Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 AM EDT Tue Jul 18 2023 Valid 12Z Fri Jul 21 2023 - 12Z Tue Jul 25 2023 ***Dangerous heat wave persists across the southwestern U.S. through early next week, and heat builds farther north across the northwestern High Plains*** ...General Overview... The main story that will make weather headlines will be the continuation of the heat wave over the Desert Southwest and building northward towards Montana and the western Dakotas in association with an anomalous upper ridge anchored over the Four Corners region. The ridge axis will initially be elongated eastward across interior portions of the Gulf Coast region through early Friday before retracting back to the west, and building northward to include more of the northwestern High Plains, and a broad trough builds from the Great Lakes to the East Coast. Meanwhile, a frontal boundary dropping south from the Ohio Valley to the Deep South will bring episodes of potentially heavy rainfall and strong storms across much of the Southeast U.S. ...Guidance Evaluation/Predictability Assessment... Similar to the past several days, the 00Z model guidance suite starts off with very good agreement and above average confidence on the synoptic scale through Sunday, and all of the guidance suggests the upper ridge axis will contract to the west as a broad trough axis builds in across the eastern half of the nation. The GFS is a little stronger with a shortwave crossing the Ohio Valley this weekend compared to the other guidance, but otherwise fits in well elsewhere. The latest 00Z GFS fits in much better with the overall consensus compared to earlier runs across southern Canada early next week. The biggest difference by the end of the forecast period is a stronger solution with the 00Z CMC regarding a trough building over the Pacific Northwest, whereas the ensemble means favor this trough to be more offshore. Overall, the degree of model uncertainty across southern Canada has improved compared to the 12/18Z model runs. A multi-deterministic model blend suffices as a starting point in the forecast process through Sunday, followed by gradual inclusion of the ensemble means to about half by Tuesday. ...Weather/Hazards Highlights... The frontal boundary that drops southward toward the general vicinity of the Interstate 20 corridor across the Deep South by this weekend is expected to be a focus for scattered to perhaps numerous showers and storms, particularly near any surface lows that develop along the boundary. There is a Marginal Risk area for flooding potential from the general vicinity of the Ozarks to the southern Appalachians on Day 4 where the best combination of forcing and instability will exist. This axis then reaches the coastal plain of the Gulf Coast region from Louisiana to northern Florida on Day 5 where a Marginal Risk is also in effect where some slow moving storms could produce a quick 1-3 inches of localized rainfall in some areas. Across the western U.S., a Marginal Risk area is valid for both days 4 and 5 from southeastern Colorado across much of New Mexico and into eastern portions of Arizona. With the core of the upper high expected to be over the Four Corner region, this will result in some advection of monsoonal moisture from western Mexico northward across the southern Rockies, resulting in isolated to perhaps scattered coverage of slow moving thunderstorms that can be expected mainly during the afternoon and evening hours. Looking ahead to Sunday and beyond, showers and storms will likely linger across the Gulf Coast region, and there are signs that some thunderstorm complexes could develop across portions of the Midwest, although confidence is limited on exactly where these will be. Isolated monsoonal showers/storms are likely to continue across parts of Arizona and New Mexico. The stubborn upper ridge across the Intermountain West/Desert Southwest will result in a continuation of the ongoing extreme heat going through next weekend and early next week, with temperatures generally running 5 to 15 degrees above average for both highs and lows from the Desert Southwest to Montana as the heat builds farther to the north, and also extending across the north-central U.S. by Monday and Tuesday. This will equate to highs well into the 110s for the lower elevations of eastern California, southern Arizona, and southern Nevada, with the potential for additional record high temperatures being established. There is some good news to report across much of the southern Plains and into the Gulf Coast region where some abatement in the intense heat is likely by the weekend as a frontal boundary heralds the arrival of more seasonal conditions compared to the past week of oppressive heat. It will still be quite hot and humid for southern Texas where heat indices may exceed 110 degrees during peak afternoon heating. Much of the eastern U.S. should generally be within a few degrees of late July averages with no major heat waves expected east of the Mississippi River. 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 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