In-situ and dropsonde data collected by the NCAR C-130 as part of the EPIC 2001 field program, along with satellite derived winds are used to document the formation of East Pacific Tropical Storm Lorena. This late season storm formed without apparent support from a larger scale easterly wave and provides and example of locally generated cyclonic vorticity over the East Pacific as described by Farfan and Zehnder (1997).

The genesis of Lorena occurred in the wake of Hurricane Juliette, and followed a series of easterly wave passages across the Yucatan peninsula and Central America and into the East Pacific. A mid-latitude trough and accompanying cold front passed through the Gulf of Mexico on September 28th and 29th, causing the winds over the Gulf and along the East Coast of Mexico to shift from the climatological southeasterlies to northerly behind the front. The northerly winds impinging on the Isthmus of Tehuantepec resulted in the formation of a northeasterly jet through the gap that extended into the Gulf of Tehuantepec.

Farfan and Zehnder (1997) have linked jets through the Isthmus of Tehuantepec to the genesis of Hurricane Guillermo (1991). In the case of Guillermo, the flow through the gap was forced by northeasterly winds ahead of a tropical easterly wave trough. Schultz et al. (1997) have described high winds through the Isthmus associated with cold fronts. However, in the case of wintertime cold fronts the air behind the front is stable, and the low level flow diverges over the Gulf of Tehuantepec. In our case, there is weak temperature gradient across the front, and the flow through the isthmus results in a narrow jet through the gap.

The presence and structure of the jet was observed using dropsonde data collected during flights of the NCAR C-130 on September 28th and 29th. During the flight on the 28th, a dropsonde at 95.3oW, 13.3oN indicated u and v wind components of about 8 m/s each from 850 mb to the surface. During the flight on the 29th a dropsonde at 96oW, 14.8oN revealed a more intense jet, with u and v wind components of about 15 m/s each at the 950 mb level and decreasing to about 5 m/s at 800 mb. Surface winds from the NASA QuikSCAT and GOES 8 visible satellite imagery allow the time evolution of the gap flow and the resulting circulation to be documented. The northeasterly flow through the gap, along with southerly winds associated with the ITCZ result in a distinct cyclonic circulation that is apparent in the visible satellite imagery and centered at 98oW, 12oN at 14 UTC on September 29th.

The disturbance developed slowly, being classified as tropical depression thirteen-E at 03 UTC on October 2nd and was upgraded to Tropical Storm Lorena at 09 UTC on the same day. The relatively slow development may be due to the lack of upper-level support associated with synoptic scale easterly waves that typically are associated with the East Pacific tropical cyclones.