Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 PM 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 through late week... ...Pattern change for east-central to eastern U.S. with cooler temperatures and cold rain and higher elevation snow... ...Overview... A long duration atmospheric river is forecast to affect the West Coast into later 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 are likely to cause flooding. Meanwhile a deep upper low will drift slowly from the Great Lakes to the Northeast U.S., leading to cooler temperatures and a cold rain/higher elevation snow for those regions. In between, an upper ridge is likely to traverse the interior West and central U.S. before the upper pattern flattens out into early next week. ...Guidance/Predictability Assessment... Model guidance begins the medium range period in fairly good agreement for important features, including upper lows over the northeast Pacific and the Great Lakes, but the details are more questionable. For example even on Thursday, there is model spread with the positioning of the Great Lakes region surface low before the western Atlantic low spins up. The 00Z EC/CMC/UKMET clustered well with a position over the Lower Great Lakes while the GFS runs as well as several of the EC-based AI/ML lows were farther west. This has impacts on low placement into late week. The 12Z models have come in with better agreement on that low placement. Then for the West, there is good agreement for the primary surface low to drift back northwest while weakening into Thursday, but another surface low approaches the Pacific Northwest from the west. There remains some spread in depth (weaker than the first one though) and placement, but the newer 12Z guidance suite seems to be coming in with better agreement for this secondary low fortunately, other than the CMC that has another low farther south. Then into the weekend, some shortwave energy may push across the northern tier. The 00Z GFS was particularly deep in developing this troughing, with the consensus showing the existence of that feature but shallower. This feature will have impacts on a possibly developing surface low in the Plains or Midwest and sensible weather like QPF and temperatures. The WPC forecast began with a blend of mainly deterministic models, decreasing the proportion of the deterministic runs with time as model spread increased in favor of the GEFS and EC ensemble means. ...Weather/Hazards Highlights... Confidence remains high for a major and prolonged atmospheric river event setting up across portions of northern California and into southwestern Oregon into Thursday-Friday. Two low pressure areas focusing a front just offshore will enhance moisture flux for significant integrated vapor transport into those areas, and the AR stalling will lead to a long duration of moderate to heavy rain amounts. Through the event, some of which is phasing into the short range period, 10 to 15 inches of QPF are forecast, and the 10 year average recurrence intervals for 48 and 72 hour periods are likely to be exceeded. Numerous areas could see flooding and burn scars will be of particular concern. A Moderate Risk remains in place for parts of northwestern California where the heaviest rainfall should focus for Day 4/Thursday. A Slight Risk extends inland even into some higher elevation areas as snow levels should be quite high on Thursday. The AR may start to move somewhat by Day 5/Friday for lower overall rain totals, so maintained a Slight Risk then, though a future upgrade may be needed once the impacts from the previous days' precipitation are more clear. The pattern will remain favorable for some precipitation into the weekend. Though there should be lesser amounts, but in some areas any additional rain may cause flooding concerns due to the very wet antecedent conditions by then. Snow levels are also forecast to fall, and some snow may become heavier in the Cascades and Sierra Nevada for the weekend. Precipitation should expand inland, including snow in the northern Rockies, with some rain and snow impacting the Intermountain West farther south into early next week. A round of precipitation (possibly snow) may occur in the northern Plains to Upper Midwest, but this is uncertain at this point. Impactful weather is also expected for much of the eastern U.S. for the latter part of the week due to the upper low traversing the Great Lakes/Ohio Valley and then Northeast. 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. Precipitation is likely with these features and on the backside, including higher elevation snow for the Appalachians and interior Northeast. The central Appalachians in particular may see several inches of snow especially on Thursday. The cold rain in the Northeast Thursday is not forecast to cause any excessive rainfall concerns since lack of instability should put a cap on rain rates and ongoing drought will mean most areas are not particularly sensitive to flooding. Breezy to windy conditions are forecast broadly from the Midwest to East in this pattern. Lake effect snow and/or rain are likely behind the lows. 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 workweek. 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 leading to warmer flow from the Gulf of Mexico, with highs reaching well 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 possibly dropping into the single digits. Tate 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