Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Oct 8 2020 Valid 12Z Sun Oct 11 2020 - 12Z Thu Oct 15 2020 ***Heavy rainfall from weakening Hurricane Delta over portions of Ohio Valley and Mid-Atlantic*** ***Major pattern change expected for the western U.S. with cooler temperatures into early next week*** ...Overview and Guidance Evaluation/Preferences... Delta will be steadily weakening and becoming post tropical by Sunday as the circulation becomes absorbed by the westerlies over the Ohio Valley. A strong cold front will be crossing the Intermountain West this weekend, and is expected to approach the East Coast by the middle of next week with another Canadian airmass settling in behind the front. By the end of the forecast period, another cold front will likely cross the north-central U.S. as the pattern continues to be amplified aloft over the north-central states. Although there is reasonable model agreement at the beginning of the period on Sunday, differences become quite apparent by Tuesday and especially by midweek, resulting in below average forecast confidence. Some of these differences regard the degree of amplification of the western U.S. trough that eventually emerges over the Plains and then the Great Lakes and Ohio Valley, with the 12Z ECMWF indicating a much stronger trough and then closed low (the latest 00Z ECMWF is not as amplified, but it is faster). The next upper level trough that tries to build in across the West by mid-week has even greater model spread, with the CMC a clear outlier indicating a much stronger trough and outside the ensemble spread, and the GFS/GEFS mean considerably faster than the ECMWF/EC mean. Taking these factors into consideration, more weighting was placed on the ECENS/NAEFS means, along with some previous WPC continuity, for the fronts and pressures for days 4-7, and slightly more GFS/ECMWF for day 3. ...Weather/Hazard Highlights... The remnants of Hurricane Delta are expected to cross the Ohio Valley and the northern Mid-Atlantic region through early Monday, while becoming absorbed by the westerlies and a frontal boundary. This is expected to result in a swath of moderate to heavy rainfall from Kentucky to southern New England, with a swath of 1-2 inch rainfall totals likely, with some higher local amounts possible that might result in some minor flooding. Across the West, a potent trough will favor a period of cooler than average temperatures along the West Coast and into the Great Basin/northern Rockies by Sunday. Precipitation will move through the West with snow likely for the higher terrain of the Cascades and the Northern/Central Rockies into Monday. This very much needed precipitation should provide some relief from ongoing fire weather concerns. Due to the relatively zonal flow/progressive pattern, areas east of the Rockies are expected to see near average temperatures due to down slope flow offsetting the cold advection, even in the wake of a cold front early next week. Mild temperatures for mid-October are also expected across the Great Lakes through Tuesday and across the Northeast next Wednesday ahead of the next cold front. No temperature extremes are currently expected. It is worth noting that given the degree of model spread, changes in future forecasts are almost a certainty, so stay tuned for updates. 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