Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 AM EDT Tue Sep 26 2023 Valid 12Z Fri Sep 29 2023 - 12Z Tue Oct 03 2023 ...MEDIUM RANGE GRIDS ARE CURRENTLY UNAVAILABLE...THE DISCUSSION BELOW DESCRIBES THEIR BASIS... ...Overview... An amplified trough is forecast for the West while increasing ridging shifts into the East. Convective rains are possible across portions of the Mid-Atlantic coast, New England coast, and portions of Minnesota which are areas that have been wetter than average this past week, while higher elevation snows settle in to the northern Rockies. ...Guidance Evaluation/Predictability Assessment... Deterministic guidance shows very good agreement with the expected flow pattern of a closing/cut off low over the West and ridging over the east-central CONUS. Blending the 12Z ECMWF/CMC/UKMET and the 18Z GFS worked well through Day 5 with inclusion of the 12Z ECENS and 12Z NAEFS means for Days 6/7. QPF grids were generally a blend of the 01Z NBM, 18Z GFS, 12Z ECMWF, and 12z Canadian. ...Weather/Hazards Highlights... A stalled front over the Florida should progress southward as a frontal wave forms to their east, progressing past Bermuda ahead of a progressive and positively tilted upper level trough. For Day 4/Friday into Saturday morning, the excessive rainfall outlook continues to have portions of Florida in the Marginal Risk as instability lurks nearby across the very warm subtropical north Atlantic and Gulf of Mexico, the 850 hPa slice of a front helps focus activity, precipitable water values at or above 2.25", and some enhanced cooler east-northeast flow aloft all conspire to potentially leading to efficient showers and thunderstorms. Since the signal for heavy rainfall across the coastal Mid-Atlantic, southern New England, and North Woods of MN has come up and there appears to be instability available along nearby fronts/convergence zones and precipitable water values are up in the 1.25-1.5" range, went ahead and added a couple more Marginal Risks to cover the excessive rainfall possibility. The risk areas generally cover regions where rainfall this past week has been a bit above average, leaving them more sensitive than usual. For Day 5/Saturday into Sunday morning, the signal for heavy rainfall across portions of the FL peninsula continues due to instability from the very warm subtropical north Atlantic and Gulf of Mexico, precipitable water values over 2.25", the 850 hPa slice of the front shifting slowly southward while helping to focus activity, and enhanced east-northeast flow just behind it advecting some cooler air aloft all potentially leading to efficient showers and thunderstorms, so added a Marginal Risk area. The risk area covers regions where rainfall this past week has been a bit above average, leaving the areas more sensitive than usual. A digging western trough will close into a cut off low centered over the Great Basin this weekend and will focus moderate precip over the Intermountain West through northern Rockies, and northern High Plains. Snow at higher elevation becomes increasingly favored with time across the Northern Rockies as the upper level low develops and it cools off aloft. Portions of central Montana appear to have some instability available to their east and enough moisture to wonder about excessive rainfall, the signal is not strong enough for a Marginal Risk at this time. Increased ridging over the center of the country and troughing across the West, will slowly shift temperatures of 10 to 15 degrees above normal east across the Central Plains through the Great Lakes with temperatures below normal expanding from the Northwest to across the West. Below normal temperatures over the Mid-Atlantic States can be expected into Friday before moderating on approach of the ridge axis. Roth 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 No significant hazards are expected over the CONUS during this forecast period. 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