Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EST Mon Nov 29 2021 Valid 12Z Thu Dec 02 2021 - 12Z Mon Dec 06 2021 ...Overview... The overall pattern will be featured by an upper-level ridge from the eastern Pacific to the Intermountain West, and quasi-zonal flow to low amplitude troughing from the Plains to the East Coast with multiple shortwave perturbations pivoting around a synoptic scale trough over southeast Canada. This may act to amplify the Eastern U.S. troughing slightly through the period. This will include an organized low pressure system tracking across southeastern Canada through Friday with a trailing front followed by a weaker low this weekend, leading to rounds of mainly light precipitation across the Great Lakes and northeastern U.S. By the weekend, showers and some storms are likely to develop ahead of a developing storm system from the Gulf Coast to the Ohio Valley. ...Guidance/Predictability Assessment... The 00Z/06Z guidance suite continues to show decent synoptic scale agreement through Friday with an upper ridge over the southwestern U.S. and a broad trough over the Northeast U.S. The only notable difference is the 06z/12z GFS continue to be much slower than the rest of the guidance on Thursday/Day 3 with organized low pressure across the Great Lakes, but eventually catch up by day 4. As the flow pattern becomes more quasi-zonal into the weekend, model spread increases some with respect to the next shortwave trough entering the northwestern U.S., as well as a weak southern stream upper low developing over northern Mexico. Enough run-to-run model variations continues to suggest a below average forecast confidence for the second half of the period. The CMC continues to be the most problematic as it becomes out of phase along the West Coast for days 6-7. At the end of the period, much of the guidance suggests amplification of the Eastern U.S. trough may lead to low pressure development near the Southeast/Mid-Atlantic on day 7, and possibly track northward along the Northeast Coast immediately following the end of the medium range forecast period (12/6 at 12z). Confidence is low in the specifics for impacts across the Northeast amidst enough timing and intensity uncertainties in the individual solutions. The WPC forecast was primarily based on the deterministic guidance blend through Friday. Thereafter, increased weighting towards the ensemble means eventually getting to a 50/50 blend of ensembles with the deterministic ECMWF and GFS. ...Weather/Hazards Highlights... Weather should be relatively quiet in terms of rainfall or snow related hazards with not much in the way of heavy precipitation currently expected through Saturday. Rounds of relatively light precipitation are forecast for the Midwest, Great Lakes, and Northeast with a progressive storm track over the area. Amounts should be most substantial in favored lake effect and possibly upslope areas, particularly downwind of Lake Ontario into the Adirondacks. Then moisture is set to increase across the Southern Plains and Lower Mississippi Valley late Friday and into the weekend, which could lead to showers and thunderstorms becoming more widespread there and then extending northeastward as the surface low over the Great Lakes becomes better organized. A shortwave into the Northwest early next week could bring another round of moderate to locally heavy rain and mountain snows to parts of western Washington and Oregon. Temperatures are expected to be above normal for this time of year across most of the nation through next weekend, with the greatest positive anomalies expected for the central and northern Plains where both highs and lows are forecast to be 15 to 25+ degrees above average for late in the week. A few locations may approach daily maximum and minimum temperature records with these mild temperatures. By the weekend, temperatures are currently forecast to moderate somewhat, but this is subject to change given forecast uncertainties. No arctic airmass intrusions are expected during this forecast period. Santorelli/Hamrick 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 Hazards: - Flooding occurring or imminent across portions of the Pacific Northwest. - High winds across portions of the Northern Plains and the Northern Rockies, Thu, Dec 2. 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