Tropical weather analysis - July 31, 2010

Eastern Atlantic tropical wave

A well-defined tropical wave located about 850 miles west-southwest of the Cape Verde Islands has become a little better organized this evening. Infrared satellite loops suggest that there is a rather concentrated area of deep convection along 8 to 9°N 32 to 33°W. There is also another area of active convection to the west of that, in the general vicinity of where ex-90L used to be.

A definite center is extremely difficult to locate tonight, even using shortwave infrared and RGB satellite animations. My best guess, which is highly uncertain, is 8N 32W. Although this system has become better organized this evening, the enormity of the cyclonic gyre with which it is embedded will likely keep any rapid consolidation to a minimum. All in all, based on CIMSS 850 mb vorticity data, this system is still poorly defined, and is embedded, as previously mentioned, in a large cyclonic gyre associated with an active ITCZ. Water vapor animations depict a large swath of very dry air just to the north of this complex weather disturbance, associated with the dry, dusty Saharan Air Layer that originates from the Sahara Desert of Africa, and gets periodically blown westward out into the Atlantic Ocean.

However, given the fact that the system will be moving generally W under the influence of a well-defined low- to mid-level tropospheric ridge, this dry air should not significantly impede development, since the steering flow will be primarily easterly, which will act to push the dry air away from the system. Latest CIMSS wind shear analysis indicates that the system is experiencing 10-20 kt of easterly to northeasterly shear, associated with a developing anticyclone just to the southwest of the Cape Verde Islands.

By about 36 hours, the system should start to gradually gain latitude and move WNW. Until then though, expect a continued W motion. By around 72 hours, a break in the Atlantic subtropical ridge is forecast to occur unanimously by all the models -- some keep it stronger, some weaker, but the general consensus is for at least a slight slackening of the south side of this ridge during this time, as a vigorous mid- to upper-level trough swings by across the mid-Atlantic states and New England.

There is still the possibility that the system could remain shallow and continue W, eventually entering the Caribbean. The NOGAPS keeps a stronger Bermuda-Azores high, which keeps the low-level flow generally easterly throughout the forecast period. This solution appears unlikely at this time, if not just for the fact that the system is likely to gradually deepen over the next couple of days.

The models are unanimously calling for tropical cyclogenesis from this feature, with the 12z CMC developing it east of the Leeward Islands in about four days. Interestingly, this particular model shows something of a Fujiwhara interaction between this system and another competing area of vorticity subsequent to this. This solution appears highly unlikely for now, but nevertheless the model has the right idea in developing this system. The 18z GFS indicates that cyclogenesis will occur right on the doorstep of the northern Leeward Islands in about four and a half days.

The 12z NOGAPS keeps the system weak, and as a consequence, it moves westward in tandem with the low-level easterly flow characteristic of the tropics. It develops it at 144 hours, while it is about 400 miles east of the central Leeward Islands. This model appears much too far south at this time, and hence I have discounted this track solution. But again, it does have the right idea with regards to eventual development from this system. Finally, the 12z ECMWF develops this system in about four days as it approaches the Leeward Islands.

Rather alarmingly, both the GFS and ECMWF foresee the synoptic scale pattern as being supportive of a stronger Atlantic subtropical ridge, strong enough to prevent the system from recurving harmlessly out to sea. While I do not like to speculate on long-range tracks, particularly with systems that do not even have a definite surface center, I am inclined to go with the solution provided by these models, based on the current large-scale pattern over North America.

Upper-level winds support continued slow development of this disturbance. I am not expecting any rapid development, however, and the NHC's current forecast of a 40% chance of tropical cyclogenesis occurring in 48 hours is a reasonable one. The earliest I would expect this system to become a tropical depression or tropical storm is Tuesday night, but more likely Wednesday afternoon.

Central Caribbean tropical wave

A tropical wave moving across the central Caribbean Sea continues to sporadically produce limited and disorganized shower activity. Upper-level winds are not conducive for development in this area, and are forecast to continue being largely zonal and strong, even as the system approaches Central America in about two days. I am not expecting any significant development of this wave, though it may bring heavy rains to Honduras, Nicaragua, El Salvador, and Costa Rica over the next couple of days as it continues moving generally W.