Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Sep 09 2023 Valid 12Z Tue Sep 12 2023 - 12Z Sat Sep 16 2023 ...Overview... Latest models and ensembles show varying degrees of upper troughing over eastern North America during the period. Within this mean trough, the most prominent feature will be an upper low forecast to close off over the Great Lakes by midweek and then lift northeastward as diffuse shortwave energy entering the western U.S./Canada eventually feeds into the trough. The Great Lakes low and surface system should produce some moderate to heavy rainfall from the Great Lakes/Mid-Atlantic into the Northeast. By the end of next week into the weekend, the track and timing of Hurricane Lee will require monitoring for any potential effects on New England and the Canadian Maritimes. Over the West, guidance disagrees over the strength/evolution of initial shortwave energy but suggests that upper ridging should ultimately strengthen over the region by late next week into the weekend. The combination of western shortwave energy and a persistent Southern Plains front will favor multiple episodes of rain and thunderstorms over the southern Plains through the period, and extending as far west as Arizona Tuesday-Wednesday. Cooler than average highs will prevail over a majority of the central/eastern U.S. while moderately above normal highs should become established over the far northern Plains and Pacific Northwest by late next week/weekend. ...Guidance Evaluation/Predictability Assessment... The primary forecast issue for the system closing off over the Great Lakes by Tuesday-Wednesday and lifting northeastward thereafter is that latest GFS runs stray to the fast side after day 3 Tuesday. Even the GEFS mean leans more to the remaining slower models/ensemble means. On the other hand, models and ensembles have been more variable and diverse with specifics of upstream western North American energy that should be feeding into the eastern mean trough late in the period. Precise day-to-day details of the eastern trough will likely play an important role in determining the path of Hurricane Lee, which guidance expects to be tracking northward along 65-70W longitude by next Saturday. The greatest guidance spread is for timing, with GFS runs being the fast extreme while the 12Z ECMWF/ECMWF mean are slowest. While east-west differences in track are much smaller relative to speed, precise longitude will be very important with respect to any potential effects on New England and the Canadian Maritimes. Continue to monitor National Hurricane Center products for the latest information on the strength/track of Lee. Meanwhile, there is a lot of spread for details of smaller scale/lower predictability energy that may be over parts of the West into mid-late week, favoring an intermediate blend. Overall the consensus evolution is toward a stronger mean ridge prevailing over most of the West late next week into the weekend. Forecast considerations led to starting with an operational model blend on day 3 Tuesday, followed by replacing the GFS component with the 18Z GEFS mean from day 4 Wednesday onward due to the GFS being fast with the Great Lakes system. Decreasing confidence in other details led to 60 percent total weight of the 18Z GEFS and 12Z ECens/CMCens means by days 6-7 Friday-Saturday, with lingering input from the 12Z CMC/ECMWF. With some manual adjustment, the forecast reflects an intermediate timing for Lee by the end of the period. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks (EROs) that cover Tuesday through Wednesday night reflect heavy rainfall/flash flood potential over two general areas, one over central/northern portions of the East and the other over/west of the southern Plains. During the Day 4 time frame, the system beginning to evolve over the Great Lakes may generate some locally enhanced rainfall from the Great Lakes/Appalachians into the Mid-Atlantic. Guidance is not fully agreeable on the exact forecast scenario QPF-wise but this area will have wet antecedent conditions from recent and near-future rainfall. By Day 5 expect a more pronounced potential for heavier rainfall over portions of the Northeast, where the slower non-GFS majority cluster for the system heading into the region merits a Slight Risk. From the southern Plains westward, shortwave energy will interact with anomalous moisture to yield areas of locally heavy rain and thunderstorms. A front persisting over the southern Plains will provide another rainfall focus. The corresponding Marginal Risk area on Day 4 extends from parts of Texas into Arizona, while the Day 5 Marginal Risk reflects a northward expansion of activity into Colorado and parts of Kansas/Oklahoma but a drier trend over Arizona. Within these Marginal Risk areas, potential exists for an eventual embedded Slight Risk depending on guidance agreement/continuity and more clarity on soil conditions, in light of some areas being fairly dry to start but expected to receive some rainfall soon before the Days 4-5 period. Some moderate to heavy rainfall may continue into eastern New England through late next week. Rainfall and other sensible weather details for New England by next weekend will depend on the track and timing of Hurricane Lee. Continue to monitor National Hurricane Center products for the latest information on Lee over the coming days. The Rain and thunderstorms over the southern Plains and Rockies should continue through the end of the week, though with a somewhat lighter/more scattered trend over the Rockies. Below normal high temperatures over the central U.S. and Midwest/Great Lakes as of Tuesday should gradually spread farther eastward, though with a moderating trend by Friday-Saturday. Best potential for highs 10-15F below normal will be during Tuesday-Thursday over the central/south-central Plains as well as the Great Lakes/Ohio Valley. Slightly above normal temperatures may hold on over far southern Texas a little beyond midweek, while the Pacific Northwest eastward to the far northern Plains may trend somewhat above normal by Thursday-Saturday. 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