Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 PM EDT Mon Aug 15 2022 Valid 12Z Thu Aug 18 2022 - 12Z Mon Aug 22 2022 ...Excessive heat threat from interior California through the Northwest/north-central Intermountain West this week... ...Monsoonal heavy rain threat continues for the Southwest and south-central Great Basin/Rockies.... ...New England coastal storm threat into Thursday... ...South/Southeast heavy rain threat through this weekend... ...Guidance Evaluation/Predictability Assessment... For the most part, the latest suite of models and ensembles remain in agreement on the larger scale pattern which features troughing over the east reinforced by a deep shortwave/closed low through the Midwest late this week, and amplified ridging over much of the West and Central U.S.. There are some continued small scale differences including timing of various features, particularly late period. The ECMWF is faster with the shortwave across the Midwest this weekend, while the CMC is quicker to cut the energy off from the main trough and hang it back into the mid-upper Mississippi Valley early next week. Out West, the CMC is notably faster with the next trough into the West Coast around Sunday-Monday. For both systems, the GFS and ECMWF seem better aligned with the ensemble means. Accordingly, the WPC forecast used a composite blend of the deterministic guidance 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 as well. The WPC QPF was based heavily on the 13 UTC National Blend of Models which seemed reasonable across all areas through the period, amidst fairly good forecast spread. Some targeted edits were made across higher impacted regions, including the Southwest and southern tier. ...Pattern Overview and Weather/Hazards Highlights... Amplified upper ridging over the West/Northwest this week will support a heat wave with daytime/overnight temperature 10-20F above normal. These temperatures are likely to produce some record daytime and overnight values. 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 in conjunction with small-scale shortwave energy to produce maily diurnally driven rain and thunderstorms. A WPC experimental medium range Slight Risk of excessive rainfall was maintained for southern Arizona for Thursday, and expanded into central/eastern Arizona and western New Mexico on Friday given enhanced QPF/ARI and an overall persistent favorable pattern. Convective activity will persist through later this week/weekend near a slow moving and wavy front across the South/Southeast within a very moist environment. The highest threat of heavy rain may be over the lower Mississippi Valley where WPC has an experimental medium range Slight Risk of excessive rainfall on Thursday/Thursday night. Organized rains will also slowly lift to the Mid-Atlantic this weekend as the front begins to lift slowly north as a warm front. To the north, there is a lingering threat for cooled wrap-around rains/enhanced winds over especially northern New England Thursday with coastal cyclogenesis ahead of a closed upper low. The deepened coastal low then departs for the Canadian Maritimes with ejection of the closed upper system. Expect some degree of organized convection to spread across the Midwest with late week/weekend digging of shortwave energy and a slowly progressive lead low/frontal passage. Elsewhere, models continue to show an increasing heavy rainfall signal across the south-central Plains to the mid-Mississippi Valley over the weekend. This seems reasonable as moisture and instability pool with return flow into a slow moving front in favorable upper diffluent flow aloft. Santorelli/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