Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Thu Jul 20 2023 Valid 12Z Sun Jul 23 2023 - 12Z Thu Jul 27 2023 ***Dangerous heat wave persists across the southwestern U.S. through the middle of next week, and heat builds farther north across the northern High Plains and eventually extending into the Midwest*** ...General Overview... The upper level flow pattern is expected to be rather amplified for late July across the continental U.S. through at least Monday with an anomalous upper ridge/closed high centered over the southern Rockies, a trough stretching from the Great Lakes to the Deep South, and a closed low off the Pacific Northwest coast. This will result in a continuance of the ongoing record-breaking heat wave across the southwestern U.S. and extending northward to the western High Plains through much of next week, and near average temperatures over the eastern states. The pattern may break down some across the northern tier states mid to late week as shortwave energy crosses southern Canada, while the closed upper high holds firm over New Mexico and portions of adjacent states. ...Guidance Evaluation/Predictability Assessment... The model guidance Sunday and into Monday remains in very good overall agreement across the continental U.S., and this has been the case for the past several days of model runs. However, there has been an increase in the level of model uncertainty across southern Canada and also across the Great Lakes and Northeast going into the middle of next week. There are differences in how quickly shortwave energy tracks eastward from the upper level vortex over the Gulf of Alaska and how that may squash the northern periphery of the ridge axis near the U.S./Canada border. The GFS over the past few runs has been faster with the closed low tracking east compared to the CMC/ECMWF, but the CMC has trended more in line with the model consensus compared to its 12Z run. The WPC front/pressures forecast was primarily derived from a nearly multi-deterministic model blend through Monday, followed by less of the 12Z CMC and a gradual increase of the ensemble means to about 50% by next Thursday. ...Weather/Hazards Highlights... The weakening frontal boundary that stalls out in the general vicinity of the Interstate 20 corridor across the Deep South by Sunday is expected to be a focus for scattered to perhaps numerous showers and storms, particularly near any sea breeze boundaries. There is a Marginal Risk area for flooding potential across the coastal plain of the Gulf Coast region from Louisiana to the eastern Carolinas on Day 4 where the best combination of forcing and instability will exist. Some slow moving storms could produce a quick 1-2 inches of localized rainfall in some of these areas. There are also signs that some thunderstorm complexes could develop across portions of the Midwest in the northwest flow pattern aloft on Sunday, and a Marginal Risk area has been introduced for that possibility. Across the western U.S., a Marginal Risk area remains valid for both days 4 and 5 from southern 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 develop from the Southeast Coast northward to include the central Appalachians, and a Day 5 Marginal Risk area is in effect for this region with the potential for some slow moving cells. Elsewhere, a fairly uneventful pattern is expected with no major areas of concern at this time. The large upper ridge/closed high over the Intermountain West/Desert Southwest will continue to have staying power through much of next week as the overall pattern here does not change that much. This will result in a continuation of the ongoing extreme heat going through next weekend and into the middle of next week, with temperatures generally running 5 to 10 degrees above average, and locally higher, 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 Tuesday and Wednesday. This will unfortunately equate to highs 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, and widespread 90s are likely for the central and northern Plains. Readings may be a few degrees lower by Tuesday-Wednesday across much of the Intermountain West, but not substantial relief. It will still be quite hot and humid for southern Texas where heat indices may exceed 110 degrees during peak afternoon heating with very high dewpoints remaining in place. Much of the eastern U.S. should generally be within a few degrees of late July averages with no major heat waves expected through next week. 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