Extended Forecast Discussion NWS Weather Prediction Center College Park MD 322 PM EDT Mon Oct 04 2021 Valid 12Z Thu Oct 07 2021 - 12Z Mon Oct 11 2021 ...Southeast/Southern Appalachians late-week Heavy Rain Threat... ...Overview... The medium range period will feature a closed upper low being slow to depart the eastern U.S. where unsettled weather is expected to prevail. Meanwhile, an upper ridge over the north-central states will give way to a couple of wavy cold fronts that is forecast to move across the western U.S. where moderate precipitation chances and high-elevation wintry precipitation are expected. ...Guidance Evaluation/Preferences... Guidance continues to offer fairly good agreement on the synoptic scale pattern across the U.S. despite the difficult-to-handle upper low meandering over the east-central U.S. into the weekend. There continues to be a gradual tendency for the models to slow the eastward progress of the rainfall ahead of the upper low across the Ohio Valley and the southern Appalachians late this week. There is also decent model agreement for a low pressure wave to form near the Carolina coasts this weekend, followed by a track toward the northeast just off the Mid-Atlantic coast. Models have come into better agreement with the timing of a developing low pressure system forecast to track across the northern Plains this weekend. A period of high-elevation wintry precipitation is indicated over the northern Rockies late Friday to early Saturday prior to the formation of the low with very good model agreement. By early next week, models indicate decent agreement for another low pressure wave forming in the central Plains as southern stream energy ejects from the Southwest/southern Rockies to the southern Plains. Finally, the next round of precipitation is forecast to move into the Pacific Northwest early next week with the approach of the next upper trough. The WPC medium range product suite was derived from a composite blend of the 06 UTC GFS/GEFS mean with the 00 UTC ECMWF/ECMWF ensemble mean. This maintains good WPC continuity as well as compatible with the latest (12 UTC) model outputs. The CMC has not been included since it deviated significantly from the GFS/ECMWF general blend. ...Weather and Potential Hazards... Widespread areas of rain and thunderstorms will accompany a frontal system as it slows across the Southeast/Mid-Atlantic this week. This should keep rainfall over parts of the Southeast/East for several days, with enhanced focus ahead of a closed upper low late this week across the southern Appalachians and into the interior Mid-Atlantic, where terrain enhancement will also be a factor. Models are trending toward drier conditions for interior new England as a low pressure wave focuses precip near or just off the Mid-Atlantic and New England coasts this weekend into early next week. A couple of developing waves will likely bring a period of rainfall over the northern and central Plains by this weekend. A wavy cold front digging slowly but robustly across the West will spread areas of rain gradually southeastward mid-later week and the weekend, eventually working across the Rockies into the north-central states. Most activity should be in the light-moderate range, along with snow at higher elevations. The West should trend cooler with time as the upper trough approaches and the cold front makes its way through the region. Daytime highs by later next week could be 5-15 degrees below normal for some locations. 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 keep daytime temperatures near normal, but also maintain decent coverage of plus 10F and greater anomalies for morning lows. Kong/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 Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Thu-Fri, Oct 7-Oct 8. - Heavy rain across portions of the Mid-Atlantic, Sat, Oct 9. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Fri, Oct 8. - High winds across portions of mainland Alaska and the Aleutians, Thu-Sat, Oct 7-Oct 9. 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