Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM 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 remains in good agreement in the overall pattern through most of the forecast period as a trough over the center of the country earlier in the period will be reinforced by additional shortwave energy, deepening over the Great Lakes as ridging over the West shifts eastward and amplifies in its wake over portions of the north-central U.S. A deep upper-level low is expected to develop in the vicinity of the Great Lakes/Interior Northeast as indicated in the last few runs of the GFS and now a supported trend in the more recent runs of the ECMWF. The CMC eventually also develops a closed low by mid-period. The spread around the exact evolution of the core of upper-level energy associated with this trough expectedly increases mid- to late period, though deep mean troughing should remain in place over Great Lakes/Northeast through the end of the period. To the West, there is a bit more disagreement with both the amplification of the ridging from the western into the north-central U.S. and the approach of upstream energy over the northeastern Pacific. Both the more recent runs of the ECMWF and GFS have varied a bit in how amplified the pattern will be, with the 00Z ECMWF more amplified and the GFS trending less amplified. The ECens mean is a bit more amplified than the GEFS mean, though these solutions at least provide a closer compromise between the more varied deterministic solutions. This will have an impact on the timing and amount of potential precipitation coming into the northwestern U.S. by early next week, with more shortwave energy overspreading the region earlier with the less amplified solutions due to the lack of ridging upstream. Finally, there is a bit of disagreement with respect to a weakness in the southern stream over the Southwest mid- to late period, but this currently looks to have little sensible weather impact due to the lack of available moisture following a strong frontal passage. The updated WPC forecast began with a composite blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS given good overall agreement. The contribution from the CMC was reduced a bit mid- to late period as the timing and evolution of the upper level low over the Great Lakes/Northeast diverged some from the ECMWF/UKMET/GFS solutions. A ramp up to a 50% contribution from the ECens/GEFS means was added by the end of the period as further spread arises with respect to the amplifying pattern over the western/central U.S. ...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. Forecast rain amounts across the guidance have been trending upward, particularly on Saturday, with upslope flow/terrain enhancement likely playing a contributing role for locally heavier totals 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 and into northern New England on Day 5/Saturday. The heavy rain threat will likely be over by Sunday but showers will linger for portions of northern New England along with lake-effect enhanced showers for the Lower Great Lakes as a deep upper-low remains overhead. 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 bring some moderate to locally heavy rain to the Pacific Northwest, eventually spreading into the Northern Great Basin/Rockies. High temperatures across the Plains into Midwest will be below average by 10-20F Friday following a cold frontal passage while one more warmer than average day is possible in the Northeast. 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 cold front(s) march east into the weekend, temperatures should cool to below average across the east-central U.S. 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 and into the High Plains early next week. 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