Vertical wind shear is the change of wind speed and/or direction with height. For tropical cyclones, wind shear is something that must be overcome if strengthening is to occur. For (minimal) tropical storm Danny, strong wind shear has persisted in keeping convection far from the storm center. When the storm center is exposed, as in the visible loop above from 28 August, the atmospheric changes that occur within the convection cannot serve to strengthen the storm center.

Wind shear can be diagnosed using satellite cloud information. It is plainly evident in the satellite loop — note how the upper level clouds are moving from the southwest — especially over the western half of the satellite loop — whereas the lower level clouds are circulating around the storm center. A diagnosis of shear from the CIMSS Tropical Weather Website shows very large shear values (>40) associated with a mid-tropospheric short wave just starting to move off the southeast coast of the United States. Values are somewhat less over Danny, and large again off to the east of Danny. As the short wave approaches Danny, shear values over Danny will increase, and chances for intensification will drop.

Other factors continue to support intensification, however, such as warm sea surface temperatures. The analysis shows ocean water temperatures in the low- to mid-80s in the vicinity of Danny. Two other features are evident in the sea surface temperature plot. The Gulf Stream shows plainly as a ribbon of warm water extending eastward from Cape Hatteras. In addition, there is a striking path of cooler temperatures off to Danny’s north and east; this is a result of the mixing and upwelling associated with the passage of Hurricane Bill last week. The energy that was lost from the warm ocean waters helped to sustain Bill’s strong winds.

Satellite-derived winds can be used to approximate Danny’s future direction. Cloud levels in visible imagery can be determined from the temperature of the cloud. Low-level steering currents (1.5-3 km) over Danny this morning were from the east-southeast whereas upper level steering currents (3-10 km) were more southerly. If Danny is a shallow feature, it might move from the east-southeast towards the southeast coast of the US, following the low-level steering. If Danny is a deeper feature, its motion should be more northerly. The forecast path from the National Hurricane Center shows northward motion, suggesting that Danny is a deep system.

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An area of disturbed weather north of the Bahamas developed into Tropical Storm Danny early on 26 August. Satellite loops of the visible imagery, above, clearly show a low-level circulation that is displaced from the area of main convection to the north and east. Until the convection and the low-level centered are more co-located, development of the system should be slow.

Observations of wind shear near the Bahamas (analysis available at the CIMSS Tropical Weather Website) show modest values near the storm, consistent with the forecast of slow strengthening.

Water vapor imagery over North America shows the environment surrounding the storm, and several features that will determine where Danny will eventually move. The environment surrounding Danny is moist — dry air intrusions into the core of the storm, a weakening influence — should be small in the next 24-48 hours. Features in the upper troposphere that may influence the path of Danny include the closed circulation over southern Mississippi and the trough dropping into the Midwestern United States. As these features approach the east coast of the United States, increasing shear may limit the strength of Danny. However, sea surface temperatures are near the seasonal peak, which favors an increase in strength.

The eventual path of Danny is difficult to predict, in part because of the disorganized nature of the storm. It should be clear, however, that interested (and/or vulnerable) parties up and down the East Coast of the United States should keep an eye on the forecast.

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The launch and post-launch checkout of GOES-14 allows a comparison of the sounder instruments on GOES-14 (see the image above) and GOES-12, the operational GOES-East (the image below). The Sounder is used to detect emitted radiation at 19 different wavelengths, in contrast to the 5 wavelengths sensed by the Imager. There is better spectral resolution on the Sounder, but spatial resolution at the sub-satellite point is limited to 8-km field-of-view separated by 10 km. At mid-latitudes, such as over the United States, resolution is about 10 km.

The GOES-14 spacecraft has a different configuration than the GOES-12 spacecraft, and as a result, the Sounder detectors are colder on GOES-14. This allows for cleaner imagery. In particular, the Sounder bands 1, 2, 12 and 15 show much less noise in GOES-14 imagery compared to GOES-12. More information on the GOES-14 instruments can be found here.

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Unseasonably cool air over the Midwest combined with warm late-Summer lake surface temperatures to sustain a line of lake-effect showers that came ashore over southern Lake Michigan on Sunday Morning. A very sharp-edged narrow cloud feature extended north-northeastward from southern Lake Michigan towards northwestern lower Michigan. Lake surface temperatures temperatures in this region, as observed by MODIS, were in the 60s. At the same time, very cool temperatures prevailed in the lower troposphere (see the plotted sounding from Gaylord, MI, from 1200 UTC 23 August here showing temperatures near 5 C at 850 hPa). The resultant temperature difference of about 13 C over the lowest 1-2 kilometers of the atmosphere helped sustain showers as shown on the Chicago (LOT) radar here. Peak dbZ values in this loop were around 24. Expect the lake-effect cloudiness to become more common in the coming months as cooler and cooler airmasses move over the relatively warm Great Lakes.

Note also the widespread valley fog over the Wisconsin River in southwestern Wisconsin and the upper Mississippi River at the beginning of the satellite imagery loop. Daytime heating is suppressed over the river valleys because energy is lost to evaporating cloud droplets, and the environment closest to the River therefore is cooler than surroundings later in the loop. For that reason, cumulus development over the rivers is delayed, and the river valleys are clear later in the loop.

Added: This case was also mentioned in Tom Skilling’s WGN Weather Center blog.

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