Tropical Storm Jelawat (Lawin)

Infrared AVN satellite imagery of Tropical Storm Jelawat from MTSAT showing very deep convection over and around the center of the storm and its proximity to the Philippines. (September 22nd, 2012 20:32UTC)

The West Pacific is notorious for being the most active basin in the world in terms of Tropical Cyclone Activity. Tropical Storm Jelawat (or Lawin per PAGASA – in the Philippines’ region of responsibility) formed about three days ago and has a very uncertain future.

The storm is over the Philippine Sea and is throwing its southwestern outer bands over the Southern half of the Philippines, potentially leading to flooding rains. According to the Joint Typhoon Warning Center, Jelawat is moving slowly to the west-southwest at about 6mph, and has top sustained winds of 70mph. It is in an area with weak steering currents and is expected to only move very slowly for the next few days.

This slow motion means that its ultimate fate is more uncertain than in a typical cyclone. However, Jelawat is expected to drift northwards in response to an approaching trough in China. This largely spares the Philippines from the worst of Jelawat, but Taiwan and the southern Japanese islands will be threatened yet again.

A tropical wave to the west of Jelawat has the potential to develop into a tropical cyclone as Jelawat moves northwards, its proximity (should it become a tropical cyclone) to Jelawat could influence its steering and make its future even more uncertain.

Satellite imagery of Post-Tropical Storm Nadine heading southeast, well south of the Azores from the US Navy Tropical Cyclone monitoring site. The National Hurricane Center is monitoring it for redevelopment while MeteoFrance is issuing marine/high seas advisories for it.

Meanwhile, in the Atlantic, after bringing tropical storm conditions and heavy rains to the Azores, Tropical Storm Nadine took an interesting path to her demise becoming a  sub-tropical storm before becoming post-tropical. It is incredibly rare for tropical cyclones to transition into sub-tropical cyclones, because the convective processes that make them tropical typically counteract and outweigh those that make them extra-tropical. A combination of shallow convection and interactions with a weak cold front allowed a pocket of upper cold air to slide in place over the storm well before the surface circulation entrained cold air from the associated cold front. This meant that Nadine briefly became sub-tropical before it took on cold air, developed surface fronts, and became post-tropical.

Regardless, Post-tropical storm Nadine’s future too is uncertain. Some forecasts have the storm being caught by an extra-tropical storm. This scenario swings the storm northeastwards towards the Iberian Peninsula which would likely bring heavy rains to Portugal and Spain. The other scenario has Nadine’s remnants being trapped by the deep layered Bermuda-Azores high and drifts the storm westwards back into the Central Atlantic – this would put Nadine back over warmer waters and give her another chance to regain tropical characteristics. As a result, the National Hurricane Center is watching Nadine’s remnants for regeneration.

In reality, these types of storms at this latitude rarely have purely tropical or extra-tropical characteristics and it is a judgement call on how to classify them. Nadine is right on the border of Tropical, Sub-tropical, and Extra-tropical and small changes in structure could mean a re-evaluation of its status.

Invest 94L, near Bermuda now is not expected to become sub-tropical as convection around it did not organize enough in time to transition from an extra-tropical storm. It is now a weakening extra-tropical low with some interesting swirls and scattered (mainly light) showers that will affect Bermuda and its marine area.

In the East Pacific, Tropical Storm Miriam has formed safely south of Mexico. It is expected to become a hurricane before heading northwestward into cooler waters and a more stable atmosphere in the cool northeast Pacific – affect no land.


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