The big weather headline for the next week — for North America at least — will surely be Hurricane Dean, a category 3 hurricane over the far eastern Caribbean Sea (see the CIMSS Tropical Cyclones site for the latest information on Hurricane Dean). Earlier today (Friday), Dean entered the Caribbean passing north of St. Lucia and south of Martinique.

The color-enhanced water vapor loop above (shown at 3-hour intervals) shows Dean approaching the northern Windward Islands of the Carribean. The thunderstorms surrounding the hurricane center are vigorous and tall, with brightness temperatures cooler than 210 K. Sea Surface Temperatures along the storm’s track get progressively warmer. As the storm approached the Caribbean, wind shear values deduced from satellite winds at 0300 UTC, 0600 UTC and 0900 UTC show mainly low values with a jump at 0600 UTC that may have slowed intensification as the storm approached the windward islands. Another factor that may have delayed intensification is the large region of dry air surrounding Dean. This is the large black region in the water vapor imagery that surrounds Dean to the north and west. Analyses of Saharan Air Layers at 0000 UTC 17 August and 1200 UTC 17 August from Meteosat 8 and GOES West show that the dry region in the water vapor imagery likely originated over the Sahara. Air masses rich in Saharan dust are known to suppress hurricane formation. Satellite data can be used to detect such dust. Satellite data from the 12 micrometer channel is compared to satellite data in the 11 micrometer channel. Where the (12-11) value is positive (that is, where the 11-micrometer brightness temperature is colder than the 12-micrometer value), dust is likely present. Both Meteosat-8 (over the Equator at 0 W) and GOES-11 (GOES-West, over the equator at 135 W) have a 12-micrometer sensor. Unfortunately, from the perspective of dust detection over the Atlantic, GOES-12 (GOES-East, over the Equator at 75 W) does not.

Now that Dean has entered the Caribbean, the dry air indicated in the water vapor imagery has eroded. Little Saharan dust is detected. Wind shear values are low, and heat content in the water is high. There is little to prevent Dean from becoming a very dangerous storm in the western Caribbean.

The visible image at 1745 UTC 17 August shows the category-3 storm (110-knot winds at the surface based on 125-knot flight-level winds). Strong thunderstorms are present in the bands that spiral out from the visible eye. The color-enhanced infrared imagery from the same time shows outflow in all quadrants. Evacuation of mass through upper-level outflow is expected if central pressure falls are occurring.

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AWIPS images of the GOES-12 water vapor channel (above) showed 3 distinct cyclonic circulation features on 17 August 2007: (1) the remnants of Tropical Storm Erin over Texas, (2) a Tropical Upper Tropospheric Trough (TUTT) over the Bahamas, and (3) rapidly intensifying Hurricane Dean over the western Atlantic Ocean (see the CIMSS Tropical Cyclones site for the latest information on Hurricane Dean).

Satellite-derived water vapor winds (or Atmospheric Motion Vectors) using GOES-12 imagery (above) were helpful in diagnosing the intensification of the TUTT circulation over the Bahamas during the 15-17 August period. Note that the TUTT circulation was evident in the upper-tropospheric 200 hPa wind fields, but not on the lower-tropospheric 850 hPa wind fields.

Since TUTT features can sometimes inhibit tropical cyclone development by introducing greater environmental shear, we examined a series of forecast synthetic water vapor channel + IR channel satellite imagery from the CRAS model (above). The CRAS water vapor image forecast suggested that the TUTT circuation would remain over the Gulf of Mexico, far to the northwest of Hurricane Dean as it intensified and moved westward across the Caribbean Sea.

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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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