Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Tue Aug 27 2024 Valid 12Z Tue Aug 27 2024 - 12Z Thu Aug 29 2024 ...A brief spell of record heat expected to spread from the Midwest to the East Coast... ...Active showers and severe thunderstorms this morning across the northern Plains and upper Midwest/Great Lakes will shift eastward into the northern Mid-Atlantic/southern New England by late Wednesday into early Thursday... ...First snowflakes of the season could reach the higher elevations of northwestern Montana on Wednesday followed by a chance of severe thunderstorms across North Dakota Wednesday night/early Thursday... The closed upper high currently centered over the mid-Mississippi Valley will be expanding eastward over the next two days, spreading into large portions of the East Coast from the Mid-Atlantic southward. This will send a quick spell of potentially record high temperatures from the Midwest into the Ohio Valley, Tennessee Valley, southern to central Appalachians and into the Mid-Atlantic. The combination of hot temperatures in the mid to upper 90s to near 100 degrees together with high humidity levels will produce maximum daily heat indices of between 105 and 115 degrees across these regions, with heat risks reaching major to extreme levels today across the Midwest into lower Great Lakes. Excessive Heat Warnings and Heat advisories are currently in effect across the mid to upper Mississippi Valley, the Midwest and southern New England, affecting nearly 61 million people, with further expansion of these warnings and advisories possible into portions of the Mid-Atlantic for Wednesday. This spell of record heat will be relatively short-lived as a cool high pressure system settling into southern Canada is forecast to send a cool and damp air mass into the Great Lakes on Wednesday and will quickly overspread New England Wednesday night, reaching into the Mid-Atlantic by Thursday morning behind a sharp cold front. Around the peripheries of the above mentioned upper high, precipitation is likely to be active along the Gulf Coast and from portions of the Southwest, northeastward into the Central Plains, upper Mississippi Valley, upper Great Lakes into northern New England in this "ring of fire". In these regions, moisture values are forecast to remain above average, supporting the potential for areas of active thunderstorms, heavy rains and flash flooding. Active showers and severe thunderstorms this morning across the northern Plains and upper Midwest/Great Lakes could result in areas of heavy rains and flash flooding. Some of these thunderstorms are expected to shift eastward into the northern New England by early Wednesday, before some additional strong to severe thunderstorms and heavy rain possible across portions of the upper Ohio Valley, central Appalachians and into the Mid-Atlantic later on Wednesday into early Thursday ahead of the sharp cold front. While much above to record high temperatures dominate portions of the central to eastern U.S. into mid week, another strong front will be pushing inland into the Pacific Northwest followed by the northern Rockies and into the northern High Plains by this evening. Much below average temperatures in the wake of this front will likely spread across the Pacific Northwest for today and into the northern Rockies/northern High Plains on Wednesday with high temperatures 10 to 20 degrees below average. There is not expected to be a lot of precipitation with this front over the Northwest, but the falling temperatures could bring the first snowflakes of the season for the higher elevations of northwestern Montana on Wednesday. By Wednesday night into early Thursday, a chance of severe thunderstorms will be moving eastward across North Dakota ahead of the strong cold front. Kong/Oravec Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php