The physical mechanisms responsible for synoptic-scale rainfall variability over Central America during the wet season (approximately May through October) are not well documented. In this study we describe the atmospheric conditions associated with wet and dry spells over Central America during the boreal summer. Wet spells, identified as periods during which rainfall is reported for at least two consecutive days at 75% or more of the stations, were determined using daily rainfall data from more than 50 stations in Nicaragua, Costa Rica, and Panama. The typical atmospheric conditions associated with the wet spells were obtained by compositing satellite imagery, available for the period 1990-1994 at 3 hourly and 16 km resolution, together with upper-air wind observations.
For the year 1997, additional information was available from the Pan American Climate Studies - Sounding Network (PACS-SONET); these observations will be used to show additional details of the windfield associated with wet and dry spells. Preliminary analyses show that, from May through October 1997, a total of 18 wet spells occurred, mainly during June and September-October. During May and July-August more dry spells were observed. Windfield composites reveal weak (intense) trade winds and middle level cyclonic (anticyclonic) shear during wet (dry) periods. Satellite imagery during 1997 suggests that cloudiness associated with wet spells has a tendency to originate in the eastern tropical Pacific. This contrasts with other years, in which tropical disturbances propagating from the east appear to be an important contributor to wet spells.