Extended Forecast Discussion NWS Weather Prediction Center College Park MD 326 AM EDT Wed Oct 23 2024 Valid 12Z Sat Oct 26 2024 - 12Z Wed Oct 30 2024 ...General Overview... Predominantly zonal flow across the lower 48 at the beginning of the period (this weekend) will give way to a more amplified pattern toward the end. A closed upper low over the Gulf of Alaska will fill and dig into the West Coast of the CONUS this weekend. This system will bring cool temperatures, rain and snow to the Northwest U.S. through early next week before developing a threat for heavy rainfall and thunderstorms over parts of the central U.S. by mid- week. A pronounced downstream ridge will generate above average temperatures across the Plains and Mississippi Valley beginning Monday. ...Guidance/Predictability Assessment... Models are in reasonable agreement regarding the overall synoptic pattern across the CONUS during the medium range period. The deterministic Euro and GFS guidance have trended slower with the digging East Pacific trough next week, while the ensemble means still have some timing spread. There's pretty good clustering of the ensembles on the depth of the trough, though. On days 3 and 4 a general model blend consisting of equal parts 12z ECMWF/CMC/UKMET and 18z GFS were utilized due to the predominantly zonal flow across the lower 48 and large ridge over the southern tier states/northern Mexico. The 12z ECE/CMCE and 18z GEFS were introduced to the blend on day 5 and continued through the end of the period to mitigate some deterministic solutions that signal the formation of a closed mid-level low over the Four Corners region by mid-week. There's some signal for another split flow pattern over the central U.S. on day 7. ...Weather/Hazards Highlights... A cold front, associated with a closed mid-level low over the Gulf of Alaska, will focus rain and high elevation mountain snow showers across much of the Pacific Northwest and Northern Rockies this weekend. Snow showers will remain confined to the northern Cascades and Olympic Mountains before expanding into the southern Cascades and Northern Rockies on Sunday, the northern Sierra and Yellowstone/Absarokas on Monday and Colorado Rockies by Tuesday. Beneath the upper trough, temperatures will be well below average. Portions of the Great Basin and Intermountain West may experience minimum temperature anomalies between 10-20 degrees below average from Monday to Wednesday. Cyclogenesis is likely to occur on the lee side of the Rockies while the digging upper-trough pulls moisture from the Gulf of Mexico beginning on Tuesday. Showers and thunderstorms may focus along a cold front slowly propagating through the central U.S.. There's some signal for a heavy rain event to unfold over parts of the Lower Missouri Valley into the Upper Midwest mid-week, due to the slow moving upper-level trough. An upper ridge over the southern and central U.S. will promote above average temperatures across portions of the Great Plains and Upper Midwest beginning this weekend. Parts of the Northern Plains and Upper Midwest will experience positive temperature anomalies between 10-20 degrees above average. Kebede 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