Extended Forecast Discussion NWS Weather Prediction Center College Park MD 239 AM EDT Tue Sep 6 2022 Valid 12Z Fri Sep 9 2022 - 12Z Tue Sep 13 2022 ***Heat wave abates to close out the week across the Western U.S.*** ***Heavy rain and potential flooding a concern across parts of the Southeast U.S. and Desert Southwest*** ...General Synopsis... The intense upper level ridge and the record setting heat wave associated with it is forecast to slowly erode and weaken as shortwave energy and upper level troughing from the north impinges upon it, thus heralding a welcome change compared to the extreme heat during the early to middle part of the week. A broad upper low/trough is forecast to be in place across the Deep South through Sunday, and this in combination with a weak frontal boundary, will tend to result in scattered to numerous showers and storms from the central Gulf Coast to the Southeast states and Appalachians. The eventual track of Hurricane Kay as it weakens also bears watching as the eventual remnants of this storm will likely result in an increase in rainfall across southern California and Arizona by this weekend. ...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. By early next week, there has been a stronger trend for a closed low to develop over the Midwest states, with the GFS farthest south with the low. Models have also come into better agreement on the next trough/surface fronts approaching the Pacific Coast later in the forecast period. In terms of QPF, the NBM 4.1 along with some of the bias corrected model was used as a baseline. The WPC fronts/pressures forecast utilized a multi-model deterministic blend for Friday and Saturday, gradually increasing use of the ECMWF and GEFS means to about half by the end of the period as individual model differences increase. ...Weather/Hazards Highlights... The intense and long duration heat wave that is encompassing much of the western U.S. and central/northern Plains is forecast to peak before the beginning of the forecast period Friday. Having said that, readings are still expected to be above normal across much of California and the Intermountain West on Friday, while an even more noticeable change in temperatures are expected to arrive for the northern Rockies and northern Plains as a strong cold front brings a welcome change to cooler conditions and even some below average readings. Meanwhile, slightly below normal highs and around normal lows are expected for the south-central U.S. into the Southeast owing to mostly cloudy skies and more widespread shower and thunderstorm coverage. Slightly below average readings are also becoming more likely across much of the central Plains and Midwest going into the weekend and Monday. In terms of rainfall potential, a meandering but weak frontal boundary is likely to stay draped along the Gulf Coast and Southeast U.S. Coast through the weekend, which combined with deep Gulf moisture and a broad upper low, is forecast to produce numerous showers and thunderstorms. There is also the potential for instances of flooding where multiple rounds of convection develop, and a Slight Risk of excessive rainfall is in effect from the Florida Panhandle to the southern Appalachians to account for this. By the weekend, a surge of increased moisture will likely arrive for the Desert Southwest in conjunction with the eventual remnants of what is now Hurricane Kay, based on recent projections from the National Hurricane Center, and a Slight Risk for excessive rainfall is also in affect for portions of southeast California and southwest Arizona on Saturday. 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