A fast-moving pre-frontal squall line passing through the upper Florida Keys developed an embedded supercell thunderstorm with attendant mesocyclone along the Florida Bay side of Upper Matecumbe Key.  The supercell thunderstorm moved east-northeast, producing a waterspout which made landfall on the bayside of Plantation Key, Islamorada.  The tornado crossed the Overseas Highway, exiting oceanside just southwest of Tavernier Creek.  The waterspout then made a second landfall oceanside in Tavernier southeast of Tavernier Creek before dissipating.

Tornado in MONROE, FLORIDA
From 15-APR-18 19:12:00 to 15-APR-18 19:20:00 (EST-5)
Location: TAVERNIER
Tornado F-scale: EF1
Property damage: 4.00K
Crop damage: 0.00K
Reported by: NWS Storm Survey

Summary:  A waterspout or funnel cloud was observed backlit by
lightning over Florida Bay shortly before making landfall as a
tornado at Indian Mound Trail of the Indian Waterways subdivision of
Plantation Key, Islamorada. Large gumbo limbo tree limbs were downed
as well as the top of a palm tree, landing 20 yards from streetside
the front steps of a residence. And adjacent residence had a rolling
driveway gate completely dislodged from its mounts, pushed into a
trailered boat and car. The tornado continue to produce mostly
damage to large limbs of hardwood trees, including gumbo limbo and
mahogany, in a path rarely exceeded two residential properties in
width toward the east-northeast. An entire gutter and downspout
system was removed from a residence. Numerous loose items, including
filled trash cans and lawn/patio furniture was moved from the
backyards and understory of homes onto front lawns and streets. Most
of the observed damage in the Indian Waterways community were
consistent with 60 to 70 mph, with possible spot values near 75 mph.
The tornado crossed a patch of mangroves and undeveloped forest, and
reached greatest intensity at San Pedro Catholic Church. There, a
large tree split at the trunk, and an old-growth mahogany was
uprooted, falling toward the east onto a portion of the structure
connecting the rectory with the main church. Fortunately, the upper
portion of the tree did not substantially damage the roof, with no
leaks observed in the downpour that followed. However, several
barrel tiles were removed from the peak ridges forming the hip roof
pattern over the rectory, as well as from the southeast edge of the
church, onto the top of the church's main entrance. Numerous large
limbs were downed toward the east, with lightweight free-standing
signs deposited northward cross the tornado path from right to left.
Estimated wind speeds were maximized in the range of 90 to 100 mph
with maximum width near 60 yards. Good impact reference made to the
observed wind impacts at this location from recent Hurricane Irma,
all having previously been removed or repaired.
The tornado continued toward the east-northeast, crossing U.S.
Highway 1 and the Old Highway, but weakened such that only large
tree limbs were downed. From this point, the tornado likely moved
out over the oceanside as a waterspout into Hawk Channel, southwest
of the oceanside entrance to Tavernier Creek.

Tornado in MONROE, FLORIDA
From 15-APR-18 19:12:00 to 15-APR-18 19:20:00 (EST-5)
Location: TAVERNIER
Tornado F-scale: EF0
Property damage: 0.20K
Crop damage: 0.00K
Reported by: NWS Storm Survey

A second path, likely a continuation as the waterspout made landfall
near Lincoln Avenue on the far southwest portion of Tavernier's
oceanside. Progressive narrowing of the damage path down to nearly
30 yards as well as a turn toward the northwest or north-northwest
was observed.  Impacts lessened from near the waterfront, including
damaged fences and large limbs,gradually down to smaller limbs and
twigs was noted. No further impacts were observed northeast of Lowe
Street.  It is likely the wind speeds 55 to 65 mph were observed,
weakening to below 50 mph roughly 3 blocks from the waterfront.