Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Wed Jul 17 2024 Valid 12Z Sat Jul 20 2024 - 12Z Wed Jul 24 2024 ...Dangerous heat and wildfire threats will build over much of the West... ...Multiple days of heavy rain are forecast for the Four Corners, South-Central U.S., Southeast, and Southern Mid-Atlantic... ...Overview... An amplified upper pattern with a strong ridge over the western U.S. and a trough over the central and eastern U.S. will persist through the medium range period. Dangerous, potentially record- setting, heat will develop underneath the ridge and persist through the weekend into next week. An upper trough will gradually nudge into the Pacific Northwest early next week, which should help reduce temperatures west of the Cascades, but interior areas will likely remain hot and dry. Cooler temperatures will also develop across much of the central and eastern U.S. underneath the upper trough and in the wake of a cold front. Multiple days of heavy rain are forecast for most of the southern tier of the nation. Monsoon conditions will persist in the Four Corners region and southern Rockies, and a slow-moving frontal boundary will stall across the south-central U.S., Southeast, and southern Mid-Atlantic. This prolonged period of heavy rain could result in flooding concerns. Tropical moisture will also work up into Florida early next week, enhancing daily shower/thunderstorm activity. ...Guidance/Predictability Assessment... Models and ensembles remain in good agreement through the medium range period. The WPC forecast blend was comprised of deterministic solutions from the 06Z GFS and the 00Z ECMWF/UKMET/CMC, with ensemble means from the GEFS/ECENS added to the blend for days 5 and 6. Excellent agreement among the models allowed for the majority of the blend to be deterministic guidance through the end of the period, which helped maintain detail of individual features in the forecast. Slightly higher weights were applied to the ECMWF to depict a slightly more robust and resilient western U.S. upper ridge amplitude over time. ...Weather/Hazards Highlights... Dangerous heat will ramp up this weekend over much of the West, with Major to Extreme HeatRisk values expected for many interior locations. High temperatures in the 90s and 100s will be common, and warm overnight lows will only provide limited relief. If the current forecast holds, temperatures may reach record high values this weekend. Heat will wane slightly for western portions of the Pacific Northwest as an upper trough and surface cold front move into the region, but temperatures elsewhere in the West are forecast to remain well above average through at least mid-week. This prolonged period of dry heat could result in enhanced wildfire danger in the West as well. Meanwhile, monsoonal moisture will linger over the Four Corners region, producing daily rounds of precipitation. Isolated to scattered instances of flash flooding will be possible, especially near steep terrain and burn scars. A Marginal Risk remains in place for this region in the WPC Day 4 and 5 (Saturday and Sunday) Excessive Rainfall Outlooks. The Marginal Risk area also includes portions of the southern/central Rockies and adjacent Plains. In the central and eastern U.S., a well-defined, wavy surface front will slowly meander, providing a focus for multiple rounds of heavy showers and thunderstorms each day through the weekend into early/mid next week. With ample moisture and instability in place, locally heavy rainfall will be possible and may cause instances of flash flooding, especially in urban and poor drainage areas and areas with repeat/training convection. Marginal Risk areas remain in place from the central/southern Plains through the Southeast to the southern Mid-Atlantic in the Day 4 and 5 (Saturday and Sunday) Excessive Rainfall Outlooks. Soils may be saturated in some areas from heavy rainfall in the short range period (days 1-3), which could enhance flooding potential as well. Additionally, an influx of tropical moisture into Florida early next week may help enhance daily showers and thunderstorms in the Southeast. Dolan 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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