textproduct: Key West

KEY MESSAGES

- Heavy rainfall is possible at times tonight. There is a marginal risk for excessive rainfall through Tuesday morning.

- Rain chances will increase further by the middle of the week as tropical moisture spreads into the area.

- Drier air associated with a cold front will arrive late week and into the weekend will bring a lower risk of excessive rainfall, though some rain chances will linger.

MARINE

Issued at 307 PM EDT Mon Sep 21 2026

A broken line of showers and thunderstorms extends from the Straits of Florida waters south of the Dry Tortugas, along and north of the island chain. Individual movement of cells has been generally to northeast. The line itself continues to wax and wane with activity but shows a general downward trend. Stronger activity will be found on the western end of this line over the next couple of hours. As we go into the late afternoon and early evening we may see a brief lull in showers and thunderstorms. Thereafter, watching for southerly flow to advect any lingering convection from Cuba northward across the Keys. This will be accompanied by a wind surge as breezes are expected to freshen to gentle to moderate during the late evening and overnight.

AVIATION

(18Z TAFS) Issued at 307 PM EDT Mon Sep 21 2026

A broken line of convection continues to hover very near the island chain and airport terminals. The general trend has been for this line to gradually dwindle. Will keep mentions of VCSH in for another couple of hours, then expecting a brief lull during the late afternoon and early evening. Watching for a renewed surge of convection later tonight originating from Cuba.

PREV DISCUSSION

Issued at 428 AM EDT Mon Sep 21 2026 The upper level low that has dominated our weather pattern over the past several days still remains stalled just off the coast of west central Florida near the Tampa Bay area, though the influence of this feature will finally start to wane by the end of the day. For now, its presence is still felt as GOES-19 satellite imagery continues to reveal a broad cyclonic flow to the cloud pattern across the Sunshine State. The 00Z KEY RAOB measured a deep southwesterly flow regime throughout the vertical column with little inhibition and abundant moisture with PWATs of 2.11 inches, though mid level temps were notably warmer than recent days at -4.7 C. Given that the proximity of the upper low is still close enough to our region to supply continued forcing for ascent, another active day of convection is generally expected to occur across the Florida Keys with today's setup possibly favoring a reverse cloud line given a somewhat more southwest flow orientation. However, this depends on if enough low level veering ultimately occurs as there might remain just enough of a southerly component to prevent an ideal alignment with the Island Chain. Should a more organized cloud line develop later today, rain rates over land communities will need to be monitored closely as forecast soundings show corfidi upshear values generally less than 10 knots, possibly enhancing the potential for back building cells and quick accumulations. It should also be noted that several recent CAM solutions are highlighting some maximum QPF bullseye values of 4-6" through tonight and while it remains to be seen if these totals will be more favored over land or offshore, there is a reasonable threat for potential hydro impacts, especially in locations that received multiple rounds of heavy rainfall in short durations. As a result, WPC maintains a marginal risk of excessive rainfall through early Tuesday morning across the entire Island Chain.

The aforementioned upper level low will open into a trough by tonight and lift away to the northeast, eventually getting absorbed into a mid latitude trough swinging across the Ohio Valley. As this occurs, remnant energy associated with this departing disturbance will lead to the development of a coastal low offshore of the Southeast US on Tuesday with the low pressure expected to then lift northeastward, deepening as it gains latitude. Eventually, it will stall and retrograde slightly as it meanders offshore of the mid-Atlantic and becoming increasingly occluded towards the end of the week. A cold front attached to this system is expected to drop southward across the central Florida peninsula by mid week with and will continue advancing further southward into late week across the south FL and into the Florida Keys. Meanwhile, ahead of this boundary, tropical moisture is forecast to surge northward across western Cuba and into the Florida Keys with peak moisture values expected to occur late Tuesday into Thursday as PWATs possibly reach as high as 2.5 inches. This moisture has its source region in the southwestern Caribbean Sea as it appears that an inverted trough feature pinches off from a larger Central American Gyre counter-clockwise circulation. This disturbance is generally expected to move westward after it crosses the Yucatan Channel as it encounters stout mid-level ridging in the Ark-La-Tex region, but the substantial increase in moisture advection expected across the Keys should yield several days of a heavy rainfall threat with likely to categorical PoPs mid week before the arrival of the aforementioned frontal boundary brings a drier air mass into the area by end of the week. However, this is not expected to be a very clean frontal passage at this time as deeper moisture doesn't completely scour out of our area so PoPs should linger in the high chance to likely range through the end of the period. While it still remains a bit early in the season for the cooler air mass changes and corresponding cold air advection that typically follow cold frontal passages, the northerly flow late in the week should help shave off a degree or two from highs and also bring dewpoints a few degrees lower, which should still be noticeable after an extended period of the typical Florida summer humidity.

KEY WATCHES/WARNINGS/ADVISORIES

FL...None. GM...None.