Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 410 AM EDT Sat Jun 22 2024 Valid 12Z Sat Jun 22 2024 - 12Z Mon Jun 24 2024 ...Heat wave focus shits from the Midwest/Ohio Valley into the Mid-Atlantic states this weekend... ...Heavy rain and flash flooding concerns across the Upper Great Lakes/Mississippi Valley Saturday and northern New England Sunday... ...Heat building across the West this weekend, monsoon-like conditions for the Desert Southwest... A heat wave will continue over much of the eastern U.S. south of a quasi-stationary boundary and under the influence of longwave ridging aloft this weekend. Forecast high temperatures Saturday will generally be in the mid- to upper 90s from the central/southern Plains to the East Coast. These temperatures remain the most anomalous and dangerous for early Summer over portions of the Midwest/Ohio Valley east to the Mid-Atlantic. Heat-related advisories and warnings are in place as humidity will bring heat index values as high as the mid-100s. A potent upper-level shortwave will help to finally push the boundary southward Sunday, bringing welcome relief to much of the Midwest/Ohio Valley, while the Mid-Atlantic continues to simmer. Numerous record-tying/breaking highs are possible. In addition, overnight low temperatures will remain to the mid- and even upper 70s, providing little relief from the heat overnight. The combination of this heat coming early in the Summer season and persisting over several days increases the level of heat stress for those without reliable air conditioning. Widespread showers and thunderstorms will continue to the north along the quasi-stationary boundary draped from New England west through the Great Lakes and into the Upper Mississippi Valley. Plentiful moisture will increase the chance for locally heavy downpours. The highest chance for potentially significant heavy rainfall will be along the boundary ahead of an upper-level wave over portions of the Upper Great Lakes/Upper Mississippi Valley Saturday. Ongoing organized storms from overnight Friday as well as the risk for more widespread, organized storms into the day Saturday has prompted a Moderate Risk of Excessive Rainfall (level 3/4) over southern Wisconsin and northeastern Iowa for the threat of scattered to numerous instances of flash flooding. A broader Slight Risk (level 2/4) covers the region. In addition, a few storms may be severe, with a Slight Risk of severe weather (level 2/5) issued by the Storm Prediction Center for the threat of some damaging winds and a few tornadoes. A locally higher threat for heavier downpours will also exist over southern New England, with a Slight Risk in place. The noted more potent upper-level shortwave arriving Sunday will help to push the boundary south and eastward, bringing higher storm chances more broadly across New England and southwestward though the Mid-Atlantic/Ohio Valley/Tennessee Valley. Stronger, very moist low-level flow interacting with the boundary over northern New England will bring the greatest chance for heavy downpours and scattered flash flooding, with another Slight Risk in place. A Slight Risk for severe weather also covers much of the same region and southwestward into the northern Appalachians/Upper Ohio Valley, where damaging winds and a few tornadoes will once again be the main threats. Upper-level ridging will begin to build over the western and then central U.S. following the shortwave passage, bringing rising temperatures across this region as well. Forecast highs Saturday across much of the interior Pacific Northwest, Great Basin, and California will be in the mid-90s to low 100s. Locally higher temperatures into the mid-100s have prompted heat-related advisories and warnings for the central California Valleys into portions of southern California. The focus over the northern tier will begin to shift eastward with the ridge on Sunday, with cooler highs in the Pacific Northwest but temperatures soaring into the mid- and upper 90s over much of the central/northern High Plains. Conditions will remain hot from central to southern California. Temperatures will still be hot but closer to average for the Desert Southwest as Monsoon-like conditions remain over the region. Highs Saturday and Sunday will range in the mid- to upper 100s, with scattered showers and storms bringing the threat for some locally heavy downpours and an isolated risk for flash flooding. Elsewhere, a pair of waves of low pressure passing over portions of the Southeast/Florida as well as south Texas will bring daily shower and storm chances. Putnam Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php