Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 PM 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 through the central U.S. with some slowing of the boundary likely across the East. 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... The latest model and ensemble guidance continues to show above normal clustering through the forecast period with the overall large scale pattern. There remain some detail differences, mainly late period, with an upper low off British Columbia which may bring some shortwave energy into the Pacific Northwest, and also shortwave energy through the broader trough over eastern Canada which may skirt the Great Lakes/Northeast. These are details that will need more time to resolve. The WPC forecast for today used a blend of the deterministic guidance the first half of the period, increasing to fifty percent of the ensemble means late period to mitigate the above differences. Overall, maintained good consistency with the previous WPC forecast. ...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 front will shift south and eastward across much of the central and eastern U.S. leading 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 the Northeast but the front should be fairly progressive limiting the overall flash flood risk . A Marginal risk continues Day 5/Thursday into the Lower Mississippi/Tennessee Valleys and into parts of the Southeast and Mid-Atlantic. An embedded slight risk was included in the update this afternoon across southeast Virginia and eastern North Carolina where the frontal boundary will be slower. The boundary may eventually stall across the region into the weekend providing focus for daily summertime convection and local heavy rain threats across the Southeast and Florida. 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.. 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. Santorelli/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