The track of Tropical Storm Erin from the CIMSS Tropical Cyclones site (above) depicts a fairly short-lived history over the Gulf of Mexico during 15 August–16 August 2007. Although wind speeds only reached minimal tropical storm intensity, the more important characteristic of Erin was the high amounts of tropical moisture tapped by the system.

AWIPS images of the GOES Sounder Total Precipitable Water (TPW) derived product (below; QuickTime animation) showed a large area of elevated TPW values ranging from 50-67 mm (2.0-2.6 in) over the Gulf of Mexico around the periphery of Erin, with TPW values greater than 40 mm (1.6 in) inland over much of Texas. Such copious amounts of moisture coupled with slow forward storm motion helped the remnants of Erin to produce very heavy rainfall (as much as 8.6 inches reported, with rainfall rates of 1-3 inches per hour — see radar-estimated storm total precipitation from Houston and Austin/San Antonio) which led to flooding across parts of Texas during the day on 16 August.

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Derechos are long-lived convectively-driven wind storms. In the upper midwest, they typically form in northwesterly flow just poleward of very warm and moist air and then surge southward. Convection overnight on 13-14 August 2007 developed into a long-lasting complex with a bow echo that propagated from north of Minneapolis to Illinois. Although wind speeds weren’t as strong in historic events such as July 1983, numerous wind damage reports nevertheless were reported to SPC.

Monday August 13 was characterized by strong moisture contrasts over the upper midwest (see the 2100 UTC dewpoint plot here; the 2100 UTC temperature plot is here). Excessive heat and humidity over the high plains extended northeastward into central Minnesota where cooler and dryer air prevailed. Predictably, this was a region for the development of strong thunderstorms, and the 0000 UTC sounding for 14 August at Chanhassen (here) showed the potential for very strong convection.

The AQUA satellite carrying the MODIS instrument flew directly over the mature thunderstorm complex at 7:50 UTC 14 August, and the color-enhanced IR window channel (10.8 micrometers) is below. Plotted on top of the clouds are the 15-minute cloud to ground lightning data. Two active regions of lightning are present; one is over south-central Wisconsin, near Madison, and a second stretches as an arc along the front of a southward-propagating bow echo over western Wisconsin, immediately in front of the coldest cloud tops. The vertex of the arc corresponds to the line of wind damage reports from SPC. This correspondence is even more striking when viewing animations of lightning data and satellite data (or the combination, at the top of this blog entry), or the radar loop with lightning superposed on top that is here. Note the striking roll-up at the eastern edge of the devloping bow echo that was captured in extreme western Wisconsin — next to the St. Croix River — in the radar image at 0554 UTC (here).

The propagating thunderstorm had a predictable influence on surface pressures. Pressures rose as the first region of lightning moved through Madison just before 08z, and again as the second region of lightning associated with the weakening bow echo moved across Dane County.

Note also the presence of a 38-knot wind gust from the southeast at 10:10 UTC as the mesolow moved through after most of the rain had fallen.

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Large wildfires continued to burn out of control in parts of Idaho and Montana (NOAA HMS product) on 13 August 2007. GOES-13 visible imagery (above; Java animation) showed smoke (from the previous day of burning) that was trapped in the valleys during the morning hours…with a transition to rapid smoke plume growth as southwesterly boundary layer winds increased and new fire activity flared up during the afternoon and evening hours.

GOES-12 visible imagery (below; Java animation) showed that smoke from previous days of burning had been transported as far eastward as Wisconsin and the western Great Lakes region on 13 August.

The extent of the eastward transport of smoke was confirmed using the MODIS Aerosol Optical Depth (AOD) product (above), with the thick smoke exhibiting high AOD values of 0.7 to 1.0 (orange to red enhancement).

MODIS true color imagery (below) showed a closer view of the smoke (hazy areas) that was drifting over Wisconsin and surrounding states during the early afternoon hours — and the IDEA aerosol trajectory forecast suggested a continuation of smoke transport aloft across the the Upper Midwest and Great Lakes regions .

The thick smoke aloft was responsible for creating a colorful (albeit somewhat muted) sunset — the photo below was taken in Middleton, Wisconsin, looking west, around 7:45 PM local time.

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GOES-11 IR imagery from the CIMSS Tropical Cyclones site (above) depicted a nice symmetric eye structure exhibited by Hurricane Flossie as it maintained Category 4 intensity in the central Pacific Ocean on 13 August 2007. Judging from the IR imagery, Flossie appeared to be an annular hurricane — annular hurricanes tend to weaken more slowly than non-annular storms of similar intensity, which may help to explain why Flossie maintained Category 4 intensity even as it began to encounter slightly cooler sea surface temperatures and increasing environmental shear. While Hurricane Flossie was forecast to weaken somewhat as it passed just to the south of the Hawaiian Islands (below), some adverse impacts (increasing winds, high surf, heavy rainfall) were still expected as the storm approached.

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