Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Mon Oct 14 2024 Valid 12Z Thu Oct 17 2024 - 12Z Mon Oct 21 2024 ...Overview... Upper troughing and reinforcing energy in the West will lead to a cool and wet pattern there, including higher elevation snow. Rain chances could increase in the southern/central Plains by the weekend. Upper ridging ahead of this trough will promote warmer than average temperatures in the northern/central Plains to Midwest as it gradually moves east, eventually displacing the chilly upper trough and surface high in the East by the weekend. ...Guidance/Predictability Assessment... Model guidance is rather agreeable on the large scale through much of the period. The upper pattern as the period begins Thursday will consist of a broad trough atop the West, a ridge axis aligned atop the Mississippi Valley and Midwest, and lingering troughing atop the Eastern Seaboard. The ridge and Eastern trough should shift slowly eastward late week, while the trough in the West is forecast to dig and create a closed upper low over the Southwest by late Friday-Saturday. Recent ECMWF runs from 12Z and the newer 00Z runs are finally on board with forming this upper low, after earlier runs maintained a phased upper trough. There is some spread in placement with the closed low as it meanders over the weekend and may slowly eject northeast by Day 7/Monday, but relatively minor spread for a feature like this in the late medium range period. Among the 12/18Z model cycle, the CMC becomes a slow/southwest outlier by the end of the period. The 12/18Z GFS and 12Z ECMWF and most AI/ML models were more agreeable, but interestingly the 12/18Z AIFS was the main outlier taking the upper low farther east of consensus as early as Saturday. Also interestingly though, the new 00Z CMC is more like these AIFS runs with an eastward/fast trend. Prefer the majority cluster farther west, but this will continue to be monitored. Guidance shows upper ridging retrograding a bit by early next week in response to this cutoff low. The WPC forecast used a multi-model blend of the deterministic guidance early in the forecast period. Given the relatively good model agreement even into early next week, was able to maintain a majority of deterministic guidance through the period though with some GEFS and EC ensemble means to smooth out the minor differences. Since recent models are more agreeable with a cutoff upper low forming and moving slowly through the Southwest, QPF trended up from the previous forecast in the south-central Rockies and High Plains, a westward shift from where in the Plains the heavier totals were previously. ...Weather/Hazards Highlights... The upper trough/low pushing across the West later week will push precipitation from the Pacific Northwest into the Intermountain West/Rockies later week, with enhanced precipitation totals in terrain. Higher elevation snow is likely, which will be the first snow of the season in some locations given the warm Fall in the West thus far. Some rain and snow could continue across the central/southern Rockies into the weekend. Meanwhile, Gulf moisture may stream in just ahead of the upper low for increasing rain chances in the central/southern High Plains late week into the weekend. While forecast details vary, recent models show some moderate to locally heavy rain totals there, but with dry antecedent conditions and rain rates not forecast to be extreme, this may not cause an excessive rain threat. Lighter rain may spread into the north-central U.S. late week too. More rounds of precipitation are likely for the Pacific Northwest into the weekend with additional impulses coming through. Meanwhile, the eastern U.S. is forecast to be generally dry late week into early next week, with the exception of parts of the Florida Peninsula. There, return flow of moisture could increase rain and thunderstorm chances especially for South Florida by Friday and perhaps spreading northward into the weekend. Models continue to vary on how much rain may fall onshore versus offshore, so no area is currently delineated in the Day 5/Friday ERO, but there may be a nonzero chance for flash flooding especially if heavy rain rates occur in the sensitive urban areas. An upper ridge aligned over the Mississippi Valley Thursday and shifting into the East Friday will promote well above normal temperatures in its vicinity. The biggest anomalies are forecast across the north-central Plains on Thursday, where highs into the 70s and 80s are 15-25 degrees above average for mid-October. Cooler than average temperatures by 10-15 degrees are expected to continue across the eastern third of the U.S. into Thursday under cool high pressure. Another round of chilly morning lows may cause frost/freeze concerns from the Great Lakes region into the Northeast/Appalachians. Gradual moderation/warming is likely there Friday and even more so into the weekend when upper ridging comes overhead, for temperatures around 5-10 degrees above normal in the northeastern tier. Then the ridge rebuilding over the central U.S. early next week will lead to warmer than average temperatures in the northern Plains to Upper Midwest once again. On the other hand, the upper trough and frontal surges in the West will cause below normal temperatures spreading across the West Coast to Great Basin/Intermountain West and Southwest particularly through late week. These colder temperatures may also lead to additional frost/freeze concerns for any still susceptible growing areas. 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