Extended Forecast Discussion NWS Weather Prediction Center College Park MD 318 AM EDT Sun Jul 14 2024 Valid 12Z Wed Jul 17 2024 - 12Z Sun Jul 21 2024 ...Hazardous Heat lingering over The South and East midweek then eases with heavy rain/convective frontal passage. Heatwave to robustly rebuild and persist this week over the West... ...Overview... Upper trough development over the eastern/northeastern Pacific this week will lead to the building of an amplified upper ridge over western North America. This sets the stage for a protracted period with widespread Hazardous Heat to include varied record values as the supporting ridge should prove slow to erode. Ample energies digging to the lee of the western ridge will carve out a mean upper trough position over the east-central U.S. as a well defined and wavy cold front acts as a elongated focus for heavy rainfall/runoff threats sinks down through the region. The last vestiges of ongoing widespread above normal to hazardous temperatures will significantly moderate over The South and East after midweek to end the heatwave with frontal passage. ...Guidance/Predictability Assessment... Model and ensemble forecast clustering through medium range time scales is better than normal, bolstering forecast confidence. A composite of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models seems to offer a solid forecast basis Wednesday-Friday with detail consistent with a pattern with above normal predictability. Opted to introduce some GEFS/ECMWF/Canadian ensemble input into the blend at longer time frames through next weekend amid slowly growing forecast spread. This strategy maintains excellent WPC product continuity and is in line with a composite of newer 00 UTC cycle guidance. ...Weather/Hazards Highlights... Monsoonal moisture will linger over the Four Corners region, which will allow for daily rounds of precipitation that could result in flash flooding, especially near steep terrain and burn scars. A Marginal Risk of Excessive Rainfall (ERO) remains planned for Day 4 (Wednesday) and Day 5 (Thursday) from Arizona to the southern Rockies/High Plains in a region with favorable upper diffluence. Favorable upper level support and a well defined/wavy surface fronts remain set to shift southward and eastward across much of the central and eastern U.S. will lead to multiple rounds of heavy showers/thunderstorms. With ample moisture and instability in place, locally heavy rainfall will be possible and may lead to instances of flash flooding. A Day 4/Wednesday ERO Marginal Threat extends from the Central Plains/Mid-South to the Ohio Valley and Mid-Atlantic/Northeast as the main front shifts southeastward. A Marginal risk continues Day 5/Thursday into the Lower Mississippi/Tennessee Valleys and down across the Mid-Atlantic and toward the Southeast. Heavy precipitation chances will further progress southward later week as this front settles south, and daily precipitation chances will persist in Florida with summertime convection from sea breeze convergence. Locally heavy rain will be possible in these areas with heavy thunderstorm activity. The ongoing heatwave with record values should finally moderate over the South and East after midweek. Hazardous heat should slowly suppress south and eastward with time as amplified troughing settles into the east-central U.S. and leads to frontal passage. Meanwhile upstream, the building of an amplified upper ridge over western North America will lead into an episode with widespread Hazardous Heat over much of the West next week to include varied record values as the supporting ridge should prove slow to erode. 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