The poleward surge of elevated Total Precipitable Water (TPW) values (50-65 mm, orange to red enhancement) associated with Tropical Storm Gabrielle could be clearly seen on an animation of MIMIC TPW covering the 48-hour period from 09 to 10 September 2007 (above). Note that toward the end of the animation on 10 September, a plume of dry continental air (TPW values of 30-40 mm, blue to cyan enhancement) was entrained into the circulation of downgraded Tropical Depression Gabrielle as the cyclone moved away from the US East Coast.

As Gabrielle began to move northeastward across the Atlantic Ocean, it traveled along the north wall of the Gulf Stream — the warm waters in that region likely helped to prolong Gabrielle’s duration at tropical depression intensity. An 8-day composite of MODIS Sea Surface Temperature (SST) data (below) revealed that water temperatures along the axis of the Gulf Stream were still as warm as 83º F (28ºC) during early September.

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The IDEA MODIS Aerosol Optical Depth (AOD) product (above) indicated that the “dry air plume” moving eastward off the US coast also contained elevated levels of particle pollution, which was supported by the large number of surface AirNOW sites (below) from Georgia to North Carolina that reported “Moderate” concentrations of particle pollution (51-100 µg per cubic meter, yellow circles).

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Tropical Storm Gabrielle made landfall over North Carolina early in the day on 09 September 2007. GOES-12 Rapid Scan Operation (RSO) visible channel imagery at 5-10 minute intervals (above) shows the low-level circulation of Gabrielle emerging from beneath a quasi-stationary cluster of convection and then moving slowly north/northeastward across eastern North Carolina during the afternoon hours. GOES-12 10.7µm IR imagery (below) depicts cold cloud top brightness temperatures of -60º to -79º C (red to black to white enhancement) associated with the convective cluster — these cold cloud tops did manage to move slightly inland across the Morehead City NC region (located near the center of the IR images) after about 15 UTC (11 am local time), and the resulting rainfall amounts along the coast were as high as 8.60 inches at Harlowe.

An AWIPS image of the 1-km resolution MODIS 11.0µm IR channel (below) shows better detail in the cloud top temperature structure compared to the 4-km resolution GOES-12 IR imagery above. An overlay of cloud to ground lightning strikes indicates that lightning activity was fairly minimal with this cluster of thunderstorms.

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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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