From the Lee Side
Summer Monsoon Advances Rapidly across India: Massive Flooding Ensues
Posted by: Lee Grenci, 1:24 PM GMT on June 19, 2013 +2
The heavy rains of the Southeast Asia summer monsoon advanced northward across India this week, causing massive flooding (as this YouTube video of an apartment building falling into a flooding river unfortunately shows). While the summer monsoon arrived in southwest India right on schedule (June 1) in the state of Kerala, it spread northward much faster than usual, as the chart below indicates. To get your bearings, the solid green contours indicate the progress of the 2013 summer monsoon (each contour is labeled with a date). You can compare this year's rapid advance to a "normal" progression, which is represented by the dashed, red contours (also labeled with dates).


The progression of the summer monsoon of 2013 (solid green contours labeled with dates) compared to normal progression (dashed red contours labeled with dates). Courtesy of the India Meteorological Department.

Keep in mind that the word monsoon can be traced to an Arabic root meaning "season." It follows that the hallmark of a monsoon is a pronounced seasonal shift in prevailing winds. So the word, "monsoon," does not is not automatically synonymous with rain. Indeed, the winter monsoon, which is marked by a prevailing wind that generally has a pronounced northeasterly component over much of India, is the country's relatively dry season (the wind blows mostly over land from a relatively dry source region). For confirmation, check out the long-term average wind vectors at 1000 mb over the subcontinent of India from December through February (wind speeds are color-coded in meters per second).

On the flip side, much of the rain that falls over India occurs during the summer monsoon. For proof, check out (below) the running tallies of precipitation at Mumbai (western India at latitude 19 degrees North; see map), Bhopal (central India at latitude 23 degrees North), and Kolkata (eastern India at latitude 22 degrees North). During the summer monsoon, prevailing winds blow decidedly onshore (here's the long-term average wind vectors at 1000 mb from June through August, color-coded in meters per second). These low-level onshore winds transport moisture inland from the Indian Ocean, the Arabian Sea, and the Bay of Bengal in response to intense heating over the subcontinent of India, setting the stage for recurrent heavy rains (idealized schematic, courtesy of, and copyright by, a World of Weather: Fundamentals of Meteorology). In effect, the intense heating of the subcontinent creates a large-scale sea breeze.


The average monthly rainfall at Bhopal, Mumbai, and Kolkata. Larger image. Courtesy of A World of Weather: Fundamentals of Meteorology.

I should point out that surface temperatures are dramatically suppressed from their May maxima (May is typically the hottest month in India). To prove my point, check out this plot of average monthly temperatures at the three Indian cities previously mentioned (Bhopal, Mumbai, and Kolkata).

I mentioned that the onset of the 2013 summer monsoon over India was right on time (June 1). There are three criteria that the India Meteorological Department follows in order to declare the official start of the summer monsoon (I'm mostly quoting from the IMD below):

1) If, after May 10, 60% of the available 14 stations in the state of Kerala (Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kasargode and Mangalore; see this map for locations) report rainfall of 2.5 millimeters of rain (roughly 0.1 inches) or more on two consecutive days, then the India Meteorological Department declares the onset over Kerala on the second day, provided conditions 2) and 3) below are also met.

2)The depth of the westerlies must be maintained up to 600 mb (roughly 4.5 kilometers above the ground) in the box bounded by the equator, Latitude 10ºN, and Longitudes 55 ºE and 80ºE. The zonal wind speed over the area bounded by Latitude 5-10ºN, Longitude 70-80ºE must be of the order of 15 to 20 knots at 925 mb.

3)Satellite-derived values of Outgoing Long-wave Radiation (OLR) must be less than 200 watts per square meter in the box bounded by Latitudes 5ºN and 10ºN and Longitudes 70ºE and 75ºE.

The first criterion (above) is pretty straightforward, and here's the bar graph of the percentage of stations reporting at least 2.5 millimeters of rain in late May and the start of June.

The 06Z re-analysis 925-mb isotachs (color-coded in meters per second) and 1000-mb wind arrows (wind direction) on June 1, 2013. Courtesy of ESRL.

The second criterion (above) relates to the arrival of the Low-Level Somali Jet (LLSJ), which is a channel of low-altitude, speedy winds that forms off the of Somalia and gets drawn eastward toward India by an onset vortex. Wow! That's a mouthful! Allow me to back-up a bit so that you can get your tropical bearings. During the warm season, the Low-Level Somali Jet, with speeds sometimes exceeding 40 knots knots at 925 mb (roughly one kilometer up), develops in May as southeasterly trade winds over the Southern Indian Ocean cross the equator and head northward along the east coast of Africa (another response to intensifying solar heating of the land masses in the Northern Hemisphere). This idealized cross section of the Low-level Somali Jet, courtesy of, and copyrighted by Penn State's online certificate program, shows the vertical structure of the LLSJ (that's the Eastern African Highlands on the left). The 06Z re-analysis of 925-mb isotachs (color-coded in meters per second) and 925-mb arrows (wind direction) on June 1, 2013 (above) clearly shows the position of the Low-Level Somali Jet over the Northern Indian Ocean and Arabian Sea. Some meteorologists have likened the Low-Level Somali Jet to an oceanic western boundary current, and perhaps I can blog about the physics involved in the LLSJ at some other time.

