Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 AM EDT Mon Oct 02 2023 Valid 12Z Thu Oct 05 2023 - 12Z Mon Oct 09 2023 ...Heavy rain and flash flooding threat could continue in parts of Texas Thursday, and may ramp up in the Northeast late week... ...Overview... An amplified upper trough and an associated wavy cold front will push through the central and eastern U.S. for the latter part of the week, bringing rain chances ahead and below average temperatures behind them. Rain and thunderstorms could be particularly heavy in the southern Plains into Thursday, while by Friday and Saturday moisture could increase in the northeastern U.S. ahead of the front and possibly lead to heavy rain. 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 large scale pattern described above, but there are some differences in the details of the trough and the timing of it moving eastward through late week. One aspect that varies among guidance is the exact evolution of the trough dependent on uncertain energies within it. Relatively minor differences like the width of the trough caused by shortwave positions affect the frontal positions and the associated QPF. For example, the 12Z ECMWF and CMC had the eastern edge of the trough and thus the front/QPF farther east than GFS runs by Friday-Sunday. By the weekend the differences affect if or when a closed low may form within the trough, which GFS runs have steadily indicated. Overall the incoming 00Z guidance shows the trough's track on the slower side, and despite there being a closed low or not depending on the model run, there looks to be a negatively tilted trough and room for moisture to tuck in just east of it. The new 00Z ECMWF does eventually form a closed low by early next week. The WPC forecast was based on a deterministic model blend particularly favoring the 18Z GFS and 12Z ECMWF early in the period, transitioning to incorporate the ensemble means to half by Day 6 and over half Day 7 as model spread increased. This blend also worked well for the upper ridge to the west of the trough, and for the next trough over the eastern Pacific that becomes a factor in the forecast by late week. Continuity from the previous forecast was reasonably well maintained, though with the 00Z guidance looking slower with the northeastern trough, the next shift may trend toward that, which would lead to slower QPF movement through the Northeast this weekend. ...Weather/Hazards Highlights... Moist and unstable inflow will meet up with the cold front pushing through the south-central U.S. on Thursday, which along with the right entrance region of the jet providing ample lift will lead to a continued heavy rain threat mainly in Texas. A Slight Risk remains in place in the Day 4/Thursday Excessive Rainfall Outlook for this convection that could produce high rain rates. The front should continue to push south with time and limit the areal coverage of rainfall, but some may linger into South Texas late week, and a Marginal Risk is delineated there in the ERO for Friday. Farther north, the cold front should track through the Midwest/Upper Great Lakes and the Ohio Valley Thursday into Friday. There is a nonzero threat of embedded heavy rain with this activity, but hopefully the frontal progression will keep showers and thunderstorms moving quickly enough not to cause flash flooding issues. However, this could change by Friday and Saturday as the tilt/placement of the trough and front may allow for Atlantic moisture to stream in and increase heavy rainfall potential as the front gets into the northern Mid-Atlantic/Northeast. A Marginal Risk is planned for the Day 5/Friday ERO across the Lower Great Lakes region and parts of the northern Mid-Atlantic and Northeast, as despite some lingering model discrepancies heavy rain seems like a good possibility, which could cause flooding issues especially in areas with low Flash Flood Guidance. Heavy rain and flooding concerns will likely continue into Saturday for New England. Much of the CONUS should dry out by Sunday with the exception of some lake effect rain showers in the Great Lakes region and any moisture lingering into Maine, southern Texas, and Florida. The Pacific Northwest though will likely see a ramp-up in precipitation chances by Monday as a front and upper trough approach. The amplified trough and the cold front will lead to cooler temperatures than average, while the upper ridges to the east and west support above normal temperatures. The Lower Great Lakes region into the Northeast should see warmer than average temperatures by 10-20F on Thursday, with highs in the upper 70s to low 80s. But as the upper trough and the primary and secondary cold fronts march east into the latter half of the week, temperatures will cool to below average across the central and then east-central U.S., eventually making their way to the Eastern Seaboard by early next week. The first frost or freeze of the season may occur in the northern High Plains behind the cold fronts late this week. In the eastern half of the U.S. lows will generally be around 5-10F below average while highs should be more like 10-15F below average, locally cooler. Meanwhile the arrival of an upper ridge across the West will first raise temperatures along the West Coast to 10-15F above average, 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