Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Nov 28 2022 Valid 12Z Thu Dec 01 2022 - 12Z Mon Dec 05 2022 ...Wet/snowy conditions expanding across the West with below normal temperatures... ...Overview... Latest models and ensemble means are consistent in showing a late week through weekend upper pattern featuring west-southwesterly mean flow between West Coast upper troughing and a Gulf of Mexico ridge. The pattern should persist into early next week over most areas but with the overall trough axis moving inland over the West with the ejection of the second of two primary shortwaves. This large scale flow will favor below normal daytime temperatures over most areas to the west of the Rockies, as well as the northern High Plains at times, while the West sees episodes of locally significant rain and higher elevation snow. Most of the eastern half of the country will be dry late this week but rainfall should increase in coverage and intensity over east-central areas during the weekend as moisture interacts with a front that a Friday-Saturday Canadian surface low brings into the region. ...Guidance and Predictability Assessment... With the departure of a strong upper shortwave and surface low pressure from eastern North America early in the period, the main focus of the forecast is on the individual shortwaves within the West Coast mean trough. While notable differences still exist, there has been some improvement in the depiction of each of two primary features. The first shortwave should be just offshore as of early Thursday and then progress inland with time, supporting a surface system that ultimately tracks into eastern Canada while pushing a trailing cold front through the East. Consensus of models through the 18Z run and what has arrived so far from the 00Z cycle suggests the GFS could be somewhat fast and the 12Z ECMWF somewhat amplified/slow. At the very least, recent ECMWF trends have been gradually faster and in turn leading to a shorter duration of potential interaction with offshore southern stream moisture/waviness that would enhance precipitation amounts along the central West Coast around Thursday-Friday. The timing differences persist for the surface system east of the Rockies, with the slower ECMWF differing the most from the ensemble means which in recent days have been the most stable guidance with this system by the weekend. The new 00Z ECMWF has indeed trended faster with this shortwave. The second system of interest will likely approach the Northwest coast by Saturday. The 12Z ECMWF was a bit on the slower side while the 12Z/18Z GFS runs in particular dropped the upper low farther south than most other guidance from late Friday into Sunday. The new 00Z GFS has favorably trended closer to consensus. Even with some of the differences along the way, by early Monday there is actually better than average agreement as the upper low opens up and moves into the West. Farther east, typical variability develops regarding the ultimate latitude of the front that is likely to stall during the weekend and become a focus for increasing rainfall. Some earlier guidance had been a little premature to develop rainfall over the Mississippi/Ohio/Tennessee Valley region but overall the models and ensembles are starting to show a more coherent signal late in the period. Teleconnections relative to the forecast mean pattern also favor enhanced precipitation potential over this region. Also of note, an upper low dropping south from the Canadian Archipelago should push a cold front into the northern tier states during the weekend. There has been some model spread/variability but a consensus model/mean approach has maintained fairly good continuity over the past day. Based on the array of guidance available through the 12Z/18Z cycles, the updated forecast started with an operational model composite to start and then incorporated modest 18Z GEFS/12Z ECMWF mean input into mid period to blunt the less desirable attributes of the ECMWF (initial West Coast shortwave) and GFS (second system). The blend transitioned toward nearly even weight of models/means late in the period. ...Weather/Hazard Highlights... The first primary system to affect the West should bring highest totals of rain and mountain snow to California late this week (northern-central areas Thursday and pushing farther south by Friday) with some moisture also extending into the northern-central Rockies. This system may incorporate some offshore lower-latitude moisture/waviness to enhance amounts but a somewhat faster trend of this system over the past day or so has lowered the potential for the heaviest totals that some guidance had been advertising. The trailing system should bring increasing rain and mountain snow to areas along the northern half of the West Coast around Friday, with best focus drifting south into northwestern California and the Sierra Nevada during the weekend. Again the northern two-thirds of the Rockies will see meaningful snow from this event, albeit with somewhat lower liquid equivalents than expected near the West Coast. Snow levels in the Northwest could decline to fairly low elevations over some areas. The Northeast should see breezy to windy conditions on Thursday as strong low pressure departs over eastern Canada, with lake effect snow gradually trending lighter with time. A wavy front reaching the north-central Plains/Upper Midwest by Friday may bring some light precipitation to the far northern tier and possibly other areas of light rain as it continues eastward. More organized rainfall could then develop over the east-central U.S. during the weekend as the front stalls and interacts with moisture in the west-southwesterly flow aloft and possibly low level Gulf inflow. A fair degree of uncertainty still exists for specifics, but there is a gradually increasing signal for the best rainfall focus to be centered over the Ohio/Tennessee/Mississippi Valley region. The moisture within the west-southwesterly flow aloft also produce some rainfall over or near the far southern Rockies but with low confidence at this time. Much of the West should see below normal high temperatures from late this week into the start of 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 High Plains (especially Montana) may see a couple separate episodes of colder readings, with highs up to 20-30F below normal on Friday and Monday. The eastern U.S. will see a brief period of moderately below normal temperatures Thursday into Friday and then a rebound to above normal readings ahead of the next front. Above normal temperatures should become more pronounced across the southern tier during the weekend and early next week. Morning lows may become particularly anomalous with some plus 20-25F readings possible over and near the southern Plains by Sunday-Monday. 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