Sunday, November 24, 2013

Tropical Storm Lehar approaches Andaman and Nicobar Islands amidst future troubles

Akshay Deoras
Severe Weather Forecaster

The newly formed Tropical Storm Lehar is approaching the Andaman and Nicobar Islands now and at the moment passing between Port Blair and Little Andaman Island as can be seen in this IR-NHC image via CIMSS taken at 1330UT,24th Nov.

At the moment, based on DVORAK estimates, Tropical Storm Lehar has peak winds of 55kt i.e about 100km/hr and over the past 6hrs has tracked NW with an increased velocity of 12kt or ~22km/hr (Previously it was tracking by about 15km/hr). Although a CDO (Central Dense Overcast) feature seems to be visible now in the IR-NHC image, Lehar has started running out of trouble as some of its vital parameters at the moment are not supporting it- although it continues to maintain its intensity and has intensified by 10kt in the past few hours.

The primary concern coming now is the sliding drier air from the North (from the mainland of India into Northern Bay of Bengal). Here is a comparison of the latest and pre latest Total Precipitable Water image-

The topmost image shows the Precipitable Water Content- an indicator of the moisture in the atmosphere some hours ago. The area to look for is North of Bay of Bengal where you can see a triangular region of purple,blue,green shades. Now see that region again in the bottommost image. Its very clear that the dry air is sliding down from the mainland into Bay of Bengal and apparently seems to be interacting with the outer rainbands of Tropical Storm Lehar. Dry air is a kind of poison to such thermodynamic systems- Tropical Storm Lehar as it chokes off the thermal engine which leads to weakening of the storm. We are monitoring this region of drier air.. The reason why this dry air is getting pushed south is being discussed. If it slides further south, within 24hrs it will interact with Tropical Storm Lehar and hence the rate of its intensification shall decrease.

The second trouble thats coming up for Lehar is the decreased lower air convergence over the last few hours. Low air convergence is an essential factor for cyclones to intensify. From the past ~25kt low air convergence, the convergence now as per CIMSS stands out to be barely 10-15kt and is offset the CDO/LLCC i.e is SE of the centre of the tropical storm.

IT does appear that over say 12hrs or so the low convergence will be decreasing significantly.
The situation at the upper level is more or less the same. The system had an excellent poleward outflow of 60kt last night which has dropped now to 30kt and the tendency seems to be decreasing ahead as well amidst a moderate wind shear of 20kt...

It does appear that Tropical Storm Lehar won't be intensifying for sometime now, say during the next 12hrs or so. It probably will be disorganise a little given the decreasing convergence. Probably the pressure at the centre which is 996mb shall rise littlebit.

The fate of Tropical Storm Lehar will entirely depend on the above parameters as it churns in a warm sea with present sea surface temperature of 29C. The subtropical ridge (STR) aloft which is steering the system is at the present towards north of the system and over the next 24hrs, the system will be riding under the STR as it becomes parallel to the latitude (non inclined) and after 72hr, the STR shall be south of the system. Unlike Cyclone Phailin, TS Lehar has some factors to non stop affect it (given above ofcourse) and later the cooler sea offshore AP-Odisha coast.

As a result, its being inferred that Tropical Storm Lehar will intensify to A MAXIMUM CATEGORY 2 CYCLONE. This intensification,rather Rapid Intensification shall start after it gets sufficient land clearance,say later tomorrow,25th Nov

Regarding the landfall position, the cone of uncertainty is still high however there are increased chances that the system will land in Andhra Pradesh (South of Vizag) : Near Kakinada on the afternoon of Thursday,28th Nov 2013. Once we have an increased confidence over the landfall area, alert map will be displayed..

Tropical Storm Lehar however shall make a landfall as a Category 2 cyclone (present estimates) with maximum winds of around 175km/hr!


No comments:

Post a Comment