Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Oct 27 2021 Valid 12Z Sat Oct 30 2021 - 12Z Wed Nov 03 2021 ...Major storm to focus a heavy rainfall/flood threat for parts of the Northeast on Saturday... ...Overview and Guidance/Predictability Assessment... The medium range period starts Saturday with a deep cyclone making its way through the Mid-Atlantic and the Northeast with active weather likely to accompany. Model guidance over the past few days has generally shown good agreement on the large scale evolution and track of this system, with some lingering uncertainties in the details. A 12Z/18Z 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 building 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 will be pushing into the Plains/Midwest 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. A reinforcing shortwave will drop into the northern Plains behind this, which should help to drive the front into the Northeast by early next week. The greatest uncertainty exists over the eastern Pacific and western U.S.. Models continue to show a compact upper low moving into the central/north-central West Coast by Saturday but the specifics still need to be worked out. Latest GFS runs have been a bit on the fast and/or south side of the guidance spread. The UKMET/CMC have gravitated closer to the more northerly ECMWF. After Saturday though, there are lingering issues with the weakening energy that remains from the upper low. 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, eventually possibly shifting southward into the Great Basin late period. This evolution will ultimately 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... Heavy rains from the Eastern U.S. cyclone will be continuing across portions of New England fueled by a sustained Atlantic moisture feed ahead of the front and anchored by the surface low. This likely will bring a flood/runoff threat of some kind to parts of eastern New Hampshire and Maine on Saturday. The front moving through the Northwest on Saturday may bring some light rain/mountain snows to parts of the Great Basin/northern Rockies, expanding into parts of the central Plains by Sunday. Some snow may be possible during the colder overnight hours even at the lower elevations of the High Plains. Confidence in the precise coverage and intensity of rain across these regions though remains low, based on the uncertainty of important details of flow aloft. Locations from the southern Plains into the Midwest and Ohio Valley and eventually the interior northeast should see expanding coverage of rainfall along the potentially wavy front 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, initial below normal daytime highs across the Southeast should moderate back towards normal behind the departing upper closed low as it lifts into the Northeast. The south central U.S. may start warm (+10-15F) on Saturday, but replaced by early next week 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. Santorelli/Rausch 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