Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM EDT Thu Sep 30 2021 Valid 12Z Sun Oct 03 2021 - 12Z Thu Oct 07 2021 ...Overview... The medium range period will feature the potential for an upper low to close off in the east-central U.S. to the south of an upper ridge/high, while at the surface a cold front moves slowly eastward and spreads showers to eastern third of the country. Farther west, troughing should amplify through the period along the West Coast and push a cold front through the Northwest and California. ...Guidance Evaluation/Preferences... Model guidance is reasonably agreeable with the overall pattern described above. Though with slightly different evolutions, models show multiple shortwaves joining over the Midwest around Monday to form a closed low as the week progresses, as upper ridging building over portions of the central U.S. could spread into south-central and southeastern Canada around midweek and separate the low from the northern stream flow. The 12/18Z GFS and 12Z ECMWF show this type of solution, and both center an upper low over the Mid-South on Wednesday/Thursday. However, the placement of the closed low has varied from run to run in each model, along with the upper ridge/high location and track. The newer 00Z GFS has an upper low closing off and diving even farther southwest toward the central Gulf Coast by midweek, with an upper high over the Midwest. The 00Z ECMWF shows a bit of a southern trend as well, with the center of the low over the Deep South. The differences in the placement reduce confidence for the exact positioning of the features, but at least the general idea for the pattern has been persistent. Saw fairly good model agreement regarding the western U.S., as an initial relatively weak southern stream low tracks from the eastern Pacific into southern California around Tuesday and then lifts northeastward and potentially gets absorbed midweek. This feature is in addition to/ahead of the positively tilted amplifying trough along the West Coast, which also shows good consistency among model guidance. Overall, the WPC forecast was based on a multi-model deterministic blend of the 12/18Z GFS runs and the 12Z ECMWF/UKMET/CMC for the first half of the period, before reducing CMC weighting due to some inconsistencies with the pattern in the East and a weaker initial western shortwave, and adding components of the GEFS and EC ensemble means by the latter part of the forecast period. The main change to the previous forecast was a faster entry of the trough and cold front into the Northwest per model trends. ...Weather and Potential Hazards... As the frontal system tracks across the central and eastern U.S., showers and thunderstorms are likely in its vicinity, shifting across the Mississippi Valley toward the Ohio and Tennessee Valleys early in the week, and spreading to the Eastern Seaboard as the workweek begins. Showers should then linger in the East underneath and on the eastern side of the upper low through at least Thursday. Some embedded heavy rain is possible, and while it is still uncertain where higher totals will occur, terrain enhancement of the rainfall in the Southern Appalachians could lead to locally heavier amounts. Meanwhile, monsoonal moisture may make its return to the Four Corners states next week supported by the leading shortwave. Tuesday currently looks to have the best chances for rainfall there. Then as the troughing and cold front come into the West Coast, showers are possible for the Pacific Northwest into northern California. Warmer than average temperatures are forecast for the north-central U.S. through the period underneath upper ridging traversing the area. Highs in the upper 70s and 80s in the Northern Plains will be 10-20F above normal for this time of year. A mild to warm Sunday is expected for portions of the East early in the week before temperatures moderate closer to normal with the cold frontal passage. The West Coast will see cool temperatures beginning around midweek as the trough approaches and the cold front makes its way through the region. 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, 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