Extended Forecast Discussion NWS Weather Prediction Center College Park MD 413 PM 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... ...Increasing chances of heavy rainfall along the western Gulf Coast Tuesday... ...Overview... Focus during the early medium-range period will be on a southern stream trough lifting northeastward across the Southern Plains which may lead to some heavier, excessive rainfall, especially near the western Gulf Coast on Tuesday. Another small scale shortwave will move through the Midwest bringing a chance for showers and thunderstorms from the Ohio Valley to the East Coast Monday. Expect an increasingly amplified upper pattern over the lower 48 by later in the week, with a deep trough digging in the West 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. 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... Model guidance showed broad general agreement early in the period with more significant differences with respect to the progression and amplification of the western trough later in the period. A cluster of solutions tended to follow the same pattern in general, leading to at least an average level of confidence in the forecast. The model blend used for the medium range product suite began with a combination of the 00 UTC ECMWF/CMC/UKMET and 06 UTC GFS which clustered around the same solution with continuity from the prior forecast. The 00 UTC CMC/UKMET quickly started to diverge compared to continuity and the 00 UTC ECMWF and 06 UTC GFS with respect to the southern stream trough lifting over the Southern Plains. Began to trend away from these solutions in the blend with the UKMET showing a broader trough and the CMC a more amplified trough. This pattern continued later in the period with the 00 UTC UKMET/CMC trending away from the 00 UTC ECMWF and 06 UTC GFS in both the amplitude and phase of the overall pattern over the CONUS. The focus turned to the evolution of the trough digging over the western CONUS later in the period. The 00 UTC ECMWF trended towards the 06 Z GFS depicting a closed low compared to their respective ensemble means. There was also a significant difference in the 06 UTC GFS compared to the 00 UTC GFS which had a more progressive and amplified trough, but was an outlier compared to the other solutions. Kept a split of the 06 UTC GFS and 00 UTC ECMWF during the later half of the forecast period with a slow ramp up of the 00 UTC ECMWF mean which does not maintain the closed low as long into the forecast. This followed continuity with the prior forecast and also allows for some uncertainty in the evolution of a deeper, less progressive trough given the varied solutions of the 00 and 06 UTC GFS. ...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. There is still a bit of uncertainty with respect to forecast QPF and thus snow amounts. QPF from the NBM tended to be higher across the region when compared to other guidance. A multi-model ensemble blend was used to reduce amounts broadly across the region. However, the individual totals for various mountain ranges, especially for southern California and the Southwest, still varied quite a bit compared to the prior forecast and individual model guidance. Thus, the details of the evolution of the progression of the trough and amount of cold air that eventually overspreads southern portions of the West will need to be closely monitored with this higher level of uncertainty. Temperatures across the West will trend much colder as the upper trough arrives and drifts over the region. In addition to the forecast precipitation, 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 is expected to 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 Monday which will bring at least a broad area of light to moderate rainfall from the Ohio Valley/Lower Great Lakes to the Carolinas and Mid-Atlantic. The trailing feature to the south may produce one or more areas of heavy rainfall over the Gulf Coast region. While there is some uncertainty in how much rain falls along/inland from the Gulf Coast versus offshore, the forecast rainfall amounts have trended upward. The coverage and intensity of the QPF across the model guidance is enough to highlight a concern for localized excessive rainfall on Tuesday. 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. Putnam/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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Mon, Oct 31. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Tue, Nov 1. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Tue-Wed, Nov 1-Nov 2. - Flooding occurring or imminent across portions of Florida. - High winds across portions of mainland Alaska, Mon-Wed, Oct 31-Nov 2. 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