Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Fri Oct 06 2023 Valid 12Z Mon Oct 09 2023 - 12Z Fri Oct 13 2023 ...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 and progress east. 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 chances will spread into the Central U.S. midweek and to the East Coast by Friday. ...Guidance Evaluation/Predictability Assessment... Model guidance is in good agreement on the overall pattern with some slight disagreements near the end of the period. The main differences are with how the deep upper level low over Southeastern Canada and the Great Lakes will interact with the approaching trough later in the week. Energy from the trough will be swept up into the low and shunt it northeast while creating an elongated low pressure area from eastern Quebec to the western/southern Great Lakes region. Several shortwaves will move around the low and through the trough during this process, which will complicate timing and positioning of surface lows that form. Models also disagree on how energy from Tropical Cyclone Lidia will move into the mid-latitudes. The GFS and CMC pull the energy north over western Mexico, but previous runs showed the energy potentially being pulled towards Texas. If this energy does get pulled east, it could enhance rainfall chances in the southern U.S. mid/late week. For this forecast cycle, a deterministic blend of the GFS, ECMWF, CMC, and UKMET was used for days 3-5 (Monday-Wednesday) with slightly stronger weight on the ECMWF and GFS. Ensemble means were added to the forecast blend beyond day 5 to create a middle of the road solution that smooths out some of the minor smaller scale differences between the deterministic model runs. ...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 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 still 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 may be considered beneficial. Precipitation chances will expand east throughout the week as a frontal system moves across the nation, reaching the Central U.S. midweek and the East Coast by Friday. Southerly winds ahead of the system will push moisture from the Gulf of Mexico into the southern U.S., which may result in locally heavy rainfall in parts of the South and Southeast Wednesday through Friday. Moisture will also spread up the East Coast by Friday, expanding the chances of heavy rainfall into the Northeast. Daily chances for showers and thunderstorms will persist over the Florida peninsula and increase through the week as a frontal boundary slowly moves north across the state. 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. Dolan 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