Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 PM EDT Fri Oct 02 2020 Valid 12Z Mon Oct 05 2020 - 12Z Fri Oct 09 2020 ...Tropical depression Twenty-five forms in the western Caribbean...heavy rain remains a threat for the southern half of the Florida Peninsula during much of next week... ...Overview and Guidance/Predictability Assessment... Global models continue to show excellent agreement on the evolution of the synoptic pattern that will influence the U.S. through the medium range period. A longwave trough over much the eastern two-thirds of the country will likely persist into the latter part of next week as upper ridging gradually loses strength over the western U.S. A couple of differences were noted in this general pattern. For the East Coast, models continue to indicate run-to-run variability regarding the potential for cyclogenesis south of New England next Monday into Tuesday. The GFS remains the fastest and flattest in forecasting the incipient upper trough. The ECMWF and CMC both show some degree of cyclogenesis south of Long Island although not as robust as it was a couple of days ago. The latest runs NAM now show a rather rapid deepening of the cyclone in the same general area. The forecast period begins on Monday with an eastern U.S. trough and an upper level ridge over the Desert Southwest and West Coast region, and the trough over the eastern U.S. is reinforced by a second shortwave and cold front arriving from the Great Lakes region. The ridge axis gradually moves east over the western High Plains and loses some amplitude as the flow becomes more zonal over the north-central U.S. towards the end of the week. For the West Coast, the ECMWF has been indicating that a slightly deeper upper vortex just off to the west of southern California will slowly move northeast into northern California and then the Intermountain region later next week. A weaker disturbance is depicted in the GFS solutions. This will help erode the upper ridge and moderate the very warm conditions across the West. The WPC morning medium range package was derived from a blend of the 00Z ECMWF/EC mean with the 06Z GFS/GEFS. The ECMWF solutions were preferred on Days 3 to 4 to handle both the West Coast and East Coast systems. The blend then places increasing weights toward the EC mean and GEFS ensemble mean from Days 5 to 7. ...Weather/Hazard Highlights... Although details remain less than certain at this time, a nor'easter-type low pressure system is expected to develop over the offshore waters of the northern Mid-Atlantic or southern New England next Monday into Tuesday. This could bring some rain and wind impacts for portions of southern New England to northern Mid-Atlantic before quickly shifting offshore. There will likely be some showers in the vicinity of the Great Lakes and the Ohio Valley through midweek as an upper level trough and a low pressure system approach from the west. Farther south in the tropics, tropical depression Twenty-five (T.D. 25) formed in the western Caribbean Sea this morning. The depression is forecast to intensify and track just north of the Yucatan Peninsula early next week. Meanwhile, a stalled front is forecast to linger across southern Florida before gradually washing out. Another tropical wave in the wake of T.D. 25 will also come into the picture as it will likely push west-northwest across Cuba. Although models have been showing run-to-run variability on handling these systems, it appears that the southern half of the Florida Peninsula will be under a continued threat of heavy showers and some thunderstorms through the medium-range period. The upper ridge persisting across the West should continue to result in hot and dry conditions, along with associated wildfire risks. Expect temperatures to be 5-15 degrees above normal from the Desert Southwest to the Northwest, and this will likely expand eastward to the central/southern Plains by the middle of next week. A cooling trend should then commence across the Pacific Northwest by next Thursday. For the eastern U.S., the cold air surge early in the week will moderate by midweek before the next surge of cold air arriving across the Great Lakes later in the week. Kong WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml