Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 PM EDT Sun Oct 01 2023 Valid 12Z Wed Oct 04 2023 - 12Z Sun Oct 08 2023 ...Heavy rain and flash flooding threat for the south-central U.S. by midweek... ...Overview... An amplified upper trough will be in place with its axis near the High Plains midweek. 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 Wednesday into Thursday. 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... The WPC medium range product suite for this coming Wednesday and Thursday was primarily derived from a blend of the 00 UTC ECMWF/UKMET and ECMWF ensemble mean that along with the 00 UTC Canadian hang back more ample upper troughing over the Southwest than the 06 UTC GFS/GEFS that seem too progressive with energies given upstream upper ridge amplitude. The ECMWF/UKMET and ECMWF ensembles are overall best clustered in this time frame across the forecast domain. Forecast spread increases Friday into the weekend and the models offer less than stellar run to run continuity with mixed ensemble support. Accordingly, prefer more compatible guidance from the 06 UTC GEFS and 00 UTC ECMWF ensemble means, This solution follows a trend with northern stream flow amplification and 12 UTC model trends remain generally in line. Overall, the pattern evolution seems to have near average predictability. WPC product continuity seems reasonably well maintained. ...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. 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 still varies with placement of any heavy amounts, but ensemble and clustering signals are growing to favor issuance of a small Slight Risk area over Texas for the Day 5/Thursday ERO to cover the threat. Some lingering rain may be possible in South Texas late week. Farther north, the cold front should track through the Midwest/Upper Great Lakes and the Ohio/Tennessee Valleys Wednesday into Friday, the into the East heading into the weekend, producing showers and 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, especially considering preference for an amplified supporting 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 and 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 midweek 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. Schichtel/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