Sunday, September 2, 2012

Severe Thunderstorm rocks Nagpur, interesting weather expected

Akshay Deoras
Severe Weather Forecaster

On the late afternoon hours of 2nd Sept 2012, a very powerful thunderstorm passed through the city of Nagpur. The Thunderstorm which arrived from the NNE directions was one of the most intense I have ever intercepted with severe winds especially the wind gusts and blowing rainfall due to the winds. The lightning also was very severe with high frequency as well

In just 30-40mins from the initiation, the rains as measured at the HQ were 33mm whereas the IMD recorded 31mm.

Pre-Warnings regarding the thunderstorm couldn't be given as the Doppler Radar of Nagpur is NOT FUNCTIONAL SINCE NEARLY A MONTH WITH THE WARNING 40KVA EXTERNAL UPS UNSERVICEABLE as the visual based warning approach is restricted to only a short corridor of around 50km where one can't estimate the key factors like the Wind Velocity, Hailcore or the rainfall rate etc.. which only a Doppler Radar can do

This raises this big question that What help is the radar costing in crores giving to the City forecasters?

One can check the radar product here 

I have been asked many times since the thunderstorm what was the reason for it and why such thunderstorm happened despite of having an almost clear morning period.

The above is the 850mb ( Roughly 4000-5000ft) above the ground atmospheric condition as analysed at 12z ( 0530PM IST) on 2nd September 2012. One can see a giant region of anti-clockwise circles or spin. Such anticlockwise spin refers to a region of low pressure for Northern Hemisphere.

Hence one can conclude that there is a broad low pressure region offshore West Bengal-Odisha region as indicated by this cyclonic spin at that atmospheric profile.

The vicinity of such lows acts like a boon for such heavy rainfall events/thunderstorms as they get moisture from such lows from the Bay of Bengal hence forward referred as the Bay Low in this text.

The next ingredient of the Severe Thunderstorm Cookbook is the energy referred as the CAPE or Convective Available Potential Energy. The parcel of air needs some energy ( A Positive Buoyancy) for lifting. The more the available energy with no factors affecting like Wind Shear, the more will be the condensation of the parcel of air thus forming a cloud type called as CB or Cumulonimbus.

The CAPE in the region ( say 100km NE of Nagpur) was near to 2000J/Kg which is a significant value in terms of the instability is concerned.

So just after 12PM IST, CB clouds started developing NE of Nagpur due to all the favoured conditions out of which we discussed a few above. The CB clouds then approached Nagpur and if we had the RADAR product, I could have confirmed how strong the thunderstorm outflow was.

But why didn't it happen before? The main reason for it is the presence of the mentioned 850mb low pressure region offshore Odisha. It basically had triggered short pulses which later developed into thunderstorms and moved SW. This condition wasnt there in the last week.

Also the above Visible image from DUNDEE showing Nagpur ( Pink dot) just after the squall line thunderstorm passed. Its very clear from the geometry of the thunderstorms that they were linearly organised and a Squall Line Thunderstorm. This must have intensified the Thunderstorm outflow!

The station 42866 which normally records the data for Nagpur did't take any data for that period so I am evaluating the expected wind speed at that moment with the help of some experts. In my opinion, it must have been atleast 70-80km/hr!

Many big trees were uprooted in almost all the parts of the city. I could record a HD video of the event along with this solo picture of the fallen branch of the tree in my area-

Here is the HD video of the Thunderstorm.. Please watch the video in full volume to enjoy the nature's fury-


In the latest news, 5 people were reported to have died as the portion of the temple in which they were collapsed on them-

India Weather Update-

The biggest concern at the moment is from a region of a Positive Vorticity at the Mid Tropospheric levels as shown in the above GFS projected ( +72hr) base 12z,2nd Sept 2012 plot. 
The region is at present near North AP offshore. Its nothing but part of the above 850mb anticlockwise spin ( top most image). Thus one can see the region of Positive vorticity approaching Central India once again by Tue-Wed period. 

In that regard, 

Heavy to Very Heavy rains are expected in Northern AP-Southern Chattisgarh 

Even Western,Central AP and Eastern Karnataka will receive heavy rains during the same period i.e Monday and Tuesday 

The rains will then move into Southern,Central Maharashtra including Nagpur on Tuesday onwards continuing on Wednesday as well and gradually spreading into Central and NW Madhya Pradesh. 

Gujarat and Sindh region will get some rains from this system and another already present there around Thu-Friday.

Considering the robust instability present in Central India, Severe Thunderstorms may re-occur and hence you must follow METD WEATHER FACEBOOK PAGE for details

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