Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 PM EDT Wed Jun 26 2024 Valid 12Z Sat Jun 29 2024 - 12Z Wed Jul 03 2024 ...Hazardous heat threats to focus across parts of the South this weekend and into early next week... ...Overview... An energetic upper level low/trough will traverse southern Canada and the northern tier of the nation and push a surface frontal system across parts of the Great Lakes and eastern U.S. this weekend. This system may bring a threat of at least locally heavy rainfall through portions of the Northeast/Mid-Atlantic back through the east-central U.S. into the south-central Plains. Meanwhile, an upper level ridge will build over the South and Southeast through early next week, and there will be an increased threat for hazardous heat for these areas under the ridge through much of the medium range period. Moisture feeding around the western periphery of this ridge also offers a wet period for the southern Rockies states. A second upper level low/trough will move into the Northwest late this weekend and push towards the Upper Midwest early next week. This system will likely bring a multi-day period favorable for heavy convective rains to parts of the Upper Midwest, particularly for Monday into Tuesday with slow progression. ...Guidance/Predictability Assessment... Model and ensemble forecast spread holds much lower than normal through medium range time scales in a pattern with above average predictability for mass fields and sensible weather/hazards. The WPC product suite was primarily derived from a blend of the 06 UTC GFS/GEFS mean and 00 UTC ECMWF/Canadian/ECMWF ensemble mean along with the compatible 13 UTC National Blend of Models and WPC continuity, albeit with more blend weighting applied to the models for better detail Saturday-Monday when shortwave timing issues are most manageable. A similar blend of latest 12 UTC guidance remains in line overall for most weather aspects. ...Weather/Hazards Highlights... As the frontal system initially over the Great Lakes progresses east by Saturday, precipitation chances will spread into the eastern U.S. and back into the Central Plains. Model guidance shows some signal for heavier rains across portions of the Northeast, but the progressive nature of the front should limit the overall flash flood threat. The day 4/Saturday Excessive Rainfall Outlook shows just a elongated marginal risk from the Northeast back into the Central Plains/Southwest. A weak surface low and slowing of the front through the Mid-Atlantic on Sunday could contribute to heavier rain potential across that region and so a marginal risk is highlighted on the Day 5 ERO for the eastern Mid-Atlantic/Carolinas. Another round of heavy rain is forecast to impact parts of the northern Plains and Upper Midwest and vicinity Monday into Tuesday as a second upper level low/trough moves across the northern tier. This system will require close monitoring given recent and forecast heavy rain, which could cause new flooding issues and exacerbate any ongoing flooding. Monsoonal type flow into the Southwest will continue the daily threat for showers and storms through much of the period. Marginal risks are in place across parts of Arizona and New Mexico for the days 4 and 5 ERO. Daily precipitation chances will also persist in the Southeast/Florida as above normal moisture lingers for the region. Hazardous heat potential will focus across the South and Southeast underneath a persistent upper ridge through the weekend into early next week. Several days of above normal temperatures (both daytime highs and nighttime lows) are forecast from the Central and Southern Plains eastwards, expanding with time into parts of the Midwest and Mid-Atlantic by mid next week. The northern tier states should be near or below normal with system passages, and some above normal temperatures may build back into parts of California and Nevada next Wednesday. Santorelli/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