Extended Forecast Discussion NWS Weather Prediction Center College Park MD 404 PM 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 start of the medium range period on Saturday will feature a deep cyclone tracking northeastward from the central Appalachians and Mid-Atlantic, likely accompanied by active weather especially within the tight gradient to the north of the system. Model guidance has maintained reasonably good agreement and continuity over the past few days and once again a consensus model blend provides a good starting point. Behind this system, model and ensemble guidance generally expects the large scale upper pattern to evolve toward a strong western Canada ridge while downstream troughing drifts from central into eastern Canada and extends into portions of the central/eastern U.S. Meanwhile a compact upper low should reach near the central/north-central West Coast by the start of Saturday followed by upstream Pacific energy approaching by Monday. Guidance is still having difficulty in resolving the ultimate fate of the initial upper low (split between getting stuck/shearing over Oregon versus continuing east-southeast; new 12Z ECMWF/UKMET/CMC side with the former) as well as what proportion of next week's Pacific energy will filter through a general mean ridge that most guidance develops over the West Coast/Interior West from late weekend through at least the first half of next week. In addition, issues with the eastern Pacific into western North America flow have a meaningful influence on the orientation of the central/eastern U.S. trough. In terms of larger-scale considerations involving 00Z/06Z guidance, the 06Z GFS and eventually the 00Z CMC provided the least appealing solutions. From early in the period the 06Z GFS showed more southwestward amplitude versus consensus for the upper trough crossing the northern tier U.S., ultimately leading to a more southern track of the upper low in Canada and slower progression for the cold front crossing the central U.S. The 06Z GEFS mean offered some support but the new 12Z GFS/GEFS adjusted back toward the majority scenario. Then later in the period the 06Z GFS was quick to erode the western Canada ridge and thus pushed the upper trough farther into the eastern U.S. than consensus. Meanwhile the 00Z CMC brought a fairly slow moving upper low into the West by next Tuesday-Wednesday--in contrast to other guidance and teleconnection relations relative to the core of positive height anomalies over north-central Canada. ECMWF runs have been bringing some energy into the West but are at least progressive enough to maintain the theme of mean ridging. The new 12Z CMC compares much better to guidance than the prior run. Downstream, recent GFS/ECMWF model runs as well as the full ensemble spread do not inspire confidence in the shape/orientation of the overall central-eastern U.S. trough, and likewise for the ultimate eastward/southward extent of the leading surface front. Latest trends appear to favor somewhat more of a positive tilt for the trough, thus pulling back the surface front somewhat by the middle of next week. Teleconnections appear to suggest fairly high sensitivity to the exact location of the positive height anomaly center over Canada, favoring a conservative approach to the forecast. Forecast considerations and guidance comparisons led to starting with a blend of the 00Z operational models for about the first half of the period followed by a trend toward the 00Z GFS/ECMWF, 06Z GEFS/00Z ECMWF means, and some WPC continuity to provide an intermediate adjustment between previous forecast latest guidance. ...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 anchored by the surface low. This likely will bring some degree of a flood/runoff threat 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 north-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 precipitation across these regions remains low, based on the uncertainty of important details of flow aloft. Locations from the southern Plains into the Midwest/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 there is a developing signal for some heavier totals over or near the southern Plains by next Tuesday or Wednesday. Pacific shortwave energy and moisture may bring one or more episodes of precipitation to central/northern parts of the West Coast Monday-Wednesday, still with fairly modest confidence in the details at this time. The Southeast will start the weekend with some highs 10-15F below normal. Expect temperatures over the region to trend closer to normal for the rest of the period as the Mid-Atlantic/Northeast system lifts away. The southern half of the High Plains will start warm (highs 10-15F above normal) on Saturday, but then an expanding area of below normal readings will settle over the central U.S. for multiple days. The chilly air should reach Montana on Saturday and then push into the central Plains and Midwest from late weekend through the first half of next week. Areas from the northern High Plains through the south-central Plains and east into the Middle Mississippi Valley/Ohio 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 Hazards: - Heavy rain across portions of the Northeast, Sat-Sun, Oct 30-Oct 31. - Flooding possible across portions of the Middle Mississippi Valley, the Northeast, and the Mid-Atlantic. - Flooding likely across portions of the Great Lakes, the Middle Mississippi Valley, the Mid-Atlantic, the Pacific Northwest, and the Northern Great Basin. - Much below normal temperatures across portions of the Central Plains, the Great Lakes, the Middle Mississippi Valley, and the Southern Plains, Tue-Wed, Nov 2-Nov 3. 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