Monthly Archives: August 2010

Ex-Tropical Storm Colin & Invest 92L…

There are several tropical systems to monitor.

Ex -TS Colin who degenerated to an open wave yesterday is still struggling but is holding it’s own. Colin fell apart due to the wind shear and the remnants are just now north of Lesser Antilles Islands. Wind shear is very high, between 20-25 knots, yet the latest satellite imagery is showing the heavy thunderstorm activity has increased. The remains of Colin is in an area that is unfavorable for any re-development. Colin will soon be under an upper-level low with plenty of dry air and high wind shear.

The latest SHIPS model forecast is a dropping of the wind shear from the 15-25 knots down to a 15-20 knot by sometime Thursday or Friday. The major models do predict that Colin will redevelop in the next 3- 4 days. What status Colin will be is still unclear. Tropical Storm is the highest it should attain but that is pushing it. A major trough of low pressure will be moving off the East coast of the U.S. and that trough will “pull” Colin to the Northwest and later out to sea. It may get close to Bermuda possibly sometime Saturday or Sunday. The reconnaissance plane has gone out late today and found that the remains of ex-Colin does not have a closed surface circulation, however they did find tropical force winds in the northeastern quadrant.

Invest 92L
Invest 92L was designated late yesterday. Invest 92L is located in the Caribbean, south of eastern Cuba and Haiti around 16° N – 72° W moving westward at 15-20 mph. The wave is over warm water with very low wind shear. There is some indication that it could develop in the next few days but is is moving very fast and maybe not have enough time to develop into a tropical depression. If it keeps the westward track it should come ashore over Nicaragua/Honduras.

And last on the list is a new system that has just come of the coast of Africa. It has very good cyclonic turning as it heads westward. This system may have problems with both moderate amounts of SAL and as it heads westward it will encounter some wind shear of 20-30 knots.

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Tropical Storm Colin

Invest 91L was reclassified as Tropical Depression Four yesterday morning and at 5:00 am today it became the third tropical storm of the season – Colin.

Although it is a tropical storm, it is very small in size. It is also very ragged in appearance. The SST’s in the area of TS Colin are in the 28° – 29° (C) range which is plenty enough for continued development but Colin’s biggest negative is the storm’s size. It makes Colin vulnerable to any mid to upper increases in either dry air entrainment or wind shear. There is some SAL but it is to the northwest of TS Colin and it is not affecting the storm right now.

Currently, TS Colin looks very sickly due to some westerly vertical shear along with the rapid forward motion (24 MPH). There is also a question whether Colin has a closed circulation or not. It is a must for a tropical cyclone to have a closed center of circulation other wise it will be downgraded to an open wave.

The forecast (short term) is fairly simple. If Colin survives the day and/or can find a way to he must do it quickly. The next 24 – 36 hours are crucial for any further development. Beyond that time frame, Colin will start to encounter an increase in the upper level winds caused by a western Atlantic trough. Any further strengthening is not indicated at this time.

Long term forecast is very difficult. Colin is on the southern side of the low/mid Bermuda high. In 48-72 hours a weakness in that ridge due to mid/upper level near the southeastern coast of the US. This should cause Colin to steer on a WNW pattern and then turn to the the NW as it slows down. At that point, the forecast is very unclear. There is no model consensus at this time. The HWRF and the GFDL models are on the right side of the guidance envelope whereas the Canadian(CMC) and GFDN models are on the left. For this forecast period, the NHC has shifted the track southwest and to the left of the previous model track.

ADDENDUM:
As expected, Colin did not last long. As of 5:00pm the NHC dropped Colin from as TS to a remnant low and no longer putting out any advisories unless regeneration occurs. There is still a lot of vorticity so regeneration may happen in a 2-3 days.

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Invest 91L

This morning an area of some very intense thunderstorms has become better organized and has now been designated as Invest 91L. The NHC has this system at a 60% chance of becoming a tropical depression by Tuesday morning. Satellite loop imagery is indicating that the intense thunderstorm activity and intensity has been increasing. The latest satellite images seem to indicate that 91L has very good cyclonic turning and may be beginning to form the surface circulation even though 91L is still within the ITCZ and still to close to the equator. This may be the beginning for 91L to break away from the ITCZ and if so then 91L may be classified as either a tropical depression or tropical storm later today or tonight.

With the SST’s in the area at around 29° centigrade and the SAL to the north of 91L along with the wind shear at 10-20 knots, further development is a distinct possibility. The only negatives for any development for 91L is the MJO, which favors downward motion over the tropical Atlantic and as stated earlier, being to close to the Equator.

Later in the week, as 91L approaches the Lesser Antilles, a strong upper-level low near Puerto Rico is forecast to bring high levels of wind shear. This would hamper any quick intensification. As usual, any long range forecasts will have changes so to state where and how strong any system will be is just speculation.

Steering layer forecasts to keep 91L on a WNW track at least for the next 1-2 days, after that a WNW – NW motion is forecast. All those in the Leeward Islands should keep a eye on 91L.

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