Extended Forecast Discussion NWS Weather Prediction Center College Park MD 424 PM 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 for timing and details. A 00Z/06Z model composite represents consensus well for this system and maintains good continuity with the previous forecasts. Behind this system the large scale flow aloft will trend toward a strong ridge over western Canada and a mean trough eventually covering the eastern half of Canada along with the northern lower 48. At the leading edge of the advancing upper trough, a frontal system reaching the Pacific Northwest by early Friday will reach the Plains this weekend. The models have recently come into better agreement with the associated upper-level shortwave as it crosses the northern tier of the U.S. and the upper trough core deepens over central Canada. The front should reach the Northeast by early next week. Greater uncertainty exists over the eastern Pacific and western U.S. A compact upper low should track into the central/north-central West Coast by Saturday but the models are still in the process of resolving the specifics. Latest GFS runs have been a bit on the fast and/or south side of the guidance spread. At least into Saturday the 12Z UKMET/CMC have gravitated closer to the 00Z ECMWF versus their old 00Z runs that were farther north. On the other hand the new 12Z ECMWF has adjusted north of the new UKMET/CMC. The complexities increase by early next week. Not only will there be lingering issues for the weakening energy that remains from this upper low, but also important detail differences for a reinforcing shortwave dropping into the northern U.S. (some combination of the two possibly supporting frontal waviness east of the Rockies). There is also uncertainty over what proportion of eastern Pacific shortwave energy may ultimately cut through a moderate upper ridge that the ensemble means build along or inland from the West Coast--which will play a significant role in the coverage and intensity of any precipitation over the West. These issues tend to have lower predictability by 6-7 days out in time, favoring a transition to a model/ensemble mean blend that provides reasonable continuity while awaiting more confident trends/clustering. ...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 front anchored by the surface low 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 snow 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. This weekend the moisture should sink southward into the Great Basin and the central Plains, with some snow possible during the colder overnight hours even at the lower elevations of the High Plains. From the weekend into early next week there is lower confidence with the precise coverage and intensity of precipitation from the West into the Plains, based on the uncertainty of important details of flow aloft. Locations from the southern Plains into the Midwest and Ohio Valley should see expanding coverage of rainfall along the potentially wavy front by early next week. Most of the rain should be on the light to moderate side but a few pockets of heavier activity could be possible. Some Pacific moisture may reach the northern half of the West Coast early next week but again with low confidence in the details at this time. For temperatures, much of the Southeast/Central Gulf region should see highs up to 10-15F below normal beneath the trough extending south from the upper low tracking into the central Appalachians by Saturday. Highs should trend back towards normal as the low lifts into the Northeast. The central U.S. will see a wide variation of temperatures over the course of the period. First an area of plus 10-15F or so anomalies for highs will drop from the northern Plains/central High Plains into the southern High Plains and then far southern Texas Friday-Sunday. This warmth will be replaced by an expanding area of below normal readings--initially over Montana on Saturday and then reaching at least as far as the central Plains and Midwest by early next week, aided by advancing surface high pressure. Areas from the northern High Plains through the south-central Plains and east into the Middle Mississippi Valley should see one or more days with highs 5-15F below climatological values for this time of year. Anomalies for min temperatures should be less extreme but could still be noteworthy for producing the first freeze in some areas from the central Plains into the Midwest around next Tuesday. Rausch/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 Hazards: - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Fri, Oct 29. - Heavy rain across portions of New England, Sat, Oct 30. - Flooding possible across portions of the Central Plains, the Mid-Atlantic, and the Northeast. - Flooding likely across portions of the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley. - Much below normal temperatures across portions of the Central Plains and Midwest, Tue, Nov 2. 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