Tropical Depression could form later this week and threaten the Leeward Islands
A tropical wave with an associated area of low pressure in the Eastern Atlantic is increasing the potential to become the next tropical depression of the season. Satellite imagery along with surface analysis charts revealed a broad area of low pressure centered near 1009 millibars at 9N-34W. Much of the convection is currently west of the low-pressure area due to easterly shear from a superimposed upper anticyclone to its northeast. Despite this, convection has increase in both spatial and intensity and has become more concentrated and organized around the low-pressure area. An ASCAT pass from last night along with RGB imagery indicated that this low-pressure area lies along the monsoon trough with west winds noted south of the system. However, there is little indications that the low-pressure area itself is closed but it should have little troubling closing off since it is not embedded in the classic “NE-SE” ITCZ, which is confluent but rather the “NE-SW” monsoon trough which is cyclonic. Upper winds, with the exception of the super-imposed anticyclone, appear conducive for development of a depression over the next few days. A depression should be declared when the low-pressure area manages to close off and convection becomes more concentrated around it, which I think, should not take more than 2 days.
Figure 1. Visible image of the Central Atlantic showing 91L's relation to the islands.
SHIPS appear to be handling the environment around 91L very well, indicating 14 knots of shear from 31 degrees (NNE) which is being observed on satellite imagery. Should we follow the SHIPS guidance, upper winds should continue to lower over the next 3 days, giving way to favourable conditions for gradual intensification and should 91L be declared a depression, could become the 2nd hurricane of the season. Through 3-5 days, shear begins to drastically increase due to encounter with the Tropical Upper Tropospheric Trough (TUTT), which will be located north of the Leeward Islands. The LGEM is the best model when it comes to intensity due to a change in the environment and so this is the guidance I am going with to have a potentially category 1 hurricane approaching the islands in 4 days, weakening as it encounters the TUTT.
Currently the steering flow is predominately westward, with a very large weakness in the subtropical ridge between 60W and 90W (30 degrees of weakness, wow). Most model guidances indicate that this weakness should influence 91L to move more west-northwest to northwest once it takes off. The exact timing of both features and the angle at which 91L leaves 10N will indicate how close the system gets to the extreme northeast Leeward Islands, the Virgin Islands and Puerto Rico. The weakness will not be strong enough to recurve 91L and it is likely to continue northwestward towards the Bahamas and the USA but this is beyond 1 week so to pinpoint a potential landfall that far on the spatial and temporal (time) scales will not be reliable. The ECMWF develops 91L and takes it on Georges-type track with probably a similar intensity. A feature that far south approaching 40W should be monitored closely.
Figure 2. Current computer model runs of 91L's track.
Interests in the Northeast Caribbean should continue to monitor the progress of 91L.
I postponed my August Outlook until a later date.