Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Wed Nov 20 2024 Valid 12Z Sat Nov 23 2024 - 12Z Wed Nov 27 2024 ***Turning much colder for the Northern Plains, and snowy conditions for the Sierra and portions of the Rockies*** ...General Overview... A quieter overall weather pattern can be expected going into the weekend after the prolonged atmospheric river event in the short range period, and the low pressure system over the Northeast U.S. will be lifting northward across southeastern Canada. The upper level flow pattern then becomes more quasi-zonal for the Sunday to Monday time period with the Western U.S. trough moving quickly inland and becoming less amplified. However, there will still be enough moisture to produce moderate to locally heavy snow from the Sierra to the central/northern Rockies, and lingering light to moderate rain for the lower elevations of the West Coast. A wavy surface front is expected to develop across the Plains with a much colder airmass moving in across the Dakotas and Montana early to mid next week, and an upper level trough likely builds back in across the northwestern U.S. by Wednesday. ...Guidance/Predictability Assessment... The 00Z models remain in good synoptic scale agreement overall with the pattern, including the exiting large upper/surface low pressure system across New England and into southeastern Canada. A blend of the latest 12Z/18Z deterministic models worked well as a starting point for the weekend, with slightly more weighting to the GFS/ECMWF. The 00Z guidance has also improved with the expected quasi-zonal flow evolution across much of the central U.S., but some differences appear by early next week in the eastern North Pacific into the West and then the High Plains. The CMC lingers the trough near the Pacific Northwest much longer than the more progressive ECMWF/GFS locations by Tuesday and Wednesday, and the CMC is also more amplified with a trough and potential closed low near the Great Lakes that does not have much ensemble or AI support, so it was not used beyond Monday. Overall model spread increases substantially by Wednesday with the overall trough evolution across the West and the Plains, so forecast confidence is low for Day 7. The ensemble means accounted for about 40-50% of the forecast blend for the middle of next week amid growing model spread. ...Weather/Hazards Highlights... There will be some abatement in the atmospheric river across California going into Saturday as the attendant cold front moves inland and the moisture flux anomalies decrease some. The best model signal for the heaviest rain is across the western slopes of the Sierra Nevada, where the Marginal Risk area will remain for the new Day 4 excessive rainfall outlook with 1 to locally 2 inches of rain by early Sunday. Nuisance level rainfall is expected for the coastal ranges of central and northern California, so no risk areas appear warranted at this time for those areas. Snow levels will be dropping in the wake of the front, and this is expected to result in heavy snow for the higher elevations of the Sierra Nevada, and moderate snow for the Intermountain West ranges and eventually the central and northern Rockies for the Sunday to Monday time period. Improving conditions are also forecast for the Eastern U.S. as the strong low pressure system slowly exits the region. There will likely be some lake effect rain/snow showers for the eastern Great Lakes and snow for the higher terrain of New England on Saturday, with breezy to windy conditions in northwest flow. Most areas should be dry by Sunday as high pressure starts building back in. Depending on eventual surface low evolution along a frontal boundary across the south-central U.S., some heavier showers and perhaps some thunderstorms could develop across portions of the Mid-South and into the southern Ohio Valley by Tuesday, and lighter rainfall extending east across the Mid-Atlantic and Northeast towards mid-week. Temperatures are expected to slowly moderate to near average across the Eastern U.S. over the weekend compared to the rather chilly conditions late this week. There is still the potential for arctic air to reach portions of Montana and into North Dakota next week with highs in the 10s and low 20s, and overnight lows in the 0s. There is still some uncertainty on how far south this arctic airmass gets, so this will be refined in future forecasts. Warm conditions should continue from southern Texas to Florida, with highs in the 70s and 80s. 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