Expect a wet weekend

Rains brought on by an upper air pattern may cause widespread damage to crops in many states

By Akshay Deoras
Last Updated: Thursday 17 September 2015

Rains brought on by an upper air pattern may cause widespread damage to crops in many states

If you have made your plan to spend the weekend outdoors, think again. The weather is likely to wash it out and, more importantly, damage winter crops in many parts of India.

An upper air pattern is all set to change the weather of almost entire India from Saturday, February 28. Precipitation during the so-called winter months isn’t a new feature for those living in north India due to frequent western disturbances. February saw a relay of four western disturbances impacting north India (till date), particularly Jammu and Kashmir and Himachal Pradesh. The readers might have come across a few forecasts mentioning the arrival of “another western disturbance”, but the weather system which is going to arrive now will be a bit different. The usual western disturbances can’t bring rainfall in multiple states but the upcoming system is going to do so.

Let’s quickly understand how this system will be different.

What is upper air weather pattern?

Earth has a big atmosphere which is composed of different layers. One such layer, closest to earth’s surface, is called the troposphere. The troposphere usually extends to an attitude of about 10 km. Its extent varies from latitude to latitude and from season to season. The significance of this layer is that all the weather systems on the earth (like low pressure systems, high pressure systems, clouds) exist in this layer and hence it’s of great importance in weather forecasting. The troposphere, too, is composed of different layers of air having different temperatures, air pressures. The phrase “upper air” simply means upper part of this troposphere, say the layer which is at an altitude of about 30,000 feet (9,000 metre or thereabout). Aircraft fly through this upper air.

Upper air has another significant aspect. Strong currents of air known as jet streams circulate across the globe, mostly around this layer. Such currents are found in both the hemispheres of the earth and they usually move from west to east over higher latitude nations such as India. These jet streams are of two forms depending on the location where they are found. A stronger jet found over very high latitude regions (typically 50°-60° north/south) like in the northern parts of the US is known as a polar jet whereas the one which is found over India (typically around 30° north/south) is known as a subtropical jet. So when one says “upper air pattern”, it refers to the pattern in the flow of these jet streams. In most of the cases, these patterns determine the weather conditions.

Zonal Flow Vs Meridional flow

Two patterns in the flow of jet stream. Courtesy- skeptical science

Two main patterns are observed in the flow of the jet stream. The red coloured pattern represents a zonal flow whereas orange indicates meridional flow. Mostly clear weather will be experienced in the regions above which the flow of jet stream is zonal. Along the orange path (the meridional flow), one can see that the jet goes up (this position is called as crest) and down (this position is called as trough). Weather forecasters keep on studying the weather charts, showing upper air conditions to monitor such crests and troughs.

Position of crests (ridges) and troughs in a jet stream’s path. Courtesy- AMS

The above image gives more details of operational significance of such a upper air weather patterns. Arrows in the image represent the path of the current of air (or jet streams) whereas ‘H’ represents a high pressure region and ‘L’ represents a low pressure region. In meteorology, such crests are called ridges.

The ridge or an elongated region of high pressure is mostly associated with clear weather but the trough or elongated region of low pressure is mostly associated with bad weather, such as thunderstorms or rains. This is because an approaching trough destabilises the troposphere and often sets the stage of a severe weather outbreak. You can see that there are two regions which are represented by dotted circles—one to the left and one to the right of ‘L’ (low pressure). The region to the right is the “hotspot” of severe weather outbreak. This is the region where the winds of the jet stream accelerate and diverge in direction. This is a favouring condition for thunderstorm development. Troughs are also known to deepen the low pressure regions situated below them.

Western disturbances are observed in the middle parts of the troposphere unlike this trough whose appearance is in the higher parts of troposphere.

Weekend’s weather pattern

GFS weather model showing trough over India on Saturday, 28th February. Courtesy- COLA/IGES

In the above image, arrows indicate the direction of winds at an altitude of about 35,000 feet (over 9,100 metre) above ground. The large purple-shaded region indicates the jet stream and the trough shape (U shape) can be seen over north-west India and adjoining Pakistan.

GFS weather model showing conditions at 5,000 feet above the ground. Low pressure over Pakistan can be seen on Sunday, 1st March. Courtesy- COLA/ IGES

An associated low pressure at about 5,000 feet (over 1,500 metre) above ground will start deepening from Friday. On Saturday, this low pressure region would be near western part of Gujarat and this will influence the winds in the lower part of the troposphere. As this low pressure moves northwards—towards Pakistan, by Sunday (see above image), massive moisture incursion from the Arabian Sea will take place. As a result, the lower part of troposphere over western, central, northern and southern India will get filled with moisture, which would favour precipitation.

Weather on Saturday

  1. Much of Tamil Nadu, Kerala, southern Andhra Pradesh and south-eastern Karnataka will get light rains.
  2. Western coast of Maharashtra (Konkan division), western parts of Maharashtra (including Pune), Vidarbha and Marathwada will receive rains towards evening/night. Some rain in Mumbai as well; up to 25mm possible in the areas around Mumbai.
  3. Western coastal line of Gujarat and western areas like Junagadh will get light rains. Light rains also expected in Daman and Diu.
  4. Eastern Madhya Pradesh also likely to receive light rains.
  5. Delhi-NCR, Haryana and adjoining parts of Rajasthan will get rains/thundershowers during the night of Saturday-Sunday.
  6. Severe thunderstorms, possibly hailstorms can be expected in parts of western Uttar Pradesh.

Precipitation to increase on Sunday

  1. Light rains can be expected in parts of Tamil Nadu and Kerala.
  2. Entire western coast of Maharashtra will get rain. Moderate to heavy intensity rainfall expected in western part of the state. Pune will also witness rains with a cold day. Moderate intensity rainfall for Eastern Maharashtra (such as Vidarbha)
  3. Heavy rains predicted for eastern Madhya Pradesh, adjoining Uttar Pradesh. Chattisgarh will also get light rains.
  4. Nepal gets heavy rains. Localised flooding possible in many areas.
  5. A cold, wet and windy day for entire north India. Heavy rains expected in Punjab, Haryana, Chandigarh, J&K, Himachal Pradesh and Uttarakhand. Moderate snowfall expected in the higher areas of western J&K, Uttarakhand and Himachal Pradesh. Due to rains, maximum temperature will stay around 16°C in New Delhi.

Calmer Monday

Reduction in rainfall predicted for Maharashtra and Madhya Pradesh. Another cold day expected in much of north India. Heavy rainfall and snowfall will continue in J&K, Himachal Pradesh. Heavy rainfall expected in Punjab, Haryana, Chandigarh and Uttarakhand. There is possibility of water logging and localised flooding in Punjab, Haryana and Chandigarh. Good rainfall expected in Delhi-NCR, resulting in another cold day.

Prediction for Tuesday, March 3

Light rains shift eastwards towards Odisha, Jharkhand and parts of West Bengal. Reduction in rains in north India and the weather will be normalising gradually.

Damage to crops

This is a period during which the Rabi crops, such as mustard, gram and lentils require dry weather and most of them are usually harvested around this time. Hence, unseasonal rainfall can significantly damage them. Wheat, too, is approaching maturity and can get adversely affected due to heavy rains. As heavy rains are expected in major wheat growing areas of India like Punjab and Haryana, significant damage to it can be expected. Plus damage to other Rabi crops also expected due to the untimely rains.

In Maharashtra, particularly the Konkan belt, this untimely rainfall will affect the productivity of mangoes as it is the flowering season. Wheat and cotton grown in other parts of the state like Marathwada, Vidarbha may also get affected due to the rains.

Akshay Deoras is an independent weather forecaster

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