Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 PM EDT Tue Aug 16 2022 Valid 12Z Fri Aug 19 2022 - 12Z Tue Aug 23 2022 ...Excessive heat threat will persist from interior California to the Northwest through the weekend... ...Monsoonal heavy rain threat for the Southwest and south-central Great Basin/Rockies into the weekend may relax early next week... ...Excessive rainfall threat for the Southern/Central Plains and Mid-Lower Mississippi Valley this weekend into early next week... ...Guidance Evaluation/Predictability Assessment... The latest model and ensemble suite are in fairly good agreement with the large scale pattern evolution into next week with ridging over the western half of the lower contiguous states (periods of weakness) and an eastern trough that becomes reinforced by a deep shortwave/closed low that tracks through the Midwest. Like previous cycles, there remains some differences in the timing, magnitude and locations of various features through the extended period. All in all, the CMC/UKMET/GFS/ECWMF provided a good starting point for the forecast and out to day 5. The GFS and ECWMF were utilized more for mid and later periods along with their ensemble means. This keeps with the forecast close to continuity from the prior issuance. ...Pattern Overview and Weather/Hazards Highlights... Much of the interior Northwest will continue to have oppressive temperatures, with daily highs ranging from the mid 90s to 100s and overnight lows in the 50s and 60s, into the weekend thus maintaining the heatwave. A vast area of from Washington/western Idaho to central/south-central California have Heat Advisories and Excessive Heat Warnings in effect through Friday. These temperatures are likely to produce some record values, mostly for overnight temperatures. The temperatures that had been 10-15 degrees F above normal will begin to relax somewhat over the weekend and into early next week as the Ridge experiences some weakening. In contrast, below normal highs are forecast across parts of east-central U.S. in the wake of a cold 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. The assiduous monsoonal moisture into the Southwest, Great Basin, and Southern/Central Rockies will continue as anomalously high moisture feeds into the Southwest and south-central Great Basin/Rockies to produce mainly diurnally driven rain and thunderstorms. This is in conjunction with embedded upper vorts and the later week/weekend larger scale influx of Pacific upper trough energy whose subsequent eastward track across the region now has more ample guidance support. The WPC experimental medium range Slight Risk for excessive rainfall remains in effect across portions of Arizona and New Mexico Friday and Saturday given enhanced QPF/ARI and an overall persistent favorable pattern that may entrain deeper moisture from a northern Mexico feature with tropical connection. There will likely be areas that have periods of intense rainfall thus the elevated threat for flooding. There is a growing guidance signal that the monsoonal pattern may relax early next week. Downstream, a heavy rainfall/flash flooding signal then emerges from the South/Central Plains to the mid-lower Mississippi Valley this weekend into early next week. The threat seems reasonable as moisture and instability pool with return flow into a slow moving front in favorable upper diffluent flow aloft, and especially with upper trough energies ejecting out from the West. Even so, the widespread NBM volumetric/areal coverage of heavy rainfall seems somewhat overdone compared to models and ensemble members. To the south, NHC also indicates that there is a growing model/ensemble signal for another tropical wave to reach northeast Mexico and southern Texas over the weekend that could moderately focus local moisture and rainfall potential. 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 by the weekend with the wavy front slated to lift slowly northward. Farther north, convection will gradually spread across the Midwest with later week to early next week digging of shortwave energy to form a closed low and with a slow to progress front. Slow cell motions and instability near the closed feature may produce runoff issues. Campbell/Schichtel 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