Weather across America

(NATIONAL) An expansive heat wave continues but relief is coming to Oklahoma and Arkansas, severe thunderstorms expected for the Mid-Atlantic and Southeast, and flash flood threats for portions of the Midwest, Northern Mid-Atlantic/Northeast and Interior Northwest.

Excessive Heat Warnings remained in effect Sunday across central and eastern Texas to the central Gulf Coast and as far north as southern Arkansas and central Mississippi. Excessive Heat Warnings are also in effect for a portion of the Florida Panhandle and Desert Southwest. Sunday's high temperatures across Texas into the Lower Mississippi Valley will again reach into the low to mid-100s with heat index values eclipsing 110 degrees during the afternoon hours. However, some suppression of the upper level ridge in place over the southern tier states will occur over the next 1-2 days as a strong storm system (by August standards) makes its way across the Midwest Sunday and toward the Mid-Atlantic and Northeast on Monday.

An accompanying cold front will drop southeastward through Missouri/Oklahoma/Arkansas through Sunday night, bringing high temperatures closer to seasonal averages in its wake for Oklahoma into Arkansas Sunday with temperatures falling below average across Oklahoma/Arkansas and the Midwest behind the cold front for Monday.  

With the related surface low tracking east from South Dakota into the Midwest Sunday, severe thunderstorms are expected to set up ahead of the cold front and near a warm front from the Ohio Valley into the Tennessee Valley. The severe weather threat will translate eastward for Monday, covering a large portion of the eastern U.S. Thunderstorms producing heavy rain will increase flash flood potential from parts of the middle Mississippi Valley Sunday into the northern Mid-Atlantic and Northeast for Monday.  

Across the northwestern U.S., a series of upper level disturbances will move into the region, bringing a threat for afternoon/evening thunderstorms and flash flood potential Sunday, with a particular focus over northern Washington into northern Idaho.