Extended Forecast Discussion NWS Weather Prediction Center College Park MD 421 PM EDT Thu Oct 28 2021 Valid 12Z Sun Oct 31 2021 - 12Z Thu Nov 04 2021 ...Overview and Guidance/Predictability Assessment... The models and ensemble means continue to show the large scale flow settling into a regime that features a strong mean ridge over western Canada, a more modest ridge over the western U.S., and a mean trough extending from the eastern half of Canada into the central and eastern U.S. Two Pacific shortwaves will likely head into western mean ridge, one early in the week and another around Thursday. The overall pattern will support an expanding area of below normal temperatures to the east of the Rockies. By mid-late week the front defining the leading side of cooler air will help to focus an area of precipitation--some of it potentially heavy--from the southern Plains northeastward. The eastern Pacific shortwaves will produce a couple episodes of enhanced precipitation along parts of the central and northern West Coast, while a fairly narrow axis of rain/snow may extend across the central Rockies/Plains early in the period. Overall the guidance shows better agreement than seen 24-36 hours ago. Early in the period there are still a couple smaller scale uncertainties. The 00Z GFS was a bit slow with the system opening up as it crosses New England late in the weekend, with more recent runs trending faster toward the consensus. Meanwhile the models have been continually challenged by the compact upper low over or near Oregon by early Sunday. Even in the new 12Z runs there is still some lingering spread for the ultimate fate of the energy that shears out, but with seemingly minimal impact on the general pattern thereafter. Then models are trending closer together for the first shortwave that arrives from the Pacific, with the weak consensus depiction appearing reasonable given that the energy is heading into a mean ridge over the West. Not surprisingly there is more spread with the next batch of shortwave energy that approaches the West Coast around Wednesday-Thursday. By next Thursday the 00Z CMC used the northern part to break down the Canadian ridge more than other guidance, favoring removal of that run from the blend. The 00Z ECMWF strayed a bit to the faster side with what reaches California but could still be used as part of a guidance average. Guidance continues to vary with the details of upper troughing east of the Rockies by the latter half of the period, which leads to increasing spread and run-to-run variability for specifics of the wavy front over the southern/eastern U.S. along with associated precipitation (especially over the East). Likely due to the aforementioned issues with the western Canada ridge, the 00Z CMC showed a more neutrally tilted eastern upper trough and suppressed surface pattern. A model/mean blend by late in the period (evenly split among the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF mean by day 7 Thursday) still led to some continuity adjustments, but less than seen in some individual solutions. ...Weather/Hazards Highlights... The two shortwaves/frontal systems expected to reach the West Coast during the period should bring a couple episodes of enhanced precipitation. The first should bring highest totals to northern California and the far southwest corner of Oregon early next week. Currently expect the second to bring highest amounts to a similar area by around Thursday but it is also possible that a meaningful amount of moisture could reach the Pacific Northwest. There is some timing uncertainty with this latter event as well. A fairly narrow band of rain and snow should set up over parts of the central Rockies/Plains early in the week, with lingering uncertainty over exact location and amounts. There is some potential for locally moderate activity for a time. An expanding area of precipitation should cover locations from the southern Plains northeastward by Tuesday or Wednesday into Thursday, with a wavy front providing some focus. The best potential for heavier rainfall extends from northeast Texas and southeast Oklahoma into parts of Arkansas/Louisiana, with decreasing confidence in specifics the farther northeastward one goes. The only notable temperature hazard is below to much below normal temperatures expanding across much of the Central U.S. during the period. A large portion of the Plains and locations farther east into the Mississippi Valley and then Tennessee/Ohio Valleys (and perhaps even the central Appalachians by Thursday) should see one or more days with highs 10-20F below climatological values for this time of year. Currently expect the axis of coolest anomalies to extend from the central Plains to along or south of the Ohio River. 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 and possibly other nearby areas around next Tuesday or so. 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 precipitation across portions of the Central Great Basin and California, Thu, Nov 4. - Heavy rain across portions of California and the Pacific Northwest, Mon, Nov 1 and Wed-Thu, Nov 3-Nov 4. - Heavy rain across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Tue-Wed, Nov 2-Nov 3. - Flooding possible across portions of the Mid-Atlantic. - Flooding occurring or imminent across portions of the Northeast and the Great Lakes. - Flooding likely across portions of the Middle Mississippi Valley, the Pacific Northwest, and the Northern Great Basin. - Much below normal temperatures across portions of the Central Plains, the Lower Mississippi Valley, the Southern Plains, the Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Tue-Thurs, Nov 2-Nov 4. - Heavy precipitation across portions of mainland Alaska, Sun-Tue, Oct 31-Nov 2. - High winds across portions of mainland Alaska, Sun, Oct 31. 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