Extended Forecast Discussion NWS Weather Prediction Center College Park MD 225 AM EDT Tue Oct 13 2020 Valid 12Z Fri Oct 16 2020 - 12Z Tue Oct 20 2020 ...Overview... A broad upper-level trough is forecast to be the dominant feature over the eastern two-thirds of the country and should spread below normal temperatures across those areas. There is increasing confidence in the potential for moderate to heavy rainfall across the Northeast for the end of the week as a surface low develops there, while a round of rain and snow is possible for the Northern Rockies and Northern High Plains. ...Guidance Evaluation/Preferences... As an upper low spins over the Hudson Bay, the general longwave pattern of an upper trough persisting across the central and eastern U.S. remains well predicted. Within the broad trough, model guidance has come into better agreement recently regarding a shortwave moving into the Northeast around Sat, helping create a surface low pressure system along a pivoting front. Then another shortwave may come into the north-central U.S. by the weekend, but some differences in its evolution remain. This affects the placement and strength of the associated surface low, but the general idea of the shortwave developing a low moving across the Northern Plains to Great Lakes is there with the bulk of the models at this point. For these features, a multi-model deterministic blend generally sufficed to minimize any particular model solution. By Mon/Tue, the wavelength of the longwave trough does start to vary between guidance. One questionable area is with the potential for ridging to the west of the trough axis. While the 12Z/18Z GFS and 12Z CMC runs show an upper ridge axis along the West Coast west of the trough, the 12Z ECMWF has flatter flow. There is plenty of variability in ensemble members there, and tonight's blend leaned toward the GEFS and EC ensemble means on Mon/Tue to handle the uncertainty. ...Weather/Hazard Highlights... A low pressure system is expected to develop over the Northeast late in the week, and bring the potential for moderate to heavy rainfall along with thunderstorms to the region. This could help with drought conditions across much of the Northeast. The higher elevations of New England could see some snow as well. A cool high pressure system coming in behind the cold front associated with that low should spread below normal temperatures by 5 to 15 degrees from the Plains to the Eastern Seaboard through the weekend. Another low and frontal boundary is forecast to come through the north-central U.S. over the weekend, which will bring even colder temperatures to that region. Highs in the Northern Plains/Upper Mississippi Valley may only reach the 30s and 40s on Sun/Mon. Additionally, a round of precipitation is forecast for the Cascades, Northern/Central Rockies, and Northern/Central High Plains through the weekend. Some notable wintry precipitation is likely in the higher elevations and could even reach lower elevations farther east. Meanwhile, the Southwest into California and the Central Great Basin should be under the influence of an upper ridge south of the main trough. Warmer than average temperatures are forecast there, and upper 90s to low 100s are forecast for the lower deserts of Arizona and California, which could be near high temperature records at some locations. Meanwhile, tropical showers are possible across southern Florida through the period. Tate 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