Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Mon Oct 12 2020 Valid 12Z Thu Oct 15 2020 - 12Z Mon Oct 19 2020 ...Overview... A broad upper-level trough is forecast to be the dominant feature over the eastern two-thirds of the country with below normal temperatures settling into the area through next weekend. Rounds of precipitation are expected particularly across the northern tier states in conjunction with relatively frequent frontal passages there. ...Guidance Evaluation/Preferences... Recent model guidance continues to show good agreement for upper troughing across the central/eastern U.S. at the beginning of the medium range period. Thus a multi-model deterministic blend was able to be used for Thu-Fri. Greater model differences arise in two areas by the weekend--first, a shortwave rounding the eastern side of the trough may help form a surface low over New England Sat. This seems to be coming into better agreement with non-NCEP deterministic models as well as many ensemble members, but GFS runs still do not show this low development and once again progress the frontal boundary farther east. Since it seems the trend is toward low formation, leaned away from the GFS suite of models for this feature. Secondly, the potential for a shortwave coming into the West around Sun remains in question as several runs of the GFS and now the 12Z CMC have come in without developing this shortwave, leaving the ECMWF generally on its own with development of this feature. The ECMWF has been persistent though and there is a large amount of ensemble spread. Thus leaned toward the ensemble means to minimize the influence of any particular model by early next week. ...Weather/Hazard Highlights... A low pressure system and its associated cold front is forecast to move across the Great Lakes region to Northeast, bringing showers and perhaps thunderstorms to those areas for the latter part of the week. Some lake enhancement of rain totals can be expected. If the aforementioned surface low develops in New England on Fri-Sat as some model guidance predicts, higher rain totals would be supported for that area as well. The cold front will end the warm temperatures across the south-central to eastern U.S. as cooler temperatures of 5 to 15 degrees below average spread from west to east across the Rockies to the Eastern Seaboard. Through early next week, 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. In the Northwest toward the Northern High Plains, a round of precipitation is possible Fri-Sat along a frontal boundary, with higher elevation snow across the Northern Rockies. Cold air is expected to be reinforced by a cold surface high across the north-central U.S. by Sun-Mon. Meanwhile, 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