Extended Forecast Discussion NWS Weather Prediction Center College Park MD 302 AM EDT Sun Jun 23 2024 Valid 12Z Wed Jun 26 2024 - 12Z Sun Jun 30 2024 ...Hazardous heat possible for southern/central portions of the Plains, Mississippi Valley, and into the Southeast for parts of next week eventually building back into the Midwest next weekend... ...Overview... An upper level ridge will remain anchored over southwestern to south-central parts of the nation through the week, leading to multiple days of above normal to possibly hazardous heat. To the north, one shortwave through the northern tier will amplify somewhat as it crosses the Midwest/East mid- to late week. The next system reaching the West Coast around Thursday should push a defined surface low pressure system into the northern Plains by late week. A third shortwave looks to enter the West again next weekend. ...Guidance/Predictability Assessment... The latest suite of guidance continues to offer overall good agreement on the large scale, and typical uncertainty in the details that could have impacts on sensible weather/QPF. Models continue to offer better agreement on the first shortwave into the Northeast late week. 12z/18z guidance runs showed some greater timing differences with the next shortwave through the northern U.S. Friday-Saturday, but new 00z runs have trended closer together. Details on energy through south- central Canada reinforcing this trough as it moves into the Great Lakes and Northeast next weekend remain uncertain and would impact overall timing. With the next system into the West on Sunday, the CMC was a little flatter than the consensus, which resulted in less ridging/heat downstream into the central U.S. and Midwest. The WPC forecast for tonight used a blend of the GFS, ECMWF, CMC, and UKMET for the first half of the period amidst sufficient model agreement. Transitioned to 50 percent of the ensemble means in the blend (with the ECMWF and GFS) late period to mitigate some of the model differences which will take additional time to resolve. Overall, this maintained good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... A cold front moving into the East on Wednesday and copious moisture and instability in a typical summertime convective pattern will support widespread showers and thunderstorms from parts of the Lower Mississippi Valley into the Northeast. There remains enough model QPF discrepancies to preclude any sort of slight risks, and the front should be fairly progressive as well, limiting the overall threat. Still, given moisture and instability, a very broad marginal risk seemed reasonable for the Day 4 Excessive Rainfall Outlook. Another defined system will reach the northern Plains by Thursday bringing a renewed threat for heavy rainfall across the north- central Plains to Upper Midwest, and a broad marginal risk was included on the Day 5/Thursday ERO period to cover this. There was enough agreement in the guidance to place a slight risk from far eastern Nebraska, into Iowa, and far southern Minnesota - an area with very wet antecedent conditions and with additional rainfall expected in the short range period as well. A heavy rainfall threat may emerge with this system farther east into the Great Lakes and Northeast Friday and Saturday as well. Anomalous moisture and sufficient instability should persist over the Four Corners states, eventually expanding into the southern Rockies and parts of the High Plains much of the period and potentially lead to some locally heavy rainfall, which could cause flooding impacts particularly over burn scars. Marginal Risks are in the outlook for Day 4/Wednesday and Day 5/Thursday for this region. Though perhaps not everywhere in the quite broad Marginal risk area will see heavy rain, slow- moving convection is a concern for these areas and warrants the risk depiction in the ERO. Elsewhere, parts of the Southeast/Florida may also see episodes of diurnally enhanced convection, but overall dry conditions/high FFGs precluded any excessive rainfall risk areas at this time. The focus for hazardous heat by mid-week will be across the south- central Plains/Mississippi Valley and parts of the Southeast where a long duration heat wave will be ongoing and heat indices near 110F for some leading to widespread major to extreme HeatRisk. The heat across the Southeast should briefly moderate on Thursday with troughing moving through, but the upper ridge will build back into the region and northward into the Midwest bringing several days again of above normal temperatures (both daytime highs and overnight lows) from the southern Plains into the Midwest and parts of the Southeast. Above normal heat is also possible across the Southwest where temperatures 105-115F are likely through much of next week. 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