Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Thu Sep 30 2021 Valid 12Z Sun Oct 03 2021 - 12Z Thu Oct 07 2021 ...Overview... The medium range pattern will take on a more blocky appearance as an upper low closes off over the east-central U.S. while an upper ridge/high builds over the northern tier states and southern Canada. A leading wavy front will spread rainfall of varying intensity across the eastern third of the country. Meanwhile an eastern Pacific upper trough should steadily amplify and become more elongated through the period, pushing a cold front into the Northwest and California. An upper low initially southwest of California will eject northeastward ahead of the upper trough and may produce some areas of rainfall over parts of the Interior West/Rockies. ...Guidance Evaluation/Preferences... Latest models and means generally agree on the general pattern evolution but multi-day continuity suggests that details of the east-central U.S. upper low are somewhat of a moving target. The most common theme in the models/means is for a south or southeast trend over the past couple days. However the full ensemble suite shows the 00Z GFS may have been too far south, while the 00Z ECMWF appears a bit east of consensus for the overall trough axis especially around day 5 Tuesday. The 06Z GFS wobbled somewhat north/west of the 00Z run, leaving a compromise of the 00Z ECMWF/06Z GFS and their respective means (along with some 00Z CMC/UKMET input early) as a reasonable starting point for the forecast. Adjustments made by the new 12Z GFS/CMC/ECMWF provide reasonable support for the preferred blend while the 12Z UKMET strays increasingly northwest of consensus after early Tuesday. The eastern Pacific upper trough nearing the West Coast has reasonable continuity but there is some spread and variability for how energy will be distributed within the feature. This leads to some run-to-run adjustments for specifics of the leading cold front and embedded waves. A model/mean blend looks good for this part of the forecast. For the upper low initially southwest of California and ejects northeastward ahead of the trough, the main consideration was to eliminate the 00Z CMC from the forecast by the latter half of the period since it became fast/weak versus other models. The new 12Z run looks considerably better and other models maintain continuity. ...Weather and Potential Hazards... Areas of rain and thunderstorms will accompany the wavy frontal system that pushes across the central and eastern U.S. during the weekend/early next week and then possibly stalls near the Southeast coast by midweek. Best focus for some areas of moderate to heavy rainfall will extend from the Great Lakes into the Tennessee/Mississippi Valleys as well as into the Northeast Sunday-Monday. Then the upper low settling over the east-central U.S. may keep areas of rainfall over parts of the East through at least Thursday. Some locations could see significant totals, whether from an episode of heavy rain and/or multi-day accumulation due to persistence of the pattern. Guidance variability and spread up to this point keep confidence in specifics fairly low but one area to monitor will be the Southern Appalachians where terrain enhancement may be a factor. Expect the upper low ejecting northeastward into the West to generate a period of rainfall over parts of the Southwest, eastern Great Basin, and parts of the Rockies mainly around Tuesday-Wednesday. The front pushing into the Pacific Northwest and northern California will spread areas of rain gradually southeastward with time. Most activity should be in the light-moderate range but a few pockets of heavier rain could be possible. The upper ridge traversing the north-central U.S. will support warmer than average temperatures across the Northern Plains and vicinity through the period. Plus 10-20F anomalies, and possibly slightly higher near the Canadian border Tuesday-Wednesday, would equate to highs in the upper 70s and 80s. A mild to warm Sunday over portions of the East will give way to highs trending toward normal thereafter with the passage of a wavy cold front. Persistence of clouds and areas of rain should maintain decent coverage of plus 10F and greater anomalies for morning lows though. The West Coast will see moderately below normal daytime highs beginning around midweek as the upper trough approaches and the cold front makes its way through the region. Rausch/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