Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Thu Oct 17 2024 Valid 12Z Sun Oct 20 2024 - 12Z Thu Oct 24 2024 ...Overview... A closed upper low atop the Four Corners into Sunday could lead to lingering snow in the south-central Rockies and cool temperatures, while moderate to locally heavy rain and some isolated flooding concerns spread into the south-central Plains. Expect the upper low to track northeast and weaken with time, producing some lighter rain. Meanwhile, precipitation is likely in the Pacific Northwest early next week with the tail end of an atmospheric river. By midweek and beyond, broadly cyclonic flow should set up across the eastern two-thirds of the CONUS while upper ridging may form over or near the West as splitting trough energy yields a trough/closed low over the eastern Pacific. Additionally, ridging that retrogrades from the Gulf of Mexico through southern Texas and Mexico will spread warmer than average temperatures to the southern states, while cold fronts will gradually cool the northern tier after that region sees very warm temperatures early in the week. ...Guidance/Predictability Assessment... Primary guidance issues continue to revolve around persistent minor timing and detail differences for the upper low ejecting from the Four Corners region and then pattern implications of the forecast separation of flow within an elongated Canada into eastern Pacific upper trough as of early next week. For the Four Corners upper low, GFS trends over the past day have brought its previous fast solution more into line with recent consensus but there is still a little waffling among other solutions. Of particular note, the 12Z CMC is a bit on the slower side of the spread (with that model tending to be slow in recent days) while the new 12Z ECMWF has also trended slower over the Four Corners and then weaker/southward over the Plains/Midwest (but closer to the guidance average at that point). Then this feature should open up/shear out as stronger northern stream energy approaches. Guidance continues to show splitting of early-week elongated Canada/eastern Pacific trough energy, with such flow separation cases frequently lowering predictability as evidenced by sometimes significant multi-run guidance spread and variability for the downstream pattern. The most agreeable aspect of this evolution is Tuesday-Wednesday closed low formation over the Pacific around 135-145W longitude and progressive Canada/northern tier U.S. shortwave energy reaching eastern North America by Thursday (supporing a leading cold front). The more contentious part of the forecast involves location/strength of upper ridging that may set up over the western U.S. or just offshore the West Coast, and details of smaller scale energy that could be left behind over/offshore of the Pacific Northwest and southwestern Canada. Latest dynamical model runs have been leaning toward a stronger upper ridge over the West, in contrast to some ECMWF/CMC ensemble members that actually depict some trough/upper low potential. Those members lead to flatter flow over the West in the resulting ECens/CMCens means. Meanwhile the 00Z/06Z machine learning models and 00Z AIFS mean depict upper ridging but on average with an axis farther west than the dynamical model cluster. Compared to the full guidance envelope, the new 12Z GFS is fairly extreme with the amplitude and eastward position of its upper ridge, as well as the eastward position of the Pacific upper low/trough. The 06Z GFS compared better to other guidance. Based on comparisons of 00Z/06Z models and means, the updated forecast incorporated an even compromise of operational runs early in the period and transitioned to an even blend of 06Z GFS/00Z ECMWF and 00Z GEFS/ECens means to reflect the most common ideas of guidance. ...Weather/Hazards Highlights... The Four Corners upper low will provide forcing for precipitation across south-central parts of the Rockies and Plains into Sunday. Snow could linger in the higher elevations of Colorado in particular, though likely decreasing in magnitude compared to late this week. Ahead of the upper low, moisture streaming in from the Pacific and the Gulf will provide an environment with PWs above the 90th if not 95th percentile for potentially heavy rain and storms across the south-central Plains. The Day 4 Excessive Rainfall Outlook maintains a Marginal Risk area centered over the Texas and Oklahoma Panhandles into the western half of Kansas, plus parts of northeastern New Mexico/far southeastern Colorado as heavy rain may cause isolated flooding threats especially if areas of high rain rates train in the same locations. Some showers will shift into the central Plains/Middle Mississippi Valley by Monday and perhaps the Ohio Valley Tuesday, though with decreasing intensity. Meanwhile, the Pacific Northwest will likely see precipitation on Sunday as an atmospheric river continues to spread moisture into the region, with generally light to moderate rain and snow amounts (lesser totals than over the Olympics in the short range). Some precipitation may linger into Monday and spread farther inland but with a continued trend toward lighter amounts. Gusty winds are possible in the Rockies as fronts push through. There is still uncertainty about specifics of eastern Pacific/western U.S. flow aloft by Tuesday-Thursday, but most guidance currently suggests that most of the West should be dry with upper ridging setting up. The fast side of the spread for an eastern Pacific upper trough/low would lead to some moisture reaching the West Coast by next Thursday or Thursday night but that scenario is somewhat in the minority at the moment. Most of the eastern U.S. should remain dry through much of the period. Easterly flow across the Florida Peninsula could bring some light showers onshore. By Tuesday-Wednesday, light rain may push across the north-central to northeastern U.S. with a front. Cooler than average temperatures by 5-15 degrees should continue into Sunday in the Four Corners states underneath the upper low, but gradually trend warmer thereafter. Overall a majority of the West should trend toward near/above normal temperatures by midweek, though with some uncertainty in light of guidance differences for the strength and location of upper ridging. Meanwhile, another upper ridge initially over the east-central U.S. and then drifting eastward will promote well above normal temperatures across the northern Plains to Upper Midwest/Great Lakes and Northeast. The Midwest can expect the greatest anomalies of 15-25 degrees above average on Sunday and Monday as highs reach the 70s. Forecast transition of the upper pattern toward broadly cyclonic or nearly zonal mean flow along with a surface cold front would moderate temperatures over the northern Plains and vicinity by midweek and into the Great Lakes/Ohio Valley/Northeast by next Thursday. A subtropical upper high remaining across the Gulf/northern Mexico/Texas into Wednesday will allow for warmer temperatures to expand across the the southern Plains into Southeast. Rausch/Campbell/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