Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM 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 with some locally heavy rainfall possible. ...Guidance Evaluation/Predictability Assessment... Model guidance has remained in good agreement on the overall pattern through much of the medium-range period (Saturday Oct 7 - Wednesday Oct 11), beginning with a northern stream trough over south-central Canada/the north-central U.S. deepening as it shifts eastward over southeastern Canada and the Great Lakes/Interior Northeast. A closed low is expected to develop and become anchored over the region for several days, likely through at least the end of the current period (Wednesday). Embedded shortwave energy may play a role in the overall gyre which differs model to model, and may have related surface weather impacts, but an overall wet and cooler trend can be expected across the region regardless. However, there is one notable shift in some of the more recent model runs noted in the prior forecast related to the track of what is now Tropical Storm Philippe. The guidance has trended towards a track that would bring Philippe into Maine/Atlantic Canada, with the post-tropical energy being absorbed into the anchored closed low. The smaller-scale nature of this feature lowers predictability at this time frame, and the evolution of the system appears complex given the likely interaction with the synoptic system over the Great Lakes/southeast Canada, though the expectation for a very wet Saturday in the Interior Northeast/New England will remain regardless. Refer to the National Hurricane Center for the latest official updated storm track for Philippe. To the West, an upper-level ridge over the western U.S. should shift a bit eastward, with ridging amplifying into the Northern Plains/south-central Canada in response to the deepening trough to the east. The 00Z ECMWF and CMC are a bit more amplified compared to some recent runs of the GFS, though the 06/12Z GFS have trended more in this direction, with the GEFS mean also in agreement. The largest spread in the guidance is related to upstream energy over the northeastern Pacific approaching and entering the Northwest early next week, with differences in the timing, track, and strength of the energy entering the Northwest both across the guidance and individually run-to-run. For example, the ECMWF has wavered back and forth in the latest few runs with a stronger, deeper trough digging along the West Coast. The most recent 00Z run trended weaker, which is more in line with the other deterministic and ensemble guidance. The spread increases into Wednesday with generally poor agreement on the energy translating through the West, either hanging back over the Great Basin (GFS) or digging over the High Plains (ECMWF), with the GEFS and ECens means expectedly in the middle of the guidance envelope. One final potential wrinkle late in the forecast period is for the remnants of a tropical system over the eastern Pacific drifting northward over Mexico into the Southwest/south Texas, hinted at in some runs of the GFS and the latest 00Z run of the CMC, which will need to be monitored over the coming days if this trend continues and predictability increases. The updated WPC forecast begins with a composite blend of the deterministic guidance (00Z CMC/ECMWF/UKMET and 06Z GFS) given good overall agreement early in the period. The contribution from the 00Z CMC is reduced mid-period given a more aggressive assessment of the energy upstream over the northeastern Pacific. A contribution from the 00Z ECens and GEFS means is steadily increased to more than half the blend by the end of the period given the notable increase in spread with respect to the handling of this energy over the northeastern Pacific after it enters the West, offering a compromise in the middle of the respective 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. Not only would this bring the potential for rainfall associated with the system directly, but the increased moisture along with the noted overlap of the cold front and upper jet may lead to locally enhanced rainfall in the interior as part of a Predecessor Rain Event ahead of the system. Upslope flow/terrain enhancement could play a contributing role in producing locally heavier totals in some terrain areas as well. Given all this, a Slight Risk remains in place for northeastern New York into parts of New England for Day 4/Saturday, and it is possible this may need to be upgraded in future outlooks with more confidence in the track of Philippe and this culmination of scenarios. 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. Locally heavy rainfall will be possible along upslope areas of the coastal mountain ranges, Olympics, and Cascades as the system draws a plume of Pacific moisture northeastward into the region. Some snow is also 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, lingering longest in vicinity of the Lower Great Lakes/Interior Northeast as the stubborn upper-low is expected to remain. 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. Putnam/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