Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Tue Oct 29 2019 Valid 12Z Fri Nov 01 2019 - 12Z Tue Nov 05 2019 1530 UTC Update... Overall, changes to the ongoing forecast from overnight were relatively minor. The forecast was updated to reflect a blend of the latest deterministic guidance (ECMWF/GFS/UKMET) during days 3-5 (Fri-Sun), with days 6-7 (Mon-Tue) based more heavily on the ECENS/NAEFS ensemble means along with continued deterministic ECMWF (which fit reasonably well within the ensemble-based consensus). The one change of note involved the clipper system crossing the northern Plains/Midwest/Great Lakes Mon-Tue, where models and ensembles showed a trend toward a slower progression. This trend was reflected in the updated WPC forecast. Ryan Previous Discussion (issued at 0657 UTC)... ...Overview and Guidance/Uncertainty Assessment... Latest models and ensemble means continue to show an amplified pattern whose large scale tends to yield better than average predictability, featuring a northeastern Pacific ridge and downstream trough whose multi-day average axis should be over east-central North America. In contrast to the short-range system affecting the lower 48 and forecast to be over southeastern Canada/New England as of early Fri--which has been plagued by high spread and variability in the guidance for multiple days--solutions cluster remarkably well in most respects through the middle of the extended period. Not surprisingly shortwave details become more uncertain toward days 6-7 Mon-Tue. Based on guidance through the 18Z cycle, a blend of the 18Z GFS and 12Z ECMWF/UKMET provided a good representation of the system near New England early Fri with a position near an average of the 18Z GEFS/12Z ECMWF means. Review the Model Diagnostics Discussion PMDHMD for updated model preferences incorporating the new 00Z guidance in the short range time frame through 12Z Fri. Behind this system the aforementioned blend also reflected consensus for a couple upstream features, a central/eastern U.S. front with anchoring surface low tracking over southern Canada/Lake Superior and an upstream wave reaching just north of the northern Plains by early day 5 Sun. The rest of the forecast trended toward an even model/mean blend consisting of the 18Z GFS/12Z ECMWF and their ensemble means, with model details beginning to diverge somewhat but still in the proverbial ballpark relative to the GEFS/ECMWF means that were quite similar to each other through day 7 Tue. This solution would have the southern Canada wave on Sun track over/near the Upper Great Lakes Mon and northeast thereafter while the trailing front pushes into the central U.S. in a fashion similar to the Fri-Sat front. The relative agreement of the GEFS/ECMWF means conceals some of the individual member spread that becomes evident over the northwestern U.S. and western Canada toward the end of the period. The spread becomes more dramatic when adding in some CMC ensembles that develop more troughing than most GEFS/ECMWF members. Over the past couple days the multi-day means by D+8 have been differing over exactly where the strongest positive height anomaly center(s) will be within an area covering the northeastern Pacific/Gulf of Alaska through the eastern Bering Sea. These differences lower confidence in determining exactly how shortwaves rounding the ridge may amplify downstream late in the period and beyond. Along and offshore the California coast there is a converging trend regarding potential for a weak upper low, whether from one individual feature or consolidation of multiple impulses. GFS/GEFS runs are still somewhat deeper than other guidance but an even blend between them and the ECMWF/ECMWF mean is reasonable given some recent trends toward the GFS idea in principle. ...Weather Highlights/Threats... The East will see a rapid drying trend while brisk to strong winds over the Northeast on Fri will slacken as low pressure lifts northeastward away from New England. Northern tier areas will see periods of mostly light rain and/or snow with a couple waves/frontal passages, including some enhancement to the lee of the Great Lakes. Specifics become more uncertain after this weekend. The front extending south from the departing Fri system will settle over the Florida Peninsula, providing some focus for occasional rain (mostly light) as easterly low level flow becomes established. Some areas over the Rockies and Plains will see temperatures 10-20F or so below normal Fri-Sat followed by readings closer to normal. Another front dropping south from Canada may bring modestly below normal temperatures during the first half of next week. Meanwhile during the weekend a modified form of the chilly Plains air will reach the East with highs/lows generally 5-15F below average. Eastern areas should return close to normal by Mon-Tue. The West will see a gradual increase in coverage of above normal highs but with anomalies tending to be around plus 10F or less. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Flooding possible for parts of the Middle Mississippi Valley, Great Lakes, and Northern Plains. - Flooding occurring or imminent across portions of the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Northern Plains. - High winds across portions of the Great Lakes and the Northeast, Fri, Nov 1. - Much below normal temperatures across portions of the Central and Southern Rockies/Plains, the Tennessee Valley, the Mississippi Valley, and the lower Ohio Valley, Fri-Sat, Nov 1-Nov 2. - Heavy precipitation across portions of the Alaska Panhandle, Fri, Nov 1. 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