Weather across America

(NATIONAL) Excessive, potentially dangerous heat continues over much of the western U.S., monsoonal thunderstorms continue across the Four Corners region with the risk for flash flooding, and a stalled frontal boundary will bring additional thunderstorm chances from the Mid-Atlantic to the southern Plains.

Widespread areas of hazardous, potentially dangerous heat continued over much of the West as a strong upper-level ridge remains in place and temperatures soar into the 100s for many locations. The heat is expected to continue into next week, focusing on portions of interior California, the Great Basin, and the northern Rockies Monday with widespread highs in the mid-90s to 100s. An upper-level low over the northeastern Pacific and accompanying surface front will bring some relief from the excessive heat to the northern Great Basin by Tuesday, with highs closer to mid-summer averages. 

However, the upper-level ridge will also shift eastward ahead of the trough, helping to spread hotter temperatures into portions of the northern High Plains, with highs into the upper 90s. Some daily record highs will be possible over the next couple days, with warm, well-above average morning lows providing little relief from the heat overnight.  Elsewhere in the West, periodic shortwave energy on the eastern side of the ridge will continue to bring daily thunderstorm chances to the Four Corners Region. 

Sufficient moisture in place will promote some locally heavy downpours with the risk for scattered instances of flash flooding. A slight risk of excessive rainfall (level 2/4) remained in place Sunday for portions of southeastern Arizona, much of New Mexico, south-central Colorado, and portions of west Texas. The upper-level ridge will shift eastward over the next couple of days, bringing somewhat drier conditions and limiting the upper-level energy, with more scattered thunderstorm chances. However, there will still be an isolated threat for flash flooding, with the greatest chance on Monday over the higher terrain of south-central Colorado into central New Mexico, where another slight risk of excessive rainfall is in place as some of these areas remain more sensitive due to burn scars.  

To the east, a quasi-stationary frontal boundary draped from the Mid-Atlantic southwest through the Tennessee Valley/Southeast and into the Lower Mississippi Valley and southern Plains will remain the focus for additional widespread thunderstorms and heavy rain over the next couple of days. Anomalously high moisture as well as expected slow storm motions will bring the greatest risk for flash flooding to the Southern Plains, with a slight risk of excessive rainfall Monday from the ArkLaTexas southwest into western Texas. Another slight risk is in effect Tuesday mainly over portions of eastern and central Texas.

Further east, more scattered, less organized storms will still pose a threat for some isolated flash flooding, with a potentially locally higher threat for urban areas. The frontal boundary will lift northward a bit by later Monday, helping to spread storm chances further north into the Interior Northeast and New England Monday evening and into the day Tuesday. Elsewhere, another frontal system passing through the Upper-Midwest will bring thunderstorm chances over the next couple of days here as well, with some moderate to locally heavy rainfall possible.