.Heat wave focus shifts to the Southeast, Mid-South, andcentral/southern Plains into midweek......Severe storms and flash flooding possible for portions of the UpperMidwest into Monday night, shifting a bit south Tuesday......Monsoon-like conditions persist for the Southwest/Four Corners Region...A broad upper-level ridge building over the west-central U.S. willcontinue to produce potentially hazardous heat over the next few days.Forecast high temperatures will continue to soar into the upper 90s acrossmuch of the Plains and Mississippi Valley and into southeastern parts ofthe Lower 48, with low 100s possible over the central Plains. Whencombined with the humidity, heat index values may reach as high as 110,prompting Heat Advisories and some embedded Excessive Heat Warnings.Meanwhile, low temperatures will mostly remain in the mid- to upper 70s,bringing little relief from the heat overnight. The arrival of this moreintense heat early in the Summer season leads to a higher level ofheat-related stress, especially for those outdoors and without reliableair conditioning available. While the north-central U.S. can expect somerelief from the heat Tuesday and especially Wednesday behind a cold front,the Southern Plains to Mid-South to Southeast will maintain potentiallydangerously hot conditions through the period, with a brief period of heatreturning to the Middle Mississippi/Ohio Valleys on Tuesday andMid-Atlantic on Wednesday.To the north, an upper-level shortwave and accompanying surface frontalsystem will move along the northern tier of the central U.S. towards theUpper Midwest. Deep moisture flowing northward ahead of the system willbring increasing storm chances through tonight. Hot temperatures alongwith the plentiful moisture will lead to strong to extreme instability,prompting an Enhanced Risk of severe weather (level 3/5) from the StormPrediction Center. Tornadoes and large hail are possible with the storms,but the primary threat is a corridor of very high wind across southernWisconsin and surrounding states. Additionally, a Marginal to Slight Riskof excessive rainfall causing flash flooding is in place for portions ofthe Upper Midwest with locally heavy downpours. The low system willcontinue eastward on Tuesday, with a cold front pushing southeastward intothe Midwest/Great Lakes region. Storms developing along and ahead of thefront could tend to repeat over the same areas as storm motions becomemore parallel to the increasingly east-to-westward oriented front. This isprompting a larger Slight Risk of flash flooding for portions of theMiddle Mississippi Valley with a Slight Risk of severe weather aligned ina similar area for Tuesday into Tuesday night. By Wednesday, thesoutheastward progression of the front will produce thunderstorm chancesfrom the Lower/Middle Mississippi Valley into the Tennessee and OhioValley to the Mid-Atlantic and Northeast.