It says heavy showers or thunderstorms will occur in few areas mainly during the daytime in North and Central Trinidad.
They said some places could see up to 30mm of rainfall within a few hours, while 10 to 15mm is likely for the rest of the country.
Winds of up to 50mph could hit Northants, while temperatures could sit at around 20C.
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On Saturday high winds are expected across the region causing disruption for outdoor events and creating a risk of falling branches and debris.
In the week starting from August 11, conditions will remain unsettled, with "brief" warm spells.
The yellow warning of rain is in place from midnight Friday (August 9) for 24 hours with forecaster predicting: "A band of heavy rain, with the possibility of some very heavy bursts, will move northwards during Thursday night and Friday".
Varying amounts of rainfall are likely across MA, with higher rainfall totals possible in a thunderstorm.
"Temperatures will mostly be around average, but brief warm spells are possible across the south and southeast".