Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Jul 12 2023 Valid 12Z Sat Jul 15 2023 - 12Z Wed Jul 19 2023 ...Hazardous to extreme heat is forecast for the entire southern tier of the U.S. through early next week and beyond... ...Heavy rain likely to focus across the central Plains as well as portions of the Northeast... ...Overview... The prevailing upper level pattern over the CONUS in the medium-range will continue to feature periodic intrusions of cool air from central Canada into the northern tier states. This pattern will tend to maintain broad troughing across the eastern two-thirds of the country with the main trough axis edges toward the East Coast with time as an anomalously strong upper ridge builds eastward across the Four Corners region. Rounds of showers and storms between these synoptic systems will likely continue across the midsection of the country, with unsettled weather moving once again into the Northeast where heavy rain is possible later in the weekend over New England. Meanwhile, the next upper trough is forecast to move through the Pacific Northwest and northern Rockies early next week, perhaps slightly weaken the upper ridge that is forecast to bring a dangerous heat wave for the entire southern tier of the U.S. especially the Southwest/California through the weekend and even well into next week. ...Guidance Evaluation/Predictability Assessment... Overall, the latest guidance remains in excellent agreement on the overall synoptic pattern through the period, though continues to offer typical differences in the smaller scale details. Some uncertainties surrounding impulses rounding through the Midwest, which have significant implications for surface features and sensible weather, including potential heavy rainfall/flooding, especially across parts of the mid-Mississippi Valley and Midwest, and into the Northeast. Model agreement continues to improve regarding the timing of the upper trough/low moving onshore into British Columbia and the Pacific Northwest early next week. Otherwise, even late in the period, there is very good agreement on an anomalously strong upper high building and holding strong over the Southwest U.S. through Tuesday, and then likely shifting back to the east next Wednesday. The WPC forecasts were based on a composite blend of the deterministic and ensemble solutions from the 06Z GFS/GEFS, 00Z ECMWF/EC mean, and 00Z CMC/CMC mean through day 5, with a higher percentage from the ensemble means for days 6 and 7. This maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Showers and thunderstorms should initially focus Saturday from the south-central Plains into the mid-South, Ohio Valley, and the Mid-Atlantic/Northeast. Given available moisture and instability, at least locally heavy rainfall seems a good bet for many locations. The best overlap in the current available guidance continues to point to the Texas Panhandle into western Oklahoma where a small Slight Risk was maintained on the day 4 ERO given very wet antecedent conditions. The rainfall axis should shift south and east by day 5/Sunday where a marginal risk includes the central and eastern Gulf Coast states for locally heavy rain associated with scattered thunderstorms. Meanwhile, the Northeast is looking at yet another round of heavy rain as anomalous precipitable water values associated with a low pressure wave stream northward ahead of a cold front. A Slight Risk was maintained on day 5 for eastern portions of New England given how susceptible parts of this region is right now due to recent very heavy rainfall. Scattered showers and embedded thunderstorms appear to linger near the East Coast through at least Tuesday as the next push of cool air from southern Canada moves into the Great Lakes. Rounds of enhanced rainfall appear possible ahead of the cold front, but this time farther north from the central High Plains toward the Midwest and into the Ohio/Tennessee Valley from early to middle of next week. Hazardous to potentially dangerous heat will impact much of the Southwest/California eastward into the Southern states (and likely beyond per latest CPC forecasts). Numerous daily high temperatures, including the all-time high temperature in Las Vegas, NV, and warm overnight low records are threatened this weekend across this region. Farther east, the main story will be oppressive humidity leading to heat indices of 105-115 (locally higher) for a large portion the South from eastern Texas into the Southeast and Florida. Again, some daily temperature records are possible. See key messages issued by WPC for additional information and graphics related to this heat wave. Much above normal temperatures across the Northwest this weekend should moderate and shift east as heights lower early next week over this region. Heat may expand northward into parts of the central Plains and Mississippi Valley and points east early next week. Kong/Santorelli WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, 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