Caribbean: NHC has 50% chance of tropical or subtropical storm development, within next 48 hours. The system will then hit cooler water and become even more energized with an upper trough. The result will be a large and powerful extratropical cyclone in the North Atlantic which likely produce far reaching swells lasting next week.
On the northern islands occasional showers with a moderate north to northwest breeze is expected… but mostly dry rest of the islands except for few seabreeze showers.
NHC is monitoring the large disturbance to the NE of the Leewards. 30% chance of sub-tropical formation in 48% or 40% within 5 days. The best chance is till next 72 hours till late Thursday. It will meander a bit but eventually move NE away from the islands.
Its effects on the islands will be some showers and possible thunderstorms with a trough/shear line extending over the Leewards and then Windwards next few days and another over the southern islands; also, slack winds causing localized sea breeze showers in the daytime. Some can be locally heavy. Seas will be locally rough in the northern islands with northerly swells, some reaching the southern islands on both Caribbean and Atlantic coasts.
Surface trough with areas of eastward moving showers(few heavy) extends over the Leewards from a complex developing low pressure to the north and northeast. Weak and variable surface winds are over the north to central Windwards leading to slow moving sea breeze showers (few mod/heavy) over eastern portions of those islands. Barbados has a flood watch. Another surface trough is around the southern islands, with occasional thunderstorms around southern Windwards & Trinidad/Orinoco River delta.
This continues for next day but less in the southern islands. Another surface trough will establish on Monday on the Leewards and the current one moves into the north Windwards then central Windwards into Tuesday.
The low pressure is to remain far to the NE of the Leewards drifting but will develop further and models have been agreeing on at least a sub-tropical storm forming(4th pic is GFS by 8am Wed) which will eventually become a powerful extra-tropical storm or according to some model runs it could possibly become even stronger hybrid hurricane force storm by weekend(5th pic GFS 2am Sat). Winds will be slack across most islands; in northern islands cooler north to northwest to west winds and over the southern islands more variable but at times westerly. Some more daytime sea breeze showers will occurs during early week.
Edit: Seas will become gentle most areas, but in the northern islands especially swells will become more frequent from early to mid-week.
Over Guyana another low level trough with upper level support is bringing scattered moderate to heavy showers and isolated thunderstorms. A jet stream is over the Windwards and the equatorial ridge has expanded over Guianas and into the Atlantic. Together with low level convergence due to the developing large low pressure, there will be more chances for slower moving moderate to heavy showers and thunderstorms on the Guyana, Suriname coastal areas.