Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 PM EDT Thu Sep 23 2021 Valid 12Z Sun Sep 26 2021 - 12Z Thu Sep 30 2021 ...Overview... The medium range period starts Sunday with amplified troughing (or a possible closed upper low) over the Great Lakes/Northeast and a compact upper low moving across the Southwest. A closed low over the Northeast Pacific will send a trough into the Western U.S. by mid-week which should amplify ridging over the center of the country. This general trough-ridge-trough pattern should maintain itself through the rest of the period as another trough moves into the West on Thursday and amplified troughing also becomes re-established over the East. ...Guidance Evaluation/Preferences... Models struggle with the evolution of a closed low/upper trough over the Great Lakes/Northeast even as early as day 3. The 06z GFS was quite a bit south with the upper low bringing it across the Northeast U.S. while the remainder kept the core of the low farther north in eastern Canada (the CMC was also farther south, but not as far south as the GFS). After the low moves through, the GFS remains much faster to lift the troughing out of the East, likely a result of weaker/faster ridging across the central U.S.. It quickly becomes out of phase with the rest of the guidance by day 6-7 which all show a more amplified ridge and renewed troughing hanging around across the East through much of the week. Out West, the guidance shows good agreement the first half of the period with initial troughing into the West with some timing/strength differences later on as a shortwave lifts through the northern High Plains and the next deep trough reaches the West Coast by Thursday. The WPC blend relied on a non-GFS compromise of the deterministic solutions with WPC continuity through day 4. After that, began to increase the ensemble means in the blend while still favoring the more consistent ECMWF with previous shift continuity. ...Weather and Potential Hazards... Rainfall from the exiting storm system in the short range forecast period should be primarily gone by early Sunday, with perhaps some lingering showers across Maine and back into the lower Great Lakes. Meanwhile, a weakening and stalled frontal boundary draped just south of the Florida peninsula will bring daily chances of diurnally-driven thunderstorms. Return flow moisture will bring increasing chances of storms to Texas by the middle of next week, where some models indicate some heavier amounts along portions of the western Gulf Coast. For the Pacific Northwest, some orographically enhanced rainfall is likely for western Washington and Oregon for early in the week as the next Pacific storm system moves inland, and high elevation snow for the Cascades after the cold front moves through. Some of this moisture should also spill out into parts of the Northern Rockies. The next round of potentially heavy precipiatation should reach the West Coast by Thursday. Much of the rest of the nation will be relatively dry, though some isolated showers and storms will be possible across portions of the Desert Southwest and the Four Corners region associated with the weak closed upper level low late this weekend into early next week. Near normal temperatures will prevail across the eastern third of the nation this weekend in the wake of the cold front passage, bringing a more fall like feel. Meanwhile, as ridging amplifies across the central U.S., above normal readings are likely across the central/northern Plains with daily highs 10 to 20 degrees above normal (and locally higher in some spots) Sunday into next week. The cold front crossing the West Coast region will likely bring below normal readings in its wake, with highs running 5 to 15 degrees below normal. Santorelli 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, winter weather outlook probabilities and heat indices are at: 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml