Tropical weather analysis - October 23, 2011

By: KoritheMan , 3:02 AM GMT on October 24, 2011

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Tropical Storm Rina

Tropical Storm Rina formed today over the western Caribbean, east of the northeast coast of Honduras. As of the latest NHC advisory, the following was posted on this system:

Wind: 40 mph, with higher gusts
Location: 16.4°N 82.2°W
Movement: NNW at 8 mph
Pressure: 1004 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)

Last visible images before sunset showed that the cyclone center was a bit exposed on the southeast edge of the convective canopy. This is likely being caused by modest southeasterly shear, diagnosed as 16 kt by the SHIPS model. A 2300 UTC SSMIS microwave overpass also revealed the exposition of the low-level center.

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

The current shear notwithstanding, the environment isn't expected to be prohibitive enough to prevent strengthening, and I am still calling for this system to become a hurricane over the next few days. Short term strengthening should be slow given current environmental trends. Water vapor imagery reveals that a large area of dry air lies to the north and south of the tropical cyclone. While the dry air to the north over the Gulf of Mexico doesn't appear to be adversely affecting the cyclone at this time, the same cannot be said for the dry air lingering to the south. The aforementioned southerly shear is ingesting dry air into the southern semicircle. While these factors should lessen as Rina gains latitude, she may eventually entrain some of the northern dry air as well, which could curb rapid intensification. At the very least, the environment should remain diffluent. While this type of shear regime is not as conducive as an anticyclonic one, it is supportive enough for at least gradual strengthening. Given that at least occasional bouts of southeasterly shear may continue impacting the cyclone throughout the forecast period, major hurricane status appears unlikely. It should also be noted that this particular tropical cyclone is rather small, and is thus prone to large fluctuations in intensity, up or down. It is also true that such storms generally have less difficulty dealing with dry airmasses because their associated circulations are too small to entrain as much dry air as the notoriously bigger ones.

Now for the fun part. Making track forecasts for October storms in the Caribbean is always fun. Water vapor imagery and synoptic steering analysis from University of Wisconsin CIMSS shows that the storm is caught between a ridge over Central America and the western Atlantic ridge. A weakness in the secondary ridge, the one over the Atlantic, is evident west of 70W. Right now, the Central America ridge is pretty far to the west, so that it isn't really allowing the cyclone to get too far west. In light of this, I feel that the National Hurricane Center forecast track is good for now, bringing the storm over the central Yucatan Peninsula on days four and five.

By day three, a significant trough is forecast to begin amplifying over the western United States. As this trough and its associated cold front pivot eastward, a large weakness should quickly manifest within the western Atlantic ridge. This evolution favors Rina turning northward into the southern Gulf of Mexico, and then gradually accelerating northeastward toward the Florida coast. This is pretty typical for October storms in this part of the world.

It should be noted that even though I anticipate Rina to eventually impact the west coast of Florida, westerly to southwesterly shear associated with the aforementioned cold front is forecast to be much too prohibitive to allow for intensification while over the Gulf of Mexico. This is also not unusual for systems coming up from the Caribbean toward peninsular Florida this late in the year.

However, since Rina is expected to be a deep system, interests in the Yucatan Peninsula should be preparing for the possibility of a hurricane impacting them in four to five days.

Watches and warnings




Invest 97L

An area of low pressure ("97L") approaching Barbados doesn't look like much now, but high cloud motions derived from satellite imagery suggest that the southwesterly shearing regime that has been impacting the system for days has finally relaxed. Dry air immediately ahead of the system may inhibit rapid development, but with upper-level winds now at more favorable parameters, this system bears watching. Although it's nothing more than a cluster of showers now, these systems can sneak up on us quickly, so we will need to closely monitor it as it moves through the Windward Islands tonight into tomorrow.

I still give this system a 30% chance of eventually developing into a tropical cyclone. Depending on how it fares through tomorrow, these probabilities may have to be increased.

Regardless of development, heavy rains and gusty winds should accompany this system as it moves through the southern Windward Islands.

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5. KoritheMan
1:11 AM GMT on October 25, 2011
Quoting BaltimoreBrian:
Another good blog Koritheman. I think Rina will be a south FL event if it comes to that. Do you see that as well or in your opinion is central Florida also in play?

Now that she is a deeper system, that's becoming more likely.
Member Since: March 7, 2007 Posts: 603 Comments: 21818
4. winter123
3:49 AM GMT on October 24, 2011
97L has been dead for at least a day...

As for Rina, it's crazy that NHC say it's moving NNW, then have the first forecast point to the WNW. Do they even pay attention to what they are doing or is it completely automated? That said, tracks will definitely shift right in the next few days. Doubt it will have any impact on Yucatan at all.
Member Since: July 29, 2006 Posts: 28 Comments: 1818
3. BaltimoreBrian
3:46 AM GMT on October 24, 2011
Another good blog Koritheman. I think Rina will be a south FL event if it comes to that. Do you see that as well or in your opinion is central Florida also in play?
Member Since: August 9, 2011 Posts: 26 Comments: 10166
2. Terradad
3:44 AM GMT on October 24, 2011
Thanks Kori! I love to read what you have to say.

Keep up the good work!

Member Since: December 21, 2009 Posts: 0 Comments: 93

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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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