Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 AM EDT Wed Oct 14 2020 Valid 12Z Sat Oct 17 2020 - 12Z Wed Oct 21 2020 ...Overview... A broad upper-level trough around a deep low spinning near Hudson Bay will remain the dominant feature across inland North America through early next week. It remains likely that moderate to heavy rainfall should occur over the Northeast through Sat, with higher elevation snow possible there. A round of snow is also expected in the Northern Rockies this weekend, with some wintry weather potentially spilling out into the Northern High Plains given cold temperatures there. ...Guidance Evaluation/Preferences... Within the broad troughing pattern, a shortwave is expected to force surface low formation across the Northeast Sat. While positioning of the surface low seems in good agreement with model guidance at this point, the 18Z GFS run did come in with a weaker low than the trend and other guidance, despite its shortwave appearing pretty energetic. Regardless, the low feature is still likely and a multi-model deterministic blend was able to be used there. Then, another shortwave dropping through western Canada into the north-central U.S. by Sun has a bit more model spread with it; despite that, a surface low clusters well just north of the Great Lakes in the deterministic guidance. Larger model differences develop Mon onward. Various additional instances of shortwave energy restructure the trough wavelength and position differently in each model. Across the northern Pacific into the West, multiple ECMWF and EC mean runs have been persistent with flatter flow Mon. The ECMWF then brings energy into the Northwest Tue-Wed, amplifying a trough there so the overall trough axis is farther west. On the other hand, the GFS suite has ridging along the West Coast in place Mon-Wed, leading to a farther east trough axis (and thus the associated cold front is also east). Mainly used a blend of the 18Z GEFS and 12Z EC means for the mass fields in tonight's forecast Mon-Wed to avoid the differences in particular models. Did not have enough confidence or compelling reason to favor one over another at this point, given the spread in the ensemble members. ...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 highest elevations of New England/Adirondacks could see some snow as well. A cool high pressure system coming in behind the cold front associated with that Northeast low will spread below normal temperatures by 5 to 15 degrees across the Plains to the eastern Seaboard through Sat. A potent cold front will cross the northern Plains to Lake Superior this weekend, reinforcing even colder temperatures over the central CONUS. Max temps of 30s and 40s should spread east across the Northern Plains to the Upper Mississippi Valley Sat-Tue, which is 15 to 25 degrees below normal. Min temps in the teens and 20s are 10 to 20 degrees below normal. Precipitation is likely with this frontal system, and notable snow is forecast for the Northern Rockies and into the Northern High Plains. There is some potential for light snow into portions of the Upper Mississippi Valley as well. Then, light to modest rain totals are possible along the frontal boundary passing through the central U.S., but the timing of the front and thus the rain is questionable. The Southwest from West Texas to California and the Central Great Basin will remain under an upper ridge through this weekend, causing dry conditions. Warmer than average temperatures are forecast there, and upper 90s to low 100s are forecast for the lower deserts of Arizona and California. The upper ridge shifts farther west late this weekend, making for more moderate temperatures by early next week. Meanwhile, tropical showers are expected across southern Florida. 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