Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue Oct 03 2023 Valid 12Z Fri Oct 06 2023 - 12Z Tue Oct 10 2023 ...Heavy rain and flash flooding threats ramp up in the Northeast late week... ...Overview... An upper trough and likely an embedded upper low are forecast to swing slowly through the Great Lakes region southward late this week into early next week, while a cold front sweeps through the eastern U.S. at the surface. Rain chances will increase along and ahead of the front, and heavy rainfall may cause flooding concerns in the Northeast on Friday and Saturday. Some rain (with lake effect enhancement) could linger into early next week underneath the upper low. Meanwhile an upper ridge should overtake the western U.S. for warmer than average temperatures and generally dry conditions, until early next week when a trough and frontal system could push into the Pacific Northwest. ...Guidance Evaluation/Predictability Assessment... Model guidance is in reasonably good agreement with the trough over the Great Lakes, with the past couple model cycles more agreeable in forming a closed low within the trough likely over the weekend. The closed low serves to slow down the movement of the feature, with only a slow drift in its placement early next week. Deterministic models do have some spread with its eventual positioning. GFS runs including the 18Z and new 00Z run having the upper low retrograde to the southwest does seem to be inconsistent with other deterministic and ensemble guidance, so the GFS was not preferred at this point. Behind the upper trough/low, models also are agreeable with an upper ridge shifting from the West Coast at the start of the period Friday gradually into the Interior West over the weekend/early next week and the central U.S. by Tuesday. Troughing coming into the eastern Pacific stemming from an Alaskan upper low shows some spread in its axis and depth but pretty minor for a feature in that region for that lead time. With the reasonably good consensus for the large scale pattern, the WPC forecast used a blend of the deterministic guidance particularly favoring the 18Z GFS and 12Z ECMWF early in the period, transitioning to incorporate the ensemble means to half by Day 6 and just over half Day 7 as model spread increased, especially lessening the GFS proportion. Continuity from the previous forecast was generally well maintained. ...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 on Friday and 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. Rain amounts look to increase by Saturday, with upslope flow/terrain enhancement likely playing a role in the Interior Northeast. Thus a Marginal Risk remains in place across the Lower Great Lakes region and parts of the northern Mid-Atlantic and Northeast for Day 4/Friday, with a Slight Risk across the Interior Northeast on Day 5/Saturday. Most heavy rain will exit by Sunday but some lingering showers are possible underneath the upper low through early next week, with lake effect rain showers likely. Farther south, the tail end of the cold front could linger into South Texas into at least Friday, sparking additional showers and thunderstorms there after a few wet days. A Marginal Risk remains in place for the Day 4 ERO/Friday. Rain amounts look to gradually lessen this weekend into early next week. Meanwhile, conditions in the West look to be dry under the upper ridge, aside from limited showers possible in the Southwest. This is likely to change early next week as a front and upper trough approach the Pacific Northwest, causing increasing precipitation chances. The amplified trough and the cold front will lead to cooler temperatures than average, while the upper ridge to the west of it supports above normal temperatures. One more warmer than average day is possible in the Northeast on Friday, while high temperatures across the Plains into Midwest will be below average by 10-20F. Lows look to be a little less anomalous Friday morning but could drop further on Saturday, with low 30s and perhaps a few temperatures in the upper 20s in the northern half of the Plains, creating the possibility of the first frost or freeze of the season. As the upper trough and the cold front(s) march east into the weekend, temperatures should cool to below average across the east-central U.S. and eventually making their way to the Eastern Seaboard by early next week. Meanwhile the arrival of an upper ridge across the West will first raise temperatures along the West Coast to 10-15F above normal, 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 into the High Plains early next week. 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