Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sat Oct 29 2022 Valid 12Z Tue Nov 1 2022 - 12Z Sat Nov 5 2022 ...Great Basin/Rockies heavy mountain snow threat next week... 19Z Update: The 12Z model guidance suite is now in strong agreement on the evolution and timing of the amplifying upper trough across the Western U.S. for the middle to end of the week. There is growing support for a closed low to develop over the Desert Southwest by Friday, and now there is above average agreement on the synoptic scale even for a Day 7 forecast next Saturday across the south-central U.S. and extending to the Southeast U.S. The latest GEFS ensemble members are slightly farther west with this trough compared to the operational GFS/ECMWF/CMC, but close enough to merit some use of the ensemble means in the forecast process. A building downstream ridge axis should be in place and result in warmer than average temperatures for early November for much of the Eastern U.S. by the end of the forecast period. The greatest degree of model spread exists across southern Canada in northern stream flow with respect to timing of smaller scale shortwaves, but it does not appear that any of these will phase with the large southern stream system through Saturday. In terms of QPF, there has been a trend for lighter rainfall across much of central/southern California compared to the previous forecast, and the potential for a heavy rainfall event along with some severe storms is on the horizon by Friday and into Saturday across portions of the central and southern Plains. Stay tuned to future forecasts as more details become discernible. The previous discussion is appended below for reference. /Hamrick ----------------- ...Overview... The dominant focus of the medium range period will be the amplifying large scale upper pattern, with a deep trough digging into and moving through the West while a ridge builds over the East. The upper trough will spread a broad area of rain/mountain snow as well as below normal temperatures across the West. Precipitation should increase in coverage and intensity over parts of the central U.S. late in the week once the western system reaches far enough eastward. However there is a decent amount of uncertainty regarding the shape and timing of the upper trough by that time, so for now the confidence in specifics of associated precipitation is fairly low. Ahead of this system, an initial Southern Plains upper trough may produce some enhanced rainfall along the western half of the Gulf Coast on Tuesday and then could bring some moisture into the East as the feature filters through the building mean ridge. A leading weak shortwave will depart from the East after Tuesday. The broad area of mostly above normal temperatures east of the Rockies through midweek will see a west-to-east erosion over the central U.S. by late in the week. ...Guidance and Predictability Assessment... The past day of guidance has offered more spread than was previously the case for the ultimate evolution of the western upper trough after midweek or so, with both the GFS and ECMWF jumping between slower closed low and faster/phased trough solutions. The majority of GFS runs have favored the slow closed low, with the GFS from 24 hours ago (28/00Z) being the primary exception. That run ended up being similar to where the 12Z ECMWF trended. There were enough progressive ECMWF ensemble members that the resulting mean trended somewhat faster/broader with the trough versus the prior two runs, but at least the southeastward amplitude of the overall trough was less extreme than in the operational run. GEFS means have tended to be close to the GFS timing from latest runs (but weaker/northward with the implied upper low). Meanwhile the CMC has trended to the closed low scenario after being more progressive earlier and the new 00Z UKMET shows better timing agreement after the 12Z run was questionably slow. Some of these differences relate to the shape of flow across the Gulf of Alaska and then southwestern Canada/Northwest U.S. The GFS and some GEFS members are the most pronounced with how much ridging extends into the northeastern Pacific and then into North America after early Wednesday. The 12Z ECMWF and some of its ensemble members are on the flat extreme. Until there is compelling evidence, favor an intermediate solution for this aspect of the forecast. The new 00Z CMC reflects such an idea. Among the 12Z/18Z guidance available for the latest forecast update, preference for the 18Z GFS/12Z ECMWF/12Z CMC cluster early in the period transitioned to a blend consisting of the 18Z GFS/12Z CMC for operational input and somewhat more 18Z GEFS relative to the 12Z ECMWF mean. The result helped to keep the forecast fairly close to continuity, with just a modest faster nudge. The new 00Z GFS/GEFS mean/CMC align with this timing fairly well. The new 00Z ECMWF has reverted back to the slow closed low scenario, greatly improving clustering for the time being. This preferred blend worked well for leading features as well, with the 12Z UKMET still out of sync for the initial Southern Plains and eastern U.S. shortwaves. Among the non-UKMET solutions, the blend helped to resolve ongoing detail differences for the shortwave emerging from the Plains, with an expected path through the building mean ridge leading to an extra degree of difficulty. ...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. The best focus for rain/mountain snow should be from the Pacific Northwest/California into the northern Rockies on Tuesday, then reaching the Great Basin/Southwest and central Rockies mid-late week. An episode of heavy mountain snow will be possible over northern-central California and across parts of the Great Basin and northern-central Rockies as depicted in the WPC Winter Weather Outlook probabilities. Uncertainty over the precise upper trough details tempers confidence in specifics of precipitation amounts and duration though. The depth of the upper trough/low is also a question mark, affecting snow levels and how far south the higher elevation snow may extend. Strong Pacific flow may drop into the Pacific Northwest toward the end of the week/next weekend, increasing precipitation over the region. At this time the southward extent of this moisture is quite uncertain though. Temperatures across the West will trend much colder as the upper trough arrives and drifts over the region. Much of the southern two-thirds of the West will likely see daytime highs 10-20F or so below normal by next Wednesday-Friday, with some nearly as extreme anomalies extending into the northern Rockies. Expect some moderation by Saturday but with many areas still below normal. Rainfall should increase in coverage and intensity over the central U.S. late next week as the western upper trough approaches, with exact timing and location of highest totals being sensitive to the progression and shape of the trough. Ahead of this system, Southern Plains shortwave energy may produce some heavy rainfall over or near the Gulf Coast region around Tuesday. There is still meaningful uncertainty with how much rain falls along/inland from the Gulf Coast versus offshore, as guidance has been oscillating in that regard. This feature could produce some areas of rainfall over parts of the East but with low confidence in the details. Locations to the east of the Rockies will be on the warm side of the amplifying pattern through at least Thursday, with temperatures of 10-20F above normal most common over the northern-central Plains/Upper Midwest. Some of these warm anomalies will extend into the Great Lakes/Ohio Valley/Northeast. Central U.S. temperatures should trend cooler by Friday-Saturday while the East remains warm. 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