ECMWF and their means. While the forecast employed a general blend, some surface details from the southern Plains northeastward still ended up having somewhat more of an ECMWF flavor--and are generally maintained in the new 00Z run. ...Weather/Hazards Highlights... Southern Plains heavy rainfall that should already be in progress may continue into Thursday, followed by some of the enhanced rainfall continuing northeastward into portions of the Ohio Valley, Great Lakes, and Northeast during Friday-Saturday. Guidance has been providing a consistent signal for an upper trough emerging from the West and moisture from Pamela interacting with a wavy front to produce the heavy rainfall over the southern Plains. However frontal wave details continue to vary significantly enough in the guidance to keep confidence low for resolving the most likely axis of potentially heavy rain farther northeast. Continue to monitor forecasts as details hopefully gain more clarity soon. By Sunday much of the eastern U.S. should be dry aside from lingering moisture over the eastern Great Lakes/New England and parts of Florida. The upper trough initially over the West will produce a brief period of rain and higher elevation snow over the central Rockies on Thursday. Models have been trending steadily higher with precipitation totals over the past couple days. Light to moderate precipitation over the northern Pacific Northwest late this week may become heavier over the Olympics and northern Cascades around Saturday with the approach of a Pacific shortwave. Expect a lighter trend heading into early next week. On Thursday the West will see significant coverage of highs 15-30F below normal with a few record cold highs possible. Such anomalies will shrink a little on Friday and more so by Saturday, with more scattered areas still seeing highs 10F or more below normal. Highs should be within a few degrees of normal over the West by the start of next week while system progression may bring moderately above normal readings into parts of the Plains by Sunday-Monday. In the East, highs will be 10-15F above normal over at least the northern two-thirds of the region Thursday into Saturday while anomalies for morning lows will be even higher, possibly exceeding plus 20F from the Ohio Valley/Lower Great Lakes into the central Appalachians. A broad area of record warm lows will be possible from the Lower Mississippi Valley northeastward. Progression of the Plains cold front into the East will erode the warmth from the west late week into the weekend, ultimately bringing near to slightly below normal highs to the East by Sunday-Monday. 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, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml