Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 AM EDT Wed Aug 17 2022 Valid 12Z Sat Aug 20 2022 - 12Z Wed Aug 24 2022 ...Excessive heat threat to persist for interior portions of California/Northwest U.S. into the weekend... ...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... ...Guidance Evaluation/Predictability Assessment... Model and ensemble solutions seem reasonably well clustered through medium range time scales with the mid-larger scale pattern evolution into next week with mean ridging with ample weaknesses over the western half of the lower 48 and an eastern U.S. trough that becomes reinforced by a deep shortwave/closed low that tracks through the Midwest. There remain some differences in the timing and magnitude of smaller scale features. A composite GFS/ECMWF/UKMET/Canadian and GEFS/ECMWF ensembles along with the National Blend of Models seems to decently mitigate these differences consistent with predictability to maintain good WPC product continuity. ...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, maintaining the heatwave. A vast area of from Washington/western 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. The ongoing monsoonal pattern will continue through the weekend as anomalously high moisture feeds into the Southwest and south-central Great Basin/Rockies to produce mainly diurnally driven heavy rains and thunderstorms. This is in conjunction with embedded upper vorts and the later week/weekend larger scale influx of Pacific upper trough energy across the region. A WPC experimental medium range Slight Risk for excessive rainfall shifts eastward across Arizona and New Mexico as fueled by enhanced QPF/ARI and a 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. 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. Later, the main heavy rainfall/flash flooding threat should emerge downstream to focus across the southern Plains and the lower Mississippi/Tennessee Valleys later through early-mid next 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. WPC QPF has trended significantly upwards from continuity given a strong guidance signal for repeat/training of cells and a ERO Slight Risk area was issued. While many locations may experience high event totals, WPC QPF volumetric/areal coverage was targeted to be somewhat lower than the even wetter 01 UTC NBM to be more consistent with models/ensembles and to limit the WPC continuity change. 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. 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