Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri Jun 30 2023 Valid 12Z Mon Jul 03 2023 - 12Z Fri Jul 07 2023 ...Major heatwave from Texas through the mid-lower Mississippi Valley to the Southeast to ease next week... ...Hot temperatures from the Desert Southwest to Interior California into Monday... ...Overview... Guidance is showing a fairly typical summertime pattern, with a lower latitude mean ridge aloft settling toward near-average strength with a couple primary features of interest farther north. The leading system will be a diffuse eastern U.S./Canada shortwave and associated wavy front, while the southern periphery of a better defined Canadian trough will progress from the northwestern U.S. into the Mississippi Valley. This latter trough will push southward and eastward with time, likely reaching the east-central states through south-central Plains by late in the week. Expect both fronts to produce episodes of showers and thunderstorms with diurnally favored rainfall also likely over the South. ...Guidance Evaluation/Predictability Assessment... For the most part, latest models and ensembles have been offering better than average agreement. The diffuse trough crossing the East has embedded smaller-scale details that have lower predictability. Thus it is likely that it will take into the short range time frame to resolve some specifics for the shortwave and associated surface front/waves. Currently the most common theme is for a weak embedded upper low to track from near the eastern Great Lakes to just north of Maine from early Monday into early Wednesday. By days 6-7 Thursday-Friday the 18Z GFS dropped its upper low to east of New England which is south of most other guidance. The new 00Z run has adjusted back much closer to the model/mean majority. Meanwhile a model/mean consensus (model composite early and then a model/mean mix as model details diverge somewhat) looks good for the upstream Canada/northern tier U.S. upper trough and leading cold front pushing south/east from the northern Plains. Behind this system, guidance agrees fairly well with the idea of a modest trough developing near the West Coast in response to a strong eastern Pacific ridge. ...Weather/Hazards Highlights... The wavy cold front progressing south and east from the northern Plains over the course of next week will likely produce multiple episodes of showers and thunderstorms, with some locally heavy. Currently the guidance signals are not quite coherent/organized enough to depict any risk area in the Day 4 Excessive Rainfall Outlook (Monday-Monday night), but there is potential for future runs to provide enough focus to suggest an area at some point. By Day 5 (Tuesday-Tuesday night) guidance generally suggests more organized activity with greater localized totals, along with at least a modest increase of available moisture. Low level upslope flow could enhance rainfall to the north of the front. Thus the Day 5 outlook plans to depict a Marginal Risk from parts of the Upper Midwest into the north-central High Plains. The western half or so of this area has had the most rainfall over the past couple weeks and would be more sensitive to additional rain. From midweek onward expect the area of showers/storms to continue moving southward and eastward, eventually reaching areas from the Great Lakes/Ohio Valley to the south-central Plains. The weaker wavy front crossing the East early next week should also produce some areas of locally heavy rainfall given sufficient moisture and instability. Guidance agreement is not exceptional, likely corresponding to the low predictability of important small-scale details, but enough support remains to keep a Marginal Risk area in the new Day 4 Excessive Rainfall Outlook. This area is centered over most of Pennsylvania and some surrounding locations, corresponding to a combination of relatively lower flash flood guidance currently and rainfall forecast in the short term before the valid day. The Northeast should trend drier mid-late week as the system lifts away but the trailing front may linger for a time over the Carolinas and provide a focus for rainfall. Meanwhile expect much of the South to see typical diurnally enhanced showers/thunderstorms of varying intensity each day. Excessive heat across the southern tier should become less widespread next week as upper ridging returns to a more climatological strength and latitude. Parts of the Southeast/Carolinas may still see highs in the mid-upper 90s early in the week along with sufficient humidity to support heat indices of 105 to slightly above 110 degrees, while the western Gulf Coast region and far southern Texas in particular may continue to see heat index values reach at least 110-115F on most days next week. Excessive summer heat over the Southwest and vicinity should continue into next Monday with the hottest locations reaching the 110-120F range and parts of the California's Central Valley exceeding 100F. After Monday, the Southwest should see temperatures moderate while the Pacific Northwest will be a localized focus for above normal temperatures. Elsewhere, high temperatures of 5-12F above normal should progress from the Upper Midwest into parts of the Northeast ahead of the initial Plains cold front. Behind the front, expect much cooler temperatures with one or more days of highs 10-20F below normal over the northern into central High Plains. Rausch 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, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw