Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM 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 produce 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. before feeding into a slow-to-depart closed low over the Northeast. There is a growing threat for New England coastal low development Wednesday-Thursday. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite of reasonably well clustered guidance from the 12 UTC ECMWF/UKMET/Canadian and the 01 UTC National Blend of Models (NBM) for Wednesday/Thursday. This solution has solid GEFS/ECMWF ensemble support for this time frame while the last few runs of the GFS have in particular not provided run to run consistent guidance for potential New England coastal system development and track. The 12 UTC ECMWF solution overall stays best clustered with the ensemble means and NBM Friday into next weekend, so used that blend to provide detail consistent with predictability amid slowly growing CONUS forecast spread. Targeted forecaster driven changes to NBM QPF were to mainly increase the QPF signature over the Upper Midwest through the period given dynamic support aloft/frontal passage and over the Northeast with the slow closed upper low and coastal storm threat. ...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 streams focuses 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 there while also increasingly spreading downstream over the Southern Plains. Elsewhere, mid-later week convective clusters should be efficent 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/weeeknd digging of shortwave energy and a slowly progressive lead frontal passage. 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