Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Mon Oct 25 2021 Valid 12Z Thu Oct 28 2021 - 12Z Mon Nov 01 2021 ...Major storm to focus a heavy rainfall/flood threat for parts of the Midwest/Southeast Thursday followed by the Mid-Atlantic and the Northeast Friday and into the weekend... ...Overview and Guidance/Predictability Assessment... An anomalously deep cyclone forecast to develop over the mid-South will likely bring a period of active weather across the eastern U.S. for the latter part of this week and into the weekend. Model guidance have generally shown good agreement on the large scale evolution and track of this system, with some expected uncertainties in its timing and details. A general model consensus appeared to work fine for this system by maintaining good continuity with the previous forecasts. This system may threaten October low pressure records across parts of the mid-Mississippi to Ohio Valley. Over the Pacific Northwest, models are in good agreement to bring a frontal wave onshore with the potential of heavy precipitation especially across the higher terrain. The GFS and GEFS tend to push the associated upper-level shortwave and precipitation farther south during the weekend than the rest of the model guidance. A gradual drying trend should follow as models are indicating that a cold high pressure system will build over western Canada and then extend southward into the northern Plains/Rockies early next week behind the upper trough. Elsewhere, models continue to indicate a possible southern stream upper low approaching California next Sat-Mon but with quite a bit of timing uncertainties. The WPC medium range forecasts were composed using an even blend of the the 00Z ECMWF/00Z EC mean, 06Z GFS/GEFS mean with a smaller contribution from the 00Z CMC and CMC mean. ...Weather/Hazards Highlights... The biggest weather threat during the medium range period will be the deep low pressure system working across the eastern third of the U.S. later this week into next weekend, bringing widespread moderate rain and areas of heavy rains. An anomalous closed upper low will slow frontal system progression and sustain moisture feed from the Atlantic to allow rainfall to expand across the Ohio Valley/Appalachians/Southeast later this week and then the Mid-Atlantic/Northeast into next weekend. Sustained moisture feed interacting with favored orographic lift should focus a possible flood/runoff threat for parts of the central Appalachians Thursday to Friday and into parts of the Northwest during the weekend. Out West, shortwave energy progressing towards the West Coast will direct a plume of moisture to support another round of heavy rain/mountain snows to mainly parts of the Pacific Northwest, but also farther inland across the northern Rockies on Thursday into Friday. Conditions should finally dry out by next weekend though as upper ridging builds over the region. For temperatures, the Midwest/South to the East Coast should remain near or more likely below normal through much of the period underneath cloudy and unstable skies. Humidity levels though should allow overnight mins to be more above normal. Behind this, upper ridging shifting across the central U.S. will initially bring a period of above normal temperatures (+10-15F+) into the northern High Plains before the next shortwave moderates temperatures back towards normal by the weekend. By Sunday into Monday, it appears that a Canadian high pressure system will bring colder than normal readings down across the northern Rockies and then into the northern and central Plains. Kong/Santorelli 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