Extended Forecast Discussion NWS Weather Prediction Center College Park MD 331 PM EDT Thu Oct 05 2023 Valid 12Z Sun Oct 08 2023 - 12Z Thu Oct 12 2023 ...Overview... As the medium range period begins Sunday, what is currently Tropical Storm Philippe is forecast to be post-tropical by then, and its energy will become absorbed into a reasonably deep upper low that looks to meander between the Great Lakes and James Bay through much of the week. The heaviest precipitation from this feature is phasing into the short range period now, but some rain could linger into early next week underneath the upper low, with lake effect enhancement likely. Farther west, initially dry conditions under an upper ridge will give way to a trough and frontal system, increasing precipitation chances first for the Pacific Northwest by Monday with some locally heavy rain possible, and spreading east as next week progresses. ...Guidance Evaluation/Predictability Assessment... Model guidance remains generally agreeable on the overall pattern through especially the first half of the period. There are some differences in the details of the upper low over the Northeast/southeast Canada and particularly with exact interactions of Philippe into this system. The 00z UKMET continued to be displaced more northeast of the concensus of the center of the low by Day 5/Tuesday, but the 12z run (available after forecast generation time) seems to have adjusted closer to consensus. The WPC forecast for today favored a blend of the CMC, GFS, and ECMWF for this system. By Tuesday and beyond, some greater differences arise with the western trough. GFS runs have been persistently a little less amplified but also quicker to progress the troughing eastward than the ECMWF and CMC which dig it into the West/Central U.S. more. Ensemble means generally follow their respective deterministic runs, but there remains a lot of uncertainty in the individual ensemble members. The 00z ECMWF may be a little too deep as it forms an embedded closed upper low, but recent CMC runs are trending in that direction too. Also some uncertainty late in the forecast period for southern stream energy possibly originating from an eastern Pacific tropical system to track into Mexico and then maybe south Texas/the Gulf, which could merge into the larger pattern enhancing rainfall totals across the Gulf/Gulf Coast/Florida around the middle of next week. With the growing differences in the deterministic guidance the second half of the period, increased usage of the ensemble means into the blend days 6 and 7. ...Weather/Hazards Highlights... The excessive rainfall threats in the Northeast will end by by Sunday as the lows and associated fronts pivot. However, showers are likely to linger underneath the upper low through much of the week, with enhanced rain amounts possible downwind of the Great Lakes. A few showers may be possible across the central/northern Appalachians as well with upslope flow. Farther west, Sunday looks to be a mostly dry day across the CONUS aside from limited showers in the Southwest and the Florida Peninsula. By Monday though, troughing and a cold front approaching and entering the Pacific Northwest should lead to increasing rain chances there. Some relatively heavy rain rates are possible due to a moderate to strong atmospheric river taking aim at the the Pacific Northwest and a coupled jet aloft providing support for ascent, along with perhaps some instability. Upslope areas of the coastal mountain ranges, Olympics, and Cascades could see higher rain totals. But recent dry conditions and it being this early in the wet season may mean not many flooding issues arise from this rain, so decided against any Marginal Risk area in the Excessive Rainfall Outlook at this time. Precipitation chances will spread eastward Tuesday and Wednesday into the Rockies and likely into the central U.S. Wednesday and Thursday, but with uncertainty in the details. Meanwhile fronts over Florida should provide a focus for rain and thunderstorms, with increasing heavy rain chances by midweek and even more so if aided by any sort of tropical moisture. Cooler than normal temperatures are likely early in the week underneath the upper trough in the east-central to eastern U.S. The coolest anomalies by 15-20F look to focus across the Great Lakes/Ohio Valley/Central Appalachians, where some locations could see the first frost/freeze for the season. Temperatures should gradually moderate closer to normal as the week progresses, lasting the longest where the stubborn upper low remains overhead. Meanwhile, the West will see above average temperatures early in the week, which will get pushed into the central U.S. through the workweek as the ridge overhead tracks eastward and gives way to troughing in the West that flips temperatures to below average (especially in terms of highs) by Tuesday-Thursday. Santorelli/Tate Hazards: - Heavy rain across portions of Florida, Wed-Thu, Oct 11-Oct 12. - Flooding occurring or imminent across portions of the Southeast. - Frost/freeze across portions of the Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Upper Mississippi Valley, Tue, Oct 10. - Frost/freeze across portions of the Great Lakes and the Upper Mississippi Valley, Mon-Tue, Oct 9-Oct 10. 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