Extended Forecast Discussion NWS Weather Prediction Center College Park MD 232 PM EDT Sun Oct 30 2022 Valid 12Z Wed Nov 2 2022 - 12Z Sun Nov 6 2022 ...Western-central U.S. system to bring heavy mountain snow to parts of the Great Basin/Rockies and possibly heavy rain to parts of the Plains... 18Z Update: The 12Z model guidance has continued to improve in regard to the upcoming pattern change across the western/central U.S. as a large synoptic scale trough amplifies, and then evolves into a closed upper low over the Desert Southwest Thursday night, and then tracks east to the central Plains by the weekend. The latest 12Z run of the CMC is now much closer to the model consensus compared to its 00Z run that was quite progressive with this low by Friday and beyond. The past couple of GFS runs are slower to eject the low out across the Plains by next Sunday, but the overall structure depicted by the GFS looks reasonable. Farther to the north across southern Canada, forecast uncertainty is higher regarding impulses in the northern stream flow, including enhanced upper level flow and shortwave energy reaching the Pacific Northwest by next weekend. The fronts and pressures forecast was primarily derived from a GFS/ECMWF/UKMET blend through Friday (since the 00Z CMC was noted as likely too progressive), followed by greater weighting towards the well clustered ECMWF/GEFS mean for the weekend. In terms of QPF, some deterministic GFS/ECMWF was incorporated along with the NBM to add detail and slightly increase QPF across the West through Friday, and then transitioned to mainly NBM for Saturday and Sunday. The previous forecast discussion is appended below for reference. /Hamrick --------------------- ...Overview... Expect strong amplification of the upper pattern to be already in progress as of the start of the period Wednesday, with a trough beginning to move into the West and ridge building over the East (with an embedded compact shortwave/low making progress toward the Atlantic). The western trough is likely to close off an upper low that should make its way into the Plains by the weekend. This system will spread a broad area of rain/mountain snow and well below normal temperatures over the West and then bring the potential for areas of heavy rainfall and strong convection to parts of the central U.S. Guidance clustering for this system has improved somewhat but there is still meaningful uncertainty over some important details. Behind this system a strong North Pacific jet should sag into the Northwest by the late week/weekend time frame, leading to potentially significant terrain-enhanced precipitation. A broad area of mostly above normal temperatures will cover areas east of the Rockies through Thursday, followed by a cooling trend over the Plains while the eastern U.S. remains quite warm. ...Guidance and Predictability Assessment... Models/means through the 12Z/18Z cycles reflected somewhat better agreement for the western-central U.S. system than in some earlier runs, with GEFS mean runs seeming to provide the most consistent idea for the trough/upper low timing (albeit with a typically much weaker strength than the operational solutions). During the first half of the period an operational model composite looked good through Thursday but then the 12Z UKMET strayed a bit faster than other guidance, favoring its exclusion by early day 5 Friday. Not surprisingly the model spread increases as the upper low reaches the Plains. By day 7 Sunday a solution between the 12Z GFS/old 00Z ECMWF (slightly west) and 12Z ECMWF/CMC (east) appeared most reasonable, fitting close to the ensemble mean trough axis. Thus the 12Z/18Z model composite transitioned to incorporate those four operational model runs (18Z GFS becoming a bit slow), with only a modest GEFS/ECens component to maintain reasonable detail of the upper low. Among the new 00Z runs, the CMC has strayed to a fast extreme while the UKMET has come back to consensus and the GFS is a bit slow like the 18Z run. That CMC run as well as a broad ensemble spread still temper confidence even though a decent majority cluster has become established. Meanwhile the GFS/ECMWF trends through 12Z/18Z seemed to show some convergence regarding the details of strong Pacific flow moving into western Canada and the Northwest U.S. late week into the weekend, with a general guidance blend providing a reasonable starting point to resolve lingering detail/position differences. ...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 mid-late week. The best focus for rain/mountain snow should be across the Great Basin and northern-central Rockies early in the period with some activity also reaching into southern parts of the West. Details over southern parts of the West will be sensitive to the exact track of the expected upper low. WPC Winter Weather Outlook probabilities reflect areas with the best potential for meaningful mountain snow in the Great Basin/Rockies. Rain should increase in coverage and intensity over the central U.S. late in the week and persist into the weekend as the western system approaches. Some rain may become heavy and strong convection is possible as well. Monitor Storm Prediction Center outlooks for latest information on severe potential. At least to the extent that rainfall does not become locally excessive, the rain will be beneficial given the ongoing drought over that part of the country. Behind this system, strong Pacific flow reaching into the Northwest toward the end of the week and next weekend should increase rain and mountain snow over the region. Guidance differences still exist but are less pronounced than they were yesterday, and there is a decent signal that this precipitation could reach moderate to heavy intensity for a time. The West will see a broad area of daytime highs 10-20F or so below normal under the forecast upper trough/low Wednesday-Friday. Double-digit anomalies should become much more scattered during the weekend (most likely lingering into Saturday over far southern locations) as temperatures moderate but most areas should remain near to below normal for highs. On the other hand, well above normal temperatures will prevail over most areas east of the Rockies through about Thursday and then the Plains will likely trend cooler while the Mississippi Valley and East remain warm. During Wednesday-Thursday the warmest anomalies (plus 10-25F) should be from the Upper Midwest into central Plains while areas from the Great Lakes/Ohio Valley into Northeast should see multiple days with temperatures 10-20F above normal during 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, 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