Extended Forecast Discussion NWS Weather Prediction Center College Park MD 434 PM EDT Tue Jul 26 2022 Valid 12Z Fri Jul 29 2022 - 12Z Tue Aug 02 2022 ...Dangerous heat wave to focus over of the Northwest/Great Basin through the weekend, but still lingers over The South... ...Excessive rainfall likely across parts of the south-central High Plains through Friday, with heavy rain threats stretching from the Southwest to the southern Mid-Atlantic into next week... ...Overview... It remains the case that 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., with high heat then set to spread to the north-central states. Heat should also linger in particular over the southern Plains and lower Mississippi Valley to the south of a wavy quasi-stationary front. This important 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. WPC issues an experimental days 4/5 Excessive Rainfall Outlook (ERO). An elongated Slight Risk area for excessive rainfall has been issued for this broad area given potential for cell training/repeat activity fueled by deep moisture and instability. Additionally, enhanced monsoonal moisture persisting well into the medium range period will continue to promote a daily threat of locally heavy showers and storms that may produce instances of flash flooding in parts of the Southwest, the southern/central Rockies and especially the south-central High Plains where a Moderate Risk area for excessive rainfall has been issued through Friday given favorable flow and longevity. All these threats and more are also highlighted in the WPC Day 3-7 Hazards Outlook product. ...Guidance Evaluation/Predictability Assessment... Deterministic models 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 some implications for excessive heat details in that region. The GFS and UKMET have been the quickest to bring this energy onshore, while the ensemble means support something a little slower, now a bit more in line with the ECMWF, some runs of the Canadian and climatology. 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 medium range product suite mainly utilized a blend of the seemingly reasonable and overall compatible GFS/ECMWF/UKMET/Canadian models Friday/Saturday, switching to the GEFS/ECMWF ensemble means thereafter to mitigate growing smaller scale embedded system variance. This maintains good WPC product continuity that is generally in line with 13 UTC National Blend of Models guidance. ...Weather/Hazards Highlights... Heat headlines expand into the Northwest/Great Basin 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. 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 closer 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. As stated earlier, 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 remains in place for 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/Schichtel Hazards: - Heavy rain across portions of northern New Mexico and southern Colorado, across the central Plains, the mid-Mississippi Valley, the Tennessee Valley, and into the southern Appalachians, Fri-Sat, Jul 29-Jul 30. - Heavy rain across portions of higher elevations of Arizona into western New Mexico, Fri-Tue, Jul 29-Aug 2. - Heavy rain across portions of the central to southern Appalachians, the Ohio Valley, and the Tennessee Valley, Sun-Mon, Jul 31-Aug 1. - Heavy rain across portions of the Mid-Atlantic, Fri-Mon, Jul 29-Aug 1. - Flooding occurring or imminent across portions of the northern Plains. - Excessive heat across portions of California into the Great Basin, the Pacific Northwest, and western Montana, Fri-Sun, Jul 29-Jul 31. - Excessive heat across the coastal plains of the Carolinas into southeastern Virginia, Fri, Jul 29. - Excessive heat across southern/eastern Texas and into the lower Mississippi Valley, Fri, Jul 29, and Mon-Tue, Aug 1-Aug 2. - Excessive heat across the mid-Mississippi Valley, Mon-Tue, Aug 1-Aug 2. 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