Record hot temperatures over portions of the Midwest/Ohio Valley & theMid-Atlantic states this weekend......Heavy rain and flash flooding concerns across the Upper Midwest Fridaynight and Great Lakes Saturday......Monsoon-like conditions for this Southwest this weekend intoMonday...Heat building across the West...A heat wave should continue for portions of the Midwest/Ohio Valley andthe Mid-Atlantic States, to the south of a stationary front. Forecast hightemperatures will rise into the upper 90s/near 100 Saturday in theMid-Atlantic, with record-tying/breaking temperatures possible.Heat-related Advisories and Warnings are in place as these temperatures,when combined with the humidity, will bring heat index values into the100-105 degree range, with values as high as 110 possible through thesouthern Mid-Atlantic on Saturday. Additionally, overnight lowtemperatures will remain in the mid-70s for many locations, at potentialrecord-tying/breaking levels, providing little relief from the heat. Theearly arrival of this heat in the Summer season, persistence over multipledays, and abundant sunshine will aggravate heat stress on those withoutreliable air conditioning. Temperatures more broadly from the East Coastto the central/southern Plains will be in the mid-90s this weekend.Meanwhile, relief continues to the north of a frontal boundary drapedthough southern New England, with highs now mostly in the 70s and low 80s. Isolated to scattered excessive rainfall/flash flooding is possible nearthe stalled frontal boundary.The greatest threat for significant heavy rainfall will be ahead of asurface wave over portions of the Upper Midwest as thunderstorm complexeslate on Friday unleash their potential.  A Moderate Risk of Excessiverainfall (level 3/4) is in effect, from southeast South Dakota to in andnear the Minnesota/Iowa border, where the highest potential for heavyrainfall/significant flash flooding exists.  Moderate to major riverflooding is also forecast to worsen.  Additional scattered instances offlash flooding will be possible more broadly across the Upper Midwest.  Anupper-level shortwave approaching from the west will help to deepen thefrontal wave and push the system eastward on Saturday, bringing the threatfor heavy rainfall/flash flooding to portions of the Great Lakessouthwestward into the Upper Mississippi Valley as an organized complex ofstorms is expected to follow the system eastward.  Some severethunderstorms will again be possible with a Slight Risk in place mainlyfor the threat of damaging winds.  As the system pushes into New Englandon Sunday, it brings an elevated risk of flash flooding to the region.