Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 AM EDT Fri Oct 18 2024 Valid 12Z Mon Oct 21 2024 - 12Z Fri Oct 25 2024 ...Overview... An anomalous cut-off low over the Great Plains will produce some scattered shower and thunderstorm activity across the region on Monday, before phasing with a northern stream trough and amplifying over the East Coast by mid-week. This will be succeeded by a ridge across the western and central CONUS through next week. A stalled trough in the eastern Pacific will contribute to a weak atmospheric river event across the Pacific Northwest early next week, followed by a closed low system and renewed potential for precip over the Pacific Northwest toward the end of the week. An anomalous warm airmass over the northern tier will shift into the Northeast through mid-week. High elevation snow possible over portions of the Pacific Northwest early and late next week. ...Guidance/Predictability Assessment... Guidance is in reasonably good agreement regarding the overall synoptic pattern across the lower 48. A general model blend consisting of equal parts 12z ECMWF/CMC/UKMET and 18z GFS were used on days 3 and 4. The GFS trended faster than the 12z guidance with the ejection of the cut-off low over the Great Plains. The 18z GEFS and 12z ECENS were introduced to the blend on day 5 to account for deterministic variance in the orientation of the phasing troughs over the Great Lakes and East Coast. The GFS and EC are favored in this blend. The CMCE is introduced to the blend on day 6 while the deterministic GFS is removed altogether due to its insistence of yet another cut-off low over the East Coast. The ensemble means and deterministic Euro appear to resolve the eastern Pacific system similarly with the ensembles carrying a reasonable amount of spread for a day 6 forecast. An 18z GEFS favored day 7 blend is used with the 12z ECE/CMCE also incorporated. ...Weather/Hazards Highlights... A Four Corners cut-off low will shift into the Central Plains by Monday, marking the end of an active weekend across the region. Some lingering showers and thunderstorms will impact parts of the Central/Southern Plains and Middle-Mississippi Valley on Monday. The mid-level low will lift far enough north that it will avoid tapping moisture from the Gulf of Mexico, which will keep PWATs, moisture advection and ultimately QPF into the mid-section of the country relatively low. Temperatures will rise 15-25 degrees above normal over portions of the Northern Plains and Upper Midwest on Monday due to ridging out ahead of the system in the Plains. A positively tilted northern stream trough will promote rain showers across the Upper Great Lakes on Tuesday and Wednesday while also dragging a plume of moisture toward the Pacific Northwest upstream of that. Things are relatively quiet on Thursday before the arrival of a Pacific system that is forecast to produce showers and potentially high elevation snow across the Pacific Northwest on Friday. Upper ridging returns over the Great Plains later in the week, which will support above average temperatures throughout the mid-section of the country heading into the next weekend. 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