Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Mon Nov 15 2021 Valid 12Z Thu Nov 18 2021 - 12Z Mon Nov 22 2021 ...Overview and Guidance/Predictability Assessment... Guidance generally agrees that progressive mean flow late this week should eventually transition toward an amplifying mean trough over the central-eastern U.S. as a ridge building over the eastern Pacific during the weekend begins to move into the West early next week. This evolution should promote a couple surges of chilly air over the central/eastern states while temperatures over the West remain near to above normal. While most solutions offer similar ideas for the large scale evolution, models and ensembles continue to be at odds over various aspects of the forecast behind the well-clustered late week upper trough crossing the East and associated cold front/trailing high pressure. Before the latest 00Z cycle it had appeared guidance was starting to cluster somewhat for the system expected to track toward the Pacific Northwest late this week--aside from the 12Z UKMET straying to the fast side and the 12Z/18Z GFS runs leaning a tad slow. Now the 00Z UKMET/CMC have adjusted to a weaker and more open shortwave that leads to a much weaker/slower/suppressed surface wave. The 00Z ECMWF is somewhat weaker/slower but on a similar track and is now more similar to the GFS. The latest manual forecast based on data through the 12Z/18Z cycles used a model composite at 12Z Thursday and then phased out the UKMET contribution. Continued uncertainty with how this system will evolve tempers confidence in details of the general area of lower pressure and one or more fronts forecast to reach the central U.S. by Saturday. Farther west, additional diffuse energy may reach areas over and southwest of the southwestern U.S. Models and ensembles vary considerably for what may become of this energy (open trough or closed low) and its location, with consecutive model runs also quite variable. In addition there are meaningful differences for timing of the upper ridge moving into the West early next week and upstream energy (12Z CMC strayed well ahead of other guidance, 12Z ECMWF a bit slow). At least the new 00Z CMC has adjusted closer to consensus. These various issues affect how amplified the central-eastern U.S. upper trough will be and the southern/eastern extent of surface fronts by the end of the period. The lack of confidence in specifics by the latter half of the period favored a fairly rapid increase of 18Z GEFS/12Z ECMWF mean input, reaching 60 percent total by day 7 Monday, with the rest composed of the 12Z ECMWF and small parts of the 12Z/18Z GFS. ...Weather/Hazards Highlights... Strong low pressure tracking over eastern Canada should bring brisk winds to areas from the Upper Midwest into the Northeast Thursday-Friday while mostly light to moderate precipitation (mostly rain) will accompany the trailing front crossing the eastern U.S. Northern and eastern portions of the Great Lakes should see a period of lake effect precipitation after frontal passage. Meanwhile the southern Florida Peninsula may experience locally moderate to heavy rainfall around Thursday as moisture interacts with a stalled front to the south and another front approaching from the northwest. Continued spread and variability in guidance for the system approaching the West Coast late this week keep confidence lower than desired for the specifics of rain and higher elevation snow that should spread across the central/northern West Coast into the northern Rockies. Upstream moisture may keep some precipitation over the northern Pacific Northwest through the weekend. As shortwave energy continues eastward, expect another area of rain to develop from the southern Plains eastward/northeastward from the weekend into early next week ahead of one or more cold fronts but again with low confidence for coverage and amounts at this time. Precipitation could change to snow over the farthest north latitudes with reinforcing cold air. Areas near the East Coast will see temperatures up to 10-15F above normal on Thursday ahead of an advancing cold front. The cooling trend behind this front will bring generally 5-12F below normal readings from the Plains into the East Thursday-Saturday. Upstream system progression may bring a brief episode of warmth from the West into the High Plains late this week, peaking with some highs 10-15F above normal over the central High Plains on Friday. Another push of cold air should extend southeastward from the northern Plains Sunday-Monday with highs 5-10F below normal. 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