Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Nov 16 2021 Valid 12Z Fri Nov 19 2021 - 12Z Tue Nov 23 2021 ...Overview and Guidance/Predictability Assessment... The overall forecast is consistent in advertising a transition from relatively flat and progressive mean flow late this week to a more amplified pattern during the weekend and early next week as a deepening upper trough heads into the East downstream from a ridge that moves into the West by Monday. The upper trough and a possible embedded low may support potentially significant low pressure that would affect portions of the East, and requires monitoring given the busy Thanksgiving holiday travel week. Meanwhile it looks increasingly likely that initially weak southern stream Pacific energy will collect into an upper low to the west/south of California. This feature may approach or reach the southwestern U.S. by next Tuesday. The expected pattern will produce two surges of chilly air over the eastern half of the country, one during Friday-Saturday and another early next week, while maintaining mostly above normal temperatures from the West into parts of the High Plains. After a lot of spread and variability over recent days, model clustering is decent but still not ideal for the system expected to reach just inland over the Pacific Northwest at the start of the period early Friday. 12Z/18Z GFS runs and the 12Z ECMWF/CMC provided the most comparable solutions through the 18Z cycle while the 12Z UKMET was weak and suppressed. The new 00Z UKMET is closer to the other models but the 00Z ECMWF has now become the weak/south extreme. Detail differences with this system's energy as well as for upstream flow digging into the amplifying mean trough maintain a fair degree of uncertainty for the specifics of one or more surface lows and frontal systems over the central and eastern U.S. from Saturday through Tuesday. This has supported a model/ensemble mean blend approach to tone down differences and run-to-run variability seen in the operational models. As for potentially stronger development near the East Coast by day 7 Tuesday, thus far there has been a decent signal for a storm system but within a broad area encompassing the Great Lakes to Mid-Atlantic through the Northeast, Canadian Maritimes, and western Atlantic. At this time an intermediate surface low position just off southeastern New England as of 12Z Tuesday provides a reasonable starting point relative to the models/ensemble means. This is somewhat closer to the ECMWF mean that has been fairly stable over its past three runs in contrast to the GEFS mean that has trended southward since yesterday (but has been steady near the southern tip of Nova Scotia in the past 12 hours of runs). The 12Z CMC mean was well offshore with the surface low but the new run has trended farther northwest for early Tuesday. The southern track of the 12Z ECMWF deep upper low near the East Coast led to some 500mb height anomalies of 5-6 standard deviations below normal while the anomalies in other models are somewhat less extreme. The new 00Z ECMWF is a tad weaker as of early Tuesday. Question marks upstream late in the period involve the next Pacific shortwave scheduled to arrive into the West around late Monday-Tuesday, along with what becomes of the upper low expected to form offshore from the southwestern U.S. Consensus indicates the 12Z CMC was too amplified with the northern stream shortwave and indeed the new 00Z run has toned it down somewhat. Prior GFS/CMC runs were fastest to eject the upper low energy into the West and the new 00Z runs have trended slower. Best model clustering favored use of the 12Z ECMWF/CMC and 18Z GFS as the basis of the updated forecast early in the period. Ongoing detail uncertainties with shortwaves digging into the eventual eastern U.S. trough and eventual straying of the 12Z CMC from consensus recommended a transition to a mix of the 18Z GEFS/12Z ECMWF means with lingering input of the 12Z ECMWF/18Z GFS. ...Weather/Hazards Highlights... The system forecast to reach the Northwest by early Friday should spread light to moderate rain/mountain snow from the northern half of the West Coast through the northern Rockies. After Friday much of the West will see multiple dry days aside from some moisture returning to the northern Pacific Northwest Sunday onward and possibly some scattered precipitation reaching southern parts of the West by Tuesday ahead of the upper low that forms southwest of California. The eastern Great Lakes could see some lingering lake effect activity on Friday. Expect precipitation to expand in coverage over the eastern half of the country from Sunday onward in association with one or more surface lows/frontal systems. It is too early to resolve detailed effects from low pressure that may be near the East Coast by next Tuesday but significant rain/snow and strong winds could be possible. At this time the relatively higher potential for meaningful snow extends from the central Appalachians through interior New England. The broad cyclonic circulation and cold air may produce some lake effect/enhancement as well. Easterly flow from late week through the weekend may promote periods of rain over the southern/eastern parts of the Florida Peninsula. Much of the eastern U.S. will see below normal temperatures Friday into Saturday with some highs 10F or more below normal on Friday. Another surge of chilly air will likely bring highs over the East back down to 5-15F below normal next Monday-Tuesday. Anomalies could trend colder depending on the ultimate depth of the upper trough crossing the region. On the other hand the central/southern High Plains may see highs up to 10-15F above normal Friday into Saturday and again next Tuesday. Expect moderately above normal temperatures over many areas from the Rockies to the West Coast through the period. 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