Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 AM EST Sun Nov 17 2024 Valid 12Z Wed Nov 20 2024 - 12Z Sun Nov 24 2024 ***Strong upper low across the Great Lakes and Northeast to result in cold rain and higher elevation snow, and prolonged atmospheric river event expected for the Pacific Northwest*** ...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 will likely meander slowly in place for a few days with a blocking high to its north across central Canada. 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 Washington for the second half of the work week, with several inches of rainfall likely for the lower elevations. 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 model differences in the expected evolution of the large scale upper trough and closed low across the Eastern U.S. by Thursday, with the ECMWF/CMC/UKMET supporting a closed low farther south across the Ohio Valley and into PA, whereas the GFS and GEFS mean are farther north and closer to the Great Lakes. However, the guidance agrees well that a surface low is likely to develop near southern New England and then slowly move out towards Nova Scotia. Model spread has also improved with the two separate lows over the eastern Pacific that will be impactful to the Pacific Northwest, although mesoscale differences remain that are typical during the day 4-6 time period. The ensemble means were gradually increased to about 50% by next Sunday amid growing model differences, while still maintaining some of the deterministic GFS and ECMWF solutions. ...Weather/Hazards Highlights... Confidence is increasing for a potentially major and prolonged atmospheric river event setting up across portions of northwestern California and into southwestern Oregon during the Wednesday through Thursday 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 during this time. 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. This will likely have additional staying power going into Friday as well 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. 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 along the Interstate 95 corridor through Thursday night. In terms of temperatures, warm and slightly humid conditions can be expected from the Mid-Atlantic southward to Florida on Wednesday ahead of the approaching cold front, which will bring a return to much cooler conditions to close out the work week. Highs should generally be about 5-10 degrees below average across the interior Eastern U.S. by Friday, mainly because of the cold upper low overhead keeping widespread cloud cover in place, and overnight lows closer to average. A gradual warming trend is likely for the Intermountain West and the Rockies after the cold conditions expected in the first half of the week. 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