Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Aug 17 2023 Valid 12Z Sun Aug 20 2023 - 12Z Thu Aug 24 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... An anomalously strong/massive upper ridge/closed high across much of the central to east-central U.S. will bring a resurgence of excessive heat through much of next week. This should also keep shortwave energy north into southern Canada with a couple systems moving through the Northeast. A vigorous upper low will be exiting New England on Sunday, with the next one coming through early 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 increase on a significant heavy rainfall/high impact event to unfold and focus across parts of the Southwest and California beginning Saturday to at least Monday with a deep lead moisture plume and favorable ingredients associated with Hilary in the tropical east Pacific as per the latest from the National Hurricane Center. ...Guidance Evaluation/Predictability Assessment... There is good agreement on the large scale regarding the pattern over the CONUS for the Medium Range period (Sunday-Thursday) with plenty of lingering uncertainty in the details. Some disagreement on amplitude of southern Canadian systems and how much such systems may erode the north side of the upper ridge over the Upper Midwest/Great Lakes as well as systems dropping into the mean upper troughing out West. The latest track guidance from the National Hurricane Center tracks Hilary just off the Baja California coast on Sunday and into southern California on Monday (likely as a Post-Tropical system by then). There is still plenty of uncertainty in the guidance on the exact track, but regardless, confidence is high for significant rainfall to impact the Southwest and into California. The WPC blend for the forecast progs tonight used the deterministic models early in the period when agreement was better, transitioning to the ensemble means (with some GFS/ECMWF inclusion for added definition). QPF for Hilary was based largely on the 01z National Blend of Models with the GFS which was most consistent with NHCs track for Hilary. ...Weather/Hazards Highlights... Deep monsoonal/tropical moisture channeling northward ahead of West Coast upper troughing and on the western periphery of a downstream central U.S. upper ridge will keep 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 significant excessive rainfall threat and confidence/model agreement is high enough for a moderate risk on the Day 4/Sunday Excessive Rainfall Outlook for the Peninsular Range in far southern California where guidance also has heavy rainfall in the Day 3/Saturday period. A slight risk surrounds this region from far western Arizona into southern/central Nevada and south-central California. For Day 5/Monday, a slight risk was drawn across much of southern/central California into southern/central Nevada. There is a notable signal for considerable rainfall across the Transverse Ranges/vicinity but continued uncertainty in Hilary's eventual track precluded the inclusion of a moderate risk at this time. To the north, moisture will interact with a frontal boundary across the Northwest/northern Rockies providing some enhancement of rainfall to that region with broad marginal risks 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 around the middle of next week. 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 should also build back into the Southeast by the early to middle of next week. Elsewhere, upper troughing out West should keep highs generally 5-15 degrees below normal with parts of the Southwest 15-25 degrees below normal especially Sunday and Monday as a result of the increased cloud cover and heavy precipitation threat. 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