Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 AM EDT Tue Aug 16 2022 Valid 12Z Fri Aug 19 2022 - 12Z Tue Aug 23 2022 ...Excessive heat threat 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 south-central Plains and Mid-Lower Mississippi Valley this weekend into early next week... ...Guidance Evaluation/Predictability Assessment... Models and ensembles remain in agreement on the larger scale upper pattern over the next week as highlighted by troughing over the East reinforced by a deep shortwave/closed low through the Midwest, and ridging with weaknesses from parts of the West to the central U.S.. There are some continued small scale differences including timing/amplitude of various features, particularly late period. A composite model blend seems to decently mitigate these differences consistent with predictability through the weekend. At longer time frames, the GFS and ECMWF overall continue to seem better aligned with the ensemble means. Accordingly, the WPC product suite was primarily derived from a composite blend of the GFS/ECMWF/UKMET/Canadian days 3-5, with a blend of the ensemble means with GFS/ECMWF days 6-7. This maintains good continuity with the previous WPC forecast. ...Pattern Overview and Weather/Hazards Highlights... Upper ridging over the interior Northwest will extend the heatwave into the weekend with daytime/overnight temperatures 10-15F above normal. These temperatures are likely to produce some record values, mostly for overnight temperatures. Meanwhile, below normal highs are expected in the east-central U.S. behind a main front. Given rainfall and cloudiness over the Southwest and south-central Great Basin/Rockies, maximum temperatures there should also tend to be below normal. Meanwhile, a large swath of the West has seen persistent monsoonal moisture over the past couple of months, and this week will continue that pattern 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. WPC experimental medium range Slight Risks of excessive rainfall are issued for 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. Even so, the widespread NBM volumetric/areal coverage of heavy rainfall seems a bit overdone overall compared to models and ensemble members. 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 somehwat overdone compared to models and ensemble members. Elsewhere, convective activity will persist mainly through later this week/weekend near a slow moving and wavy front that will persist across the South/Southeast. Pooled high precipitable water values support runoff issues with heavy local downpours. 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. Local NBM QPF has seemed too light with this system over the past few days, but continues to gradually trend wetter. 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