Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 PM EDT Tue Sep 26 2023 Valid 12Z Fri Sep 29 2023 - 12Z Tue Oct 03 2023 ...Overview... An amplified trough is forecast to develop over the western U.S. while increasing ridging shifts into the East. Convective rains are possible near a lingering front along southern New England coast Friday into Saturday but with a high degree of uncertainty. Meanwhile, portions of Minnesota could receive heavy rain ahead of a front where it has been wetter than average this past week. As the amplified trough is forecast to close off into a large upper low in the western U.S., higher elevation snows can be expected over portions of the northern Rockies, while moderate to possibly locally heavy rains are forecast for the lower-elevations from areas of the Great Basin to portions of Montana Sunday and Monday. ...Guidance Evaluation/Predictability Assessment... The ensemble means from the GEFS, ECMWF, and CMC show rather good agreement with the synoptic pattern evolution over mainland U.S. through Day 7. Deterministic guidance from the GFS and ECMWF are rather similar as well, except near the East Coast where differences in the placement of the heavy rain near the southern New England coast are quite noticeable. The ECMWF, as well as the EC mean, has been favoring a more northward intrusion of the heavy rain into southern New England compared with the GFS/GEFS, which may be attributed to a stronger upper trough that tends to amplify a surface inverted trough/front together with formation of low pressure waves. Meanwhile, agreement remains very good with the closing/cut off low over the West and ridging over the east-central CONUS. The CMC shows a weaker surface ridge and upper trough along the East Coast but similar to the EC and GFS elsewhere. A 40% blend of the 00Z ECMWF/00Z EC mean, 40% 06Z GFS/06Z GEFS, and 20% from the 00Z CMC/CMC mean was used to composed this morning's WPC medium-range forecasts, with higher percentages from the ensemble means for Days 6/7. ...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 Marginal area has been introduced over the southern High Plains on Day 5 given better model agreement for convection to develop ahead of a lee-side trough. 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. Kong/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