Aletta holding onto life; 92E slowly organizing; Watching the Atlantic for trouble

By: TropicalAnalystwx13 , 2:13 AM GMT on May 18, 2012

Tropical Depression Aletta is clinging to life this evening despite the presence of moderate wind shear and dry air. In fact, an objective analysis from the Cooperative Institute for Meteorological Satellite Studies (CIMSS) reveals high south-southwesterly wind shear in excess of 25-30 knots affecting the system. In addition, water vapor satellite images show Aletta is located on the eastern periphery of a large area of very dry air. However, the system has continually firing deep convection in excess of -60 ┬░C atop a well-defined center of circulation. It is uncertain how long convection will continue to fire, but Advanced Dvorak Technique numbers show that Aletta may very well be on the fringes of tropical storm intensity.

Figure 1. Late evening infrared imagery of Tropical Depression Aletta.

Forecast for Aletta
Tropical Depression Aletta has began a northward turn due to the effects of a weakness passing to its north across the US Central Plains. As this weakness continues eastward, Aletta should continue a northward motion and eventually turn towards the northeast. The latest SHIPS model continues with high wind shear in excess of 20 knots throughout the rest of the forecast period and a very dry atmosphere characterized by 700-500 mb Relative Humidity values below 50%. These conditions would typically act to weaken a system rapidly, but given how Aletta continues to build deep convection in such an environment argues for a more gradual weakening trend. The official National Hurricane Center forecast dissipates the cyclone in roughly 36 hours or so; I believe it will take slightly longer than that. Some of the global models are predicting that after Aletta degenerates into a remnant area of low pressure it could be absorbed within a larger circulation (Invest 92E). Considering how weak the low pressure area would likely be, a Fujiwhara effect is not feasible.


INIT 18/0000Z 30 KT 35 MPH
12 HR 18/1200Z 30 KT 35 MPH
24 HR 19/0000Z 25 KT 30 MPH
36 HR 19/1200Z 25 KT 30 MPH
48 HR 20/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
72 HR 20/1200Z 20 KT 25 MPH...POST-TROP/REMNT LOW
96 HR 21/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
120 HR 21/1200Z...DISSIPATED

Figure 2. Rainbow imagery of Invest 92E.

Invest 92E slowly organizing
Invest 92E is gradually becoming better organized in the East Pacific this evening. Disregarding the recent dissipation of deep convection, which is likely due to the effects of D-MIN, the last visible satellite images of the day revealed a slightly better defined area of low pressure as opposed to its appearance earlier this morning. An objective analysis from CIMSS reveals high wind shear of 20-30 knots. However, given its satellite appearance, this may have analyzed a good deal too high. Relative Humidity values lie in excess of 75% which is very favorable for continued organization.

The forecast for 92E
As aforementioned, 92E lies within an environment favorable for continued development. In fact, a majority of the models show the invest attaining tropical storm status within the next four days. The latest National Hurricane Center Tropical Weather Outlook gave 92E a Medium chance, 40%, of becoming a tropical cyclone within the next 48 hours (5 PM PDT Saturday). All things considered, I believe the disturbance has a slightly lower chance, 30%, of becoming at least a tropical depression by Saturday evening. While difficult to forecast, the latest SHIPS model forecast gives 92E a respectable 26% probability of undergoing rapid intensification of at least 30 mph. I wouldn't rule it out at all either after the weekend.

The long term track of 92E is unknown. Many of the global models, such as the ECMWF and GFS, continue 92E west-northwest/northwestward over the next few days. However, as a strong weakness builds over the US Central Plains, a turn towards the north and then northeast is predicted. How far west the disturbance gets before turning determines whether or not it makes landfall on the Mexican coastline the first time. If 92E does not manage to get very far before feeling the effects of the weakness, the chances for landfall are heightened. On the flip side, if the disturbance can get very far west, its chances of making landfall are reduced as a ridge of high pressure will likely build in behind the trough. Regardless, residents living along the Mexican coastline need to be on high alert for 92E as many of the models, GFS and ECMWF included, intensify "Bud" into a hurricane before landfall.


INIT 18/0000Z 25 KT 30 MPH
12 HR 18/12000Z 25 KT 30 MPH
24 HR 19/0000Z 25 KT 30 MPH
36 HR 19/1200Z 25 KT 30 MPH
48 HR 20/0000Z 25 KT 30 MPH
72 HR 20/1200Z 25 KT 30 MPH
96 HR 21/0000Z 30 KT 35 MPH
120 HR 21/1200Z 30 KT 35 MPH

Watching the Atlantic
The global models continue to hint on the development of a hybrid low pressure area off the coast of the Carolinas over the weekend as an upper level low becomes stacked with a low pressure area at the surface over the Gulf Stream. It doesn't appear particularly likely that a strong system would develop, but a weak area of low pressure that brings heavy rainfall and gusty winds along the coasts of South Carolina, North Carolina, and Virginia specifically is definitely feasible. I am giving this area a low chance, 10%, of becoming a tropical cyclone over the next 48 hours.

After several weeks of waiting on the GFS model's solution to verify, an area of low pressure has finally developed in the Gulf of Honduras. Little to no convection obscures this low pressure area due to high wind shear in excess of 20 knots and the effects of D-MIN. Unfavorable conditions are expected to continue for the next few days as an upper level low stays parked along the northern Gulf Coast. It is possible that wind shear could lower by the middle of next week as an anticyclone moves across Central America, but land interaction will likely be a big inhibitor for anything tropical that tries to form. As an upper level trough passes to the north of the disturbance, a northeastward track will likely be induced, tracking the low and associated heavy rainfall across Florida and the Bahamas. Due to strong westerlies, anything that exited the Caribbean would likely be sheared to part quickly. I am giving a low chance, ~0%, of this low pressure becoming a tropical cyclone over the next 48 hours.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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6. MAweatherboy1
10:30 AM GMT on May 18, 2012
Nice job :)
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5. KoritheMan
5:21 AM GMT on May 18, 2012
Excellent update, Cody. We definitely need to watch 91E.
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4. PedleyCA
4:12 AM GMT on May 18, 2012
Nice one.
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3. ajcamsmom2
3:17 AM GMT on May 18, 2012
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2. AllStar17
2:25 AM GMT on May 18, 2012
Very nice job!
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1. pcola57
2:25 AM GMT on May 18, 2012
Thumbs up TA :)
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Small town USA guy. Politics nerd. Soccer fan. Interested in eyewalls, deformation zones, and hook echos.

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