Extended Forecast Discussion NWS Weather Prediction Center College Park MD 448 PM EDT Fri Oct 06 2023 Valid 12Z Mon Oct 09 2023 - 12Z Fri Oct 13 2023 ...Increasing heavy rain threats for the Midwest and Gulf Coast/northern Florida by mid to late next week... ...Overview... A deep upper level low is forecast to park itself over southeastern Canada and the Great Lakes and churn in place through most of next week. A nearly stationary surface low will create precipitation chances near the Great Lakes early in the week with some lake effect enhancement possible. In the West, a trough will move onshore early in the week diving through the West with some chance for closed upper low development over the Rockies around Thursday. This feature will drive a surface frontal system east across the country through the week. Precipitation chances will be greatest on Monday and Tuesday in the Pacific Northwest where locally heavy rainfall may be possible, then precipitation will spread into the Central U.S. midweek and to the East Coast by Friday. A frontal boundary lifting through the Gulf and Florida will increase precipitation chances across Florida/the Gulf Coast as well by mid to later in the week. ...Guidance Evaluation/Predictability Assessment... Model guidance is in good enough agreement Monday-Wednesday on the overall pattern across the CONUS to use a equal blend of the latest deterministic guidance. After Wednesday, uncertainty increases as the Western trough dives across the Intermountain West (south of a blocky upper high over central Canada) and attempts to close off an upper low over the Rockies which progresses into the Central Plains. There is still a lot of run to run variability in the details of this, but models have been trending more towards the presence of this feature the past few runs. The ensemble means offer a good middle ground solution, but the 06z GEFS mean was notably faster than the rest of the guidance with the surface low next Friday well on the eastern edge of the guidance envelope. The WPC blend for days 6-7/Thursday-Friday used a blend of the deterministic GFS, ECMWF, and CMC along with the ECENS mean late in the period which offered good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... The threat for excessive rainfall is forecast to remain low Monday and Tuesday. The only area of concern is the Pacific Northwest where moisture will stream onshore ahead of an approaching frontal system. Locally heavy to possibly excessive rainfall will be possible at times due to the combination of surface lift and upper level dynamic support for ascent, and upslope areas of the coastal mountain ranges, Olympics, and Cascades will likely see the highest rainfall totals. A Marginal Risk of Excessive Rainfall area was considered, but was not introduced for either day at this time because the system seems quite progressive, a stretch of dry weather will precede this event, and it is early in the wet season. These factors will all limit the flash flood potential. Much of the Pacific Northwest is still experiencing drought conditions, so much of this rainfall could be more beneficial than hazardous. Precipitation chances will expand east throughout the week as a frontal system moves across the nation, reaching the Central U.S. by around midweek. A warm front lifting through the east-central U.S. will provide focus for possibly heavy rain from the Central Plains into the Midwest Wednesday-Friday. Meanwhile, a separate warm front lifting through the Gulf of Mexico and the Florida Peninsula brings a threat for heavy rainfall to mainly the Florida Panhandle and northern Florida starting Wednesday. This may be aided by an area of low pressure in the Gulf of Mexico and possible remnant tropical moisture. Temperatures will be cooler than normal during the first half of next week from the Midwest to the Northeast underneath the upper low. High temperatures are forecast to be 10-15 degrees below average, and overnight lows will be cold enough for frost to form in parts of the Upper Midwest and Great Lakes region. Above average temperatures are forecast to start the week across much of the Central U.S. and the West, but temperatures will drop through midweek as the frontal system moves through. Santorelli Hazards: - Heavy rain from the eastern portion of the central Plains into portions of the Midwest, Wed-Thu, Oct 11-Oct 12. - Heavy rain across portions of the Gulf coast, the Southeast, and into South Carolina, Wed-Fri, Oct 11-Oct 13. - Flooding occurring or imminent across portions of northeastern Florida. - Frost/freeze across portions of the northern Plains into the upper Midwest, Mon-Tue, Oct 9-Oct 10. 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