The Low-Level Somali Jet is typically drawn eastward toward India by an onset vortex, which is a type of subtropical cyclone characterized by a strong circulation in the middle troposphere (600 mb is a fairly standard pressure level for charting the onset vortex...hence the reference in IMD's second criterion above). Below is the 06Z GFS model analysis of 600-mb streamlines on June 1, 2013, showing the 600-mb footprint of the onset vortex over southern India (note the cyclonic circulation over this region; larger image). I should note here that cyclonic vortices in the middle troposphere occur more frequently in June around the time the summer monsoon begins and when the monsoon advances northward along India's west coast. These mid-tropospheric circulations occur much less frequently later in the summer-monsoon season. At any rate, these mid-level subtropical circulations help to draw the Low-Level Somali Jet eastward, setting the stage for the summer monsoon to get underway and spread northward and eastward across the India subcontinent.


The 06Z GFS model analysis of 600-mb streamlines on June 1, 2013. Courtesy of Penn State.

With regard to the third criterion, low values of outgoing long-wave (infrared) radiation go hand in hand with low cloud-top temperatures during the warm season. In other words, low OLR values indicate the tops of tall convective clouds...thunderstorms that can produce the notorious heavy rains associated with the summer monsoon. Check out the analysis below that represents the average OLR values for the period, May 31-June 6, 2013. I circled, in white, relatively low OLR values surrounding southern India (between 160 and 200 Watts per square meter). More specifically, here is the bar graph showing the timely decrease (May 23 - June 1) in outgoing long-wave radiation over the latitude-longitude "box" referenced by the India Meteorological Department (revisit the third criterion listed above). To gain a better sense for the link between low OLR values and high convective cloud tops, I present the 00Z infrared and and water vapor images on June 1, 2013, which indicate thunderstorms in the vicinity of Kerala.


The average values of outgoing long-wave radiation from May 31 to June 6, 2013. Courtesy of ESRL.

The bottom line that I want readers to take away from this blog is that the word, monsoon, does not automatically equate to rain. There are monsoons that produce only a modicum of precipitation (the winter monsoon in Southeast Asia, for example). Moreover, this year's summer monsoon in Southeast Asia has advanced like gangbusters.

Lee
Categories:Southeast Asia Summer Monsoon Tropical Meteorology
Updated: 1:46 AM GMT on June 20, 2013   Permalink | A A A
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The El Reno, OK Tornadoes
Posted by: Lee Grenci, 4:25 PM GMT on June 10, 2013 +8
Despite a slow start to the 2013 tornado season, the number of headline tornadoes in May has kept me really busy thinking about instructive ways to present these events to my faithful Wunderground readers. The last one on my list is the EF-5 twister that ripped through central Oklahoma on May 31, 2013.

Our fearless leader, Jeff Masters, suggested I write a blog about the anticyclonically rotating tornado that formed in concert with the cyclonically rotati...
Categories:Severe Weather Mesoscale Forecasting
Updated: 12:04 PM GMT on June 12, 2013   Permalink | A A A
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The Long-Lived, Nearly Stationary Tornado in Ottawa County, KS
Posted by: Lee Grenci, 5:55 PM GMT on June 05, 2013 +7
These days, it seems like the slow movement of any weather system gets attributed to a blocking high-pressure system, whether one exists or not. The criteria for blocking highs are pretty clear-cut (see my previous blog on blocking highs). Yet, these criteria too often seem to get watered down and diluted to the point that blocking highs have become a default explanation for any slowdown in weather systems.


The long-lived tornado in Ottawa County, K...
Categories:Severe Weather Mesoscale Meteorology Opinion
Updated: 12:40 PM GMT on June 06, 2013   Permalink | A A A
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The Moore Tornado
Posted by: Lee Grenci, 3:23 PM GMT on May 28, 2013 +12
On Tuesday, May 21, in the aftermath of the EF-5 tornado that devastated Moore, Oklahoma, the previous day, I watched Dr. Greg Forbes of The Weather Channel give a live interview standing in front of some of the damage. At the time, the media referred to the twister as an EF-4 tornado, but Dr. Forbes pointed to a heavy propane tank that had been ripped from its moorings and then thrown a distance of a quarter mile, providing evidence, Dr. Forbes stated, that support...
Categories:Severe Weather Mesoscale Meteorology
Updated: 12:09 AM GMT on May 29, 2013   Permalink | A A A
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The Granbury Tornado
Posted by: Lee Grenci, 7:20 PM GMT on May 19, 2013 +8
If you listen or read carefully to any of the media's typical account of the development of tornadic supercells (thunderstorms with rotating updrafts), you'll get the overall impression that strong winds aloft generate the strong vertical wind shear that favor supercells. Such impressions are not always true. Indeed, the supercell that spawned the EF-4 tornado that ripped through Granbury, Texas, on Wednesday evening, May 15, 2013, was a compelling example.
Categories:Severe Weather
Updated: 10:19 PM GMT on May 20, 2013   Permalink | A A A
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Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.

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