Extended Forecast Discussion NWS Weather Prediction Center College Park MD 438 PM EDT Wed Sep 07 2022 Valid 12Z Sat Sep 10 2022 - 12Z Wed Sep 14 2022 ...Heavy rain and potential flooding a concern across parts of the Southeast U.S. and Desert Southwest... ...Overview... The West will see steady progress toward a pattern change bringing cooler conditions compared to the intense heat wave currently in progress over the region. Moisture associated with what will eventually be the remnants of Hurricane Kay will push inland across much of southern California, Nevada, and western Arizona, likely producing an area of enhanced rainfall into the weekend. Some of this moisture in less pronounced form should reach the Great Basin and Rockies as a mean trough axis sets up near the West Coast. Meanwhile the guidance agrees in principle that a northern Rockies/Plains upper trough should close off a low that tracks over the Midwest and Ohio Valley/Great Lakes region. This evolution along with the initial wavy surface front that eventually becomes an occluded system will likely produce an area of enhanced rainfall, while a broader shield of rain should spread over the eastern U.S. as this system interacts with an area of moisture already producing rain and thunderstorms over the Southeast and vicinity. Between the eventual troughs over the West Coast and east-central U.S., upper ridging should progress through the West and into the Plains. ...Guidance Evaluation/Predictability Assessment... Trends over the past day have generally been gravitating closer to the 00Z ECMWF/ECMWF mean for the path of the east-central U.S. upper low, leaning to the southwestern half of the prior envelope. The 06Z GFS/GEFS mean have adjusted somewhat southwest of their 00Z runs but are still northeast of the ECMWF while the 12Z GFS/GEFS have nudged a little farther south. On the other hand, the 00Z CMC was a southwest extreme but the new 12Z run adjusted to the 00Z ECMWF and the 12Z UKMET has wobbled a little north with its upper low. Thus a 06Z GFS/00Z ECMWF compromise tilted a bit more to the ECMWF provided a reasonable depiction of this feature. The developing mean trough aloft near the West Coast will consist of an ejecting East Pacific upper low and then upstream energy that feeds into the overall trough. There is typical spread with details for how height falls push into the West early-mid week, with a model/ensemble mean blend providing good continuity. ...Weather/Hazards Highlights... The first strong cold front of September will bring quite a change to the Rockies and western High Plains into the Upper Midwest for the upcoming weekend. Highs should be only in the upper 50s to upper 60s for many of these areas on Saturday, which is 10-20 degrees or so below normal for this time of year for eastern Colorado/Wyoming and western Nebraska. This will be a welcome relief compared to the heat wave earlier in the week. Meanwhile clouds and rainfall from Kay will bring cooler conditions over the Southwest U.S. The Pacific Northwest will see above normal temperatures on Saturday (plus 10-20F anomalies) with this warmth progressing eastward through the northern Rockies/High Plains Sunday-Monday, but readings will not be as extreme as this past week. The cool air over the central U.S. this weekend will modify as it continues eastward under the forecast upper trough/low, bringing highs a few degrees below normal early-mid week. Clouds/moisture should keep eastern U.S. morning lows 5-10 degrees or so above normal. In terms of rainfall potential, a weakening front near Gulf Coast and Southeast U.S. Coast, in combination with a weak upper trough/low through the weekend, will interact with deep Gulf moisture to produce numerous showers and thunderstorms for the Southeast U.S. on Saturday. There is also the potential for instances of flooding where multiple rounds of convection develop, and a Slight Risk of excessive rainfall is depicted across the southern Appalachians to reflect this possibility. Expect a surge of increased moisture across the Desert Southwest and California by the weekend in conjunction with the eventual remnants of what is now Hurricane Kay, based on recent projections from the National Hurricane Center. A Slight Risk for excessive rainfall is also in affect for portions of central/southern California on Saturday. The coverage and intensity of rainfall farther north over the western U.S. is still a bit uncertain, as moisture will be anomalously high but instability is more limited and upper troughing that eventually develops off the West Coast is fairly modest. Currently expect some rainfall to extend into the Great Basin and north-central/central Rockies by early-mid week. Farther east, a wavy front pushing into the south-central Plains/Upper Great Lakes as of early Saturday may produce one or more bands of locally moderate to heavy precipitation. Specifics will take some additional time to resolve. Expected closing of an upper low over the Midwest/Great Lakes/Ohio Valley could maintain an axis of enhanced rainfall in its vicinity. The overall upper trough anchored by this closed low and the leading surface system may interact with the moisture initially over the South to spread a broader shield of rainfall over the eastern U.S. from Sunday through the first half of next week. It is likely there will be some embedded bands of heavier rainfall but it may take into the short range time frame before the details come into clearer focus. Rausch/Hamrick Hazards: - Heavy rain across portions of the southern California into the Sierra Nevada, the upper Midwest, the upper Tennessee Valley, as well as the Southeast including northern Florida and into southern Appalachians, Sat-Sun, Sep 10-Sep 11. - Heavy rain across portions of the southern Mid-Atlantic coast, and upstate New York, Mon-Tue, Sep 12-Sep 13. - Flooding occurring or imminent across portions of the lower Mississippi Valley. 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml