Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Thu Jun 27 2024 Valid 12Z Sun Jun 30 2024 - 12Z Thu Jul 04 2024 ...Hazardous heat threats to focus for parts of the South and vicinity this weekend and well into next week... ...Overview... The main weather pattern for next week over our fine nation should continue 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. The best chance for heavy rains will be across the Upper Midwest Monday- Tuesday, lingering through the holiday, 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... Model and ensemble forecast spread continues a trend of being 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 for the Sunday-Tuesday timeframe from a composite blend of the 06 UTC GFS and 00 UTC ECMWF along with the compatible 13 UTC National Blend of Models (NBM) and WPC continuity. Opted to switch the blend to best clustered guidance from the GFS/ECMWF and GEFS mean along with the NBM for Wednesday and the Fourth amid slowly growing, but still manageable timing variances. Newer 12 UTC guidance overall remains in line. It should be noted that the latest ECMWF ensemble mean was unavailable for this product cycle given computer issues. ...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. Guidance has reasonably trended less progressive with this feature early next week up over the western Atlantic. 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 also remains for 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. There is uncertainty in the details, but a fairly broad slight risk remains for the Day 5/Monday ERO, particularly given recent and forecast heavy rain, which could cause new flooding issues and exacerbate any ongoing flooding. The Storm Prediction Center also indicates a threat for severe weather to monitor. Ample showers and storms that offer local heavy downpours with runoff issue potential look to continue southward and eastward as well into the Mid- Mississippi Valley/Central Plains and Great Lakes/Ohio Valley Wednesday into Independence Day. Monsoonal type flow into the Southwest will continue the daily threat for showers and storms through much of the period. Marginal risks remain 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. This pattern is likely to persist into next Tuesday/Wednesday as well given the lingering and deepened moisture channel on the upper ridge western periphery. Daily precipitation chances will also persist in the Southest/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/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