Extended Forecast Discussion NWS Weather Prediction Center College Park MD 505 PM EDT Tue Jul 11 2023 Valid 12Z Fri Jul 14 2023 - 12Z Tue Jul 18 2023 ...Hazardous to extreme heat is forecast for much of the Southern U.S. and into the Southwest through early next week and beyond... ...Heavy rain concerns likely to focus from the central Plains eastward to portions the Northeast... ...Overview... The prevailing upper level pattern over the CONUS in the medium-range will feature periodic intrusions of cool air from central Canada into the northern tier states. This pattern will tend to anchor an upper-level trough near the Eastern Seaboard as a warm upper ridge centered over the Southwest is forecast to build further inland toward 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 a couple rounds of enhanced rainfall are possible. Meanwhile, a potentially dangerous heat wave is forecast to overspread the entire southern tier of the U.S. and especially the Southwest through the weekend and even well into next week. ...Guidance Evaluation/Predictability Assessment... The latest guidance remains in reasonable agreement regarding the overall synoptic pattern across the CONUS during the medium range, though continues to offer typical differences in the 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. Upper ridging over the Northwest early in the period should shift east with time as a closed upper low over the northeastern Pacific moves into British Columbia early next week. The CMC has gone back-and-forth between a fast and a slow solution with this closed low. The GFS and ECMWF also had a similar timing issue but not as severe as the CMC. The ensemble means are in much better agreement in this regard. Otherwise, even late in the period, there is relatively excellent agreement on an anomalously strong upper high building and holding strong over the Southwest U.S. and the upper low near Hudson bay dropping slowly southward and grazing the upper Great Lakes next Tuesday. The WPC forecasts this morning were based on a composite blend of 40% 00Z ECMWF, 40% 06Z GFS, and 20% from the 00Z CMC through day 5, with increasing contributions from the ensemble means late period from the 00Z EC mean and the 06Z GEFS. ...Weather/Hazards Highlights... A frontal boundary dropping slowly southward through the Midwest this weekend will be the focus for more daily rounds of showers and storms. Given available moisture and instability, heavy rainfall seems a good bet for portions of the Mid-Mississippi Valley into the Midwest. The best overlap in the current available guidance points to the southern Missouri to Kentucky region, where a Slight Risk was placed on the Days 4 Excessive Rainfall Outlook. The northern portion of this front will move into the Northeast with rounds of additional rainfall. A low pressure wave tracking up the East Coast could help focus an initial round of heavy rain late Friday into early Saturday across central to southeastern New England. A Slight Risk was maintained on the day 4 ERO for the region given how sensitive to heavy rain this area already is. There could be another round of enhanced rainfall moving into New England later in the weekend associated with another coastal low pressure wave. More rounds of enhanced rainfall are possible for New England into early next week as the upper trough is forecast to amplify. Meanwhile, near and along the trailing portion of the front, rainfall should drift farther south into parts of the south-central Plains to the mid-South and the Northeast. A broad marginal risk was maintained on the day 5 ERO near and ahead of the entire boundary, with a Slight Risk being introduced across Oklahoma into the Texas Panhandle where models are in good agreement for the next impulse to eject from the central Rockies and interact with the front. Elsewhere, showers should linger early next week across parts of the Gulf Coast into the Southeast and Mid-Atlantic region. Hazardous heat is expected to expand across the southern tier through at least this coming weekend (and likely beyond per latest CPC forecasts) as the upper ridge strengthens over the Southwest and into much of the South. Much above normal temperatures will expand from the southern High Plains into the Southwest and California, with potential for numerous record daily daytime highs and warm overnight lows possible. 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. Elsewhere, a cold front dropping south from Canada should bring temperatures down to 5-15F below average over parts of the northern and central Plains through Sunday with some moderation early next week as upper ridging tries to build in. Kong/Santorelli Hazards: - Heavy rain across portions of the Central/Southern Plains, the Middle/Lower Mississippi Valley, the Tennessee/Ohio Valley, and the Northeast, Fri, Jul 14. - Heavy rain across portions of the Central/Southern Plains, and the Lower Mississippi Valley, Sat, Jul 15. - Flooding possible across portions of the Northeast and the Northern Plains. - Flooding occurring or imminent across portions of the Northeast and the Southern Plains. - Flooding likely across portions of the Northeast. - Hazardous heat across portions of the Central/Southern Plains, the Central/Southern Rockies, the Lower/Middle Mississippi Valley, the Central Great Basin, the Tennessee/Ohio Valley, the Mid-Atlantic, California, the Southeast, the Southern Appalachians, and the Southwest, Fri-Tue, Jul 14-Jul 18. 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