Tropical weather analysis - August 7, 2010

Tropical Storm Colin

Tropical Storm Colin continues to struggle to intensify. Visible satellite animations reveal quite nicely the partially exposed surface center along the western end of the deep convection. As of the 5:00 PM AST NHC advisory, maximum sustained winds are 40 mph, the minimum central pressure at 1012 mb, and the movement toward the NNE at 7 mph. Though the overall environment is undeniably more moisture-laden than it has been over the last couple days, there is still dry air evident to the west of the system, slowly being entrained (albeit to a lesser extent) into the western semicircle of the circulation thanks to a persistent but gradually weakening upper-level low a couple hundred miles to the southwest of Colin. Deep convection is shapeless and not very impressive or organized, according to infrared satellite animations.

Both CIMSS and satellite-derived data (low cloud motions) suggest that, though Colin is still disorganized, the vertical shear it has been battling over the last two days has significantly abated, with an expansion of convection finally occurring along the western portion of the circulation. Colin is well-embedded within a large weakness in the Atlantic subtropical ridge. The models suggest a generally N motion for the next 36-42 hours, followed by a NE motion. All in all, I agree with the NHC's current forecast track, and residents across Bermuda should continue monitoring the progress of the tropical cyclone over the next 24 hours or so, until it safely passes the island. Heavy rain squalls should begin affecting the island tonight and into the early morning hours on Sunday.

Watches and warnings

A tropical storm warning is in effect for:

Bermuda

A tropical storm warning means that tropical storm force winds are expected in the warning area within the next 36 hours.

All in all, I would expect 2 to 4 inches of rain to fall across the island, along with tropical storm force winds. Wind gusts could reach or exceed 50 mph. Upper-level winds appear favorable for Colin to intensify until about 48 hours, when upper-level winds will become unfavorable. Additionally, the cyclone will be passing over sub-26C SSTs by about this time. Extratropical transition should be complete by 60-66 hours. None of the models, save the GFDL and HWRF, make Colin a hurricane. These two models may be seeing some baroclinic intensification, since they bring it to hurricane status late on day two or early day three, when Colin is expected to be extratropical.

Residents living along Atlantic Canada should also follow the progress of Colin over the next couple of days, though I am not expecting a landfall here. I am also not expecting Colin to become a hurricane, even as an extratropical cyclone.

Invest 92L

A weak tropical wave moving across Belize (dubbed " Invest 92L" by the National Hurricane Center) remains highly disorganized, and is not a threat to develop into a tropical depression today. By tomorrow, however, it will have emerged into the Bay of Campeche, where it will need to be monitored for potential development. Upper-level winds appear to be only marginally conducive for development in the Bay of Campeche. I am not expecting any significant development of this system prior to it moving inland across northeast Mexico or extreme south Texas in about 48 hours.

Invest 93L

Another, more well-defined tropical wave, is located in the far eastern Atlantic Ocean about 1200 miles west-northwest of the Cape Verde Islands. This wave was designated "Invest 93L" by the National Hurricane Center on Thursday. Based on the last of the visible satellite images, my best estimate for the center position is 20N 40W. Because this wave is already at a rather high latitude for an easterly wave, eventual recurvature out to sea is a probable, nearly certain scenario. Though the overall low- and mid-level circulation is fairly well-defined, deep convection associated with this system is currently minimal, and confined to the north of the wave axis.

In addition, water vapor imagery depicts mid-level dry air around the system. This is the main inhibiting factor against development, though moderate southwesterly shear of 10-20 kt may also act to slow any immediate additional development of the system. Upper-level winds appear favorable for additional slow development of this disturbance as it moves WNW at around 15 to 20 mph over the next several days. By day four, vertical shear could increase a bit over the system, as it loses the upper-level anticyclone the GFS keeps over it for the first few days. The biggest inhibiting factor, as previously mentioned, will be dry air over the central Atlantic. Though this air is not nearly as dry as we've previously seen this season, it is certainly dry enough to inhibit any rapid development, even if upper-level shear is generally favorable.

Given all of these parameters, I expect 93L to continue to slowly develop, possibly organizing into a tropical depression tomorrow night or Monday morning. This system does not presently appear to be a pose a threat to any land areas, with the possible exception of Bermuda, and even that's iffy. By day four, another upper-level trough, albeit weaker than the last, will be approaching from the west, once again eroding the western Atlantic subtropical ridge, which will allow for a slowing of the forward speed by 72 hours, a turn to the NW by 96 hours, and a gradual bend to the N and NE thereafter.

It is possible that this system could become a hurricane, though if it wants to do so, it has a bit of work ahead of it. I'll peak the system as a 65 mph tropical storm.

Cape Verde Islands tropical wave

Another tropical wave, located southwest of the southern Cape Verde Islands, is also a threat to develop. This system could actually pose an eventual threat to land areas, so we need to carefully monitor the progress of this system over the next several days. Upper-level winds greatly favor development of this system, with a uniform 200 mb easterly flow foreseen by the GFS. None of the models are presently developing this system, but perhaps this is because the wave was not initialized all that well, if at all, within these models. As the wave's surface signature increases, the models may latch onto development. We'll have to see. This system has a better shot at ultimately becoming a hurricane than 93L does.

Gulf of Mexico disturbance

Most of the models have been predicting that a cutoff low will branch off in the northeastern Gulf of Mexico from the longwave trough recurving Colin in about 72 hours. The upper wind environment appears favorable for anything that might try and develop here. The eventual track of such a system is uncertain, but a landfall from Destin to New Orleans is probable at this juncture. Residents in these areas should closely monitor the progress of this potential system over the next several days.

Regardless of development, it appears that some locally heavy rainfall will be occurring over portions of the northeastern and northern Gulf Coast beginning in about 72 hours.

The models are not currently indicating much of a surface reflection with this feature, though they do depict it fairly well in the low- to mid-levels of the atmosphere.