Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Oct 28 2022 Valid 12Z Mon Oct 31 2022 - 12Z Fri Nov 04 2022 ...Pacific Northwest heavy rain lingering into Monday followed by Great Basin/Rockies heavy mountain snow threat... ...Overview... Expect an increasingly amplified upper pattern over the lower 48, with a deep trough reaching the West by next Wednesday-Friday while mean ridging builds over the East. The upper trough will spread a broad area of rain and mountain snow, along with well below normal temperatures, across the West. Rain may develop over the parts of the Plains by late week. Meanwhile a weak shortwave crossing the East early in the week will support some rainfall while a trailing southern stream feature could produce areas of rain near the Gulf Coast and over parts of the East but with low confidence in the details. Within a broad area of mostly above normal temperatures east of the Rockies, the warmest anomalies should generally be centered over the northern-central Plains and Upper Midwest. ...Guidance and Predictability Assessment... There is decent agreement and continuity with the amplifying western upper trough in principle, but still some important differences in details. The most agreeable cluster among the 12Z/18Z runs consisted of the 18Z GFS/12Z ECMWF and their ensemble means. These solutions were in the middle of the spread (with recent ECMWF runs trending toward the slower GFS) while the means have at least hinted at some potential for flow separation as the operational runs depicted a closed low. GFS runs are starting to exhibit some waffling though, with the 12Z run on the slow side and the new 00Z version straying faster by late in the week. The UKMET/CMC are more open/progressive with the upper trough, due in part to being faster than other guidance with upstream flow from the Bering Sea/Gulf of Alaska. Farther east, the models have been varied with the details of an axis of shortwave energy extending from the Midwest into northwestern Mexico as of early Monday. Trends of most models/ensembles over the past couple days have been toward greater separation, with the northern part crossing the East Monday-Tuesday while the southern part tracks across the southern tier at a slower pace, ultimately filtering through the building eastern U.S. mean ridge around Wednesday-Thursday. The new 00Z UKMET has a questionably slow/deep depiction of the northern shortwave and is a slow extreme with the southern one. The weak nature of the energy, and potential for the southern feature to track through a developing mean ridge, would typically lead to moderately low predictability. For the latest manual forecast, guidance comparisons led to using a 12Z/18Z model composite (with more emphasis on the GFS/ECMWF relative to the UKMET/CMC) early in the period followed by a transition toward a model/mean blend of the 18Z GFS/12Z ECMWF and their ensemble means. The blend split the models/means evenly by day 7 Friday. ...Weather/Hazard Highlights... The upper trough amplifying into the West and leading surface frontal system will bring a period of wetter and colder weather to the region. The combination of dynamics and moisture should spread locally enhanced precipitation southeastward with time, from the Pacific Northwest/far northern Rockies into California/Great Basin and then the Southwest U.S. and central Rockies. Pacific Northwest activity on Monday will be a somewhat tempered continuation of heavy weekend precipitation over the coastal ranges/Olympics and Cascades. Southeastward progression of the moisture shield will bring a threat for heavy mountain snow into northern-central California and across the north-central Great Basin/Rockies as depicted in the WPC Winter Weather Outlook probabilities. Temperatures across the West will trend much colder as the upper trough arrives and drifts over the region. Much of the southern half to two-thirds of the West will likely see daytime highs 10-20F below normal by next Wednesday-Friday, which such anomalies may be more scattered over northern areas. Rainfall may increase in coverage and intensity over the central U.S. late next week as the western upper trough approaches, with exact timing sensitive to progression of the trough. There should be two primary areas of rain ahead of this system. One will be with the shortwave/surface system emerging from the Midwest early in the week, with mostly light to moderate totals. The trailing feature to the south may produce one or more areas of heavy rainfall over the Gulf Coast region but low-predictability details will be important for determining how much rain falls along/inland from the Gulf Coast versus offshore. Areas to the east of the Rockies will be on the warm side of the amplifying pattern for most of next week, with temperatures of 10-20F above normal most common over the northern-central Plains/Upper Midwest. Some of these warm anomalies will extend farther east at times, while cooler air should start to reach the Plains by Friday. 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml