Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Tue Jul 16 2024 Valid 12Z Fri Jul 19 2024 - 12Z Tue Jul 23 2024 ...Dangerous heat and wildfire threats will build over much of the West this week into next week... ...Monsoonal conditions continue for the Four Corners states, and heavy rains are forecast for the South, Southeast, and Southern Mid-Atlantic... ...Overview... An amplified upper pattern will be highlighted by a strong ridge across the western U.S. and a trough over the east-central U.S. through the medium range period. An upper trough will work its way into the Pacific Northwest by early next week, but progress will be slow due to the strong downstream ridge. Dangerous heat will develop underneath the ridge, and temperatures may reach record values. In addition to dangerous heat, low humidity/dry fuels and breezy winds will result in an increased threat of wildfires by late week for portions of the Pacific Northwest. Better moisture will focus far to the south across the Southwest/Four Corners region and southern Rockies where monsoonal conditions will likely continue into next week. Underneath the trough in the east-central U.S., a slow moving frontal boundary will sag into the south-central U.S., Southeast, and Mid-Atlantic. This wavy and well defined boundary is forecast to stall and create a prolonged period of heavy rain that could result in flooding concerns given the deep moisture pooled along the boundary. Areas north of the boundary will see a break in the heat as cooler and drier air moves in from Canada. ...Guidance/Predictability Assessment... Model guidance remains in good agreement through the medium range period. A general model blend of the deterministic GFS/ECMWF/CMC/UKMET and the 13Z National Blend of Models was used to create the forecast for Friday through the weekend. Higher than normal agreement in the details has led to increased confidence during the first half of the period. Details become a bit more uncertain early next week, which is expected, so ensemble means were added to the forecast blend to smooth out smaller differences. ...Weather/Hazards Highlights... As the heatwave wanes in the East, a new heatwave will take hold of much of the West later this week. Heat will ramp up this weekend into early next week, 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. Per the Climate Prediction Center, significantly above-normal temperatures are forecast to persist across portions of the interior West into the week two period. 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 Day 4 and 5 (Friday and Saturday) Excessive Rainfall Outlooks. The Marginal Risk area also includes portions of the southern/central Rockies and Adjacent High Plains. In the central and eastern U.S., a well-defined, wavy surface front will slowly shift south, providing a focus for multiple rounds of heavy showers and thunderstorms each day through the weekend into early 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. A Marginal Risk area is in place in the Day 4 and 5 (Friday and Saturday) Excessive Rainfall Outlooks from the Gulf Coast states through the Southeast to the southern Mid-Atlantic, with an embedded Slight Risk area in the Carolinas on Day 4 (Friday). The risk of flash flooding will be highest in the Carolinas where favorable moisture, instability, and upper level jet dynamics will align. Soils in this region may also be saturated from forecast rainfall in the Day 1-3 period. Dolan/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