Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Sep 06 2022 Valid 12Z Fri Sep 09 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... ...Overview... The intense western U.S. upper level ridge and its associated record setting heat wave should be eroded to some extent by the start of the period early Friday as an upper trough reaches the northern Rockies. Weaker ridging that remains over parts of the West will merge with a progressive Northeast Pacific ridge to bring a brief period of warm to hot temperatures into the Northwest, followed by a cooling trend as weak mean troughing aloft nears the West Coast early next week. Moisture from Hurricane Kay (currently forecast to track just west of Baja California) will also bear watching as it will likely increase rainfall over parts of southern California and Arizona by this weekend. Meanwhile a broad trough/upper low should be in place over the South through Sunday. This feature and a wavy surface front will likely support scattered to numerous showers and storms from the central Gulf Coast to the Southeast states and Appalachians. Guidance shows an improving signal for northern tier upper trough energy to form an upper low that may track over the Midwest/Great Lakes during the first half of next week. This system could produce some enhanced rainfall in its vicinity as well as spread a broader shield of rain over the East as the overall trough interacts with the moisture initially over the South. ...Guidance Evaluation/Predictability Assessment... From the large scale perspective, the most notable guidance consideration is the improved signal with respect to an upper low expected to form over the Midwest around Sunday-Monday and then track into or near the Great Lakes by Tuesday. Up to this point the ECMWF/ECMWF mean have been more consistent in showing this evolution, or at least a fairly amplified upper trough. The 06Z/12Z GFS runs have fallen in line with this scenario, as have the last couple CMC runs. The new 12Z UKMET strays a bit southwest of consensus by early Monday (as did the 00Z GFS). The 12Z ECMWF has adjusted farther south with its upper low late in the period, highlighting the continued uncertainty for specifics. The GEFS mean may still be lagging a bit on the degree of flow separation but is closer for the overall upper trough. Farther west, some spread develops regarding height falls moving into the West ahead of a weak upper low ejecting from the eastern Pacific as well as with possible upstream energy. An intermediate solution between the faster 06Z GFS/00Z CMC and slower 00Z ECMWF/GFS as well as 06Z GEFS/00Z ECMWF means. Also note that the 00Z GFS strayed out of phase with consensus over southern Canada late in the period. Forecast considerations led to an operational model composite for the first half of the period, with greatest weight on the 00Z ECMWF and 06Z GFS. Then the blend starting to incorporate some 06Z GEFS/00Z ECMWF mean input along with the ECMWF/GFS/CMC, with somewhat more ECens relative to the GEFS given better definition of the former over the East. ...Weather/Hazards Highlights... The intense and long duration heat wave that is encompassing much of the western U.S. and central/northern Plains should peak before the beginning of the forecast period Friday. However still expect above normal temperatures across much of California and the Great Basin on Friday (with some plus 10-20F anomalies over parts of California). Warm to hot temperatures should shift more into the Pacific Northwest for the weekend followed by gradual moderation from west to east. The northern-central High Plains will see highs up to 15-20F below normal Friday-Saturday, a pronounced change from the hot temperatures expected during the short range time frame. Less extreme cool highs will spread into the central/east-central U.S. by the first half of next week under the upper trough/low forecast to travel over these areas while clouds and rainfall will tend to promote below average highs over the South/Southeast. In terms of rainfall potential, a meandering weak frontal boundary should remain along the Gulf Coast and Southeast U.S. Coast through the weekend and interact with deep Gulf moisture and a broad upper low 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 depicted from the Florida Panhandle to the southern Appalachians to reflect this possibility. 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 onward. Meanwhile a surge of increased moisture will likely arrive into the Desert Southwest 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 southeast California and southwest Arizona on Saturday. The coverage and intensity of rainfall farther north over the western U.S. is still a question mark, as moisture will be anomalously high but guidance instability is not particularly impressive and upper troughing that eventually develops off the West Coast is fairly modest. Rausch/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