During the cooler half of the year cold surges from the interior of North America frequently advance equatorward with northerly winds and pressure rises and are often associated with significant precipitation in the subtropics. Cold surge precipitation in Mexico, Central America, and the Caribbean is investigated by means of climatological, composite, and case study analyses using radiosonde, surface, ship, satellite, precipitation, and the NCEP-NCAR reanalysis datasets. Wet and dry composites and case studies will be presented.

For the cold season (October-March) of 1977-98 327 cold surges were found of which 25-45% produced 30-40% of the cold season precipitation. Cold surge passage at Brownsville, Texas and Veracruz, Mexico brought a peak in precipitation to eastern Mexico and the Caribbean and a minimum in precipitation in western Mexico. A maximum in precipitation occurred a day later in Central America and two to three days later in western Mexico as the flow shifted favorably behind the cold surge high.

In the wet composites and wet case study, the mutual interaction of the midlatitudes with the tropics created the setting for precipitation in the subtropics. A progressive, amplified pattern set the stage for a strong subtropical jet (STJ) at low latitudes. A deep potential vorticity trough acted to tap tropical moisture and to produce a tropical plume. A deep trough at 200 hPa was associated with a low-level inverted trough in the easterlies and inflow of moisture into eastern Mexico.

In the dry composites, zonal and stationary patterns failed to perturb the tropics and were associated with dry surge events. In the dry composites and dry case study, a strong ridge at 200 hPa across Mexico and Central America indicated a large area of subsidence. The STJ in the dry cold surges was either poleward of the subtropics or was located offshore in the Pacific. An inverted trough failed to form in the easterlies and low-level moisture remained in the tropics.