Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Sep 7 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*** ...General Overview... A pattern change will be underway across much of the western U.S. and the northern Plains/Midwest as a strong cold front heralds a change to much cooler temperatures for the weekend, compared to the intense late season heat wave that is currently ongoing. The upper trough that is accompanying this front is expected to evolve into a closed upper low across the Ohio Valley and Great Lakes region going into early next week, with a ridge axis near the East Coast blocking its eastward progression. Moisture associated with what will eventually be the remnants of Hurricane Kay is expected to advect inland across much of southern California, Nevada, and western Arizona, which will be a welcome change from the ongoing drought and miserably hot conditions. There will likely be another trough approaching the Pacific Northwest late in the forecast period. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite initially has a good overall synoptic scale depiction of the overall pattern through early Sunday with the typical mesoscale model differences. As the upper low builds in across the Midwest by Monday, the GFS is northeast of the well clustered ECMWF/CMC solutions, and this continues going into Wednesday as well. The CMC develops a stronger trough in the northern stream across Canada by Wednesday, but is otherwise fairly close to the model consensus with the trough near the West Coast region. In terms of QPF, the NBM along with a little of the operational 12Z ECMWF was used as a baseline. The WPC fronts/pressures forecast utilized a multi-model deterministic blend through Monday, gradually increasing use of the ECMWF and GEFS means to about half by the end of the period as individual model differences increase, with less weighting towards the GFS early next week. ...Weather/Hazards Highlights... The first strong cold front of September is forecast to bring quite a change to the Rockies eastward to the western High Plains and extending into Minnesota for the upcoming weekend. Highs are expected to be only in the upper 50s to upper 60s for many of these areas on Saturday, which is roughly 10-20 degrees 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. Significant abatement of the heat is also expected across California and Desert Southwest, with much of this cool down attributable to clouds and rainfall associated with a weakening Tropical Storm Kay by that time. By Sunday into Monday, temperatures warm up again to slightly above average levels across the northwestern U.S., but not to the same magnitude as this past week. Across the eastern U.S., readings should generally be within 5 degrees of climatological averages for highs, while overnight lows near the East Coast should be about 5-10 degrees above normal most nights. 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. A surge of increased moisture is expected to be in place 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. Elsewhere, a wavy front pushing into the Central Plains/Upper Great Lakes late this week into the weekend 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 could maintain an axis of enhanced rainfall in its vicinity. The overall upper trough anchored by this upper 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 into early next week. Hamrick 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