Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Jul 19 2023 Valid 12Z Sat Jul 22 2023 - 12Z Wed Jul 26 2023 ***Dangerous heat wave persists across the southwestern U.S. through early next week, and heat builds farther north across the northern High Plains*** ...General Overview... The main story that will continue to make weather headlines will be the continuation of the intense heat wave over the Desert Southwest and then building northward towards Montana and the Dakotas in association with an anomalous upper ridge anchored over the Four Corners region through the middle of next week. 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 Gulf Coast region and the Florida Peninsula, and this will also serve to retract the upper ridge west to the west and result in some abatement of the ongoing heatwave across the Deep South states. ...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 about Tuesday across the continental U.S., and near average confidence across southern Canada. 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 and remains in place through early next week. There are some modest model differences by next Wednesday, with the CMC a little stronger with a shortwave across the Midwest, the ECMWF a little more suppressed with the ridge axis near the Canadian border, and the GFS stronger with a trough near the Pacific Northwest coast. Overall, the degree of model uncertainty across southern Canada has improved compared to yesterday's model runs. A multi-deterministic model blend suffices as a starting point in the forecast process through early Monday, followed by gradual inclusion of the ensemble means to about 40-50% by mid-week. ...Weather/Hazards Highlights... The frontal boundary that drops southward toward the general vicinity of the Interstate 20 corridor across the Deep South by Saturday is expected to be a focus for scattered to perhaps numerous showers and storms, particularly near any surface lows that develop along the boundary and sea breeze boundaries. There is a Marginal Risk area for flooding potential across the coastal plain of the Gulf Coast region from Louisiana to northern Florida on Day 4 where the best combination of forcing and instability will exist. Some slow moving storms could produce a quick 1-3 inches of localized rainfall in some of these 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 Corners 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 Monday, scattered showers and storms will likely linger across the Gulf Coast region, although not expected to be as widespread. There are also signs that some thunderstorm complexes could develop across portions of the Midwest in the northwest flow pattern aloft, although confidence is limited on exactly where these will be. Isolated monsoonal showers/storms are likely to continue across parts of the Four Corners region. 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 unfortunately 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 and record long streaks of 110+ degree readings for some Arizona locations. Readings may be a few degrees lower by Tuesday-Wednesday across much of the Intermountain. 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 expected 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