A Tehuantepecer event is in full swing this morning, with winds analyzed by the GFS of around 60 kt at 850 hPa blasting into the Gulf of Tehuantepec. The Tehuantepecer is a local type of mountain gap wind primarily produced in the wintertime as cold airmasses surging southwards in the Gulf of Mexico result in a large pressure gradient between the Bay of Campeche and Gulf of Tehuantepec, forcing what can be a violent surge of wind through the Chivela Pass on the Isthmus of Tehuantepec. The diminishing Coriolis force southward of this mountain gap allows the outgoing channel of wind to continue outwards for what can be hundreds of kilometers.
This particular event played out in such fashion, with a dense, cold air mass surging south over the Gulf of Mexico yesterday -- the same airmass currently bringing anomalously cold temperatures to much of the continental US. The cross-isthmus pressure gradient may have been ever so slightly strengthened by nearby Tropical Storm Pilar, though the weak intensity of that storm and its small barometric footprint suggest its impact on this event was fairly insubstantial.
The area of clear skies in the Gulf of Tehuantepec directly downstream of this gap wind event is no coincidence. Winds upstream of the isthmus are forced upwards by topography but then isentropically descend downslope on the other side of the isthmus, adiabatically warming and forcing relative humidity values to plummet within the planetary boundary layer.
