Heat wave focus shifts from the Midwest/Ohio Valley into theMid-Atlantic states this weekend......Excessive Rainfall and Severe Weather concerns across the Upper GreatLakes/Mississippi Valley tonight followed by New England Sunday......Heat expands from West into Plains early next week, monsoon-likeconditions for the Desert Southwest...A heat wave will continue over much of the eastern U.S. south of aquasi-stationary boundary and under the influence of longwave ridgingaloft this weekend. Forecast low temperatures Saturday will generally bein the mid- to upper 70s from the central/southern Plains to the EastCoast. These temperatures remain the most anomalous and dangerous forearly Summer over portions of the Midwest/Ohio Valley east to theMid-Atlantic. A potent upper-level shortwave will help to finally push theboundary southward Sunday, bringing welcome relief to much of theMidwest/Ohio Valley, while the Mid-Atlantic continues to simmer. Numerousrecord-tying/breaking highs are possible. In addition, overnight lowtemperatures will remain to the mid- and even upper 70s, providing littlerelief from the heat overnight. The combination of this heat coming earlyin the Summer season and persisting over several days increases the levelof heat stress for those without reliable air conditioning.Widespread showers and thunderstorms will continue to the north along thequasi-stationary boundary draped from New England west through the GreatLakes and into the Upper Mississippi Valley. Plentiful moisture willincrease the chance for locally heavy downpours. The highest chance forpotentially significant heavy rainfall will be along the boundary ahead ofan upper-level wave over portions of the Upper Great Lakes/UpperMississippi Valley tonight. Ongoing organized storms from overnight Fridayas well as the risk for more widespread, organized storms into the daySaturday has prompted a Moderate Risk of Excessive Rainfall (level 3/4)over southern Wisconsin and northeastern Iowa for the threat of scatteredto numerous instances of flash flooding. A broader Slight Risk (level 2/4)covers the region. In addition, a few storms may be severe, with a SlightRisk of severe weather (level 2/5) issued by the Storm Prediction Centerfor the threat of some damaging winds and a few tornadoes. A locallyhigher threat for heavier downpours will also exist over southern NewEngland, with a Slight Risk in place. The noted more potent upper-levelshortwave arriving Sunday will help to push the boundary south andeastward, bringing higher storm chances more broadly across New Englandand southwestward though the Mid-Atlantic/Ohio Valley/Tennessee Valley.Stronger, very moist low-level flow interacting with the boundary overnorthern New England will bring the greatest chance for heavy downpoursand scattered flash flooding, with another Slight Risk in place. SPCupgraded their Slight Risk to an Enhanced Risk over parts of central NewEngland/much of New Hampshire and Vermont, where damaging winds and a fewtornadoes will once again be the main threats.Upper-level ridging will begin to build over the western and then centralU.S. following the shortwave passage, bringing rising temperatures acrossthis region as well. The focus over the northern tier will begin to shifteastward with the ridge on Sunday, with cooler highs in the PacificNorthwest but temperatures soaring into the mid- and upper 90s over muchof the central/northern High Plains. Conditions will remain hot fromcentral to southern California. Temperatures will still be hot but closerto average for the Desert Southwest as Monsoon-like conditions remain overthe region. Highs on Sunday and Monday will range in the mid- to upper100s, wi