Tropical weather analysis - September 13, 2011

By: KoritheMan , 4:41 AM GMT on September 14, 2011

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After a lengthy break brought on by a cold, I am back to forecasting. Sorry for the delay, thanks for waiting!


Tropical Storm Maria remains weak as of the latest NHC advisory:

Wind: 50 mph, with higher gusts
Location: 23.5°N 68.2°W
Movement: N at 9 mph
Pressure: 1003 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)

The satellite signature is not very impressive, with the center along the northwestern mass of the convection.

Figure 1. Latest infrared satellite image of Tropical Storm Maria, courtesy of NOAA's Satellite Services Division (SSD).

The relatively flat outflow pattern on the western side suggests some residual westerly shear due to a nearby upper trough. This has been a persistent feature throughout Maria's lifetime, and synoptic analyses suggest that it is still going fairly strong, albeit weaker than the last several days. Nevertheless, this feature is slowly lifting northeast, which could transition the upper flow pattern to southwesterly, which would be more conducive to intensification (but still not ideal).

The GFS continues to insist on allowing a reduction of the vertical shear over the next day or so, which could allow for some modest restrengthening. By about 48 hours, however, southwesterly shear is forecast to increase dramatically in advance of a deep-layer trough now moving through the Ohio Valley, which should put an end to the intensification process. In addition, Maria is likely to pass over Katia's cold wake that she upwelled a week or so ago, and although objective SST analyses indicate that water temperatures have recovered slightly in this region, they are still only marginally warm, about 26 to 26.5C, which is barely above the threshold generally needed for tropical cyclogenesis.

Having said all of that, I think the NHC's current forecast of 65 mph is reasonable, although it remains to be seen if the shear will ever decrease as forecast, and if it does, if Maria is able to ever regain any structural integrity.

High amplitude southwesterly flow associated with the aforementioned trough is likely to pull Maria north-northeast, then northeast, out to sea. However, it could pass within about 100 miles or so west of Bermuda on Thursday afternoon, which would be close enough to bring tropical storm conditions to that island. Consequently, a tropical storm warning has been issued for that island.

The global models suggest that Maria will become extratropical in about two and a half days, and then merge with the large extratropical low on Saturday.


Elsewhere in the tropics, the global models, including the reliable GFS and ECMWF, continue to predict the formation of a tropical depression in the eastern Atlantic over the next five days. This system is very likely (>90%) to end up recurving out to sea, probably even east of Bermuda.

Climatology suggests an end to the Cape Verde season is coming soon, and possible Ophelia may be our final one. Then we will need to start looking closer to home for development. In fact, the global models already hint at a more favorable pattern for homegrown development, with large scale troughing over the central US, and ridging downstream of that, something we have not seen much of this season. However, run to run variability and the long-range prospects of such an event make trying to predict its specifics meaningless.

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1. traumaboyy
6:58 AM GMT on September 14, 2011
Thanks Kori!!
Member Since: August 18, 2008 Posts: 0 Comments: 2266

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.

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