African Wave 91L Worth Watching; Record Quiet Spell Ends in Western Pacific

By: Dr. Jeff Masters , 3:11PM,GMT on September 8,2014

A tropical wave (91L) that emerged from the coast of Africa on Sunday is headed west to west-northwest at about 15 mph. Satellite images show 91L has a moderate amount of spin and heavy thunderstorm activity, but these thunderstorms are poorly organized. The disturbance is embedded in a moist air mass, has moderately warm (SSTs) of 27.5°C (82°F) beneath it, and is experiencing light wind shear. These conditions favor development. The 8 am EDT Monday run of the SHIPS model predicted that wind shear would remain light to moderate ( 5 - 15 knots) for the next five days, favoring continued development. However, development will be slowed by the fact that the atmosphere at mid-levels of the atmosphere (between 500 - 700 mb) should grow steadily grow drier. Two of our three reliable computer models for predicting tropical storm formation predict development of 91L over the next five days. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odd of development odds of 30% and 60%, respectively. A trough of low pressure expected to push off the U.S. East Coast early next week should induce a more northwesterly track for 91L next week, and the disturbance does not appear to be a long-range threat to the Lesser Antilles Islands. It remains to be seen if 91L will be a long-range threat to Bermuda, the U.S. East Coast, or the Canadian Maritime Provinces late next week.


Figure 1. MODIS satellite image of tropical wave 91L off the coast of Africa at approximately 8 am EDT Monday September 8, 2014. Image credit: NASA/GSFC.

Record quiet spell ends in Western Pacific
A remarkable month-long hiatus in tropical cyclone activity in the Western Pacific finally came to an end on Sunday, when Tropical Storm Fengshen formed a few hundred miles south of Japan. Prior to Fengshen, the last named storm to form in the Western Pacific was Tropical Storm Nakri, which formed on July 29 (according the Japan Meteorological Agency, the official agency responsible for typhoon warnings) or August 2 (according the U.S. Joint Typhoon Warning Center.) Typhoon Genevieve, which crossed the Dateline into the Western Pacific on August 7, originated in the Eastern Pacific, so doesn't count as a named storm originating in the Western Pacific. According to the archive of Japan Meteorological Agency Western Pacific typhoons kept at Digital Typhoon, August 2014 marks the first August since records began in 1951 that a tropical storm did not form in the Western Pacific. During the period 1951 - 2013, an average of 5.6 named storms formed in the basin during August. The previous record low for August was two named storms, which occurred in both 1979 and 1980. The lack of activity this August in the Northwest Pacific was due, in part, to an unfavorable phase of the Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days. The MJO caused dry, sinking air to predominate in the Western Pacific during August. Thanks to a busy July, the Western Pacific has seen 14 named storms so far this year, which is only one behind average.


Figure 2. MODIS satellite image of Tropical Storm Fengshen south of Japan, taken at approximately 03 UTC Monday September 8, 2014. At the time, Fengshen had top sustained winds of 65 mph. Image credit: NASA.

Jeff Masters