Tropical weather analysis - May 17, 2013

By: KoritheMan , 5:41 AM GMT on May 17, 2013

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Perhaps Simon will do better in 2014. In a bit of a surprise, westerly shear, associated with a large upper-level trough several hundred miles northwest of the tropical cyclone, began to increase over Alvin this morning. This shear has exposed the center to view, with Alvin generating only disorganized convection in broken bands southeast of the low-level center. Props to the ECMWF and CMC which anticipated this development several days ago.

As of the most recent NHC advisory, the following information was contained on the cyclone:

Wind: 40 mph, with higher gusts
Location: 9.8°N 110.8°W
Movement: WNW at 14 mph
Pressure: 1006 mb

Alvin is extremely disorganized, and I am honestly having trouble distinguishing it from another ITCZ disturbance to the west.

Figure 1. Latest infrared satellite image of Tropical Storm Alvin. Image credit: NOAA's Satellite Services Division (SSD).

An earlier ASCAT pass suggested that the circulation had decayed into a sharp surface trough. Low cloud lines on shortwave infrared imagery are certainly not suggestive of an existing closed surface circulation, and evening microwave data reveals that even the mid-level center is beginning to lose definition. Westerly shear continues to affect Alvin, and the global models forecast this to continue throughout the forecast period. The GFS still stubbornly assumes that the upper tropospheric flow will improve and become mildly diffluent, but even that model is starting to recognize the presence of westerly shear within its 850-200 mb forecast fields. Given the hostile environment, Alvin is expected to degenerate into a remnant low within the next 48 hours, although it could occur much sooner if a closed circulation definitely does not exist. I will generously keep a distinct vorticity maximum throughout the five day forecast period, but I am not expecting Alvin to last that long after dissipation.

Alvin remains south of a weak mid-level ridge over southern Mexico. This is the primary steering mechanism for now, but the global models forecast a gradual turn to the west in about two or three days as Alvin becomes a shallower vortex and responds to the lower-tropospheric flow. Compared to yesterday, the model consensus is actually a consensus, and this increases confidence in my track forecast. I am well to the south of the National Hurricane Center forecast track as well as the model consensus given the likelihood that the cyclone no longer has a closed circulation. I could have gone even farther south, but at least some continuity with the models is desirable. Alvin -- or more likely its post-tropical remnants -- are anticipated to turn southwestward near the end of the forecast period while the mid-level center moves northward to northeastward ahead of the trough near Baja.

5-day intensity forecast

INIT 17/0300Z 35 KT 40 MPH
12H 17/1200Z 35 KT 40 MPH
24H 18/0000Z 30 KT 35 MPH
36H 18/1200Z 30 KT 35 MPH
48H 19/0000Z 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 20/0000Z 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 21/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 22/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW

5-day track forecast

My 5-day forecast track for Alvin.


There is the potential for long-range development in the Caribbean or western Atlantic if the long-range pattern in the GFS verifies, with a high over the southern United States, which will tend to focus convergence and upward motion farther south in tandem with the arrival of the upward MJO into this area of the world. I will probably not touch on this much, however, until the timeframe and pattern become more certain.

There is also a tropical wave moving westward across the central Atlantic south of 10N approaching 45W. While upper-level winds are not favorable for development in the Atlantic, this wave will need to be monitored for potential development when it enters the eastern Pacific in about a week.

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1:38 AM GMT on May 23, 2013
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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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