Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Nov 10 2024 Valid 12Z Wed Nov 13 2024 - 12Z Sun Nov 17 2024 ... Another atmospheric river to bring heavy rain and mountain snow threats to parts of the Northwest midweek... ...Overview... A fairly progressive pattern is forecast through later week and into next weekend. A somewhat amplified trough will push from the central to eastern U.S. Wednesday-Friday, with increasing chances for this trough to tap into tropical Gulf moisture and cause heavy rainfall for portions of the southeastern quadrant of the country midweek, with light to moderate much-needed precipitation farther north in the Midwest to Mid-Atlantic/Northeast. Upper ridging on either side of this trough will promote periods of warmer temperatures across the eastern half of the nation through the period. Meanwhile out West, an atmospheric river will impact parts of the Northwest around midweek bringing heavy rain and snow threats, with additional rounds of precipitation later in the week with trough reloading along the coast. This positively tilted trough is forecast to set up over the West while ridging builds atop the East next weekend. ...Guidance/Predictability Assessment... While model guidance is reasonably agreeable with the overall pattern, even early on there are some model differences regarding the timing and amplitude of upper troughs and ridges that have sensible weather impacts. First, the trough with its axis over the Plains to start the period Wednesday continues to show basically two camps: slower and deeper non-NCEP guidance (ECMWF, CMC, EC mean, and AI/ML models, along with the UKMET that may even be too slow), and faster and shallower NCEP guidance (GFS, GEFS mean, and the tail end of the NAM's forecast). The slower and deeper trough leads to more moisture inflow from the Gulf for heavier rainfall in the southeastern CONUS. With the bulk of the guidance indicating a slower/deeper trough, and considering the GFS's propensity toward faster trough progression, favored more of an ECMWF/CMC type of solution with this trough. For the most part, newer 12Z non-NCEP models have trended a bit shallower/faster with the trough though (GFS/GEFS are still farther east). So there is still some uncertainty with the details but perhaps models are converging toward a middle ground solution. Models show the general idea of an upper ridge behind this trough relatively well, but there is more spread with the next trough coming into the West. Differing distributions of vorticity within this trough lead to model differences with possible northern/southern stream separation by the weekend, which affects the trough progression. The 00Z ECMWF was particularly aggressive in depicting a closed low well offshore of Mexico in the southern stream but the 12Z ECMWF has backed off on this. The WPC forecast favored the ECMWF with lesser proportions of the GFS, CMC, and ensemble means early in the period. Gradually increased the proportion of ensemble means in the blend to half Day 6 and more Day 7 amid increasing spread especially with the second trough. ...Weather/Hazards Highlights... A modest atmospheric river will continue to impact the Pacific Northwest into northern California into the beginning of the period Wednesday. Above normal moisture and some instability along the coast could lead to heavy rain that may create minor flooding concerns especially considering there is another AR set to impact the region in the short range period. Thus a Marginal Risk remains in place for western Washington/Oregon and into northwest California on the Day 4/Wednesday ERO. Snow may be heavy in the mountains, particularly the Cascades as the week progresses, with some modest snow amounts into the northern Rockies as well. After Wednesday, precipitation should trend lighter as the moisture moves inland, but some lingering precipitation along the coast may be possible as weaker energy reloads into the larger scale trough. Meanwhile additional snow is possible in the northern Rockies to perhaps the northern High Plains. Tropical Gulf moisture streaming ahead of a central U.S. trough and cold front, as well as coastal front interaction and possibly even some remnant energy from what was Rafael, could lead to heavy rain from the central Gulf Coast to Mid-South Wednesday. Current model differences limit confidence in placement of the heaviest QPF though. Models including the 00Z ECMWF and recent AIFS/CMC/UKMET runs showed good clustering in the heaviest rain over northern/central Mississippi and vicinity, but the new 12Z ECMWF coming in farther south near southern Alabama/Florida Panhandle limits confidence in that position. Thus maintained the broad Marginal Risk for the region but expect locally excessive rain totals. If and when model guidance becomes more agreeable, this setup does seem favorable for an embedded Slight Risk in future forecasts. Farther north, light to moderate precipitation is forecast across the Midwest. Rainfall should pivot into the East Thursday while lightening overall. Most of this precipitation looks to fall as rain given the warmer than normal temperatures, but some ice and snow are possible in higher elevations of the Interior Northeast, though this forecast showed a decrease in forecast precipitation there. While system progression will provide some day-to-day variability, most of the lower 48 to the east of the Rockies should see around normal to above normal temperatures much of the week. Meanwhile lingering snow cover in the central/southern Rockies will promote cooler than normal highs there through Wednesday or Thursday while the West otherwise should trend colder with more amplified troughing moving into the region late week and into next weekend. Tate/Santorelli 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