Extended Forecast Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Wed Aug 17 2022 Valid 12Z Sat Aug 20 2022 - 12Z Wed Aug 24 2022 ...Significant monsoonal excessive rainfall threat for the Southwest and south-central Great Basin/Rockies into the weekend may relax early next week as hazard focus shifts to the Southern Plains then Lower Mississippi/Tennessee Valleys... ...Excessive heat threat to persist for interior portions of California/Northwest U.S. into the weekend... ...Guidance Evaluation/Predictability Assessment... Confidence is above average for a multi-day heavy rain event that spreads from the Southwest into the Southern Plains that will result in scattered to widespread areas with threats for flash flooding given near soil saturation conditions in the short-term period, heavy rain over repeated locations prior to, and into the beginning of the extended periods across the Southwest, before shifting eastward. A majority of the guidance show 7 day areal averages of 3 to 7 inches; with isolated maxes possibly climbing into the upper singles/lower double digits over the course of the week. Overall, the latest models continue to handle the mid-larger scale features well. The GFS/UKMET are trending on the faster/eastern edge of the cluster which would support QPF increases a little further south/east than the previous forecast. The ECWMF/CMC are still within the cluster but are favoring a slightly slower/western position within the cluster. The small nuisances will likely affect the exact locations of the maximum QPF over the Southwest and as it translates eastward into the Southern High Plains, both camps have an axis with amounts of 2 to 5+ inches. The The preferred WPC blend used a composite of GFS/ECMWF/UKMET/Canadian initially with slightly more emphasis on the GFS and GEFS/ECMWF ensembles along with the National Blend of Models. ...Pattern Overview and Weather/Hazards Highlights... Strong, diurnally driven thunderstorms will continue to overspread the Southwest, the Great Basin and Southern Rockies thanks to multiple mid/upper-level impulses and over-abundant, pooled monsoonal moisture and influx of Pacific moisture into the region. Local downpours are expected and at times, over very sensitive areas such as urban areas and slot canyons, that will likely result in rapidly changing and very dangerous situations as water rushes off. WPC has hoisted an experimental day 4 Moderate Risk (level 3/4) for portions of southeast Arizona and southwest New Mexico and expanded the Slight Risk across central and eastern New Mexico. Some locations across southern Arizona and New Mexico could surpass 25-year ARI. The monsoonal pattern may relax early next week. Meanwhile, NHC also indicates that another tropical wave may reach northeast Mexico and southern Texas over the weekend that could focus local moisture and rainfall potential. The heavy rain and flooding threat will shift to the Southern Plains, Lower Mississippi River and into the western reach of the Tennessee Valley for the upcoming week. The threat seems formidable as moisture and instability pool with return flow into slow moving/wavy fronts in favorable upper flow as upper trough energies eject out from the West. Models are suggesting a strong signal for repeat/training of cells and have shifted the QPF axis a little south from the previous run. WPC has an experimental day 5 Slight Risk for excessive rainfall spanning from eastern New Mexico to Arkansas. Showers and thunderstorms are expected to persist into the weekend along a slow-moving, wavy frontal boundary over the South and Southeast. There may be local areas that have heavy downpours thanks for very moist air pooled over the region. Organized rains will also slowly lift to the Mid-Atlantic over the weekend with the wavy front slated to lift slowly northward. Farther north, convection will gradually spread across the Midwest this weekend/early next week as digging shortwave energy forms a closed low. Slow cell motions and instability near the closed feature offer potential for local downpours with runoff issues. The heatwave over the Pacific Northwest will drag on through the weekend, with daily highs ranging from the mid 90s to 100s and overnight lows in the 50s and 60s. A vast area of from Washington/western and southern Idaho to central/south-central California already have Heat Advisories and Excessive Heat Warnings in effect through Friday. These temperatures are likely to produce some record values, mostly for overnight temperatures. Temperatures should begin to relax somewhat through the weekend and into early next week as the ridge weakens. In contrast, below normal highs are forecast across parts of east-central U.S. in the wake of a wavy front. Given the cloud cover and rainfall over the Desert Southwest and across the southern portions of the Great Basin and Rockies, daily temperature readings near or below normal are expected. Campbell 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/Southern Plains, the Middle/Lower Mississippi Valley, and the Tennessee/Ohio Valley, Sun-Tue, Aug 21-Aug 23. - Heavy rain across portions of the Southern Rockies/Plains, and the Southwest, Sat, Aug 20. - Flooding occurring or imminent across portions of the Southern Plains. - Flooding likely across portions of the Southern Plains. - Excessive heat across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat, Aug 20. - Heavy rain across portions of mainland Alaska, Sat-Mon, Aug 20-Aug 22. 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