Unusual development of a tropical or subtropical cyclone is possible southwest of Baja California during the next few days. The National Hurricane Center (NHC) issued a special Tropical Weather Outlook on Thursday afternoon for a weather system southwest of Baja California. NHC indicated that there is a 50% probability of development of a tropical or subtropical cyclone during the next five days. No tropical cyclone or subtropical cyclone is known to have developed over the Eastern North Pacific Ocean in January. The record extends back to 1949, but it is most complete for the era of Geostationary Operational Environmental Satellites (GOES) which extends from 1970 to the present.
A broad area of low pressure was located about 1300 miles (2100 km) southwest of the southern tip of Baja California. Clusters of showers and thunderstorms were occurring in parts of the low pressure system. Visible satellite images were not showing evidence that the showers and thunderstorms were forming into rainbands. There was a broad area of low pressure, but there was no evidence of a distinct low level center of circulation.
The broad area of low pressure will move through an environment somewhat favorable for the formation of a tropical or subtropical cyclone during the next few days. It will move over water where the Sea Surface Temperature is near 27°C, which means that there will be enough energy in the ocean to support a tropical cyclone. An upper level low is northwest of the broad surface low pressure system. The upper level low is producing strong southwesterly winds which are blowing across the top of the broad low pressure system. Those winds will cause moderate vertical wind shear, which will inhibit the formation of a tropical cyclone. If the winds weaken, a tropical cyclone could form, but if the wind shear remains stronger, a subtropical cyclone could develop. As mentioned above, the National Hurricane Center indicates that there is a 50% probability of development of a tropical or subtropical cyclone.
The broad area of low pressure is forecast to move slowly toward the north during the next several days. If a tropical or subtropical cyclone develops with taller thunderstorms, then the southwesterly winds blowing around the upper low will steer the cyclone toward Baja California. The system could bring gusty winds and heavy rain to Baja California and northern Mexico in a few days.
A broad area of low pressure has formed over the Eastern North Pacific Ocean south of El Salvador. The environment is favorable for the development of a tropical cyclone. The National Hurricane Center (NHC) has designated the low pressure system as Invest 90E. NHC is indicating that there is a 50% probability of the formation of a tropical depression during the next 48 hours and an 80% probability of formation during the next five days.
At 2:00 p.m. EDT on Monday the center of Invest 90E was located at latitude 7.9°N and longitude 89.5°W which put it about 370 miles (595 km) south of San Salvador, El Salvador. Invest 90E was stationary. The maximum sustained wind speed was 30 m.p.h. (50 km/h) and there were wind gusts to 40 m.p.h. (65 km/h). The minimum surface pressure was 1008 mb.
More thunderstorms formed on Monday around a broad area of low pressure south of El Salvador. Visible satellite imagery suggested a broad counterclockwise rotation of the area of thunderstorms, but there were no indications of a well developed core of the circulation. Several smaller centers of rotation were visible within the broad low pressure system, but these may be transient features. The broad area of thunderstorms was producing upper level divergence. The divergence is pumping out mass and if that continues, then the surface pressure will decrease.
The area of low pressure is in an environment favorable for tropical development. It is moving over water where the Sea Surface Temperature is near 30°C. The low is sitting underneath an upper level ridge and the winds in the upper level are weak. There is little vertical wind shear and there is nothing to inhibit upper level divergence. The circulation should continue to organize, which is why the probability of formation of a tropical cyclone is high.
The area of low pressure is in an area where the steering currents are weak and Invest 90E was nearly stationary on Monday. A high pressure system northeast of the broad area of low pressure system is expected to strengthen. When that happens, the high is likely to steer the area of low pressure toward the northwest.
May 15 is the official start of the hurricane season for the Eastern North Pacific Ocean. The Sea Surface Temperatures (SSTs) should be cooler than they were in 2015, since the El Niño is dissipating. That could produce a hurricane season in which the activity is nearer to normal. In a normal year we might expect 13-17 named storms, 7-10 hurricanes and 3-4 major hurricanes. Of course, other environmental factors including the location and strength of atmospheric systems will also determine how active the season is.
The National Hurricane Center (NHC) is currently monitoring Invest 90E which is about 950 miles (1530 km) south-southwest of the southern tip of Baja California. There is a well defined low level circulation in Invest 90E, but there are no deep thunderstorms. NHC is giving Invest 90E a 20 percent chance of developing into a tropical cyclone during the next five days.