Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM 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 this week. ...New England coastal storm threat into Thursday... ...South/Southeast heavy rain threat this week. ...Guidance Evaluation/Predictability Assessment... Models and ensembles remain quite agreeable on predicting the amplified upper air pattern across the mainland U.S. over the next week. Ample smaller scale differences remain, but overall forecast spread seems to have decreased with recent guidance runs. Accordingly felt comfortable using a composite blend of the GFS/ECMWF/UKMET/Canadian and GEFS/ECMWF ensemnble means along with the National Blend of Models (NBM). ...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 have 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 rain and thunderstorms, particularly in the afternoon and evening hours. Issued a WPC experimental medium range Slight Risk of excessive rainfall for southern Arizona for Thursday and Friday given enhanced QPF/ARI and overall persistent favorable pattern. The 01 UTC NBM has significantly trended rain amounts upward across the Southwest, but to mitigate some of the continuity change WPC QPF also included a third of WPC dayshift continuity based from yesterdays 13 UTC NBM. Convective activity will persist through later week/weekend near a slow moving and wavy front across the South/Southeast within the deeply moist precipitable water values. 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 Thursday into Friday. Targeted an increase in NBM QPF there Friday into Saturday. Organized rains will also slowly lift to the Mid-Atlantic this weekend with wavy front return flow. There is 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 organized convection will spread across the Midwest with late week/weekend digging of shortwave energy and a slowly progressive lead low/frontal passage. NBM QPF has trended upward, but was once again increased across this broad region given upper support and frontal/low passage. Increasingly organized convective rains are slated to spread out 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. However, given increased uncertainties at these long forecast time frames, opted to mitigate some of the significantly increased 01 UTC NBM QPF areal coverage/amounts. 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