Tropical weather analysis - August 8, 2010

Colin

Tropical Depression Colin degenerated into an open trough of low pressure earlier this afternoon. Regeneration is not expected as the system moves NNE to NE within the southwesterly flow associated with a deep-layer trough exiting the east coast of the United States. Colin, or rather, what's left of it, will ultimately make extratropical transition and subsequently become absorbed into said trough over the next several days. Some rain showers and gusty winds are still possible across Bermuda, but nothing significant. However, some isolated tropical storm force wind gusts, specifically along higher elevations, have been reported on the island.

Invest 93L

Invest 93L, located several hundred miles west-northwest of the Cape Verde Islands, continues to display signs of organization. Shortwave infrared satellite animations suggest that the center is located at 23N 46W, moving WNW. Earlier this afternoon, visible satellite animations clearly depicted a closed surface circulation, albeit exposed from the deep convection due to strong vertical shear. This is still the case this evening, with the well-defined surface center (which is closed) displaced well to the southwest of the pulsating deep convection. 93L continues to be embedded within a relatively dry environment, evidenced on water vapor loops. This, in conjunction with the aforementioned vertical shear, analyzed at 10-20 kt by data from the University of Wisconsin CIMSS, should preclude any rapid development over the next day or so.

This system has been slow to develop, which wasn't really unexpected given the meager environment in which it is currently embedded. But I must admit, this system has certainly developed slower than I thought it would. I thought for sure that by tonight or early tomorrow morning, it would have been labeled a tropical depression.

Additionally, CIMSS vorticity data suggests a vertical decoupling of the low- and mid-level circulations. This is undoubtedly due to the aforementioned shear, which is being induced by an upper-level low several hundred miles west of Invest 93L. All of these negative factors notwithstanding, however, vertical shear has decreased over the system during the last 12 hours. Vertical shear is forecast to ease somewhat over the next several days as the upper low deamplifies and retreats slightly northward. In fact, the 18z GFS suggests that the system will move in tandem with a weak upper-level anticyclone currently centered just to the south of the disturbance. The GFS foresees 93L losing the anticyclone after about 60 or 66 hours. Though shear will increase over the system subsequent to this, it will still be weak enough to allow slow intensification, because the system will probably be moving in tandem with the upper flow beginning in about 84-90 hours.

I expect a continued WNW motion, followed by a gentle bend to the NW with a deceleration in the forward speed beginning in about 48-54 hours as the subtropical ridge weakens with the approach of the powerful longwave trough that recurved Colin. Thereafter, a gradual turn to the N is expected late on day two, followed by a swing to the NE with a gradual increase in forward speed expected late on day three or very early day four.

None of the models significantly intensify the system, and most barely depict it as a perturbation. Though I will still forecast gradual intensification, peaking the system at 65 mph, the models could certainly be onto something given how hesitant this system has been to develop since its designation as an invest. I will continue to carefully assess this possibility.

Invest 94L

An area of disturbed weather located just offshore the Florida east coast was designated "Invest 94L" this afternoon by the National Hurricane Center. The poorly defined center is estimated to be about 75 miles east of Vero Beach, moving slowly to the SW. Surface, buoy, and ASCAT observations suggest that a broad but closed circulation may exist at the surface. Surface pressures are somewhat low in the area, but not overly so, and are not significantly falling at this time.

94L is currently experiencing 20 kt of northwesterly shear induced by the longwave trough currently recurving Colin. This has resulted in a vertical decoupling of the low and mid-level circulation centers, with the latter being displaced about 35-50 miles southeast of the low-level center. In addition to the shear, dry air is being entrained into the circulation due to the aforementioned northwesterly shear. These factors should inhibit any significant development over the next day or two.

Current steering/model data suggests that 94L will be slow to move inland, and most of the models do not bring it inland over central or eastern Florida until around 36 hours. This is certainly possible, and this will be a difficult storm to forecast in the near-term due to the weak steering regime in the region. It is entirely possible that this system could perform an anticyclonic loop over the next day or so. However, I will instead forecast a general, but quite slow (5 mph or less at times) motion to the SW, in agreement with the models and the overall synoptic scale steering pattern.

Thereafter, the system is forecast to pick up speed and emerge into the Gulf of Mexico. This is where the forecast begins to get easier. Most of the models take this system inland over eastern or southeastern Louisiana sometime on Thursday, though many of them do not show much of a surface reflection. This is likely due to the fact that it doesn't appear to have more than a couple days over water after emerging into the Gulf.

After 36 hours, the upper-level winds will relax significantly as upper ridging replaces the current upper troughing. Furthermore, because 94L will be moving WNW, in tandem with the forecast upper-level winds at day two and beyond, it is a good possibility that this system will try and undergo tropical cyclogenesis while moving across the eastern and northern Gulf of Mexico.

I give this system a 40% chance of eventually becoming a tropical cyclone, and residents from Destin to Beaumont should carefully monitor the progress of this system over the next several days. Though at this time, based on the models, it appears that southeast Louisiana is the most likely target for an eventual landfall.

Regardless of development, heavy rainfall will continue spreading over portions of central and south Florida over the next day or two, potentially causing some localized flooding.