Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Sat Oct 02 2021 Valid 12Z Tue Oct 05 2021 - 12Z Sat Oct 09 2021 ...Southeast/Southern Appalachians/Mid-Atlantic Heavy Rain Threat with a Blocky Closed Low... ...Overview... Models and ensembles herald an amplification of the medium range weather pattern next week. The flow should become more blocky 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 and closed upper trough/low support will spread rainfall of varying intensity and duration 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. A lead upper low initially offshore of California will eject northeastward ahead of the upper trough and may produce some areas of rainfall over the Southwest/Four Corners region/Rockies. ...Guidance Evaluation/Preferences... Guidance generally agrees on the overall pattern amplification, but show varability with pesky embedded features. For the upper low dropping from the Ohio Valley to the Southeast, placement continues to vary some from run to run. Meanwhile, an amplifying eastern Pacific upper trough nearing the West Coast and leading closed low ejection from offshore CA offer reasonable larger scale continuity, albeit with increasing forecast spread for how the energy will be distributed and also overall timing of the trough axis into late next week. The WPC medium range product suite was primarily derived from a composite blend of the reasonably compatible runs from the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models for Days 3-5 (Tue-Thu) and WPC continuity. This composite has good ensemble support. Growing model forecast spread suggest a quick transition toward the ECMWF ensemble mean days 6/7, with only limited input from the GFS/ECMWF/Canadian models. ECMWF ensembles are more amplified than the GEFS/NAEFS and that seems to better fit the overall increasingly blocky nature of the flow. The latest 00 UTC guidance offers similar variances and more detail differences, but a blend would remain in line. ...Weather and Potential Hazards... Areas of rain and thunderstorms will accompany a wavy frontal system that slows across the Southeast/Mid-Atlantic next week, with enhanced focus as a closed upper low works over the region. This should keep rainfall over parts of the Southeast/East through much of next week, with an uncertain but gradual focus from the Southeast into the Mid-Atlantic. The guidance continues to signal that some locations could see some significant local totals, whether from an episode of heavy rain and/or a 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 will be a factor. A lead upper low ejecting northeastward into the West is expected to generate a period of rainfall over parts of the Southwest, Four Corners, and parts of the Rockies mainly around Tuesday-Wednesday. A subsequent and wavy cold front digging slowly but robustly into the West will spread areas of rain gradually southeastward with time into mid-later next week. Most activity should be in the light-moderate range, but a few pockets of heavier rain could be possible along with snow in the higher elevations. 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. Downstream upper ridging traversing the north-central U.S. will support warmer than average temperatures across the Northern Plains and vicinity next week with some plus 10-20F anomalies that equate to highs in the upper 70s and 80s. Persistence of clouds and areas of rain with the emerging closed upper low/trough and lead frontal system over the Southeast/East should moderate daytime temperatures, but should maintain decent coverage of plus 10F and greater anomalies for morning lows. Schichtel 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