Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM 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 and into next week... ...Monsoonal conditions continue for the Four Corners States; and heavy rains 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 to 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 later week for portions of the Pacific Northwest in particular. Better moisture will be located far to the south with focus 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 broad pooled plume of high precipitatble water air. Broad areas north of the boundary will see a break in the heat as cooler and drier air comes from Canada. ...Guidance/Predictability Assessment... Guidance spread through medium range time scales again seems lower than normal. A blend of best clustered guidance from the 18 UTC GFS and especially the 12 UTC ECMWF/UKMET along with the 01 UTC National Blend of Models seems to offer a solid forecast basis Friday through the weekend with reasonable detail consistent in a pattern with above normal predictability. Recent differences over the east/northeast Pacific have eased. Opted to add GEFS/ECMWF ensemble mean data into the blend to replace the UKMET that is not available at these longer time frames. WPC product continuity is well maintained and in line with newer 00 UTC guidance composites. ...Weather/Hazards Highlights... A big weather story will be the building and persistence of a major heatwave over much of the West this week and into next week. The NWS HeatRisk product is showing widespread Moderate to locally Major to Extreme HeatRisk values developing mid-week and strengthening this weekend, and record-setting temperatures will be possible. This heat wave is forecast to continue into the week two period, per the Climate Prediction Center outlook, and hot temperatures could result in a prolonged period of enhanced wildfire danger as well. Meanwhile, 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 remains in the Excessive Rainfall Outlook (ERO) for Day 4/Friday and Day 5/Saturday from the Four Corners region to the south-central Rockies/Plains with favorable upper diffluence, instability, and moisture. Favorable upper level support and a well defined/wavy surface front will shift southward across 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. The Day 4/Friday and Day 5/Saturday EROs show a slowly southward shifting Marginal Risk area from east Texas to the South/Southeast and southern Mid-Atlantic. An embedded Slight Risk area introduced for Day 4/Friday focuses the threat across the eastern Carolinas given pooled high precipitable water values in a pattern with highly favorable upper jet right entrance region support. There is also a risk of short range heavy rains to provide lead wet soils. The wavy boundary will meander over the region this weekend into next week, providing focus for daily summertime convection and local heavy rain for the South/Southeast and Mid-Atlantic. 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