Marine Weather and Tide Forecast for Big Coppitt Key, FL

DISCUSSION
Issued at 423 AM EDT Mon Aug 17 2026 Radar scans from KBYX this morning picked up a few cargo ships moving in the Florida Keys. This is an example of radar ducting, when the radar pulse bends back towards the surface and encounters a non meteorological object. This phenomenon highlighted that the pre-sunset environment over the Florida Keys this morning was extremely stable. Deep layers of dry air associated with the Saharan Air Layer (SAL) are maintaining a robust inversion across the Keys waters, thus the anomalous radar propagation. The implication for today's weather is, once again, mostly sunny skies and warmer than normal temperatures. Temperatures along the island chain early this morning are in the lower to mid 80s, though winds are light enough for the interior locations of the larger islands (Key West, Big Pine Key) to radiatively cool to near 80 degrees. However, dew points remain in the upper 70s.
Despite the dry air aloft, the surface will still feel muggy.
Breezes are not expected to provide too much relief today either.
While stations along the reef are measuring light to gentle breezes, later today the winds are expected to slacken. In sum, brace for another muggy day in the Keys.

Upon zooming out to the synoptic perspective, two features will define weather across the Keys for the first half of the week. The first one is the aforementioned SAL. The immediate effects of the SAL are the mostly clear skies and lack of shower coverage across the Keys waters. Unfortunately, the absence of cloud cover means nothing will prevent the islands from heating rapidly during daytime hours. The second feature is that a col region is on track to set up over the Florida Keys. Lower pressure over the eastern US means a weaker subtropical ridge axis over the Gulf and Southern Florida, and thus light to gentle breezes across the Keys waters. Recent statistical model runs suggest periods of light and variable winds may be observed through Thursday or Friday at the latest.

When will this pattern break? The Saharan Air Layer is expected to wash out during the middle of the week, but that does not mean showers will redevelop en masse. Environmental winds are still in a pattern that would direct deep layer moisture away from the Florida Keys. Model ensembles indicate lower pressure and mid level troughing will persist over the Eastern United States through the end of the week. For better or worse, there is medium to high confidence that hot and muggy conditions with light to gentle, occasionally varaible, winds will persist through the week and into next weekend.

MARINE
Issued at 423 AM EDT Mon Aug 17 2026 No watches, warnings, or advisories are in effect for the Florida Keys waters. From synopsis, a col region is setting up across the Florida Keys early this week. Generally gentle east to southeasterly breezes will become light and variable at times during the week. Westerlies are possible later this week in the event the col region is forced south towards Cuba. A Saharan Air Layer will keep showers suppressed during the first half of the week.

AVIATION
(00Z TAFS)
Issued at 423 AM EDT Mon Aug 17 2026 VFR conditions will persist at both EYW and MTH for the upcoming TAF period. A layer of Saharan dust will keep shower and cloud coverage low. Near surface winds will start to slacken before sunset, and variable conditions are likely at MTH tonight.

PRELIMINARY POINT TEMPS/POPS
Key West 92 81 92 82 / 0 0 10 0 Marathon 90 81 90 83 / 10 0 10 0

KEY WATCHES/WARNINGS/ADVISORIES
FL...None.
GM...None.