Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 PM EDT Mon Oct 14 2024 Valid 12Z Thu Oct 17 2024 - 12Z Mon Oct 21 2024 ...Overview... Deep upper troughing with a likely closed low in the West will lead to a cool and wet pattern there, including higher elevation snow and frost/freeze concerns in some of the valleys. Heavy 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 continues to be agreeable on the large scale pattern evolution through much of the period, but some uncertainties still in the timing and details of individual systems. Most notable system during the medium range will be a shortwave diving into the West early period, with models now showing agreement on a deep closed low developing over the Southwest by this weekend and eventually lifting north and east early next week. Through the 00z run, the CMC was more progressive and displaced east of the GFS, ECMWF, and UKMET, but the new 12z run today did jump west. 00z GFS was quickest to lift the system northward Sunday-Monday but the new 12z run did slow down. Good agreement on an upper ridge building back in across the East again behind a departing upper trough/low, but how far westward this ridge retrogrades early next week is dependent on the evolution of the western/central upper low. WPC forecast for today preferred a non-CMC blend early in the period, which was more west with the low and most consistent with the previous WPC forecast. Leaned more heavily on the ensemble means late period with lingering and increasing uncertainties. ...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 potential, particularly for sensitive burn scar locations. Lighter rain may spread into the north- central U.S. late week too along the front. 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 north of a stationary front 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/valley locations. Santorelli/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