A heat wave begins to settle in over the Great Lakes, Ohio Valley andthe Northeast today, continuing through midweek...Significant heavy rain/flash flooding threat for the Texas Gulf Coastlater Tuesday and especially Wednesday associated with Potential T.C.One.Heavy rain and severe thunderstorms possible Tuesday/Wednesday from theUpper Midwest into the Central Plains...Late-season wet snow will persist across the high-elevations of thenorthern Rockies for the next couple of days...An active week is in store across the U.S. as a heat wave builds over theGreat Lakes/Ohio Valley/Northeast, multiple areas of flash flooding andsevere weather impact the central U.S., and even some late season snowcontinues in the northern Rockies. A strong upper-level high will continueto build in over the Great Lakes/Northeast, bringing the first significantheat wave of the season to the region. Forecast highs Tuesday andWednesday will reach into the mid- to upper 90s, even well into northernNew England. Widespread, numerous record-tying/breaking high temperaturesare possible. Additionally, morning lows will remain in about the mid-70s,at record-tying/breaking levels, providing little relief from the heatovernight. The early arrival of this magnitude of heat, the duration,abundant sunshine, and lack of relief overnight will increase the dangerof this heatwave beyond what the exact temperature values would suggest.This is especially true for those without adequate air conditioning, whichbecomes more of a concern for locations further north that are not asaccustomed to periods of persistent, intense heat.Further north, additional rounds of showers and thunderstorms are expectedas the influx of moisture continues to interact with a wavy frontalboundary draped across portions of the northern/central Plains and UpperMidwest. A broad area of showers and thunderstorms will continue throughMonday evening along and ahead of the front as it slowly lifts northward.A Slight Risk of Excessive Rainfall is in effect over portions of thenorthern Plains/Upper Midwest where the available moisture, as well asimpressive upper-level dynamics associated with an approaching trough, maylead to some heavier rain rates (2" per hour) and scattered flashflooding, especially in urban areas. A Slight Risk of severe weather(level 2/5) is also in effect from the Storm Prediction Center, spreadingfurther westward into the northern/central High Plains, for some largehail, damaging winds, and a few tornadoes. The front will begin to pushback southeastward as a cold front Tuesday ahead of the approaching troughbefore stalling out through the Middle Mississippi/Missouri Valleys aheadof the eastern ridge by Tuesday night. To the west, the unseasonably strong upper-level system will also continueto bring an intrusion of much cooler, below average air into portions ofthe northern/central Plains/Rockies and Great Basin. Forecast highs onTuesday will be in the 50s and 60s in the northern Rockies/High Plains,with some low 70s into the Great Basin. In addition, forecast morning lowson Tuesday in the low to mid-30s throughout much of the Great Basin haveprompted Frost/Freeze-related Warnings and Advisories. Temperatures shouldrecover a bit Wednesday for the Great Basin and northern Rockies whileportions of the central Plains drop into the 60s following the cold frontpassage. Areal showers and thunderstorms with some moderate rainfall willalso linger over the northern Rockies/High Plains Monday evening and intothe day Tuesday before tapering off by later Tuesday night. Winter-weatherrelated Warnings and Advisories remain in effect as moderate to heavy snowcontinues for higher mountain elevations. While most of the accumu