Friday, August 13, 2010

Rain Coming Soon

A few days ago we had a tropical wave come through here in eastern Guatemala and nothing happened. The reason that nothing happened is the wave did not have significant moisture to work with. A pressure disturbance passing through relatively dry air doesn't trigger any activity. But when a tropical wave travels with or passes through a moist air mass, it can trigger precipitation. The extremely heavy rainfall we got a week and a half ago resulted from such a coincidence.

I've been watching this coming for several days but things can change unpredictably in the tropics and I wasn't a sure if the moisture would get here in time for the wave or if the moist air mass would stay mostly south of us. But it's looking like we have another moisture maximum combined with a wave arriving Saturday night, so be prepared. I expect significant nighttime rainfall to continue for several nights after that but the heaviest should be Saturday night.

For those of you who are into weather forecasting here in the tropics, one of the resources I use is the GOES-E water vapor image here:

http://weather.unisys.com/satellite/sat_wv_east.html

Note that what is shown in the image is NOT clouds. White indicates high moisture content, black indicates very dry air, and grayscale in between.