Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 AM EDT Sun Oct 01 2023 Valid 12Z Wed Oct 04 2023 - 12Z Sun Oct 08 2023 ...Heavy rain will cause flash flooding concerns in the south-central U.S. around midweek... ...Overview... As the medium range period begins Wednesday, an amplified upper trough will be in place with its axis over the High Plains or so. Moist and unstable inflow ahead of the trough and an associated wavy cold front should lead to widespread heavy rain and flash flooding across the southern Plains and perhaps into the Lower Mississippi Valley particularly on Wednesday, with some heavy rain possibly lasting later in the week on a more limited scale. Farther north, the cold front marching eastward should help cause some rain to push across the east-central and eastern U.S. through the latter half of the week, as well as spread cooler than normal temperatures behind it after a warm spell in the Great Lakes/Ohio Valley into the Northeast around midweek. ...Guidance Evaluation/Predictability Assessment... Operational models and ensemble means agree on the general idea of gradual eastward progression of the amplified ridge-trough-ridge configuration but there are notable detail uncertainties that complicate the forecast of sensible weather over some regions. One aspect now phasing into the short range period Tuesday but also affecting Wednesday is a faster surface low track into south-central Canada, which has reduced the QPF compared to a day ago. At least model guidance has converged better with that at this point. Another thorn in the side of the forecast the past couple days has been the potential for some energy to hang back in the southwestern U.S. by around Thursday. The 12Z CMC and UKMET and their newer 00Z runs have been consistent in showing that, while the 18Z GFS did so in weaker form. ECMWF runs had not favored this...up until the currently incoming 00Z run that indicated this possibility. Fortunately though, even though recent 00Z models still have some differences with this energy, at least the shortwave does not look to be terribly strong, and the track of the main trough axis is not as affected. The 12Z CMC and UKMET did end up with a faster main trough track compared to the more agreeable ECMWF/GFS and their means. That 12Z CMC/UKMET at least seemed like a minority solution to lean against for this aspect of the forecast. But will continue to monitor this potential for the southwestern shortwave that remains uncertain. Meanwhile, recent models have stabilized a little better with the evolution of how energy over and upstream from the Gulf of Alaska feeds into the mean trough based on the character of the eastern Pacific ridge drifting toward the West Coast. But models with stronger shortwaves like the 18Z GFS show some brief dampening of the northern periphery of the ridge leading to somewhat broader downstream troughing. The 00Z guidance seems to be somewhat more agreeable but with some variations remaining, especially with the CMC. These differences affect the positions of the leading cold front reaching the East Coast and a secondary cold front traversing the central U.S. through late week. Overall the WPC model preference was for an operational model blend for Day 3/Wednesday, but quickly took out the 12Z CMC and UKMET and replaced them with the 18Z GEFS and 12Z ECens means. Increased the percentage of the means to just over half by Days 6-7 to limit too much individual model influence with their continued variations in the details of the flow. ...Weather/Hazards Highlights... The south-central U.S. will be an area of focus for widespread heavy rain especially on Wednesday, as the right entrance region of the jet will provide ample lift, and plenty of instability is in place just ahead of the cold front. A Slight Risk remains in place there in the Day 4/Wednesday Excessive Rainfall Outlook, as despite the drought conditions, high rain rates that could overcome the dryness are likely. Generally minor adjustments were needed compared to the previous issuances of the Slight Risk, and a Marginal Risk continues to cover some of the seemingly lower-probability UKMET/CMC solutions that indicate heavy rain farther northwest. By Thursday, the cold front will push southeast and clear out rain potential on its backside, but could still provide forcing for convection along and ahead of it into southern and eastern parts of Texas and into the Lower Mississippi Valley. Model guidance is quite variable with rain amounts and placement of any heavy amounts, so have a Marginal Risk for the ERO to cover any threat while awaiting better model agreement. Some lingering rain may be possible in South Texas late week. Farther north, the cold front should track more progressively through the Midwest/Upper Great Lakes on Wednesday, across the Ohio/Tennessee Valleys around Thursday, and into the East by Friday, producing showers and perhaps thunderstorms along and ahead of it. This rain should overall be lighter than farther south, but may be enhanced in the Northeast especially in favored terrain, as the front could begin to tap Atlantic moisture within southerly flow to increase rainfall, depending on the specifics of upper troughing by late week. The amplified trough and the cold front will lead to cooler temperatures than average while the upper ridges to the east and west will support above normal temperatures. The Interior West should see cooler temperatures than average especially through midweek, with with lows 5-10F below normal but highs around 10-15F below average, but warming to near normal for the latter part of the week. Meanwhile warmer than average temperatures by around 15-20F above normal are forecast for the Midwest into the Great Lakes and the Interior Northeast, with highs in the upper 70s to low 80s. Some daily records for highs/warm lows may be possible Wednesday. As the cold front and upper trough march east into the latter half of the week, temperatures will cool to below average across the central and then east-central U.S., limiting the above normal temperatures to the Northeast. A trailing front may reinforce the cooling trend by Friday-Sunday, and there could be concerns for the first frost or freeze of the season to occur in the northern High Plains. Approach/arrival of the eastern Pacific upper ridge should raise temperatures along the West Coast to above average, by as much as 5-15F, and some record warm lows may be possible. Tate/Rausch 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