Extended Forecast Discussion NWS Weather Prediction Center College Park MD 354 PM 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 and eventually extending into the Midwest*** ...General Overview... Guidance agrees well upon a persistently strong Four Corners upper high with a ridge extending north into southern Canada, while a downstream eastern U.S. upper trough achieves its greatest depth/amplitude during the weekend before trending somewhat more shallow next Monday-Wednesday. The northeastern Pacific/northwestern North America pattern should take on a Rex block appearance, with some height falls possibly reaching the northwestern U.S. as the Pacific upper low drifts toward/into British Columbia. The Four Corners upper high/ridge should support continuation of the intense heat wave over the Desert Southwest and a rebound of heat over the southern Rockies/High Plains after the weekend, while the pattern evolution will likely help to spread hot temperatures into the northern Rockies/High Plains during the weekend and extending farther eastward into the Midwest next week. Meanwhile, a frontal boundary settling into the South will bring episodes of potentially heavy rainfall and strong storms across much of the Gulf Coast region and the Florida Peninsula, and also help to temper the ongoing southern tier heat wave. ...Guidance Evaluation/Predictability Assessment... Forecast issues within the agreeable large scale pattern have not changed much over the past day. Guidance is gradually trying to converge for the specifics of shortwave energy and associated surface evolution affecting the Northeast into the weekend, while predictability is low for trailing shortwaves that will influence specifics of the wavy front that should linger over the western Atlantic into the southern tier states through the first part of next week. Details are still somewhat ambiguous and variable for shortwave energy dropping into the western side of the mean trough, tempering confidence for details of a front that may reach the Midwest/Great Lakes early next week. In differing ways the 06Z/12Z GFS runs are somewhat on the extreme side of guidance in being faster to bring the Pacific upper low/surrounding flow into western North America. The 06Z GFS was the more extreme of the two in terms of height falls reaching the northwestern U.S. Based on the 00Z/06Z guidance comparisons, the updated forecast started with a 00Z/06Z operational model blend early in the period. 06Z GFS issues over the northwest favored swapping in the 00Z version for GFS input by days 5-7 Monday-Wednesday, while adding in some GEFS/ECens/CMCens mean guidance during the latter half of the period helped to downplay lower confidence details that arise by that time. ...Weather/Hazards Highlights... The wavy front settling into the South this weekend along with any sea breeze boundaries should provide a focus for showers and thunderstorms that may produce some locally heavy rainfall due to their slow movement or repeating over the same area. The Days 4-5 Excessive Rainfall Outlooks that cover the Saturday-Sunday night period depict a Marginal Risk area for flooding potential from the central Gulf coast region to northern Florida/Southeast Coast where the best combination of moisture, forcing, and instability will exist. An embedded Slight Risk area could eventually arise if shorter term guidance clusters sufficiently in terms of location and QPF magnitude, but for now the signal does not appear strong enough to introduce such an area yet. 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. After Sunday expect lingering showers and thunderstorms across the Gulf Coast region to become more scattered with time as the front to the north weakens. Meanwhile one or more episodes of showers/storms will be possible over the Midwest and eventually extending into the East due to a combination of shortwaves within the upper trough and a front pushing southeast from Canada. It will take additional time to resolve coverage and totals from this potential activity. Isolated monsoonal showers/storms are likely to continue across parts of the Four Corners region. The Four Corners upper ridge will maintain the extreme heat over the Southwest through the weekend and early next week. Temperatures of 5-15 degrees above average for both highs and lows over the Desert Southwest as of Saturday may decline a bit to 5-10 degrees above normal for highs thereafter but this heat will remain hazardous. 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 and record long streaks of 110+ degree readings for some Arizona locations. Meanwhile temperatures over the southern High Plains/Rockies should rebound to 5-10F above normal after a brief time near normal on Saturday. Farther north, the Intermountain West into northern Rockies should see highs/lows 5-15F above normal during the weekend followed by gradual moderation as the northern area of anomalous heat shifts its focus from the northern High Plains (already starting by the weekend) toward the Midwest. By next Wednesday, highs from the northern half of the Plains into parts of the Midwest should be 5-15F above normal (90s to around 100). These may approach or exceed the highest temperatures of the summer so far at some locations. Parts of the South should see a reprieve from this week's intense heat as a frontal boundary stalls over the region. It will still be quite hot and humid for southern Texas where heat indices may exceed 110 degrees during peak afternoon heating and the Florida Peninsula may see hazardous heat indices as well. Most areas east of the Mississippi River should generally be within a few degrees of late July averages with no major heat waves. Rausch/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 - Hazardous heat across portions of the Lower Mississippi Valley, the Central Great Basin, the Southern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Southeast, the Pacific Northwest, and the Southwest, Sat-Wed, Jul 22-Jul 26. - Hazardous heat across portions of the Northern Rockies and the Northern Great Basin, Sat-Sun, Jul 22-Jul 23. - Hazardous heat across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Rockies, the Southern Plains, and the Southwest, Mon-Wed, Jul 24-Jul 26. - Hazardous heat across portions of the Central Plains, the Lower Mississippi Valley, the Northern Plains, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, and the Southern Plains, Tue-Wed, Jul 25-Jul 26. - Hazardous heat across portions of the Northern Plains and the Northern Rockies, Sun-Wed, Jul 23-Jul 26. - Hazardous heat across portions of the Southeast and the Lower Mississippi Valley, Sat, Jul 22. 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