Extended Forecast Discussion NWS Weather Prediction Center College Park MD 142 PM 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. The models and guidance show fairly good agreement on the large scale evolution during the medium range period, but continue to offer plenty of variability in the details. Models have trended a little weaker with a storm expected to track towards the Pacific Northwest late this week, with the UKMET the weakest and quickest to eject that energy into southwest Canada/far northern U.S.. After day 4, uncertanity in the evolution of the shortwave as it crosses into the Central U.S. this weekend and eventually amplifies over the East early next week is high as several pieces of energy rotate through the trough to help amplify and reinforce the troughing as it enters the East next Monday. This tempers confidence in the details of the general area of lower pressure and one or more fronts crossing the central into Eastern U.S. this weekend. There is enough ensemble support to mention potential development of a deep cyclone to impact parts of the Northeast or New England early next week/just beyond the medium range period. However, the model spread for timing and intensity of this feature remains too high to be able to talk about specific impacts this may (or may not) have on the busy Thanksgiving holiday travel week. The WPC forecast used a general model blend of the 06z GFS with the 00z ECMWF and CMC as the basis for the forecast days 3-4. After this, added and gradually increased contributions from the ensemble means to help mitigate some of the harder to resolve detail differences. This also maintains decent continuity with the previous WPC forecast. ...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 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 the 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. Santorelli/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