Extended Forecast Discussion NWS Weather Prediction Center College Park MD 453 PM 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. Fast-moving shortwave troughs rounding the base of the broad upper trough will bring rounds of precipitation particularly across the northern tier states with relatively frequent frontal passages there. Some wintry precipitation appears to be in the offing across the central Rockies, the upper Great Lakes, and northern New England by next Sunday. ...Guidance Evaluation/Preferences... Model guidance continues to indicate two areas of forecast uncertainty in the U.S. during the medium range period. The first area to watch is over New England for late this week into the weekend where the ECMWF has been consistently indicating the likelihood of cyclogenesis while a more progressive pattern has been indicated by the GFS. The most recent runs of the GFS and GEFS have trended in the direction of the ECMWF. The CMC, on the other hand, now indicates cyclogenesis much farther to the south off the Mid-Atlantic coast which appears to be an outlier. However, the latest (12Z) ECMWF has trended toward that direction, although not as far south as the CMC. The other area of concern is over the Pacific Northwest where a shortwave developing in western Canada could dip further down into the western U.S. as indicated by the ECMWF. The last couple of runs from the ECMWF have backed off on the southward plunge of this feature more in line with the GFS solutions. The WPC medium-range prognostic charts were derived from a general compromise of the 00Z ECMWF/EC mean and the 06Z GFS/GEFS. The 00Z CMC was excluded from the blend since it appears to overdevelop both of the systems of concern. By Day 6 and 7, mainly the ensemble means from the EC and GEFS were used to handle the uncertainty. ...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. It appears that heavy rainfall is possible over New England from Friday to parts of Saturday as a low pressure system is forecast to develop and intensify. The trailing cold front from the cyclone 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. 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. Some meaningful wintry precipitation appears to be possible across the central Rockies, the upper Great Lakes, and northern New England next Sunday. 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. Kong/Tate WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: Hazards: - Heavy precipitation across portions of the Northern Rockies, Sun, Oct 18. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Fri, Oct 16. - Flooding possible across portions of the Southeast and the Southern Appalachians. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Appalachians. - Flooding likely across portions of the Southeast and the Southern Appalachians. - High winds across portions of the Central Plains and the Southern Plains, Thu, Oct 15 and Sat, Oct 17. - Much above normal temperatures across portions of the Central Great Basin and California, Thu-Sat, Oct 15-Oct 17. - Much below normal temperatures across portions of the Upper/Middle Mississippi Valley, the Northern/Central Plains, and the Northern Rockies, Sun-Mon, Oct 18-Oct 19. 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