Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 341 AM EDT Thu Aug 01 2024 Valid 12Z Thu Aug 01 2024 - 12Z Sat Aug 03 2024 ...Dangerous heat engulfs much of the country as one heatwave continues from the Southern Plains to the Southeast/Mid-Atlantic and another begins in the West... ...Severe weather and flash flooding expected for portions of the Midwest Thursday... ... Locally heavy monsoonal shower and thunderstorm chances continue for the Southwest... Widespread, dangerous heat will unfortunately be the main weather story for much of the country this week as one heatwave continues for the Plains, Middle/Lower Mississippi Valley, Southeast, and Mid-Atlantic, and another heatwave begins over portions of the West. A strong upper-level ridge will remain in place over the southern-tier of the U.S. the next couple days, allowing for high temperatures to reach into the low 100s over portions of the Southern Plains/High Plains, the upper 90s to low 100s for the Lower Mississippi Valley, and the upper 90s for the Southeast into the southern Mid-Atlantic. High humidity values for areas east of the High Plains will bring heat indices into the 105 to 110 degree range, potentially as high as 115 for parts of the Lower Mississippi Valley, with many areas under heat-related advisories or warnings. The combination of hot temperatures/high heat indices, as well as very warm morning lows only dropping into the mid- to upper 70s, will be dangerous to anyone without access to adequate air conditioning. Temperatures warming into the upper 80s/low 90s with periods of higher humidity will lead to some muggy, potentially dangerous conditions for portions of New England as well, particularly on Friday. An approaching cold front will bring relief to northern portions of the Southern Plains and Middle Mississippi Valley on Friday, with the expectation the heat wave will begin to wane for most locations through the weekend. In the West, an upper-level ridge will also begin to build northward, sending high temperatures across much of the northern tier of the West 10-20 degrees above mid-Summer averages. Forecast highs Thursday are into the upper 90s/low 100s for the inland Pacific Northwest/northern Great Basin and mid- to upper 90s for the northern Rockies/High Plains. Temperatures will get even hotter on Friday, with highs into the low to mid-100s for most locations. Many near record-tying/breaking highs are possible. Similar to areas further east, many heat-related advisories and warnings have been issued as this heat will also reach dangerous levels for the general public. Smoke from area wildfires will also continue to plague parts of the region, resulting in hazy conditions and poor air quality, and the risk for more wildfires will increase as the hot, dry conditions settle in. An upper-level low/accompanying surface frontal system traversing the northern side of the ridge over the southern tier of the U.S. will bring another round of showers and thunderstorms to the Midwest Thursday. High surface moisture leading to very strong instability will support intense thunderstorm development through the afternoon across the warm sector ahead of an approaching cold front, with lingering outflow boundaries from overnight convection helping to trigger individual and clusters of storms. Sufficient shear with the passing upper-level wave will lead to the threat of severe weather, with a Slight Risk (level 2/5) from the Storm Prediction Center covering Illinois, Indiana, Kentucky, and southwestern Ohio, mainly for the threat of some damaging winds. The threat for some intense downpours given the high moisture/strong storms, and potential for some more widespread, organized clusters of storms, will lead to some heavier rainfall totals and the risk for flash flooding. Many of these locations have seen recent heavy rainfall given repeated rounds of organized storms passing through, leading to wetter antecedent conditions more sensitive to any additional rainfall. A Slight Risk of Excessive Rainfall (level 2/4) covers much of same region from northeastern Illinois southeast through Indiana/western Ohio and into eastern Kentucky. Some slow moving storms producing heavy rainfall under the passing upper-low may also lead to some flash flooding, with the Slight Risk extending northwest into southern Wisconsin. Some more isolated storms will be possible further east into the Appalachians and Mid-Atlantic Thursday. Additional storms will be possible across the region Friday as the system shifts eastward, with a greater chance of storms spreading into the Appalachians and the Mid-Atlantic. Some isolated severe storms and instances of flash flooding will be possible. A lingering upper-level weakness will lead to scattered showers and thunderstorms over Florida and portions of the Southeast Thursday, with moderate to locally heavy rainfall possible, especially over the Florida Peninsula. More widely scattered storms will remain possible Friday. Persistent Monsoonal conditions over the Southwest will continue to bring daily shower and thunderstorm chances. Sufficient moisture across the region will lead to the threat for some locally heavy downpours and an isolated flash flooding, particularly over terrain sensitive areas such as burn scars. A lingering frontal boundary will lead to some storms over portions of the southern High Plains as well on Thursday, and a subtle upper-level wave along the edge of the upper-ridging will bring storm chances northwestward into portions of California Friday, particularly in vicinity of the Sierra Nevada. Similar to areas further north, forecast high temperatures more broadly in the region will be trending hotter and above average, with 90s and low 100s for most locations outside of the California Coast, and mid-100s to low 110s for the Desert Southwest. Putnam Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php