Tropical Weather

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.

Satellite image - Tropical Storm Colin

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.

Vertical Shear Map

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.

Latest Model Tracks

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.

Colin - now a remnant low
Tropical Weather

Was Invest 95L anything tropical?

Did Invest 95L ever meet the criteria to make it a depression or tropical storm? I say NO – it never met the criteria and was nothing but a rain maker with some significant winds.

Tropical Cyclone:
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center.

It the time of 1800 (zulu) – 95L had been a frontal boundary. At around 2100 -it detached from the front so it is now on it’s own. Under NHC criteria it requires the organized deep convection to persist for 12-24 hours. Only then if it still meets the criteria – the NHC may classifying a system as a Tropical Cyclone.

Just looking at the radar imagery at the time – it did not have a fully closed circulation. Also the convection was all SW of the “center”. Some people will argue these points and that’s fine – we all have are own feelings. But just using the NHC criteria guide lines  – 95L was just nothing but a  wet and windy storm.