MyFoxHurricane.com blog

Danielle fluctuates/New Depression Forming

Posted by Andy Johnson under Daily updates

Danielle underwent a rapid strengthening phase overnight Monday and became a Category 2 hurricane on the Saffir-Simpson Hurricane Wind Scale at 5AM Tuesday morning. During the day, however, a weakening trend occurred and winds  decreased back to tropical storm strength at 5pm. At 11pM Tuesday, Danielle was back to huricane strength. As I mentioned in my blog yesterday, Danielle had been insulated from the large dry air mass situated to the west of the circulation. For a while the two areas were moving in tandem but today Danielle caught up with the drier air mass and I believe that is the primary reason she has weakened as drier air has been entrained into the circulation. There are several signs of evidence for the dry air intrusion. Water vapor imagery has continued to show an uncharacteristically large area of dry air (at least for the tropics in August) to the west of Danielle. In addition, the TRMM (Tropical Rainfall Measuring Mission) Satellite has shown a closed eyewall on the east side of Danielle but a partially open eyewall on the west side of Danielle. The TRMM satellite is a polar orbiting satellite that allows us to see rainfall rates. This satellite is especially useful in tropical ocean regions where there are no radars available. Due to the curvature of the earth, radar ranges are limited but satellites are able to view a much larger area. The erosion of the western eyewall is evidence of some drier air. The secondary reason for weakening is due to increased southwesterly shear. Southwesterly winds of 20 knots at the northern edge of the surface circulation of Danielle have begun today and some shear as high as 30 kts is located in the northern outflow region of the hurricane.

In the satellite image below notice that most of the high cirrus outflow is on the east side of Danielle, which is consistent with southwesterly winds aloft. Invest 96L: is close to becoming a tropical depression and is unusually that is was so strong even when it was over land in Africa. There are signs of a weak circulation and it is bringing gusty winds to the Cape Verde Islands. Usually, storms intensify west of the Cape Verde Islands so the Islands themselves do not get gusty winds. The station closest to the circulation center is Praia, Cape Verde.  At 11:00 AM today (local time) they reported a NE wind gusting to 24 MPH. At 5:00 PM (local time) they reported a SE wind at 14 MPH. The veering wind indicates that the center passed south of Praia and is now located to the southwest. It is interesting to note that observer at Praia, Cape Verde recorded widespread dust at 11:00 AM (only partly cloudy at 5:00 after the passage) and Mindelo, Cape Verde has reported widespread dust all day with winds out of the NE gusting to 27 MPH. Mindelo is further northwest of Praia. Another strong convective cluster is getting ready to move off of the African coast and may put even more of a damper on the extensive dust that has been blowing off of the continent.

A small low in the NW Gulf is causing above normal tides along the west coast of Central Florida due to a long fetch of southwesterly flow. Tides were running 2 to 4 feet above normal around Cedar Key Tuesday afternoon.

An concentrated area of convection persists in the southwest Caribbean Sea but without signs of organization or a surface circulation.

The image below is an enhance dview of Danielle taken by the GOES weather satellite. Compare this image with the images from my blogs over the past few days. Notice the magenta area which contains the coldest cloud top areas. The magenta area has shrunk since yesterday. Note that even the red areas are mostly to the east of the circulation center which is consistent with southwesterly shear.

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 The Saharan Air Layer (SAL) McIDAS image below shows the location of Danielle (to the west), Invest 96L in the middle and a new convective cluster over Africa (to the right).  The rightmost cluster could develop once it emerges off of Africa, as well. Even though the SAL is extensive, if you compare it to a few days ago or a week ago, there is less dust, at least at lower latitudes (from 10° to 20° N). Most of the African waves that develop into hurricanes emerge from 10° to 20° N.  As we say today from the report from Praia, Cape Verde, the dust diminished once the tropical wave passed to the west.

The image below shows upper level winds overlaid on a satellite image. The arrows shows the direction of wind and the circular yellow lines show the speed. Wind shear over the northern part of Danielle is between 20 and 30 knots and she is headed more into that unfavorable area for development.

The TRMM satellite image below shows the erosion of the western eyewall as the rain pattern is not concentric around the eye. There is a solid area of convection on the east side of the eye but not on the west.

Long range models are taken with a grain of salt but can show general trends. The long range GFS model below shows Danielle recurving but the next system (Earl?) possibly making it further west than Danielle is expected to move by next Monday.  

Tropical Storm Frank has remained offshore of the Mexican coast. The forecast cone shown below is now totally offshore. As a result, tropical storm warnings were dropped for Acapulco. Hopefully, cruises going to the Mexican Riviera can be re-routed effectively around this storm.

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