Weather Across America

(NATIONAL) Dangerous and record-breaking heat continues to intensify from the Plains to the East Coast, multiple rounds of storms to bring the threat of severe weather and flash flooding to portions of the northern Plains, Mississippi Valley, and Great Lakes region, and a monsoonal pattern continues for the Four Corners region with some locally heavy downpours and isolated flash flooding possible.

A broad heatwave from the Plains to the East Coast will continue over the next couple of days, becoming particularly intense for many of the urban centers in the Southeast, Ohio Valley, and along the East Coast. Forecast highs range from the low to mid-90s in New England west through the interior Northeast and into portions of the Great Lakes, the mid-90s to low 100s from the Mid-Atlantic/Southeast west through the Middle/Lower Mississippi Valley, and the low to mid-100s over portions of the central and southern Plains. 

Widespread heat-related advisories and warnings are in effect as the combination of hot temperatures and humidity brings heat indices into the 105-110 degree range. Numerous record-tying/breaking highs are possible along the East Coast and into portions of the Ohio Valley and Southeast. Very warm overnight low temperatures generally in the mid- to upper 70s, also near record-tying/breaking levels, will provide little relief from the heat overnight. Most of the populated urban centers of the Southeast, Ohio Valley, and along the East Coast will be at a major (level 3/4) to extreme (level 4/4) risk from this heat, as conditions will be dangerous and potentially deadly for those without adequate access to effective cooling and hydration. 

A cold front will bring relief to portions of the central Plains, Middle Mississippi Valley, and Great Lakes Tuesday while the intense heat will unfortunately continue into at least mid-week for areas to the south and east. In the West, after a brief break from the heat following last week's heat wave, hotter temperatures will begin to return Tuesday over portions of the Pacific Northwest, with highs into the upper 80s to upper 90s, and in the northern Great Basin, with highs in the mid 90s to low-100s.  Multiple rounds of storms are expected in vicinity of a frontal boundary pushing southward through the northern Plains and upper Mississippi Valley/Great Lakes this (Sunday) evening and once again on Monday. 

Plentiful moisture, moderate to strong instability, and strong westerly flow aloft will favor the potential for severe weather, particularly for one (or more) organized convective systems producing damaging winds, as well as some large hail and a few tornadoes. For the rest of the day and into Sunday evening, the Storm Prediction Center has outlined a Slight Risk (level 2/5) of severe weather over portions of the Great Lakes and Ohio Valley as well as the northern Plains. Then, on Monday, there is an Enhanced Risk (level 3/5) for portions of the Upper Mississippi Valley into the Great Lakes where a greater threat of more widespread, significant damaging winds will exist. 

In addition, anomalously high moisture leading to heavy rain rates, as well as the tendency for storms to follow a similar path in parallel with the boundary, will bring an increasing flash flood risk as antecedent conditions become increasingly wet. There is a Slight Risk of Excessive Rainfall (level 2/5) for portions of the Upper Mississippi Valley into the Great Lakes through Sunday evening and once again on Monday. The front will push eastward towards the Interior Northeast and southward into the Ohio Valley, Middle Mississippi Valley, and Central Plains on Tuesday. 

Additional rounds of heavy rain-producing thunderstorms, once again having the tendency to cluster and repeat over the same locations in parallel with the front, will bring another risk of scattered flash flooding, particularly for portions of the Ohio Valley southwest through the Middle Mississippi Valley where another Slight Risk of Excessive Rainfall is in effect. Additional isolated instances of flash flooding, as well as some damaging wind gusts, will be possible with storms elsewhere along and ahead of the front.  

Daily thunderstorm chances will remain in the forecast over portions of the Four Corners region as a Monsoonal pattern remains in place. Anomalously high moisture flowing into the region will lead to the chance of some locally heavy downpours, and the risk of isolated flash flooding, particularly for any burn scars. Elsewhere, some daily scattered thunderstorm chances are also expected over portions of the Southeast and Florida Monday and Tuesday.