Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Aug 14 2022 Valid 12Z Wed Aug 17 2022 - 12Z Sun Aug 21 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 Wednesday/Thursday... ...Overview... Upper ridge building over the West/Northwest will support an excessive heat pattern over the region this week. Farther south, the persistent ridge and less certain embedded impulses will also continue to support widespread monsoonal moisture streaming into an unsettled Southwest and south-central Great Basin/Rockies. The downstream digging of shortwaves/height falls to the lee of the ridge will periodically reinforce upper diffluent flow down over the central U.S. to focus periods of heavy rain/thunderstorms most likely down the lower Mississippi Valley near and south of a wavy front. Across New England, there continues to be a threat of coastal cyclogenesis Wednesday-Thursday where model uncertainty remains high. ...Guidance Evaluation/Predictability Assessment... Model ensemble means remain quite agreeable with one another on predicting the relatively amplified upper air pattern across the mainland U.S. through next weekend. A slow-to-depart closed low over the Northeast midweek continues to be a feature where models indicate the greatest amount spread and run-to-run variability. There continues to be the potential for the closed low to interact with Atlantic moisture passing over the Gulf Stream to trigger coastal cyclogenesis. Rainfall amounts are highly sensitive to the intensity and track of the cyclone as well. Across the mid-South, models are in relatively good agreement on the placement of an area of enhanced rainfall ahead of a low pressure wave and a stationary front. Some north-south model differences were noted regarding the placement of the rainfall axis from the Deep South to the East Coast late this week. For the monsoonal rains over the West, models have generally been indicating a slow decreasing trend on the QPF through the end of the week. Finally, uncertainty in the amplitude of the next shortwave dipping into the northern Plains continues to result in some model spread regarding the associated timing and placement QPFs later this week. The WPC medium-range forecasts were composed based on a consensus of 40% from the 06Z GFS/GEFS, 40% from the 00Z EC/EC mean, and 20% from the 00Z CMC/CMC mean, together with the 13Z NBM, transitioning to mostly a consensus of the ensemble means by Day 7. ...Weather/Hazards Highlights... As the upper ridge expands over the West/Northwest this week, expect 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. A WPC experimental medium range Slight Risk of excessive rainfall is in place for the south-central Rockies/High Plains through midweek as additionally supported by digging shortwave energy/upper diffluence and terrain lift. In this pattern, convective activity will persist through later week/next weekend near and south of a slowly progressive cold front across the Deep South to the Southeast, with the highest threat of heavy rain down the lower Mississippi Valley. The enhanced rainfall is forecast to slowly work its way across the East Coast by next weekend with a coastal wavy front. Elsewhere, mid-later week convective clusters should be efficient rainfall producers within the deeply moist precipitable water values of the Lower Mississippi Valley/Gulf Coast states given upstream history and thunderstorms will also spread and organize across the Southeast with wavy and slow moving frontal boundaries in a moist pooled airmass underneath the favorable right entrance region of the upper jet. Meanwhile, a threat of a multi-day rain and wind event is increasing for much of the coastal Northeast, from New Jersey up to Maine Wednesday into Thursday with coastal cyclogenesis ahead of a closed upper low. Periods of heavy wrap-around rains are also a distinct possibility over eastern New England. There is also a developing opportunity for organized convection from the Dakotas/Upper Midwest to the Great Lakes with the late week/weekend digging of shortwave energy and a slowly progressive lead frontal passage. Kong/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