Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Tue Jul 26 2022 Valid 12Z Fri Jul 29 2022 - 12Z Tue Aug 02 2022 ...Dangerous heat wave continues across parts of the Northwest through the weekend... ...Excessive rainfall likely across parts of the central High Plains on Friday, with heavy rain threats stretching from the Southwest to the southern Mid-Atlantic into next week... ...Overview... An upper level ridge will be parked over the southern and western parts of the country through much of the period while somewhat amplified troughing slides across the Midwest/Ohio Valley and into the Northeast. Dangerous heat will continue into next weekend across the Northwest U.S. while portions of the south-central Plains should finally see a period of modest relief to the recent heat wave thanks to a meandering surface front. This quasi-stationary front will also be the focus for a multi-day, heavy rain event from the south-central Plains to the southern Mid-Atlantic with excessive rainfall likely in some places. Additionally, enhanced monsoonal moisture persisting into the 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 especially the southern/central Rockies and High Plains. ...Guidance Evaluation/Predictability Assessment... The latest runs of the deterministic and ensemble means remain fairly well clustered with the large-scale pattern and evolution of the expansive ridge and the trough digging across the Midwest/Great Lakes and into the Northeast. Some model differences remain in regards to the distribution and timing of individual energies within the trough but for the most part these differences fall within the normal degree of spread. Saturday and beyond the trough over the Northeast will be reinforced as multiple impulses move through and the guidance continues to offer plenty of uncertainty in the details. Out West, there are lingering questions in the timing of a shortwave entering the Northwest, which would have implications for early period excessive heat in that region and how long it sticks around. GFS and UKMET are the quickest to bring this energy onshore, while the ensemble means support something a little slower like the ECMWF and CMC. The models suggest the overall pattern may become more amplified by the middle of next week with troughing over the West and East sandwiching a building heat dome over the Four Corners and into the Plains. The WPC blend utilized a blend of the deterministic models through day 4, with increasing influence from the ensemble means thereafter, along with smaller portions of the ECMWF and CMC for a little added system definition. This maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Heat headlines will shift to the Northwest during the medium range period as temperatures should finally moderate more towards normal (for late July at least) for the recently heat stricken south-central U.S.. Meanwhile, excessive heat will continue into the weekend across the Northwest with highs in the low to mid 100s for the Columbia Basin, and even 90s expected as far west as Portland and Seattle. These much above normal temperatures combined with little to no relief overnight from the heat, could create a dangerous heat threat for many across the region. Some daily records for highs/warm lows could also 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 suggest by early next week temperatures could once again bounce back to normal as the ridge shifts into the northern Plains. A building heat dome over the central part of the nation by around next Tuesday, may once again bring another period of above normal heat to much of the Plains states. A quasi-stationary boundary draped from the south-central Plains to the southern Mid-Atlantic will continue to focus a threat for heavy to excessive rainfall with some areas likely to see repeated rounds of heavy rainfall increasing the risk the for localized flash flooding. The days 4 and 5 experimental EROs have expansive slight risks depicted along this boundary from the central Appalachians/southern Mid-Atlantic to the central Plains. Activity along this boundary may sink slightly southward with time, continuing through next weekend for some regions in especially the Tennessee Valley. Meanwhile, persistent monsoonal moisture will continue to support diurnally favored showers/storms with some locally heavy to excessive rainfall over a very soggy Four Corners region into the middle of next week. Models show increasing potential that this moisture plume may interact with the central Plains front, enhancing rainfall totals farther north and east into parts of the central and southern Rockies and High Plains. A moderate risk was introduced on the day 4 ERO from far northeast New Mexico, southeast Colorado, and into far western Kansas in a region likely to receive very heavy rainfall at the end of the short range period as well. Santorelli 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