Extended Forecast Discussion NWS Weather Prediction Center College Park MD 338 PM EDT Wed Aug 16 2023 Valid 12Z Sat Aug 19 2023 - 12Z Wed Aug 23 2023 ...Significant Southwest U.S./California Excessive Rainfall/Runoff Threat fueled by deep tropical moisture in advance of Hilary... ...Heat wave expanding across the central to east-central U.S. this weekend through early-mid next week... ...Overview... A very active weather pattern is set to emerge. A vigorous shortwave with embedded closed low will shift out of the New England this weekend as an anomalously strong/massive upper ridge/closed high expands across the central to east-central U.S. bringing a resurgence of excessive heat through at least the middle of next week. Troughing over the West coast looks to remain intact through the period renewed by shortwaves out of the Gulf of Alaska and into southwest Canada. Confidence continues to to increase on a heavy rainfall and potentially high impact event to unflold and focus across parts of the Southwest and California Saturday to Monday associated with a deep lead moisture plume and favorable ingredients in advance of the forecast approach of extratropically transitioning Hurricane Hilary out from the tropical east Pacific as per the latest from the National Hurricane Center. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from the 01 UTC National Blend of Models along with added information from WPC/NDFD continuity and the most compatible GEFS/ECMWF ensemble means whose composite seems to provide a reasonable forecast basis across much of the nation, but in particular seems most in line with the latest NHC Hilary forecast. ...Weather/Hazards Highlights... Deep monsoonal/tropical moisture channeling northward ahead of East Pacific/West Coast upper troughing and on the western periphery of a downstream central U.S. upper ridge will increasing make much of the interior West then northern Rockies wet this period. Heaviest rainfall amounts will especially focus over parts of the Southwest U.S./southern California, but remain locally uncertain as dependent on the ultimate track of Hilary. However, models do continue to trend towards a signficant excessive rainfall threat and confidence/model agreement is high enough to maintain a slight risk across across portions of southeast California, southern Nevada, and western Arizona on both the days 4 and 5 (Saturday and Sunday) WPC Excessive Rainfall Outlooks. Heavy rainfall may also lift across south-central California as well into early next week. To the north, moisture will interact with a frontal boundary across the Northwest/northern Rockies providing some enhancement of rainfall to that region. Broad marginal risks are depicted on the Excessive Rainfall Outlooks. Much of the rest of the country looks to remain mostly dry under the expansive upper ridge, with the exception of Florida and the Gulf Coast as tropical moisture shifts westward across Florida and the Gulf of Mexico. This could also bring some rainfall into southern Texas next TuesdayWednesday. A heat wave will build over the southern and central U.S. into the Midwest with daytime highs 10-20 degrees above normal in some places. This equates to daytime highs near 100 degrees possible as far north as the Upper Midwest states, with daily record highs possible, and some locations in the Midwest seeing their hottest day of the year so far. Humidity levels will send heat indices into the 100s for many. See key messages issued by WPC for additional details and graphics related to this heat wave. Heat may also build back into the Southeast by the middle of next week, and possibly beyond per the latest forecasts from the Climate Prediction Center. Elsewhere, upper troughing out West should keep highs generally 5-15 degrees below normal with parts of the Southwest 15-20 degrees below normal especially Sunday and Monday as a result of the increased cloud cover and heavy precipitation threat. Schichtel/Santorelli 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