Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 2:07 AM GMT on May 15, 2013
A broad area of low pressure centered over the eastern Pacific about 650 miles south of Acapulco, Mexico continues to be a threat to eventually become a tropical cyclone.
The center is a little easier to locate this evening, possibly due to a somewhat better definition in the surface wind field compared to yesterday (as per low-cloud lines on last light visible satellite imagery). There is still not much evidence of vertical shear over the system, and both the SHIPS and GFS keep a light shear environment over the system throughout the forecast period (five days). I should note that the upper flow in the GFS has been inconsistently anticyclonic and diffluent; either scenario would favor intensification, but the former would be more beneficial in breeding a hurricane. There is a well-established equatorial outflow channel in the western semicircle, and a well-defined poleward outflow channel in the northern semicircle due to a narrow mid- to upper-level trough trailing from an upper low over northwest Mexico. There is some indication that southerly to southwesterly shear could increase a little in about five days, but there is not enough evidence to settle on this yet. Either way, the system is likely to become a tropical cyclone in a few days, and it seems likely to become the first tropical storm of the season. However, the circulation is still not well-defined as denoted by some recent ASCAT data and SSMIS data. There is, however, a formative curved band in the eastern semicircle, and overall the cloud pattern looks less disheveled than yesterday.
Figure 1. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD).
A weak mid-level ridge over the Gulf of Mexico is being maintained to the north of the system for now, which is imparting a mostly westward motion. The global models forecast a turn to the west-northwest over the next 24-48 hours as broad upscale troughing amplifies over the western United States. A southeastward-moving upper low off the Pacific northwest should provide some additional amplification to the trough over northwest Mexico over the next couple of days, which should allow for a more northwestward motion in about 72-96 hours. There is some disagreement amongst the models as to how far north the system goes as it comes under the influence of the trough, with the CMC and NAVGEM (old NOGAPS) actually forecasting the trough to bypass 90E, which causes the system to slide westward underneath a building mid-level ridge near the end of the forecast period. The GFS shows more amplification of the trough, a pattern that would favor a long-range threat to southern Baja. Since the guidance has a tendency to flip back and forth with systems in their formative stage, I will go with the trend of the previous forecast and maintain a poleward motion out to day five, but not nearly enough to pull the system toward the coast.
Probability of development in 48 hours: 40%
The CMC continues to show the development of a secondary system behind 90E, which goes on to become a tropical cyclone. I still don't see solid evidence of a tropical wave, but ASCAT wind barbs show what appears to be a cyclonic wind shift around 6N 91W; visible satellite and CIMSS 850 to 700 mb vorticity data confirmed a broad cyclonic rotation signature at this general latitude all the way east to about 87W as well, so there is at least something to monitor as possible catalyst for another area of low pressure to form in a few days.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.