Gabrielle was declared a Subtropical Storm by the National Hurricane Center late in the day on 07 September 2007, and reached Tropical Storm status in the western Atlantic Ocean on 08 September. While some convection was associated with Gabrielle, GOES-12 IR imagery from the CIMSS Tropical Cyclones site (above) indicated that the convection was located well to the northwest of the circulation center; GOES-12 visible imagery (below) showed that the circulation center of Gabrielle remained exposed during much of the day.

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Many meteorological eyes have been watching a system in the western Atlantic during the past couple of days. This GOES-12 visible image includes a circulation near 28 N, 70 W (more easily seen in the animation above) that could be the kernel of the next tropical system in the Atlantic.

There are several notable features in the loop. First, there is an obvious shear line at the surface running southwest-northeast in the loop. North of the shear line, persistent northeasterlies are obvious, as cloud features (including a very nice example of an expanding outflow boundary at the northern edge of the domain) steadily propagate to the southwest. In the southeast corner of the domain, surface feature motion is characteristic of southwesterly flow. Convection is promoted in the convergent zone between these two airstreams. The mesoscale circulation evident in the center of the domain has persisted for several days. Such circulations can develop into tropical systems if the energy that is released in the convection can be restricted to a small central region. When that happens, pressures at the surface can fall, accelerating the wind, which acceleration enhances evaporation of vapor from the sea surface, and that enhanced vapor then supports further convective development.

In the present case, very strong shear (as diagnosed here by CIMSS-generated cloud-drift winds) at the southernmost end of a strong mid-latitude trough have displaced significantly the mid- and upper-level convective towers downstream of their boundary layer roots. This displacement is evident especially in the convection on the southern edge of the circulation later in the loop above. As long as the convection, and associated latent heating, is displaced downshear of its boundary layer roots, a tropical depression is unlikely to form. Shear is forecast to relax as the mid-latitude trough pulls away from the system (Note the low shear values just to the west of the system in the mapping of shear here). If the region of low shear overspreads the lower-tropospheric circulation before the circulation erodes, then a tropical depression will likely form.

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Hurricane Felix rapidly intensified from a Category 4 to a Category 5 storm off the northern coast of Colombia late in the day on 02 September 2007 (becoming the second Category 5 hurricane of the 2007 Atlantic Basin season). A GOES-12 IR image from the CIMSS Tropical Cyclones site (above) shows a well-defined eye surrounded by a ring of cold cloud top brightness temperatures (-70º to -76º C on the corresponding AWIPS IR image) at 22:15 UTC.

The eye structure was even more spectacular on an animation of GOES-12 visible channel imagery (below). The NOAA Hurricane Hunter aircraft crew reported that the eye exhibited the so-called “stadium effect“ on their reconnaissance flight, and this stadium effect is apparent near the end of the visible image animation (when low the sun angle illuminated the eye region from the side).

A plot of the CIMSS Advanced Dvorak Technique (below) reveals the trend of very rapid intensification of Hurricane Felix late in the day on 02 September. A National Hurricane Center discussion stated that the central pressure of Felix dropped at a rate of 3.4 millibars per hour in a 7 hour period.

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GOES-12 visible channel images (above; Java animation) showed a linear mesoscale convective system developing across parts of Iowa, Minnesota, and Wisconsin on 28 August 2007. Note the picturesque shadows cast by the individual cumulonimbus towers building in northeastern Iowa between 21:00 and 22:00 UTC. The larger cluster of thunderstorms in north-central/northeastern Wisconsin produced several reports of hail (up to 1.0 inch in diameter) and wind gusts of 60-80 mph (SPC storm reports). An AWIPS image of the MODIS 11.0µm IR channel around 19:09 UTC depicted cloud top brightness temperatures as cold as -79º C (-110º F) in north-central Wisconsin, with numerous cloud to ground lightning strikes.

AWIPS GOES-12 sounder Derived Product Images (DPI) of Convective Available Potential Energy (CAPE), Lifted Index (LI), and Total Precipitable Water (TPW) at 17:00 UTC (below) indicated that instability and moisture were increasing within a narrow zone along and just ahead of a wavy frontal boundary that was advancing slowly southeastward through the region. Isolated CAPE values were in excess of 4000 J/kg, LI values were less than -8º C, and TPW values were greater than 50 mm (2.0 inches) in the general area where new convection was seen to develop rapidly in north-central Iowa about 4 hours later on the GOES-12 visible images above.

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