Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 AM EDT Thu Jun 27 2024 Valid 12Z Sun Jun 30 2024 - 12Z Thu Jul 04 2024 ...Hazardous heat threats to focus across parts of the South this weekend and into next week... ...Overview... Medium range period continues to feature a seemingly repeating pattern of active and progressive northern stream shortwaves through the northern states as a strong ridge meanders over the South, eventually building back into the East and West Coasts. Best chance for heavy rains will be across the Upper Midwest Monday- Tuesday, and across the Southwest with a continuous moisture feed around the western periphery of the ridge. Hazardous heat threats will focus across much of the southern Plains to Southeast most of the period, with some threat building into parts of the Southwest and interior California by mid to later next week. ...Guidance/Predictability Assessment... Models and ensemble guidance continues to show above normal agreement through most of the period for the overall large scale pattern, with some uncertainty in the details of individual systems which would have larger impacts on sensible weather/QPF. A general model blend seemed to suffice as a starting point for now for the WPC forecast tonight. Greater uncertainties begin to emerge by Wednesday- Thursday next week with a building ridge over the Western U.S.. The ECMWF is strongest with this, which holds a closed low back over the Gulf of Alaska longer. The CMC is weakest with the ridge which allows that low to drop south and its associated energies to impact the Northwest. Obviously this has bigger implications on potential heat threats out West late period. Ensembles and Machine Learning models suggest a trend towards ridging (weaker than the ECMWF), so the late period WPC forecast was based more on the ECMWF and GFS along with ensemble means. Overall, maintained good continuity with the previous WPC forecast through mid next week. ...Weather/Hazards Highlights... A weak surface low and slowing of a front exiting the East Coast on Sunday could contribute to heavier rain potential across portions of the Mid-Atlantic/Carolinas. A marginal risk remains in place for the Day 4/Sunday Excessive Rainfall Outlook. Showers and storms should also focus along a boundary back into the central and northern Plains ahead of the next system. Given some increased sensitivity and soil moisture, a marginal risk was added to the Day 4 ERO across parts of the northern Plains as well. Heavy rain potential should increase in coverage for the Upper Midwest and vicinity Monday into Tuesday. Lots of uncertainty still in the details, but a fairly broad slight risk was included for the Day 5/Monday ERO particularly given recent and forecast heavy rain, which could cause new flooding issues and exacerbate any ongoing flooding. Showers and storms look to continue eastward into the Great Lakes/Midwest Tuesday-Wednesday as well. 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, with potential for upgrades to a slight in subsequent updates depending on short range rainfall. 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 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/heat may build back into parts of California towards the end of the period. 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