Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 PM EDT Tue Jul 23 2024 Valid 12Z Fri Jul 26 2024 - 12Z Tue Jul 30 2024 ...Hazardous heat to shift into the northern Plains... ...Showers/storms with locally heavy rainfall expected over the Four Corners and from the western Gulf Coast into the southern Mid-Atlantic/Southeast... ...Overview... In the big picture, the latest forecast guidance continues to show the evolution of the large scale pattern moving toward a typical summertime pattern. This will feature main weather system tracks across the northern tier of the U.S. and southern Canada. Mean troughing is expected to persist over the West Coast while a building/merging ridge over the Southeast into the Southern Plains will hold strong. This type of weather pattern will bring anomalous heat to the northern Plains while excessive rainfall is possible along the Gulf Coast into the Southeast as well as the monsoonal moisture areas over parts of the West. ...Guidance/Predictability Assessment... Today's model guidance showed relatively good agreement through the period, with the best clustering for the western U.S. troughing that is expected to persist for much of the period. Meanwhile, while the models, including the ensembles and Machine Learning (ML) guidance, show a building ridge over the southern/southeast U.S. by Day 5-7, the exact placement/strength and amplitude of it is still a bit uncertain and models do show some variability with. The models are slow/sluggish to dissipate/evolve the bagginess and weak vort over the Central U.S. early on in the period as well as a lingering trough off the East Coast. By early next week though, there is reasonably good agreement that the ridge over the Southern U.S. reasserts itself and perhaps centers over the S. Plains areas while the northern tier experiences a fast parade of storm systems, likely to keep some level of troughing from the northern Plains into the western/northern Great Lakes. Forecast confidence began the period above average to average but by Day 7 settled on mostly average given the typical summertime pattern and variability. The WPC model blend preference included mostly ECMWF/GFS/UKMET/CMC at the beginning (Day 3) but incorporated a fair amount of ensemble means by Day 5 through Day 7. ...Weather/Hazards Highlights... The threat of excessive rainfall will continue over the course of the period, with the Day 4 Excessive Rainfall Outlook featuring a Slight Risk over portions of the coastal and Piedmont regions of the Carolinas, where the combination of additional heavy rainfall over wet soil/ground conditions could result in scattered instances of flash flooding. Elsewhere, a deep moisture plume will exist across portions of Texas Gulf Coast through the Southeast U.S. Additionally, a continuation of monsoonal moisture will bring a localized flash flood threat to much of the Four Corners region into portions of the Intermountain West. Across portions of southern Arizona into New Mexico, forecast rainfall amounts and the amount of moisture/instability may necessitate a future upgrade to Slight Risk, but confidence on exact placement of heavy rainfall and potential rain rates wasn't high enough to introduce at this time. On Day 5 (Saturday 12Z to Sunday 12Z), broad Marginal Risks (Level 1 of 4) exist for portions of Four Corners/Rockies, much of the Southeast and Mid Mississippi Valley and across the far northern Plains where a passing weather system may lead to robust convection and locally heavy rainfall. Temperatures will generally be above normal (5-10F) across much of the northern tier, especially the northern Plains where the experimental HeatRisk is showing a large area of Moderate to Major areas (Level 2-3 of 4) Friday through Sunday. The expected temperature anomalies would yield highs in the upper 90s to low 100s. Some of this heat should spread eastward across the Great Lakes/Northeast by Sunday- Tuesday, with highs 5-10F above normal (upper 80s to around 90). Much of the West will likely see near to slightly below normal highs through the period, with the exception of the Southwest which should see some lingering above normal readings through Friday and possibly Saturday. The wet pattern across the southern tier late this week into the weekend will keep most areas below normal for highs, especially southern/eastern Texas. The rest of the southern tier should trend closer to normal by early next week while southern Texas may remain a bit on the cool side. Rausch/Taylor 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