Extended Forecast Discussion NWS Weather Prediction Center College Park MD 145 AM EDT Wed Jul 12 2023 Valid 12Z Sat Jul 15 2023 - 12Z Wed Jul 19 2023 ...Hazardous to extreme heat is forecast for much of the Southern U.S. and into the Southwest into next week and beyond... ...Heavy rain concerns likely to focus from the central Plains and Gulf Coast into portions of the Northeast... ...Overview... The pattern over the CONUS during the medium range period should trend slightly less amplified with time. A deep upper low anchored near Hudson Bay on Saturday should eventually drop south and east skirting the Upper Great Lakes early next week before it moves into southeast Canada. Small scale energies rounding the base of the trough through the central Plains and mid-South will help to maintain general troughing across the Midwest and Northeast with additional rounds of rain expected across these regions as well. Ridging will progress from the Northwest into the north-central U.S. as an upper low moves into British Columbia with lower heights across the Northwest next week. Meanwhile, to the south, a potentially dangerous heat wave is forecast to overspread the entire southern tier of the U.S. and 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. Better agreement tonight on the upper low into British Columbia, but some question on another potential low by day 7 just off the coast. 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 tonight were based on a composite blend of the deterministic solutions through day 5, with some inclusion of the ensemble means days 6 and 7 with the GFS/ECMWF. This maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Showers and storms 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, but still a lot of uncertainty exactly where. The best overlap though in the current available guidance points to the Texas Panhandle into western Oklahoma where a small slight risk was maintained on the day 4 ERO given very wet antecedent conditions. Rainfall should shift south and east by day 5/Sunday and once again the Northeast is looking at yet another round of heavy rain as anomalous precipitable water values stream northward along a cold front. A slight risk was added to the day 5 ERO for eastern portions of New England given how susceptible parts of this region is right now due to recent very heavy rainfall. Farther south, also included a slight risk on the day 5 ERO across parts of the Gulf Coast and Lower Mississippi Valley with a broad marginal risk surrounding these areas including the Southeast into the Mid-Atlantic. Another cold front into the central Plains/Midwest early next week may bring additional rounds of rain and storms across the mid-Mississippi Valley/Midwest/Ohio Valley. Showers may linger through at least Tuesday as well across parts of the Mid-Atlantic and the Northeast. 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 and warm overnight low records are threatened this weekend and early next week 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. 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