Extended Forecast Discussion NWS Weather Prediction Center College Park MD 431 PM 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 northern High Plains*** ...General Overview... From Friday into early next week expect the anomalously strong upper high initially over the Desert Southwest to shift into the Four Corners region while the ridge extending northward into the Interior West/northern Rockies amplifies a bit as it shifts toward the northern High Plains. In response, the downstream upper trough over the eastern half of the lower 48 should amplify and sharpen into the weekend. This pattern will maintain the Desert Southwest heat wave as a significant weather story through the period while expanding the heat farther north/northeast through the Interior West and into the northern Rockies/Plains. At the same time, the amplifying eastern upper trough will help to suppress the upper ridging and associated very hot and humid conditions forecast to extend along and just north of the Gulf Coast region through late week. The surface front ahead of the upper trough will support episodes of potentially heavy rainfall and strong storms over portions of the South. Finally, an upper trough should approach the Pacific Northwest early next week as the anchoring Pacific upper low drifts toward northern British Columbia. ...Guidance Evaluation/Predictability Assessment... The latest models and ensembles continue to show good agreement with the large scale pattern evolution with some embedded detail uncertainties. Within the eastern upper trough, shortwave specifics are small enough in scale to have low predictability in the medium range time frame but will be meaningful on a more localized basis, in particular for the Mid-Atlantic/Northeast low pressure and frontal system Friday into the weekend and then a front that may drop into the Midwest/Great Lakes during the first half of next week. A blend/mean approach provides a reasonable intermediate solution where differences exist. Meanwhile guidance has been a little more erratic with details over the North Pacific into the Pacific Northwest and western Canada. Stray solutions (00Z GFS, 12Z UKMET most recently) have been progressive enough with upstream flow to result in ejection of the upper low drifting over the northeastern Pacific, while the 00Z CMC was on the amplified/fast side with its trough crossing the Northwest by day 7 Tuesday. The remaining majority cluster has favored a slower upper low/trough. A 00Z/06Z operational model blend early followed by a transition to a combination of models and ensemble means (06Z GEFS/00Z ECens) provided a good representation of the most likely scenario for significant features. ...Weather/Hazards Highlights... The frontal boundary that drops southward from a southern Mid-Atlantic through southern Plains orientation into the South late week into the weekend should be a focus for scattered to perhaps numerous showers and storms, with any waves providing additional enhancement. Alignment of the front may promote west-east training of convection over some areas at times. The updated Day 4 Excessive Rainfall Outlook covering the Friday-Friday night period plans to introduce a Slight Risk area centered over the eastern half of Tennessee, where there is a favorable overlap of significant rainfall expected in the short term and best guidance signals for additional heavy rainfall potential during Day 4. The surrounding Marginal Risk area extends from the general vicinity of the Ozarks to parts of the interior Southeast. The axis for potentially heavy rainfall should reach the central Gulf Coast region through northern Florida and parts of Georgia/South Carolina on Day 5 (Saturday-Saturday night), with only a Marginal Risk maintained in light of high flash flood guidance values and some lack of clustering for QPF details. Shorter term guidance may ultimately help to resolve areas with greater rainfall potential. 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 shift into the Four Corner region, some monsoonal moisture may advect 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. Meanwhile low pressure and an associated frontal system may bring some showers and thunderstorms to the Northeast in the Day 4 time frame, with wet ground conditions but some model/ensemble scatter for details leading to a Marginal Risk area across that region. 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 from the Midwest eastward as a decelerating warm front crosses the central Plains and a cold front drops south from Canada into the Midwest/Great Lakes. 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 through this weekend and early next week, with 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 especially Friday-Saturday. Temperatures generally 5 to 15 degrees above average for both highs and lows will likely build farther north through the Great Basin into northern Rockies, and then extend into the northern Plains by early next week while highs over the Great Basin could start to moderate a bit next Tuesday if an upper trough gets close enough to the Pacific Northwest. Meanwhile the front dropping into the southern tier should help to suppress the intense heat from the southern Plains into the Gulf Coast region by the weekend. It will still be quite hot and humid over southern Texas and parts of the Florida Peninsula where heat indices may approach or exceed 110 degrees during peak afternoon heating. On the cool side of the front, highs may be up 10-15 degrees below normal over the central Plains on Friday. 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. 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 Great Lakes, the Northeast, the Southern Appalachians, and the Ohio/Tennessee Valley, Fri, Jul 21. - Hazardous heat across portions of the Lower Mississippi Valley, the Southern Plains, the Northern Rockies/Great Basin, California, the Southeast, and the Pacific Northwest, Fri-Sat, Jul 21-Jul 22. - Hazardous heat across portions of the Lower Mississippi Valley, the Northern/Central Great Basin, the Northern/Southern Plains, the Northern/Central Rockies, California, the Southeast, and the Southwest, Fri-Tue, Jul 21-Jul 25. - Hazardous heat across portions of the Southeast, the Southern Appalachians/Plains, the Lower Mississippi Valley, and the Tennessee Valley, Fri, Jul 21. - Hazardous heat across portions of the Northern Plains/Rockies, Sat-Tue, Jul 22-Jul 25. 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