textproduct: Key West

KEY MESSAGES

- Well above normal rain chances (around 60-70%) will persist across the Florida Keys through the weekend and into early next week.

- Heavy rainfall is expected at times. There is a marginal risk for excessive rainfall through Monday morning.

- While breezes will be light to gentle (5 to 10 mph) throughout the weekend, gusty winds and locally hazardous seas are expected in the vicinity of thunderstorms.

PREV DISCUSSION

Issued at 306 AM EDT Sat Sep 19 2026 Broad cyclonic flow aloft is essentially in place across the entire state of Florida as the well-advertised upper level low continues to gradually retrograde across the central FL peninsula. Meanwhile, mid to upper level ridging is in place across the Ark-La-Tex region and this feature will prevent the upper low from making meaningful westward progress in the coming days as the upper low is expected to emerge into the eastern Gulf waters by tonight and linger in this region into early next week. The influence of this feature across the Florida Keys is evident in the latest 19/00Z KEY RAOB as winds are generally from the WSW from about 650 mb and higher with a mainly easterly wind profile below this layer into the low levels. In addition, the 500 mb temperature sampled from this sounding was -7.4C, which places this value within the 25th percentile per SPC sounding climatology. While these mid level temperatures are impressive for mid September, the core of the cold air aloft (around -10C) will remain north of our region closer to the center of the upper low but we are certainly close enough to still expect enhancements to mid-level lapse rates at times. Regarding the evolution of convection today, there are a variety of messy solutions on the suite of CAM guidance as one would expect in an environment of deep moisture with favorable synoptic ascent aided by the presence of the upper low and enhanced divergence aloft. The main limiting factors appear to be more on the thermodynamic side with instability values not expected to be too impressive with models showing MLCAPE values in the 500-1000 J/kg range, despite the cooler mid level temperatures. Additionally, a dry slot circulating around the upper low may try to punch into the region from the west later today. Thus, there may be a bit of lull in convection by the evening hours, especially if the atmosphere becomes very worked over before the arrival of drier air. Regardless, if a lull in activity does occur this evening, it should be short-lived as there will be plenty of lingering boundaries around to allow for convection to remain a concern going into the overnight hours. Locally heavy rain and isolated strong wind gusts will remain the primary hazards with any of the more robust activity that is able to materialize.

The upper low should generally stall somewhere off the coast of the Tampa Bay area later in the weekend and place the Florida Keys on the eastern side of the circulation. This should favor a more deeper southerly wind profile and combined with better positioning for vorticity advection, additional rounds of convection will continue to be closely monitored. Heavy rainfall and localized flooding will remain the main concern even as storm motions should pick up somewhat later in the weekend. As a result, WPC maintains a Marginal risk of excessive rainfall throughout the weekend and into early next week across the Florida Keys with PoPs remaining in the high chance to likely range throughout this time.. By Tuesday, models show the upper low will finally be opening into a trough and becoming absorbed by a mid latitude trough through mid week, though there are some signals that the remnants of this feature could aid in the development of a coastal trough/low off the Southeast US coast before this system lifts northward later in the week. While model solutions differ in the positioning and strength of this system, it may be able to send a cold front southward into central FL and possibly south FL by mid to late week. Meanwhile, as this occurs, a surge of tropical moisture from the Caribbean is forecast to potentially push into the the region, which appears to be related to the Central American Gyre (CAG). While the specific forecast details remain poor at this time, there continues to be some ensemble support with about a 10-20% chance of PWATs greater than 2.5 inches reaching the Florida Keys. However, with the upper level pattern across the southern US featuring ridging in place across TX/LA, this may be enough to steer the deeper moisture feed to the west of our region but uncertainty still remains high at this time so trends will continue to be monitored closely.

MARINE

Issued at 1056 AM EDT Sat Sep 19 2026 Scattered showers and thunderstorms will grow in coverage and intensity through the afternoon and evening hours. Any stronger thunderstorm will be capable of producing gusty winds, as well as suddenly building and confused seas.

From synopsis, light to gentle breezes will continue through the weekend with winds mostly out of the east on Saturday and gradually shifting to the south to southeast into Sunday. In addition, an increasingly wet weather pattern will develop across the Florida Keys coastal waters through early next week, as an upper-level low retrogrades westward across the central Florida Peninsula and eventually slides into the Gulf later in the weekend. As a result, scattered to occasional numerous showers and thunderstorms will be a daily occurence, with gusty winds, lightning strikes, blinding downpours, and locally hazardous seas being the main hazards mariners could encounter in the vicinity of this activity. The upper-level low is forecast to move out of the area early next week, with winds becoming more southerly and breezes remaining mainly light to gentle, though occasional periods of moderate breezes could occur at times. Rain chances will also continue to remain elevated into next week, as deep moisture lingers across the Florida Keys coastal waters in the wake of the departing weather disturbance.

AVIATION

(18Z TAFS) Issued at 144 PM EDT Sat Sep 19 2026 VFR conditions continue at this hour at both EYW and MTH. Despite the lull in activity, convection developing over South Florida is expected to surge towards the Keys later this afternoon, and a TEMPO group has been utilized at both sites to account for that TSRA potential. In addition, cloud line development may support VCSH as early as 19Z. Convection should diminish late this evening with prevailing VFR conditions expected to return overnight. Additional redevelopment may occur near the terminals early tomorrow morning. Amendments will be utilized as needed.

PRELIMINARY POINT TEMPS/POPS

Key West 90 78 90 79 / 50 50 50 70 Marathon 88 79 88 80 / 50 50 50 70

KEY WATCHES/WARNINGS/ADVISORIES

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