Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Tue Nov 29 2022 Valid 12Z Fri Dec 02 2022 - 12Z Tue Dec 06 2022 ...Wet/snowy conditions across the West with below normal temperatures... ...Overview... Guidance shows progressive west-southwesterly mean flow aloft over most of the lower 48, between a ridge over the Gulf of Mexico and an upper low that drops from northern into central Canada. Mean troughing that becomes established near the West Coast mid-late week should finally push inland early next week. This pattern will likely lead to below normal daytime temperatures over most areas to the west of the Rockies, as well as a couple episodes of well below normal temperatures over the northern Plains. Expect some areas in the West to see significant rain and higher elevation snow. Shortwaves ejecting from the West Coast mean trough could generate a couple vigorous systems to the east of the Rockies, producing snow over the northern tier and playing a role in a developing area of potentially significant rainfall over the east-central U.S. ...Guidance and Predictability Assessment... Among the 12Z/18Z guidance, GFS runs were a bit on the faster side of the spread (though not to an extreme degree) for the leading shortwave ejecting from the West while the CMC was on the slow side versus other models/ensembles to a greater degree. This left the GFS/ECMWF/UKMET as the best cluster for this feature. Over the past day, the models have trended noticeably stronger for the surface low that tracks across the north-central Plains into the Upper Great Lakes Friday-Saturday. In the 00Z cycle both the GFS and CMC have trended favorably toward the average of previous guidance. For the next system dropping down toward the West Coast late week through the weekend, the GFS/ECMWF have converged on an upper low track that reaches the coast near or just south of the Oregon/California border. UKMET runs have been a bit north of that. The 12Z CMC became noticeably flat with the trough (which has been a tendency of the model and its ensembles over recent days) while the new 00Z run has adjusted much closer to consensus. Minus the earlier CMC/CMCens runs, there has been better than average clustering as the second upper feature opens up and moves through the West, ultimately supporting a well-defined surface system reaching the Midwest/Ohio Valley/Great Lakes area by day 7 next Tuesday. A model/ensemble average looks good for the initial representation of this system given typical predictability that far out in time. Finally, guidance agrees that the shape and progression of the upstream ridge should encourage the next bundle of digging shortwave energy to take a farther east trajectory into the western U.S. early next week, with typical spread and variability for specifics. This trailing energy may have some influence on the east-central U.S. system. With the 12Z CMC comparing poorly to other guidance in multiple ways, the updated forecast blend emphasized the 18Z GFS and 12Z ECMWF/UKMET for about the first half of the forecast and then transitioned to a combination of the GFS/ECMWF and their ensemble means. ...Weather/Hazard Highlights... A leading wavy front pushing into the West late this week should bring some terrain-enhanced rainfall to parts of the southern half of California as well as some snow to portions of the Great Basin and central Rockies. Trailing low pressure drifting southeast toward the central West Coast into the weekend will likely spread locally moderate to heavy rain and higher elevation snow into the Pacific Northwest and then California, with some of moisture also extending into the Great Basin/central Rockies. Meanwhile, flow aloft ahead of both systems may carry in some added moisture from the eastern Pacific, leading to areas of rainfall across Arizona and New Mexico. Some of this activity could be on the locally moderate to heavy side but confidence is low in the details at this time. The southern half of the West should trend drier by next Monday or Tuesday as upper troughing moves farther inland but some precipitation may persist over the Northwest/Rockies. These systems may produce gusty winds over some areas. Areas east of the Rockies should see a fairly active pattern as features eject from the West. Latest trends have been for a stronger system to track from the north-central Plains through the Upper Great Lakes around the end of the week, producing a band of snow across the northern tier and a brief period of strong winds. Farther south, some light rain may initially accompany the trailing front as it pushes into the eastern U.S. However the front should stall and then return northward as a warm front ahead of the next system emerging over the Plains/Midwest, serving as a focus for increasing rainfall over the middle latitudes of the east-central U.S. Sunday onward. Both low level flow from the Gulf and the west-southwesterly flow aloft should contribute moisture for this event that could produce areas of moderate to heavy rainfall. Some rain may extend back into the Plains as well. The aforementioned trailing system may produce another area of snow across the northern tier early next week, with totals and wind strength dependent on the ultimate depth of the system. Much of the West should see below normal high temperatures from late this week into early next week, with most locations 5-15F below normal. There may be a few isolated daily records for cold morning lows/daytime highs near the West Coast late this week. The northern Plains should see a couple separate episodes of colder readings. The first will feature highs 15-30F below normal on Friday mainly over Montana into North Dakota. The next push of similarly cold anomalies should expand across a larger portion of the northern Plains next Monday-Tuesday. Southern/eastern areas will tend to see near to above normal temperatures aside from a cool start to the day over the East on Friday, with various areas of plus 10-15F anomalies likely during the weekend into early next week. The southern tier should see multiple days of plus 15-25F anomalies for morning lows. Also the central Plains may see highs up to 10-20F above normal on 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