Extended Forecast Discussion NWS Weather Prediction Center College Park MD 407 PM 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*** ...Overview... A strong and persistent upper ridge/high anchored over the southern Rockies will keep hotter than usual temperatures across the Southwest into the southern Plains. Away from the upper high, the amplified ridge-trough pattern over the lower 48 as of Sunday should trend steadily less amplified over the course of next week as a northeastern Pacific upper low (part of a Rex block configuration including an Alaska/northwestern Canada ridge) tracks into western Canada and flow to its south becomes more progressive across southern Canada and the far northern tier of the U.S. The initial eastern trough should become just a modest weakness by next Thursday. This pattern evolution will likely spread a northern area of heat from the northern Rockies/High Plains toward the Midwest. Some diurnally favored showers and thunderstorms will be possible over the Four Corners states and fronts/upper shortwaves may produce some episodes of convection over the eastern half of the country, but overall the coverage of heavy rainfall threats should be less than average for the middle of summer. ...Guidance Evaluation/Predictability Assessment... Within what continues to be an agreeable pattern evolution among the guidance, the most contentious part of the forecast still involves what happens to the northeastern Pacific upper low and its surrounding flow after early next week. GFS runs over the past day have become rather consistent in being on the faster side of the guidance spread for the upper low by days 6-7 Wednesday-Thursday, and while the 12Z run has slowed a bit versus 00Z/06Z runs, the 12Z UKMET is similar to the 12Z GFS and this scenario is within the realm of ensemble possibilities. ECMWF/CMC runs are still somewhat slower with the upper low. Either way, consensus suggests that a weak trough may linger near the Pacific Northwest coast into next Thursday. Farther east, the models/means agree that the southern Rockies upper ridge should gradually extend its reach farther eastward across the south-central Plains as the eastern trough aloft fills. By midweek the 12Z CMC is slower to weaken the trough versus other guidance, with recent trends actually leaning toward slightly quicker weakening of the trough. A 00Z/06Z operational model blend early and a transition to an even model/ensemble mean weight by day 7 Thursday provided a good reflection of consensus early and downplayed details that have lower confidence late in the period. ...Weather/Hazards Highlights... The combination of the front likely stalling over the South and localized sea breeze boundaries may help to focus scattered to numerous showers and storms near the Gulf Coast region and Southeast through at least late weekend, with coverage/totals likely decreasing somewhat thereafter. The Day 4 (Sunday-Sunday night) Excessive Rainfall Outlook maintains a Marginal Risk area from the central Gulf Coast to southeastern Coast. By Day 5 (Monday-Monday night), the localized heavy rainfall threat over the southern tier should be confined more to the Southeast Coast but approaching shortwave energy could promote localized heavy rainfall farther north over parts of the Mid-Atlantic/Appalachians. A Marginal Risk encompasses these areas. Meanwhile diurnally favored convection may be locally heavy over portions of the Four Corners states during Days 4-5, possibly expanding northwest into some of the Great Basin by Day 5 in association with moisture and shortwave energy flowing around the western periphery of the southern Rockies upper ridge. The outlooks depict a Marginal Risk area on both days. A front dropping into the Midwest/Great Lakes by early next week and likely weakening as it continues eastward, along with a separate front over the Plains, may produce showers and thunderstorms at times through the period. Some of the rainfall could be locally heavy but guidance signals with respect to location and QPF magnitude appear too diffuse to depict any risk areas in the outlooks through Day 5 for now. Shorter term guidance may eventually provide enough focus to define regions with relatively greater heavy rainfall potential. The Four Corners states will likely see diurnal showers/storms continue through the end of the forecast period next Thursday, with the same being the case for the Florida Peninsula. The strong upper ridge over the southern Rockies should keep temperatures 5-10F above normal over locations from the Southwest into the southern High Plains through the period. Even if the anomalies over the Southwest will be a little less extreme than in the shorter term, the heat will continue to be hazardous into next week. Flow around the upper ridge will also extend hot temperatures (5-15F above normal) into the Great Basin into northern Rockies/High Plains by Sunday. The Great Basin should trend closer to normal thereafter, while the next work week should see northern tier heat spreading form the northern Plains into the Midwest with highs 10-15F above normal. Flattening of the upper flow over the East may eventually allow for plus 5-10F anomalies to reach the Great Lakes/Ohio Valley/Northeast by next Wednesday-Thursday. Shower/thunderstorm uncertainties temper confidence in some temperature details though. Away from the Southwest, the highest heat index values (possibly reaching or exceeding 110F during peak afternoon heating) should be over southern Texas/western Gulf Coast and the Florida Peninsula given the high dewpoints over those areas. The Pacific Northwest will tend to see highs near or moderately below normal due to the proximity of upper troughing. 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 Hazards: - Heavy rain across portions of the Central Gulf Coast, Sun, Jul 23. - Hazardous heat across portions of the Lower Mississippi Valley, the Central Great Basin, the Northern Plains, the Southern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Southeast, the Pacific Northwest, and the Southwest, Sun-Wed, Jul 23-Jul 26. - 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 Tennessee Valley, the Middle Mississippi Valley, the Southeast, and the Southern Plains, Tue-Wed, Jul 25-Jul 26. 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