Extended Forecast Discussion NWS Weather Prediction Center College Park MD 223 AM EDT Wed Sep 29 2021 Valid 12Z Sat Oct 02 2021 - 12Z Wed Oct 06 2021 ...Overview... The medium range period will begin Saturday with a deep upper low still influencing the Northeast, with multiple shortwaves tracking across the central U.S. Some semblance of a Rex block may develop by later in the period as an upper high tracks across southern Canada as low pressure in the mid/upper levels gets trapped to its south over the Midwest. ...Guidance Evaluation/Preferences... Model guidance has been consistent for several model cycles regarding an upper low centered over southeastern Canada over the weekend with its core lifting out to the Northeast early next week. Shortwave troughs across the southern/central High Plains, the Northern Rockies, and the Northern Plains (Saturday positions) have some differences, but along the line of what is expected for small scale features in the medium range period. The main area of concern from a model diagnostics standpoint remains the central U.S. beginning early next week. The aforementioned shortwaves may combine with energy splitting off and tracking westward from the main upper low, which will likely develop troughing and potentially a closed low generally over the Midwest. This is in conjunction with an upper ridge building across the Great Basin/Rockies through Monday and possibly forming a closed high as it shifts eastward over south-central Canada Tuesday, blocking the low from the northern stream. At the surface, model consensus suggests a low pressure/frontal system could push southeastward across the Great Lakes and Ohio Valley through the early part of next week. While there are some model differences in the formation and orientation of all these features, a consensus blend of the 12/18Z GFS, the 12Z ECMWF, and their ensemble means seemed like the best approach for the WPC fronts/pressures forecast. ...Weather and Potential Hazards... Moisture spreading into the central and eastern U.S. will continue to cause generally moderate rain ahead of the cold front expected to track from the Plains/Mississippi Valley over the weekend into the Ohio Valley, Appalachians, and Eastern Seaboard by the early part of the workweek. Rainfall totals could also be enhanced in the vicinity of a stationary front extending east of the surface low in the Ohio Valley. Additionally, some moisture may return to the southern Rockies by Monday/Tuesday, while scattered showers and thunderstorms are possible for southern Florida through the period. Warmer than average temperatures are forecast for the Midwest and Ohio Valley especially in terms of lows, but even highs should be around 10 degrees above normal on Saturday, though are likely to moderate to near normal by Monday. The Northern Rockies/Northern Plains can expect highs around 10 to 15 degrees above average for the first half of next week underneath the upper high forming. Meanwhile, temperatures a few degrees below normal are forecast for the Four Corners states and into southern/central parts of the Plains through the period. Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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