Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 PM EST Sun Nov 17 2024 Valid 12Z Wed Nov 20 2024 - 12Z Sun Nov 24 2024 ...Strong, long-duration atmospheric river will take aim at northern California/southwestern Oregon for midweek into late week... ...Pattern change for east-central to eastern U.S. with cooler temperatures and cold rain and high elevation snow... ...Overview... A long duration atmospheric river is forecast to affect the West Coast this week as upper and surface lows/frontal systems direct a strong moisture plume into northern California/southwestern Oregon in particular, where several inches of rain and mountain snow are likely. Meanwhile a deep upper low developing across the Great Lakes and Northeast U.S will meander slowly in place later this week with a blocking high to its north across central Canada. This will result in cooler temperatures, 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 the interior Northeast. Between these two regions, an upper ridge is likely to build across the Rockies and eventually across the Plains by next weekend, warming up temperatures in the interior West and southern Plains. ...Guidance/Predictability Assessment... Model guidance begins the medium range period in fairly good agreement for important features, including a deep surface low somewhere in the 940s to 960s in the eastern Pacific offshore of Vancouver Island Wednesday, with its cold front directing the AR into the West Coast. Models show this low gradually weakening as it moves back northwest into Thursday, giving way to another low with more spread in depth (weaker than the first one though) and placement approaching the Pacific Northwest Friday into the weekend. The newer 12Z guidance suite seems to be coming in with better agreement for this secondary low fortunately. A model blend seemed reasonable initially, but there was increasing spread by late week into the weekend. Namely, the 00Z CMC was farther offshore with an upper low and directed the AR farther north into the Pacific Northwest given its mean flow from the southwest rather than west. Meanwhile, the 06Z GFS brought in a notable shortwave into the Northwest Saturday and tracking into the north-central U.S. by next Sunday. This was dismissed as an outlier; however, the newer 12Z GFS held on to an evolution like this and the 12Z EC is not too off either, so this feature will continue to be monitored and may necessitate changes in future forecasts. Farther east, models are reasonably agreeable with the upper and surface lows moving across the Midwest to East. The 00Z GFS became the main outlier by late week as its lows were displaced to the southeast of consensus. Otherwise a model/mean blend seemed reasonable. The WPC forecast began with a blend of the 00Z ECMWF/CMC/UKMET and 06Z GFS. Decreased the proportion of the deterministic runs (especially those other than the ECMWF) with time as model spread increased especially in the West, in favor of the GEFS and EC ensemble means. ...Weather/Hazards Highlights... Confidence remains high for a potentially major and prolonged atmospheric river event setting up across portions of northwestern California and into southwestern Oregon during the Wednesday through Friday time period. Enhanced moisture flux ahead of a meandering front and at least two low pressure areas will result in enhanced orographic lift and likely produce some 4 to locally 8 inch rainfall totals during this time. Slight Risks remain in place for both Days 4/Wednesday and 5/Thursday in the ERO. Burn scars will be of particular concern for flooding. By Thursday/Day 5, snow levels are forecast to rise with an influx of warm air, so extended Thursday's Slight Risk farther inland/into higher elevations where rain on snow may exacerbate flooding concerns. The long duration of the AR stalling could make this event be on the higher end of the Slight Risk range, with multiple days of rainfall compounding flooding impacts. Additionally, at least marine areas should see strong winds through Wednesday given the deep eastern Pacific low. The AR should finally start to move and weaken into late week and next weekend, bringing precipitation farther south while snow levels lower into the Cascades/Sierra Nevada, and farther east with some chance for snow to extend into the northern High Plains. 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 over the Midwest and slowly making its way across the Great Lakes/Ohio Valley and then Northeast as the week progresses. Surface low pressure will track through the Great Lakes region and give way to surface cyclogenesis near the Mid-Atlantic and southern New England in a Miller B type evolution. Some strong winds are likely in the northern Plains Wednesday behind the first surface low, with breezy to windy conditions broadly from the Midwest to East in this pattern. Precipitation is also likely with these features. Some locally heavy rain is possible in Florida for midweek with moisture streaming in ahead of the tail end of associated cold fronts. Coastal areas of the Mid-Atlantic and Northeast including the Interstate 95 corridor should also see some modest to perhaps locally heavy rain Wednesday-Thursday. This did not seem to necessitate any ERO areas as lack of instability should put a cap on heavy totals, and ongoing drought will mean most areas are not particularly sensitive to flooding. 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. 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 that 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 as the ridge builds in after the cold conditions expected in the first half of the week. Tate/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