Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sat Jul 27 2024 Valid 12Z Tue Jul 30 2024 - 12Z Sat Aug 03 2024 ...Hazardous heat to shift from the south-central Plains early period to the Northwest later next week... ...Overview... Guidance continues to show the pattern evolving toward a strengthening southern Rockies/Plains upper ridge which will build back and amplify across the West later next week. This should help set up a prolonged period of hot weather over many areas with the most anomalous heat expected over central parts of the Plains early to midweek. Warmer than normal temperatures are forecast to expand north toward the U.S./Canadian border later in the week, despite shortwave energy moving through the Midwest and eventually developing mean troughing over the eastern U.S. that will support a broad area of showers/thunderstorms drifting across the eastern half of the country. ...Guidance/Predictability Assessment... Model guidance is at least agreeable on the large scale regarding the initially southern tier ridge shifting and building over the West later next week. The main model uncertainties are with the pattern to the north, with timing and strength differences of shortwaves even early in the forecast period. The model spread in the details made it difficult to determine outliers, but thought the 00Z CMC was a bit fast with an initial Great Lakes shortwave and a Northwest shortwave into Tuesday, so favored other guidance instead. Then by the latter part of the week, the 00Z ECMWF (and the newer 12Z ECMWF as well) were on the slower side of the envelope with a north-central U.S. shortwave and a surface low/frontal system. Even the ECMWF-based AI/machine learning models were faster, so this provided confidence to lean away from the slower operational EC solution. Another feature to watch by the end of the period is the potential for a tropical cyclone to approach the forecast area. Models have shown ample spread in the placement and even the existence of this low. The deterministic ECMWF and the EC ensemble members have been showing it most consistently. Operational runs have been tracking the tropical low to the east of Florida but some of its ensemble members take it west of Florida into the Gulf of Mexico. Recent GFS runs had been showing a tropical low as well (into the Gulf), until the 12Z GFS drops it completely. The EC-based AI/ML models vary from not having a low, to having a weak one on either side of Florida, while the AIFS is similar to the operational ECMWF. CMC runs are slower but show a Gulf low. Overall, something to watch but low confidence of what ends up occurring. The WPC forecast used a deterministic blend early in the period, with less weighting for the CMC and eventually eliminating the ECMWF going forward in time, in favor of the GEFS and EC ensemble means. ...Weather/Hazards Highlights... A wet pattern is likely from the Midwest to the Eastern Seaboard next week with ample moisture and instability in place as well as mean troughing aloft and surface frontal systems for scattered thunderstorms. Thus the Days 4-5 Excessive Rainfall Outlooks covering Tuesday- Wednesday night show fairly broad Marginal Risks across parts of the Midwest to the Northeast. Models continue to show a focus for potentially multiple rounds of storms affecting eastern parts of the Ohio Valley into the central/southern Appalachians, which is also an area with relatively low Flash Flood Guidance, so an embedded Slight Risk remains in place there for Tuesday. By Wednesday, model guidance has trended drier in that region, so removed the Day 5 Slight Risk from the Appalachians that was in the previous issuance. However, model guidance is keying in on a likely mesoscale convective system with heavy rain rates moving on the cusp of the instability gradient in northwest mean flow that should affect parts of the Midwest during that period. There is some placement spread with this, but the antecedent wet conditions for much of the area and the heavy rainfall potential seems to warrant a Day 5/Wednesday Slight Risk across the Midwest, though the area will likely be adjusted in future forecasts if/when models converge. Much of the north-central U.S. to the eastern third of the country should remain generally unsettled the rest of the week as a upper trough is slow to budge out of the region while it generally weakens. Elsewhere, monsoonal convection over the Southwest should stay confined to southern areas through midweek, and Marginal Risks remain reasonable for the Days 4-5 EROs. Chances for convection may expand northeastward into later week depending on the shape of the southern Rockies/Plains ridge and upper level energy rotating around its western periphery. Some rain is forecast for the Northwest into Tuesday with an initial shortwave. Also on Tuesday, above normal precipitable water values should lead to widespread rain/storms across the Florida Peninsula, so a Marginal Risk still looks on track there for the Day 4 ERO before moisture lessens midweek. The forecast pattern evolution will support an expanding area of warmer than average temperatures across the Lower 48. The most significant high temperature anomalies should be over the Central Plains early on where some locations should see multiple days with highs 10-15F or so above normal and high temperatures broach 105F. The experimental HeatRisk likewise reflects an expanding area of Moderate to Major risks of heat-related impacts through next Wednesday, and even some pockets in the Extreme category in the south-central Plains. An incoming front will cause some easing of the heat late next week in that region. Much of the West will likely see near to slightly below normal highs through the first half of next week before the strengthening ridge pushes temperatures above normal by next Thursday, and more significantly so next Friday into Saturday with lower elevations seeing 100F+ high temperatures. The Ohio Valley to Mid-Atlantic and Northeast should also trend warmer with time next week. Tate/Santorelli 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