Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Oct 04 2023 Valid 12Z Sat Oct 07 2023 - 12Z Wed Oct 11 2023 ...Heavy rain and flash flooding threats likely in the Northeast Saturday due in part to Philippe... ...Overview... An upper trough and likely an embedded upper low are forecast to swing slowly through the Great Lakes region and south this weekend. At the surface a cold front will sweep through the eastern U.S., while ample moisture streams in ahead of it, and the low circulation associated with Philippe (which should be post-tropical by then) could track near or into Maine. This combination will lead to heavy rain and flooding concerns in the Northeast on Saturday. Then some rain could linger into early next week underneath the upper low, with lake effect enhancement likely. Meanwhile an upper ridge should overtake the western U.S. for warmer than average temperatures and generally dry conditions through Saturday, though early next week a trough and frontal system looks to push into the Pacific Northwest and farther into the interior West as next week progresses. ...Guidance Evaluation/Predictability Assessment... Model guidance has been persistent in showing an upper-level trough axis across the east-central U.S. at the start of the period Saturday, and the energy within the trough looks to form a closed low within it atop the Great Lakes over the weekend and also help the trough to become negatively tilted (which can help the system pull in more Atlantic moisture). The 12Z CMC closed off the low farther north compared to the bulk of other guidance. Some small differences are noted with the positions of the associated surface cold front, which affects the axis of heavy QPF. One change over the past day is that models have shown a marked trend toward a farther west track for current Tropical Storm Philippe. The National Hurricane Center as well as much of the deterministic and ensemble guidance indicate that Philippe should become post-tropical but its extratropical low could track into New England or Atlantic Canada. The mean position looks to be into Maine. This would provide another swath of heavy rain to the Northeast. The low and energy are likely to get absorbed into the southeastern Canada/northeastern U.S. system on Sunday. Models show the core of the upper low could drift slowly north through the first half of next week, with some variations in the centroid position, but the ensemble means showed good agreement so preferred their placement in general. The 12Z CMC retrograded the upper low to the west by Day 7/Wednesday, which was not preferred as it was not supported by other guidance, but the 00Z CMC looks more reasonable. Farther west, models agree on the general idea of an upper ridge atop the West tracking toward the Rockies early next week. GFS runs have been more supportive of a shortwave over the Great Basin over the weekend, but these differences do not seem to have much sensible weather impact. The ridging will get compressed by energy and troughing coming across the eastern Pacific into the West early next week, and this trough has some variations in the model guidance. GFS runs remain on the less amplified side with the troughing, so more shortwave energy overspreads the Northwest earlier, in contrast to the incredibly amplified 12Z ECMWF which was slower. The newer 00Z ECMWF backed off some on this amplification and now is more consistent with other guidance on the timing of the trough and frontal system. The WPC forecast began with a multi-model deterministic blend favoring the 12/18Z GFS and 12Z ECMWF. Incorporated the GEFS and EC ensemble means in the blend by Day 5/Monday and increased their proportions fairly quickly through Days 6-7 given the increasing deterministic model spread in contrast with their steadier solutions. ...Weather/Hazards Highlights... As the upper trough and forming embedded low track across the Great Lakes late week, Atlantic moisture is likely to flow into the northeastern U.S., leading to increasing rain totals especially by Saturday along and ahead of the cold front. Instability may be somewhat limited, but the forcing for ascent will be strong with the right entrance region of the jet overhead, and precipitable water anomalies are forecast to be at least above the 75th if not the 90th percentile. Additionally, the low pressure system that is currently Tropical Storm Philippe, but is forecast to be post-tropical by that time per the National Hurricane Center, has shown a westward trend over the past day and is currently expected to track into Maine or surrounding areas. Given all this, a Slight Risk remains in place for northeastern New York into parts of New England for Day 4/Saturday. Upslope flow/terrain enhancement could play a contributing role in producing locally heavier totals in some terrain areas. The heavy rain threat will likely be over by Sunday, though there is a small chance it could persist in Maine, but showers will linger for portions of northern New England along with lake effect/enhanced showers for the Lower Great Lakes underneath the deep upper low. Elsewhere, much of the country should be dry through the weekend, aside from some lingering light rain possible in South Texas and some limited showers in the Southwest. But the Pacific Northwest should start to see increasing precipitation chances early next week as a front and upper trough approach, and precipitation is forecast to spread east with time into the interior West. Some snow is possible in the higher elevations of the Cascades and northern Rockies. Cooler than normal temperatures will be in place across the central U.S. on Saturday, with the northern half of the Plains at risk of its first frost or freeze of the season. These below average temperatures are forecast to shift east early next week under the upper trough/low behind the cold front. Lows around 10-15F below normal and highs of 10-20F below normal are likely across the east-central U.S. and reaching the Eastern Seaboard. These look to gradually moderate towards midweek. Meanwhile, the initial upper ridge across the West will raise temperatures along the West Coast to 10-15F above normal on Saturday, with some record warm lows possible. The shift/expansion of the ridge farther east should lead to warmer than average temperatures across the northern Rockies and into the High Plains early next week. But the West Coast is expected to flip to below average especially in terms of highs by Tuesday-Wednesday as the next trough and low approach. 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