Tropical weather analysis - May 13, 2012
This just keeps getting better and better. In a surprising turn of events, the Atlantic has decided to spin something up, as well. A non-tropical gale area ("92L") has developed about 400 miles southwest of the southern Azores. Sometimes it's easy to miss these small subtropical entities in light of other, more flagrant observations. Satellite and scatterometer data indicate that this system possesses a large, well-defined circulation at the surface. In addition, this low is already producing winds to tropical storm force, mainly in squalls to the east of the center. These winds are probably at least partially attributable to a steep pressure gradient between the low and the Bermuda/Azores ridge, whose axis lies to north.
Figure 1. Latest infrared satellite image of Invest 92L, courtesy of NOAA's Satellite Services Division (SSD).
Synoptic data indicates that 92L is embedded within a large cyclonic circulation over the north Atlantic. This circulation extends all the way down to at least 850 mb. In theory, this would tend to cap development potential, but 92L is not a tropical entity. Water vapor and CIMSS vorticity imagery shows that 92L is sitting beneath an upper low that encompasses this cyclonic circulation. This is promoting a low shear environment, hence the relative symmetry of the cloud pattern. The associated shower activity appears to be getting a little better organized in recent frames.
Sea surface temperature analyses from NOAA's AOML division shows 92L sitting right in the middle of cold waters, about 19C.
Figure 2. Atlantic sea surface temperatures (SSTs) as of May 11, 2012, courtesy of NOAA's AOML research division.
Long-term satellite loops show that 92L has barely moved, except for perhaps 30 miles to the north. This is reflective of the weak steering regime which surrounds the storm. There is a wide diversity in the respective model tracks, between the westernmost GFDL which crawls the system northwest, and the easternmost GFS, which moves the system generally east-northeast. This kind of disagreement usually implies weak steering, so little movement of this disturbance is forecast for the next few days, save for perhaps a continued northward drift. More likely, overall motion will continue to be slow and erratic as 92L attempts to loop around the large cyclonic circulation in which it is embedded, only to get tugged back to the west by the subtropical ridge.
I hate forecasting these slow-moving storms. Fortunately, it appears that nature has seen fit to grant me a reprieve, as a positively-tilted upper trough currently stretching from the Bahamas to Atlantic Canada slowly amplifies over the next 48 hours. Unfortunately, it appears as if this trough will not be in a hurry to pick up 92L, and it will likely have to wait for reinforcements in the westerlies emanating from Quebec. Even then, recurvature will be painfully slow, and 92L could meander near the Azores for several days.
Any development from 92L will be subtropical.
Probability of genesis in the next 48 hours: 40%
An area of disturbed weather continues about 550 miles west-southwest of Acapulco. The associated shower activity has become better organized during the day.
Nighttime imagery, particularly in light of the ongoing convective burst, has made finding a center rather difficult. My best estimate is 10.4°N 103.9°W, which is right in the middle of the convection. However, WindSat was unavailable, and ASCAT inconclusive, so the actual center could be anywhere within a half a degree north and west of where I have it. Undoubtedly, visible satellite images will shed some light in the morning. It should be noted that there appears to be a vorticity center northwest of the convective mass, and if that is correct, 90E is highly disorganized. My forecast is predicated on the assumption that the low-level center is in that convective ball.
The system is currently underneath a budding anticyclone, and the GFS forecasts this anticyclone will strengthen over the next 48 hours. That, combined with 29 to 30C SSTs which the system currently sits over, would tend to favor significant intensification. Thus, if current trends persist, I would expect a tropical depression sometime tomorrow. It is possible that a period of rapid intensification could ensue, but I am not willing to forecast this for two reasons: the first being that we have little skill in anticipating these events, and also because the system still lies close to the ITCZ. If rapid intensification does happen, it is not out of the question that this low could come close to hurricane strength.
After 48 hours, the system loses its upper-level anticyclone as it approaches the mid-oceanic trough characteristic of the central Pacific waters. By Tuesday, the system will be heading into very strong westerly shear, which should retard the development process and initiate weakening. It should be noted that should the system stay farther south, it would likely avoid the worst of the shear, and would survive longer due to warmer sea surface temperatures.
As the models predicted, a fast-moving upper low approaching 35N has lifted out, and is now approaching southern California. Consequently, the frontal zone that was attendant to this feature has lost its upper support, and is thus dissipating. This is shown nicely on water vapor imagery, where the middle- to upper-tropospheric flow is becoming more zonal. The sudden rebuilding of the Pacific ridge has prompted me to shift my forecast a little to the south of where it was yesterday. I anticipate a slow northwestward motion in the short-term, followed by west-northwest, then west with an increase in forward speed. The westward turn should occur in about 36 hours. Some of the models try to turn the system toward Baja in the long-range, but this is contingent on a vertically deep system. Nevertheless, there does appear to be some troughing in that area within the global model forecast fields around that time, so depending on the amplitude of this trough, this possibility will need to be assessed.
Most of the models lose the circulation in about five to six days.
Probability of genesis in the next 48 hours: 60%
The global models, mostly the GFS and ECMWF -- you know, the important ones -- continue to forecast the possibility of a secondary tropical cyclone developing to the east of Invest 90E in about a week. This system is forecast to originate in the Gulf of Tehauntepec. However, given the large surge of monsoonal moisture likely to envelope the eastern Pacific/western Caribbean during this time, the potential certainly exists for a cyclone to spin up on the Atlantic side. Regardless, it appears that a large fetch of tropical moisture associated with a probable area of disturbed weather over this area of the world heralds heavy rainfall for south Florida. This region is under a severe drought, so any rains will be beneficial.
Also, I held off on mentioning this yesterday, but the GFS, and the ECMWF forecast that a cold front forecast to move off the eastern US next week could serve as the nucleus for a tropical storm. Such a storm would very likely move out to sea, well-embedded within the mid-latitude westerlies.