Wednesday, September 16, 2009

Thundershowers line up along coasts, parts of peninsula

The monsoon trough continued to run close to the foothills of the Himalayas on Tuesday but the unsettled weather marked by widely separated thundershowers ruled the roost in the south peninsula. The monsoon has been vigorous in south interior Karnataka during the 24 hours ending Tuesday morning, according to an update.


While the alignment of the monsoon trough indicated little or nil activity over northwest India, a weather-maker north-south trough ran down along the country's eastern corridor. The India Meteorological Department (IMD) update said that the latter trough cut a wedge from Bihar to south Tamil south coastal Tamil Nadu through Jharkhand, Orissa and coastal Andhra Pradesh. Moisture-laden easterlies empty their contents into this trough raising the moisture levels, which combine with the ambient temperature to set up thundershowers and cloudbursts in the region. This is being replicated along the west coast as well, resulting in localised heavy falls, the likes of which have been reported from Mumbai as well as parts of western Maharashtra. This is expected to continue till such time as the dry climes pushing in from the extreme western parts of the country bring the monsoon withdrawal line following not too far from behind. The IMD has not given any hint as to the advance of the `Big Dry' since the known meteorological events associated with the event have not materialised in their entirety.


The withdrawal of the monsoon is more gradual than its onset and is characterised by the reduction in overall rainfall; decay of the anti-cyclonic circulation established over the Tibetan Plateau during monsoon; and the reappearance of the upper-level westerly jet stream south of the Himalayas. On ground, parts of Rajasthan and adjoining Gujarat are already witnessing elevated mercury levels even as thundershowers line up over the peninsula. The IMD said in its update that the subdued monsoon over northwest Rajasthan had driven up day temperatures appreciably to markedly above normal to reaching 40 deg C and above at many places. The US National Centres for Environmental Prediction (NCEP) indicates that the drying is obvious over these regions, which would gradually extend into adjoining Madhya Pradesh during the week ending September 22. Over the peninsula, activity emanating from southern Bay of Bengal will grow into adjoining southeast coast and the rest of the peninsula. Almost the entire west coast and adjoining regions will benefit, with showers tending to concentrate over Kerala and adjoining interior Tamil Nadu. The week from September 22 to October 1, conventionally the last active monsoon week over the mainland, would see a surge in thundershower activity over south Kerala and adjoining southern Tamil Nadu. In the east, Gangetic West Bengal is expected to come under thundershowers during this phase.


This projected itinerary would mean that monsoon withdrawal would not be complete by the September-end and will get delayed until sometime into October. On Tuesday, the IMD traced the cyclonic circulation persisting from overnight to over east Uttar Pradesh. The presence of a trough in the westerlies would bring isolated thundershowers over the extreme north. The IMD has forecast scattered to fairly widespread rainfall with isolated heavy falls over south peninsular India during the next four days. Scattered to fairly widespread rainfall is also likely over the North-Eastern States during the next two days and increase thereafter. Satellite pictures on Tuesday showed convective clouds over south Andaman Sea, southeast Bay of Bengal, Assam, Meghalaya, Nagaland, Manipur, Mizoram and Tripura. A warning valid for the next two days said that isolated heavy rainfall is likely over Karnataka, coastal Tamil Nadu, Kerala and Andaman and Nicobar Islands. Fairly widespread rainfall activity with isolated heavy falls is likely over south peninsular India and the North-Eastern States.

Hurricane Fred - Update

NASA's QuikSCAT satellite captured this view of Hurricane Fred on September 10, 2009, shortly after 4 p.m. Atlantic Daylight time. At the time, Fred had winds of about 150 kilometers per hour (90 miles per hour), making it a Category 1 hurricane according to the National Hurricane Center. This image shows the wind structure of the storm. The strongest winds are purple, and the weakest are blue. Like all tropical storms, Fred has a core of strong winds surrounded by gradually weakening winds. The storm exhibits a distinctive "s" shape-a structure left over, perhaps, from its time as a Category 4 storm.

QuikSCAT is a scatterometer that measures wind speed by bouncing microwave pulses off the ocean's surface and recording the echo. An ocean tossed by strong winds will scatter the energy differently than a calm ocean, so the measurement provides an indication of both wind speed and direction. However, since hurricanes are relatively rare, scientists have not calibrated the data to accurately report wind speeds greater than 50 knots (about 60 miles per hour or 90 kilometers per hour). Heavy rain also distorts the ocean's surface, making wind speeds more difficult to gauge. As a result, this image shows relative wind speeds rather than providing an accurate reporting of actual wind speeds.

According to a bulletin released by the U.S. National Hurricane Center at 5 p.m. Atlantic Standard Time September 9, 2009, the eye of Hurricane Fred hovered over the Atlantic Ocean some 955 kilometers (595 miles) west of the southernmost Cape Verde Islands. With maximum sustained wind speeds of 185 kilometers (115 miles) per hour, Fred was a Category Three hurricane. The National Hurricane Center reported, however, that the storm had weakened slightly over the previous several hours, and was expected to continue weakening over the next 48 hours.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA'sTerra satellite captured this true-color image of Hurricane Fred at 8:55 a.m. Atlantic Standard Time (12:55 UTC) on September 9, 2009. Spanning hundreds of kilometers, Fred travels northwestward over the Atlantic Ocean.