Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Sat Oct 3 2020 Valid 12Z Tue Oct 6 2020 - 12Z Sat Oct 10 2020 ***Tropics becoming active again with Tropical Storm Gamma over the southern Gulf of Mexico*** ***Periods of heavy rain for portions of Florida near a stalled front*** ...Overview and Guidance/Predictability Assessment... Broad cyclonic flow is expected across the north-central U.S. and the Northeast through the end of the week with a large scale trough extending southward from Canada. An upper level ridge will initially be present across the western third of the U.S. at the beginning of the forecast period Tuesday before gradually flattening as the westerlies over the northern half of the nation become more zonal. There will likely be a developing trough just off the West Coast by next weekend as the pattern changes over the eastern Pacific. The models and ensembles are in excellent agreement through about Wednesday, particularly across the central and eastern U.S. regarding the upper trough and the shortwave perturbations rotating around it, bringing reinforcing cold frontal passages from the Great Lakes to the East Coast region. By Wednesday night, more noteworthy model differences emerge west of the Rockies as the upper ridge starts to break down, and the evolution of the eastern Pacific trough. The GFS in particular is considerably more amplified with the trough building much farther south than the ensemble consensus, whilst the CMC and ECMWF both indicate a potent trough but keep it farther north, more in line with the ensemble means. This would likely have significant ramifications on the forecast by the end of the forecast period and slightly beyond, with changes likely in the forecast as things become more certain. The WPC fronts and pressures forecast was primarily derived from a blend of ECMWF/CMC/GFS through midweek, before transitioning to a greater contribution from the ensemble means by Friday and Saturday along with some previous WPC forecast continuity. ...Weather/Hazard Highlights... After the cold front and offshore surface low move away from the East Coast, another pristine airmass will settle in across much of the eastern U.S. for early in the week with Canadian high pressure briefly building in before the next cold front arrives from the northwest. This next front probably will not have much moisture available, so most areas should have light to moderate showers at most, followed by another autumnal airmass settling into the region to end the week. Overall, temperatures should be near normal to about 5 degrees below normal for most places. A gradual moderating trend is likely by next weekend as the upper trough lifts out and more southerly flow develops after the surface high moves offshore. The opposite should hold true across the Intermountain West and the western High Plains, with highs running about 5-15 degrees above average and the positive anomalies shifting east from mid to late in the week. A return to cooler conditions with increasing rain chances is appearing likely by next Saturday for the Pacific Northwest in association with the approaching eastern Pacific trough. Farther south in the tropics, Tropical Storm Gamma has formed in the western Caribbean Sea. This storm is forecast to intensify some and track over the northern Yucatan Peninsula and then likely over the Bay of Campeche by early next week. Meanwhile, a stalled front is forecast to linger across South Florida before gradually washing out. Another tropical wave in the wake of Gamma will also cross the western Caribbean and potentially the Gulf of Mexico by next weekend. Although the models have been showing some run-to-run variability on the handling of 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 much of the medium-range period. Hamrick 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