Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Mon Nov 18 2024 Valid 12Z Thu Nov 21 2024 - 12Z Mon Nov 25 2024 ***Strong, long-duration atmospheric river will take aim at northern California/southwestern Oregon for the end of the week*** ***Pattern change for east-central to eastern U.S. with cooler temperatures and cold rain and higher elevation snow*** ...General Overview... A deep upper level low developing across the Great Lakes and Northeast U.S will make weather headlines to close out the work week. This system will likely meander slowly in place for a few days with a blocking high to its north across central Canada, and then should finally lift north across northeast Canada by Sunday. This will result in a cold rain from the Ohio Valley to the East Coast, and early season accumulating snow for the central Appalachians and the higher terrain of New England. Out West, an unsettled weather pattern across the eastern Pacific is expected to result in a prolonged atmospheric river event from northern California to western Oregon for the second half of the work week, with several inches of rainfall likely for the lower elevations, and snow makes a return to the Sierra Nevada. Between these two regions, an upper level ridge is likely to build across the Rockies and eventually across the Plains by next weekend. ...Guidance/Predictability Assessment... There are still some modest model differences in the expected evolution of the large scale upper trough and closed low across the Eastern U.S. by Thursday, but generally close enough to support a blend of the deterministic guidance through Friday. This also holds true for the surface low that develops near southern New England and then slowly moves north towards eastern Quebec. Model spread has also improved with the two separate lows over the eastern Pacific that will be impactful to the Pacific Northwest, but more differences appear by Sunday and into Monday. At the time of the fronts and pressures generation, the 12Z CMC became more out of phase with the model consensus by the second half of the forecast period, so it was dropped for Sunday and into Monday, and more in line with the GFS/ECMWF and their respective means. The ensemble means were gradually increased to about 2/3rds by next Monday amid growing model differences, while still maintaining some of the deterministic GFS and ECMWF solutions. ...Weather/Hazards Highlights... Confidence is high for a prolonged atmospheric river event setting up across portions of northwestern California and into southwestern Oregon during the Thursday through Friday time period. Enhanced moisture flux ahead of a meandering cold front and two separate low pressure areas will result in enhanced orographic lift and likely produce some 4 to locally 8 inch rainfall totals by Friday night. Given the duration of the event, a Slight Risk area is in place for both Days 4 and 5 across these areas, including the coastal ranges and extending to the coast. A Day 4 (Thursday) Moderate Risk area is now planned for Del Norte and Humboldt counties in northwestern California, owing to heavy rainfall and saturated grounds in the days leading up to this time period, and highly anomalous integrated water vapor transport oriented nearly orthogonal to the terrain. There will likely be some abatement going into Saturday with the front slowly making its way inland across the region. Snow levels will initially be high, but should fall going into the weekend, including much of the Sierra range where heavy snow is becoming more likely. Impactful weather is also expected for a large portion of the Eastern U.S. for the latter half of the work week. This will be the result of a large upper low closing off and inducing surface cyclogenesis near the Mid-Atlantic and southern New England. Cold air aloft will be sufficient to support accumulating early season snow for the higher terrain near the Great Lakes and interior Northeast, and especially for the central Appalachians where some totals in excess of six inches are a good possibility. There may also be some lake effect snow in the wake of this storm system, although temperatures will likely be marginal and this could mix with rain at times. Rainy and breezy to windy conditions are expected for the major cities in southern New England through Thursday night. Temperatures will turn noticeably colder across the Eastern U.S. in the wake of a strong cold front and the upper low moving in overhead by Thursday, with highs expected to be about 5 to 15 degrees below average to close out the work week. Given widespread cloud cover, overnight lows will be closer to average with less of a diurnal range for many areas. A warming trend is forecast across the Southern Plains going into the weekend as an upper level ridge builds in and warmer flow from the Gulf of Mexico, with highs reaching into the 80s for the southern half of Texas. Signs of an arctic airmass may start to be noticed for parts of Montana and North Dakota by next Monday, with overnight lows dropping below 10 degrees potentially. 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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw