Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Tue Oct 26 2021 Valid 12Z Fri Oct 29 2021 - 12Z Tue Nov 02 2021 ...Major storm to focus a heavy rainfall/flood threat for parts of the Mid-Atlantic and the Northeast Friday into Saturday... ...Overview and Guidance/Predictability Assessment... An anomalously deep cyclone, capable of threatening October low pressure records for some places in the Ohio Valley, will be tracking into the Ohio Valley and Mid-Atlantic by the start of the medium range period on Friday. This 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 over the past few cycles has generally shown good agreement on the large scale evolution and track of this system, with some lingering uncertainties in its timing and details. A general model consensus continues to work well for this system and maintains good continuity with the previous forecasts. A frontal system will progress from the Pacific Northwest on Friday, into the Central U.S. this weekend. The models have recently come into better agreement with the associated upper-level shortwave as it skirts the northern tier of the U.S. and deepens over central Canada. A second shortwave dropping into the Northern Rockies/Plains behind this one will reinforce the cold frontal boundary driving it into the Midwest, Ohio Valley, and south. There is generally good agreement on the large scale, but enough uncertainties in timing and details to warrant more support from the ensemble means in the WPC forecast by days 6-7. Elsewhere, models continue to indicate a possible southern stream upper low approaching California this weekend, with some lingering uncertainties. This feature should weaken over the Great Basin as the next trough slides eastward towards the West Coast early next week. There is again plenty of uncertainties regarding timing and details of the upper system and the associated surface front that a trend towards the ensemble means is best for now (complimented by the deterministic GFS and ECMWF). ...Weather/Hazards Highlights... The biggest weather threat during the medium range period will be the deep low pressure system working across the Eastern U.S. later this week into next weekend, bringing widespread moderate to heavy rain. By Friday, the heaviest rainfall should be impacting much of the Mid-Atlantic and into the lower Great Lakes, lifting into the Northeast by Saturday. A sustained Atlantic moisture feed ahead of the surface front will likely focus a possible flood/runoff threat across parts of these regions. The front moving through the Northwest will continue organized rainfall and mountain snows across parts of the northern Rockies/Pacific Northwest on Friday, with conditions finally beginning to dry out by next weekend as upper ridging builds behind the departing trough. Moisture should sink southward this weekend into the Great Basin to the central Plains, with some snow possible during the colder overnight hours, even at the lower elevations of the High Plains. Rainfall should begin expanding along the front by early next week from parts of the southern Plains into the Midwest and Ohio Valley. For temperatures, much of the Southeast/Central Gulf region should be as much as 10-15 degrees below normal underneath of the upper low, but should trend back towards normal as the low lifts into the Northeast. A period of above normal temperatures across the Central U.S. to begin the period will be replaced by an expanding area of below normal values by late this weekend and early next week as surface high pressure filters into the region. Daytime highs from the northern High Plains into the south-central Plains and mid-Mississippi Valley could be 5 to 10 degrees below climatological values for this time of year. 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