Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sun Nov 14 2021 Valid 12Z Wed Nov 17 2021 - 12Z Sun Nov 21 2021 ...Overview and Guidance/Predictability Assessment... Models and ensembles continue to show a progressive pattern with broad cyclonic flow across most of the lower 48 while an upstream ridge builds over the eastern Pacific next weekend. This regime will favor some day-to-day variability in weather conditions and a trend toward moderate temperatures, generally within 10F on either side of normal. Guidance remains fairly well clustered for a strong southern into eastern Canada storm system and trailing cold front sweeping through the eastern half of the country mid-late week. Predictability remains lower than desired for details behind this system but some common ideas are starting to take shape and hopefully solutions make additional progress in converging soon. Specifics over the eastern Pacific Wednesday-Thursday and continuing eastward across the lower 48 thereafter have been particularly troublesome in recent days. Over the past 24 hours the models have been converging toward varying degrees of interaction among amplifying energy originating from the Bering Sea/Aleutians/North Pacific and a weakening shortwave to the south to produce a surface system that reaches somewhere along the northern half of the West Coast by late Thursday or so. This is better than previous days when an isolated model run had a system reaching the coast while the rest weakened it well offshore. However there is still a wide range of possibilities for strength and track...00Z GFS strongest/northward and 00Z UKMET weakest (flat with its shortwave), while northward trends in the 18Z/00Z GFS and 00Z CMC/00Z ECMWF leave the 12Z ECMWF on the southern side of the envelope now. Not surprisingly ensemble members also show a lot of latitude spread. The 00Z model trends may suggest a northward adjustment may be in store after the current forecast made use of the 12Z GFS/ECMWF/CMC/UKMET from Wednesday into early Friday. The relative improvement in clustering for a meaningful eastern Pacific into western U.S. shortwave after midweek has brought guidance closer together for the lower 48 pattern by mid-late period. In particular the GFS is now holding its shortwave energy far enough west to join the ensemble means and most other models that have been showing high pressure over the eastern U.S. and western Atlantic versus a low pressure system that some earlier GFS runs had depicted. The most common theme in the guidance at the moment is for a frontal system to reach the Plains by Saturday and continue eastward into Sunday. The ridge that builds off the West Coast during the weekend may support elongation of upper troughing into the Southwest U.S., which would lead to some deceleration of the front over the southern Plains. The 00Z ECMWF has shifted noticeably farther west with the upper ridge versus continuity. The forecast during the latter half of the period trended to a 40-50 percent total weight of the 18Z GEFS/12Z ECMWF means with the rest composed of the ECMWF/CMC and 12-18Z GFS runs (whose input diminished late due to increasing differences relative to consensus). ...Weather/Hazards Highlights... The vigorous surface low tracking across southern into eastern Canada mid-late week will bring a period of strong winds to northern tier areas, especially the Northern Plains into Wednesday and perhaps to some degree locations farther east thereafter. Expect mostly light to moderate rain to develop along the trailing cold front over the east-central U.S. around Wednesday and progress eastward/northeastward into Thursday. Highest totals should extend northeastward from the south-central Mississippi Valley. Chilly flow behind the system may produce a period of lake effect precipitation, mainly over the upper and eastern portions of the Great Lakes. The southern Florida Peninsula will likely see increasing rainfall mid-late week as moisture interacts with a stalled front to the south. Easterly low level flow should lead to highest totals over eastern areas. Meanwhile it looks increasingly likely that a Pacific system reaching the northern half of the West Coast after midweek will produce organized precipitation over central/northern parts of the West. Uncertainty over the strength and track of the system maintain lower than desired confidence in details of precipitation coverage and magnitude as well as any wind effects near the coast. The energy from this system may support a central U.S. front that could start to produce some rain by next weekend. Northern parts of the Pacific Northwest may see another episode of precipitation next weekend. Overall expect temperatures across the lower 48 to reach fairly seasonable levels by next weekend with many locations seeing highs within a few degrees of normal. Before then, the warm sector ahead of the front crossing the eastern half of the country Wednesday-Thursday will bring a brief period of warmth to the South and East as temperatures reach up to 10-15F above normal (perhaps even warmer for morning lows on Wednesday). Colder air behind the front will spread from the Plains into the East mid-late week with readings generally 5-10F or so below normal. Parts of the High Plains could rebound to scattered plus 10F anomalies 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml