Tropical weather analysis - May 14, 2012
A non-tropical area of low pressure centered in the far eastern Atlantic about 470 miles south-southwest of the southern Azores is no longer a threat to develop. The system lost much of its thunderstorm activity earlier in the day, and, the current resurgence notwithstanding, this low has already peaked, and will slowly fizzle as the cyclonic circulation that's sitting over it moves away, taking with it the system's upper support.
Probability of genesis within the next 48 hours: 10%
The Pacific, on the other hand, remains active, with an area of disturbed weather centered about 600 miles south-southwest of Manzanillo continuing to generate showers and thunderstorms. Convection diminished earlier in the day, possibly as a result of dry air intrusion, but is on the rebound again. Low cloud motions indicate that the circulation associated with this disturbance is gradually gaining definition, and with favorable upper-level winds in the path of this system for at least the next day or so, this low has the potential to become a tropical depression.
Figure 1. Latest infrared satellite image of Invest 90E, courtesy of NOAA's Satellite Services Division (SSD).
The upper flow over the system is not anticyclonic, but is at least diffluent enough to allow strengthening. This theme will soon change, as the GFS forecasts the system to run into a wall of westerly shear by Tuesday afternoon. This shear appears to be attributable to the persistent mid-oceanic trough which normally dominates the central Pacific. Incidentally, this is also why Hawaii generally remains free of tropical cyclones.
Water vapor and large scale imagery indicates that the Pacific ridge is building westward ahead of 90E. With no obvious reasons why this ridge should weaken, this pattern favors a west-northwest to westward motion over the next five days. By Tuesday evening, weak southwesterly flow is forecast to reestablish over Baja California, so that enough of a weakness will be present in the ridge to nudge the system unclimatologically slowly. It's worth noting that the GFDL still turns 90E toward Baja by midweek, but this is predicated on a deeper system responding more fully to the trough draped across that area in the models. All of the models show this weakness, but most don't respond to it since they keep 90E a relatively weak system. Given the narrow window of opportunity in which the system has to strengthen, I tend to agree with this.
The global models lose the circulation in about five days, but this could occur sooner given the magnitude of westerly shear the system is forecast to encounter.
This low is unlikely to become anything more than a weak tropical storm.
Probability of genesis within the next 48 hours: 60%
The global models continue to insist on another tropical cyclone forming to the east of Invest 90E, in the Gulf of Tehuantepec. This is forecast to occur in about five days. Satellite imagery and MIMIC-TPW imagery from CIMSS indicates a large surge of moisture has manifested in this region. There is not enough evidence to justify labeling the area of rotation in the southwestern Caribbean a tropical wave. Either way, a large area of rainfall and embedded thunderstorms stretches across the Caribbean and far eastern Pacific from about 7-8°N 75°W westward to about 100°W, and the eastern end of this activity could serve as the instigator for a tropical storm.
A large monsoonal circulation over this portion of the world is typical this time of year, and is often times how an early-season tropical storm gets going. Some of the models are responding to this pattern by hinting that the western Caribbean will see development as well. Given current satellite trends, and the forecast of a northeastward-moving tropical storm in the GFS, it is possible that a situation similar to Alma/Arthur in 2008 will transpire, leading to the formation of Alberto in the western Caribbean. It is also possible that this will be a separate entity. Regardless, both areas will need to be watched carefully, as the potential does exist for tropical storm formation in this region.
The models are depicting the possibility of a weak tropical cyclone developing off the southeast or mid-Atlantic coast in about four days. The weak cold front/trough currently over the southern United States is forecast to deamplify and become quasi-stationary over the eastern seaboard during this time. With strong upper-level shear, development isn't terribly likely, but the quick forward motion of the system as it accelerates into the westerlies could allow it to outrun the shear.