Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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 regarding the overall pattern through the early part of next week, though some differences remain with the exact interactions of Philippe and the upper/surface lows early in the period. By Monday it is all expected to have merged and the placement of the features become a little more clear. The 12Z UKMET though looked to be too far northeast of consensus with the center of the upper low especially by Day 5/Tuesday, and was aggressive with some shortwaves around the southern side of the low that other models had more merged into the trough itself. Favored a blend of the 18Z GFS/12Z ECMWF and a lesser component of the 12Z CMC early in the period instead. This forecast blend also worked for the ridge to the west and the eastern Pacific to Northwest trough. By Tuesday and beyond, some greater differences arise with the western trough. GFS runs have been persistently on the less amplified side with the trough, for some initially minor timing differences of the cold front. The evolution of the troughing and energy become more questionable going forward into Wednesday and Thursday. GFS and GEFS runs progress the trough eastward more quickly than the ECMWF suite that digs it into the West. The CMC suite seems more like the ECMWF, especially the CMC mean, though the details vary with the deterministic runs. Given typical model biases, favored the slower EC-type solutions to some extent for the mass fields and the QPF blends. Did think the 12Z ECMWF may have dug too deeply by Day 7/Thursday as it forms an embedded upper low, and the newer 00Z run did have the low a bit farther east. Somewhere close to the 12Z EC mean seemed like the best position for the trough. Another potential wrinkle late in the forecast period is for southern stream energy possibly originating from an eastern Pacific tropical system to track into Mexico into into the Southwest/south Texas, which could merge into the larger pattern but this remains highly uncertain at this time. With the growing differences in the model guidance, especially the deterministic runs, ramped up the proportion of ensemble means in the forecast blend to over half by Day 7, especially favoring the ECens mean. ...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 rain totals increasing especially by midweek. 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 (around 10-15F below normal) for lows are forecast across the Southeast, while below average highs by 15-20F look to focus across the Great Lakes/Ohio Valley/Central Appalachians. 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. Tate 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