Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 with some isolated flooding concerns spreads into the south-central Plains. The upper low is forecast to track northeast and weaken as the week progresses with 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 atop the eastern two-thirds of the CONUS while upper ridging may form over the West in response to ample energy across the eastern Pacific. Additionally, ridging retrograding from the Gulf of Mexico through southern Texas and Mexico will spread warmer than average temperatures to the southern states, while cold fronts gradually cool the northern tier after a warm early week period there. ...Guidance/Predictability Assessment... Recent guidance is generally agreeable with the pattern described above. There are some minor timing differences with the ejection of the Four Corners upper low. The 00Z CMC seems to be a slow/farther west outlier with the upper low even as the period begins Sunday and maintains a slightly slower feature while recent GFS/EC runs are more agreeable, along with the AI models. By around Tuesday the upper low's timing may also depend on the arrival of northern stream energy pushing into the north-central U.S., but this is showing better consensus in recent guidance. Farther west, a southwest to northeast oriented mean trough stretching from southwestern Canada into the Pacific early next week shows some spread with its evolution, dependent in part on energy distribution within the trough and additional shortwaves diving in. Model guidance from the last few cycles has indicated that an upper low is likely to close off well offshore, perhaps between 140W and 150W longitude, but creeping eastward by Wednesday-Thursday. This leads to the possibility for mean ridging to set up over the West Coast to its east, and promoting broad cyclonic flow to the east of that ridge over generally the rest of the lower 48. There are uncertainties with possible shortwave energy near the Pacific Northwest around Wednesday lingering after that upper low farther west pinches off. Many deterministic and AI models show indications of this feature, but both the 12Z and newer 00Z ECMWF are among the most aggressive with it. Regardless of how agreeable the majority cluster may be at the moment, these flow separation cases can still produce significant variability during the extended range. The WPC forecast was based on a blend of the deterministic 18Z GFS and 12Z EC/UKMET/CMC early in the period, with decreasing proportions of the deterministic models as the period progressed in favor of the pretty agreeable GEFS and EC ensemble means. ...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. A Marginal Risk remains in place centered over the Texas and Oklahoma Panhandles into the western half of Kansas, 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/Mid-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 modest rain and snow amounts. Some precipitation may linger into Monday and spread farther inland but with lesser amounts. Gusty winds are possible in the Rockies as fronts push through. There is still uncertainty about specifics of eastern Pacific/West Coast flow aloft by Tuesday-Thursday, but most guidance currently suggests that the Northwest will see a drier trend by then with upper ridging setting up. The eastern U.S. is forecast to be generally 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 are forecast to continue into Sunday in the Four Corners states underneath the upper low, but gradually warm after that. Meanwhile, upper ridging ahead of the upper low will promote 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 region by next Thursday, while a subtropical upper high across the Gulf/northern Mexico/Texas will allow for warmer temperatures to expand across the the southern Plains into Southeast. 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