Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Fri Jun 18 2021 Valid 12Z Mon Jun 21 2021 - 12Z Fri Jun 25 2021 ...Heat intensifies over the interior Northwest then spreads to the Plains next week... ...Wet tropical remnants of "Three" exits the Carolinas Monday then off the East Coast... ...Trailing frontal heavy rains across the South to the East Coast early next week... ...Guidance/Predictability Evaluation... Guidance this morning is in fairly good agreement on the overall evolution of the synoptic pattern across the U.S. through the medium-range period. A general blend of the 00Z ECMWF/EC mean, 06Z GFS/GEFS, and the 00Z CMC/CMC mean yielded solutions that agree very well with WPC continuity. The ECMWF and UKmet continue to predict a much better-defined circulation associated with Potential Tropical Cyclone "Three" than the GFS as it is forecast to exit the Carolinas into the Atlantic on Monday. There is a general model trend to push the cold front faster through the eastern U.S. by next Tuesday. Toward the latter part of next week, there appears to be emerging model tendency for overrunning rain/convection to track across the Midwest ahead of a warm front. ...Weather Highlights/Hazards... The record-breaking and dangerous heat over the Southwest/California is expected to moderate by the middle of next week, as temperatures are is forecast to return to near or slightly below normal for these areas. However, heat is expected to intensify early next week from the interior Northwest to the northern part of the Great Basin toward the northern/central Rockies, before spreading into the northern and central Plains by the middle of next week, and may continue into late next week. Meanwhile, Potential Tropical Cyclone "Three" is forecast by the NHC to track inland across the lower Mississippi Valley this weekend. The remnants of the system is expected to bring a period enhanced rainfall across the Carolinas on Monday before tracking northeastward off the East Coast as mainly a maritime threat. Early next week upper support, frontal approach and Gulf moisture/instability return in the wake of the tropical system will support periods of strong convection with heavy downpours across the already wet South that would lead to additional runoff issues, especially with lead training. Generally progressive frontal focusing rains will then spread through the Ohio Valley, Appalachians and the Eastern Seaboard, lingering over the Southeast/Florida midweek with the trailing front. Upstream, additional digging upper energies and return frontal support may combine to favor a moderate convective rain focus from the eastern Plains and north-central Mid-Mississippi Valley/Midwest later next week ahead of a warm front. Kong/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, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml