Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 AM EDT Thu Jun 29 2023 Valid 12Z Sun Jul 02 2023 - 12Z Thu Jul 06 2023 ...Major heatwave from Texas through the mid-lower Mississippi Valley to the Southeast to ease next week... ...Hot temperatures from the Desert Southwest to Interior California... ...Excessive rainfall threat from the Ohio Valley to the north-central Appalachians Sunday/Monday... ...Overview and Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived through Independence Day from a blend of reasonably well clustered mid-larger scale guidance of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with the 01 UTC National Blend of Models. This solution has good overall ensemble support, albeit with ample local convective focus differneces. Opted to then replace the UKMET/GFS with the more compatible GEFS/ECMWF ensemble means into the composite blend amid growing mid-later next week forecast spread. This replacement also removes from the equation less supported GFS tropical interactions into the Gulf of Mexico and subsequent central Gulf Coast very heavy rainfall. Overall, WPC product continuity is reasonably maintained with the blending process tending to mitigate smaller scale guidance differences consistent with individual system predictability. Latest 00 UTC guidance remains generally in line. ...Weather/Hazards Highlights... A main ejection of upper trough energy from the Midwest through the Northeast over into the extended holiday weekend will focus showers and strong to severe storm chances with a wavy frontal system with focus across the Ohio Valley and north-central Appalachians along with a threat for locally heavy rainfall and runoff issues. WPC Day 4/5 Excessive Rainfall Outlook "Marginal Risks" areas seem reasonable given instability/moisture pooling and potential for repeat/training of cells. Antecedent rainfall and additional influence from northern stream energy are also in the mix. The overall convective rain focus should shift east and sink into the south/southeast and become less organized early-mid next week along a trailing/stalling front then re-emerging return flow. Upstream, enhanced instability and convective precipitation chances will increase and shift southeastward across the north-central Rockies/Plains to the mid-upper Mississippi Valley through early to mid next week ahead of the next main frontal system set to drop in from Canada in approach of an amplified west-central Canadian upper trough/height falls. Excessive heat to start the weekend from southern/southeast Texas into the Lower Mississippi Valley and Southeast/Florida will briefly expand northeastward into the Carolinas through Monday before a general downward trend towards summer averages outside of south Texas. The heat will be oppressive with widespread high temperatures in the mid-90s to low 100s which, when combined with the humidity, will lead to heat indices in the 110-115 degree range, locally even a bit higher. Excessive Summer heat will also overspread portions of the West this weekend, with the central California Valleys seeing highs of 105-110 degrees, upwards of 10-15 degrees above average. The Desert Southwest will experience a notable heating this weekend as well with max temps 5 to 10 degrees above average which means low to mid 110s for these hot environs south from Las Vegas and west from Phoenix. Elsewhere, above average high temperatures in the upper 80s to mid 90s along the northern tier of the central U.S. this weekend will shift eastward into the Upper Midwest/Great Lakes region ahead of a cold front, which will bring much cooler temperatures in the 70s to portions of the central/northern High Plains for the July 4 holiday. 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, experimental excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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