Extended Forecast Discussion...amended NWS Weather Prediction Center College Park MD 353 AM EDT Fri Aug 18 2023 Valid 12Z Mon Aug 21 2023 - 12Z Fri Aug 25 2023 ***Hilary to bring continued excessive rainfall threat from southern California into the Great Basin on Monday*** ***Major heat wave expected across the Central to Southeast U.S. through the middle of next week*** ...Synoptic Overview... An impressive 600dm upper ridge will be anchored in place across the Midwest and Central Plains for the beginning to middle of next week with widespread excessive heat and humidity accompanying it. This ridge will likely start to decay some going into the end of the week and then gets displaced towards the south as shortwave energy impinges upon it from the north, but the heat and humidity will likely continue for much of the central and southern states. The other story making weather deadlines will be the anomalous surge of deep tropical moisture associated with the track of Hilary moving north across the Desert Southwest and extending to the California coastal areas, and then northward across much of the Intermountain West, with an elevated potential for flooding. Across the Northeast U.S., and upper level trough is expected to be in place through at least Wednesday and this will shield the region from the excessive heat that is expected across the Central U.S., and an increase of moisture across the northern Gulf will fuel more widespread coverage of showers and storms across southern Texas going into Tuesday. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite indicates good overall synoptic scale agreement across the forecast domain, with the highest overall confidence with the huge upper ridge over the Central U.S. early in the week, with multiple models indicating a rare 600dm contour over the Midwest. The main difference early on is the much faster CMC solution with the track of Hilary across southern California, but otherwise it fits well elsewhere across the nation. Looking ahead to the middle of the week, the CMC is displaced to the west of the model consensus across the Pacific Northwest with the position of the closed low, and it also extends the ridge axis from the Plains northward into south-central Canada. The GFS temperatures with the upcoming heatwave are likely too high across much of the Midwest and even across parts of the Great Lakes and Ohio Valley, with the ECMWF and ensemble mean temperatures closer to what is expected to evolve. The forecast track for Hilary brings a weakening tropical storm to southern California by early Monday morning, and quickly becomes a remnant low by Monday night. The WPC blend for the fronts/pressures incorporated a nearly deterministic model blend through early Wednesday, and then reducing the CMC and slowly transitioning to about 40% of the 18Z GEFS/12Z ECENS ensemble means for Thursday and Friday. ...Weather/Hazards Highlights... Heavy rainfall will continue to be a threat across central and southern portions of California on Monday, and also including portions of south-central Nevada, as the moisture plume steadily lifts north across this region. There is a Slight Risk of excessive rainfall for the Day 4 period Monday, with the majority of the rain expected from 12Z Monday to 00Z Tuesday. Some isolated 2-3 inch rainfall maxima are possible across portions of the central southern Sierra, and 1-2 inches for the Transverse Mountains. If the system slows down more than expected, a future upgrade to Moderate Risk may eventually become necessary in later updates. Farther to the north, tropical moisture will interact with a frontal boundary across the Northwest/northern Rockies providing enhancement of rainfall, and therefore a Slight Risk is in effect for Day 4 from eastern Oregon to northwestern Montana. Most areas farther east are forecast to remain mostly dry under the expansive upper ridge, with the exception of the western Gulf Coast as tropical moisture advects westward (with a Marginal Risk on Day 5 for southern Texas). A major heatwave is expected across much of the Midwest and extending across the central and southern Plains through about Thursday with the massive upper ridge in place and suppressing cloud development. Widespread 100+ degree high temperatures are expected from southern South Dakota and points southward, and some overnight lows may stay above 80 degrees, especially from central Texas to the Florida coast. These readings across the Midwest are on the order of 10-20 degrees above late August averages, with the core of the anomalies centered mainly over Iowa through mid-week. There will likely be some abatement to the later summer heatwave across the Midwest by Friday as the ridge is suppressed to the south. The opposite holds true from California to Nevada and western Utah with temperatures running 15 to 25 degrees below average owing to expansive cloud cover and widespread rainfall, perhaps setting some record low max temperatures across this region. Temperatures slowly moderate going into the second half of the week across this region, whilst still remaining slightly below average. Hamrick 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw