Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Thu Jul 18 2024 Valid 12Z Sun Jul 21 2024 - 12Z Thu Jul 25 2024 ***Heat wave continues across the West, and showers and storms with heavy rainfall expected from the Four Corners to the Southeast*** ...General Overview... It remains the case that an amplified upper pattern with a strong ridge over the western U.S. and a broad upper trough over the Plains and Midwest will persist through next Thursday. Dangerous, potentially record-setting, heat will develop underneath the western ridge and persist through the weekend and into next week. Monsoon conditions will persist across the southern Rockies, and a quasi-stationary front will likely persist across the south- central U.S. to the southern Mid-Atlantic. A multi-day pattern with scattered storms with heavy rainfall is forecast for much of the southern tier of the nation as a result. Tropical moisture will also work into Florida early next week to enhance daily shower/thunderstorm activity. ...Guidance/Predictability Assessment... The 12Z model guidance suite maintains generally above average model agreement for the Sunday through Thursday time period, with a general deterministic model blend sufficing as a starting point in the forecast process. There are some modest differences that emerge towards the middle of next week, with the ECMWF displaced some to the northwest with the trough axis across the Midwest and the CMC slightly more amplified, but still good enough to have about 20% each of the ECMWF/CMC/GFS and about 30-40% of the ensemble means. ...Weather/Hazards Highlights... Dangerous heat will ramp up this weekend over much of the West, with major to extreme Heat Risk 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 by 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 next week. This prolonged period of dry heat could also result in enhanced wildfire danger. 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 (Sunday and Monday) Excessive Rainfall Outlooks. A Slight Risk area is now planned for the northern New Mexico Rockies for Day 4 (12Z Sunday-12Z Monday) owing to heavy rainfall expected in the 72-hour time period prior, and better model agreement for higher totals across this region. The Marginal Risk area also includes portions of the southern and central Rockies and adjacent High 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 and well into 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 repeated/training convection. Marginal Risk areas remain in place from the central/southern Plains through the Southeast to the southern Mid-Atlantic in both the Day 4 and 5 (Sunday through Monday night) Excessive Rainfall Outlooks. A Slight Risk area is now planned for the Day 5 period across portions of central Texas where the 12Z deterministic model signal is now in better agreement on an axis of enhanced 2-4+ inch totals, and there will likely be some noteworthy rainfall in the same general area the day prior. Additionally, an influx of tropical moisture into Florida early next week may enhance daily showers and thunderstorms in the Southeast. Hamrick/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 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