Extended Forecast Discussion NWS Weather Prediction Center College Park MD 329 PM EDT Mon Jul 25 2022 Valid 12Z Thu Jul 28 2022 - 12Z Mon Aug 01 2022 ...Dangerous heat wave over the south-central U.S. to moderate by medium range, while heat builds across the Northwest into next weekend... ...Excessive rainfall likely for parts of the central Appalachians into Saturday, with heavy rainfall possible also extending as far west as the Central/Southern Rockies... ...Overview... A persistent ridge will be parked over the western U.S. through the course of the extend period while somewhat amplified troughing slides across the Midwest/Ohio Valley and into the Northeast. Sweltering heat will increase/spread across the Northwest U.S. while portions of the south-central Plains will experience modest relief to the recent hot temperatures thanks to a meandering surface front. This front will be also be the focus for a multi-day, heavy rain event. Areas of excessive rainfall remains likely in places along this boundary, particularly the central Appalachians, but also farther west into the Tennessee/lower Ohio Valleys and Central Plains. Additionally, enhanced monsoonal moisture lasting from the short to medium range period will continue to promote daily risks of locally heavy showers and storms that may produce instances of flash flooding in parts of the Southwest and southern/central Rockies and High Plains. ...Guidance Evaluation/Predictability Assessment... The latest runs of the deterministic and ensemble means remain fairly clustered with the large-scale pattern and evolution of the expansive ridge and the trough digging across the Upper Midwest/Great Lakes around midweek and shifting into the Northeast by the weekend. Some model differences remain in regards to the distribution and timing of individual energies within the trough but fall within the normal degree of spread. Saturday and beyond the trough over the Northeast will be reinforced as multiple impulses move through. The main upper ridge axis across the Northwest should shift east in favor of a couple of weak shortwaves entering the Pacific Northwest by around Saturday-Sunday with varying differences in the details. There is some indication that a stronger shortwave may advance into the Northwest by the beginning of next week. The WPC preferred blend began with a general model blend using the ECWMF/GFS/UKMET/CMC that transitioned to the ECWMF/GFS/CMC/EC ensemble mean/GEFS mean beyond Day 5. ...Weather/Hazards Highlights... A vast portion of the south-central U.S. has been excessively hot this summer but temperatures are forecast to trend closer to normal for late July as a cold front settles into the region. In contrast, temperatures will continue to rise across much of the Northwest and Interior West as the upper ridge strengthens over the West. Multiple days of excessive heat will overspread the Northwest, with some locations likely to see highs as much as 10-20F above normal. This equates to temperatures in the low to mid 100s for the Columbia Basin, with 90s expected to reach as far west as Portland and Seattle. Some daily records for highs/warm lows could be set across interior portions of the Northwest. There is some forecast uncertainty with the timing of when the temperatures will moderate again, but current forecasts show some lowering of the temperatures, though still above normal, by next weekend, while temperatures of about 10F above normal shift into the northern Plains. With the frontal boundary boundary taking up residence across the south-central states and providing a focus for convection, much of this region along with the parts of the Mid-Atlantic will have multiple days of showers and thunderstorms. There will be areas that will see repeated rounds of moderate to excessive rainfall that will increase the risk the for localized flash flooding, especially across the Mississippi/Tennessee Valleys and into the Central Appalachians. The days 4 and 5 experimental EROs have expansive slight risks depicted along this boundary from the central Appalachians to the central Plains. A moderate risk was identified across portions of West Virginia and far eastern Kentucky on the day 4 ERO, which covers the potential for continued heavy rainfall over an already soggy area likely to receive heavy rainfall in the short range as well. Activity along this boundary may sink slightly southward with time, continuing through next weekend for some regions in especially the Tennessee Valley. Given several days of rainfall over the same area, this likely will raise concerns for flooding as flash flooding. Rainfall should slide across portions of the Northeast ahead of a more progressive cold front on Thursday, though linger across portions of the southern mid-Atlantic for a couple of days. Persistent monsoonal moisture will continue to support diurnally favored showers/storms with some locally heavy rainfall over the Four Corners states through this week. Across parts of Arizona in particular, anomalous moisture lingering over likely wet soil from activity in the short range period could lead to additional flash flooding there, with another uptick expected on Friday and prompting the introduction of a slight risk on the day 5 ERO. The moisture plume is also expected to interact with the central Plains front, enhancing rainfall totals farther north and east into New Mexico and Colorado midweek, with increasing potential for flooding and flash flooding. Slight risks remain in place for the days 4 and 5 EROs in this area. Campbell/Santorelli Hazards: - Heavy rain across portions of the Four Corners, Southern Rockies, and Front Range into the Great Plains, Thu-Fri, Jul 28-Jul 29. - Heavy rain across portions of the Central/Southern Plains, the Mid-South, the Central/Southern Appalachians, the Tennessee Valley, and the Ohio Valley, Thu-Mon, Jul 28-Aug 1. - Flooding occurring or imminent across portions of the Northern Plains. - Excessive heat across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Thu-Sun, Jul 28-Jul 31. - Excessive heat across portions of the Southeast, the Southern Mid-Atlantic, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Thu-Fri, Jul 28-Jul 29. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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