Sunday, July 29, 2012

Keeping Tabs On Tropics

July will shape up as a quiet month in the tropics, with no tropical storms developing during the month for the first time in a July since 2009 (El Nino year).  Given the active start we had in May and June, our quiet July has put us back onto the same pace as 2008 and 2011 for tropical development to this point (with four named storms by July 28th in both years).   As of now, the tropics are still generally quiet with one tropical wave approaching the Caribbean Islands (bottom left of the graphic below) and a second wave at about 35 degrees West longitude, and a third wave just exiting off of Africa.

Part of the problem with getting tropical formation in July has been the persistence of dry air in the atmosphere in the Central Atlantic, which limits tropical development as moisture rich air is limited thanks to the influence of the Saharan Desert in Africa pushing a mass of dry air westwards in the mid level trade winds. The presence of dry air chokes off thunderstorm development and prevents it from taking place. It also doesn't help that the upper level winds in the Atlantic were largely unfavorable in July as well...this has changed somewhat in the lower latitudes around 10 Degrees North but farther north (around 20 N) shear is still a major issue.

The wave near 55 W (the one approaching the Caribbean) has a chance of perhaps getting to the east of Bahamas by week's end. Computer modeling from the Euro and the GFS occasionally shows this disturbance tracking through the Caribbean and across Hispanola. Development isn't likely to occur assuming it takes a track over Hispanola thanks to the presence of mountains on the island. However, if it does make it to the east of the Bahamas it could provide more rain for Florida (if it bends back towards the East Coast), or perhaps simply bounce parallel to the East Coast thanks to the positioning of a mid level trough in the atmosphere just offshore.  This entity probably doesn't into anything overly nasty -- it could become a weak tropical cyclone but wind shear looks to be an issue off the East Coast this weekend so if it were to develop it probably wouldn't amount to much in terms of intensity.

The aforementioned trough will result in an unsettled week of weather for us in Philadelphia, typical for early August...with varying chances of storms in the forecast each day.  The latter half of the weekend may present a pretty solid chance for thunderstorms with a relatively sharp frontal boundary pushing towards the East Coast.