Friday, 11:55 a.m.
For a change, it will actually feel like summer in most of the East this weekend. The humidity that was sent packing yesterday in the wake of a cold front is already charging across Iowa and Illinois and will spread across the Ohio Valley into the East tonight and tomorrow. This will set the table for a summery weekend in much of the country, particularly from the southern Plains and Mississippi Valley to the East Coast.
Examine the 500mb forecast for this evening across North America:
The upper-level trough that pulled the cold front through the Northeast and mid-Atlantic states late Wednesday and Wednesday night is long gone, and in the process it has pulled Cristobal into the cool waters of the North Atlantic.
Meanwhile, an upper-level ridge has popped over the Carolinas. The south to southwest flow aloft on the back side of this ridge is helping to drive a warm front into the Ohio Valley, and behind it the tropical air is free-flowing. If you look at the chart a little more closely, you'll see an avenue for the true tropical moisture over the western and northern Gulf of Mexico to come northward through eastern Texas and Louisiana over the next 24 hours, focusing on the Mississippi Valley before becoming more diffuse heading into Sunday as it slips into the Ohio Valley.
Farther north, the upper-level trough coming across the Plains is generating some showers and thunderstorms over the central Plains into in the Midwest. Some of these thunderstorms may become severe across parts of Iowa, eastern Minnesota and Wisconsin late this afternoon and tonight.
While another upper-level ridge over the Southwest, really, off the southern California coast, is fostering some typical late summer heat over the southern Rockies into southern California, another trough is swinging into the Northwest, pushing another cold front through Washington and Oregon. It has limited moisture associated with it now, but that should change late tonight and tomorrow as the front moves into Montana and Wyoming. This front bears watching over the weekend, as it will run into increasingly hot and humid air that may be the rocket fuel for severe thunderstorms. If you look at the 12z Aug. 29 NAM surface forecast for Sunday night, you can get the distinct impression a nasty line of thunderstorms will be on the march into the Midwest:
Many other models have the full support of this notion, so it is likely to be more than just noisy late Sunday and Sunday night from parts of Kansas and eastern Nebraska up into Iowa and Minnesota.
Once the front passes, cooler air will empty out of the interior Northwest and western Canada into the northern Rockies and northern Plains. There will be a hint of fall in the air across the region, with temperatures some 4 to 8 degrees below average for the end of August and beginning of September.
In contrast, the heat and humidity that's been brutal across parts of the southern Plains and lower Mississippi Valley of late will expand into the Ohio Valley then into the East. Clouds, showers and thunderstorms will keep the heat from being as intense in these areas as it has been farther south and west, but the dew points will be decidedly up as summer tries to take a land grab, if you will, during the weekend.
The trough lifting out of the Plains this afternoon and tonight will be much weaker by Sunday, so the front tied to this feature will also be weak. That means as it moves through the Great Lakes and Ohio, there will be little change behind it. The dew points may come down a wee bit, but if there's more sunshine, then the impact will be negligible with higher actual air temperatures. By the time the front limps into the East, it will have an opposite impact - it will mean increased heat and humidity as the weekend progresses. By Monday, here's the net effect of all this in terms of departures from normal:
The trailing front coming across the Midwest Monday will also have lessening impacts as it heads downstream. There may be a little downturn in the heat and humidity across the Great Lakes into New England behind it, but with a stronger upper-level ridge in the offing for late next week into the start of next week, some of the highest readings of the summer may be achieved late next week and the start of next weekend from the Ohio Valley to the Northeast.
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