textproduct: Key West

KEY MESSAGES

- Heavy rainfall is expected at times today. 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.

DISCUSSION

Issued at 1008 AM EDT Mon Sep 21 2026

A stormy morning for the Southeast Gulf waters with an occasional flare-up near the island chain. The convection to our north is slowly creeping south but there remains a high amount of uncertainty as to how far south it will get. The hi-res models which initialized the line of convection further south than it's origin would stall it across the Keys today, especially the Lower and Middle Keys. This on top of higher tides could make flooding possible for the afternoon.

Unfortunately, only time will tell how this plays out but did not feel confident enough to go out with likely PoPs for today. Have nudged down slightly to high-end chance for now. We will be turning more southerly through the day which would act an inhibiting wind direction for convection. This however, is mostly dictated in the lower levels (below 700 mb). At 700mb and higher we have the moisture, but an upper level low sits to our northwest. This displaces the Keys from the best source of lift and therefore is driving a lot of the uncertainty as to what will happen later today with convection.

MARINE

Issued at 1008 AM EDT Mon Sep 21 2026

An upper-level low lingers in the eastern Gulf waters through tonight but will finally lift away from the area on Tuesday. However, weather conditions will continue to remain unsettled throughout most of the week as tropical moisture from the Caribbean spreads northward into the Keys into the middle of the week. As a result, rain chances will remain elevated with daily scattered to numerous showers and thunderstorms. The main hazards that mariners can expect with this activity will be a daily occurence with gusty winds, lightning strikes, blinding downpours, and locally hazardous seas. Breezes will generally remain light to gentle for most of the week but brief periods of moderate breezes may occur at times, mainly in the evening and overnight hours. In addition, winds will be out of the south to southeast throughout most of the week but winds will start to shift more northerly by late week and into the weekend as a frontal boundary gradually pushes southward across coastal waters of the Florida Keys. Seas will also start to build by the end of the week and into the weekend as northeasterly swell increases in the wake of the frontal boundary, especially in the Straits of Florida.

AVIATION

(15Z TAFS) Issued at 1008 AM EDT Mon Sep 21 2026

VFR conditions can be expected outside of any passing convection. Uncertainty is high that convection will develop either over the island chain or remain just to the north. Therefore, am trying to be judicial about VCSH/TS until the picture becomes more clear and will amend TAFs as warranted.

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.

PRELIMINARY POINT TEMPS/POPS

Key West 89 79 90 79 / 50 60 70 70 Marathon 88 80 88 80 / 40 60 60 70

KEY WATCHES/WARNINGS/ADVISORIES

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