textproduct: Key West

KEY MESSAGES

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

- Heavy rainfall is expected at times. There is a marginal risk for excessive rainfall through the middle of next week.

- 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.

AVIATION

(18Z TAFS) Issued at 115 PM EDT Fri Sep 18 2026 After a lull in activity, scattered showers and thunderstorms will develop after 18z, continuing into the evening in the vicinity of the terminals. Activity will be slow moving, resulting in bouts of sub-VFR ceilings and visibilities. Another lull is expected for much of the overnight hours, before regeneration of activity occurs thereafter. Have included general VCSH for the latter half of the forecast period, with low confidence in that forecast. VFR conditions are expected outside of any shower or thunderstorm activity with surface winds around 5-8 kts today out of the northeast to east.

PREV DISCUSSION

Issued at 301 AM EDT Fri Sep 18 2026 The synoptic picture reveals an anomalously strong upper level ridge (600 dam) that remains centered around the lower MS Valley and an upper level low is positioned in the Atlantic waters off the Cape Canaveral area. Throughout the day, this feature will retrograde westward as it tracks across the central Florida peninsula before eventually shifting into the Gulf over the upcoming weekend. Meanwhile, surface high pressure off the coast of New England has an associated ridge axis extending southwestward into the Southeast US, which is placing the Florida Keys in an easterly low-level flow pattern. However, while the flow has been mainly easterly throughout the entire atmospheric column in recent days, there will be a noticeable change in the mid to upper level wind pattern today as the cyclonic flow influence associated with the aforementioned upper level low becomes more apparent across the region with winds aloft becoming more southwesterly as the upper low tracks by to our north. The biggest change that will result from these opposing wind vectors at different levels is that storm motions will likely be more slow-moving and erratic than the westerly storm motions observed in recent days, which enhances the potential for heavier rainfall and localized flooding issues, particularly in locations that receive heavier downpours for extended durations. As a result of these concerns, WPC has placed all of the Florida Keys in a Marginal risk of excessive rainfall for the next several days given that this upper level low will be the main feature that will influence our local weather pattern throughout the weekend and into next week. This also means that rain chances each day will be above climatological normal values, pretty much peaking in the 50-60% range through early next week. Regarding severe weather potential, the core of the cooler mid level temperatures (-9C to -10C at 500 mb) is forecast to stay just north of our area in central FL but 500 mb temps may still fall to around -7C/-8C across the Keys, which is still only between the 10th and 25th percentile for mid September so this should favor some enhancement to mid-level lapse rates at times and the development of stronger cores aloft. Should this materialize, stronger wind gusts can't be ruled out if some of this activity can take advantage of the increase in buoyancy despite the background shear values being on the weaker side, though deep-layer shear values will increase a touch by later in the weekend and into next week (sfc-6km near 20 kts) as the upper low meanders in the eastern Gulf. Overall, an unsettled weather pattern is anticipated to end the week and throughout the weekend with persistent forcing for ascent remaining in place across the region, helping to maintain elevated rain chances, localized flooding concerns, and occasional strong wind gusts as daily scattered to occasional numerous showers and storms are forecast to blossom across the Keys each day.

As the new week arrives, some question remains as to when this upper level low will depart the region as the GFS maintains the upper low in the eastern Gulf into Tuesday before eventually lifting if northward into mid week but the ECMWF opens it into a trough earlier in the week. Regardless of either scenario, deep moisture is generally expected to remain across the Florida Keys well into next week as southerly flow lingers in the wake of the departing upper low. As we approach the end of the forecast period next week in the mid to late week time frame, forecast models show ridging aloft building across the central and southern Plains with an upper trough/low around the Great Lakes region, likely driving a frontal boundary gradually towards the Southeast US by late week. Meanwhile, attention may also need to be focused to the south of the Florida Keys with some potential for a surge of tropical moisture approaching from the Caribbean. This moisture surge appears to be related to the Central American Gyre (CAG) and would align with a climatologically favored time of the year as these features typically become more established in early Fall. That said, the synoptic details are highly uncertain still but there is some ensemble support with this moisture surge at this time with both the GEFS and EPS showing about a 10%-20% chance of PWATs in excess of 2.5 inches by the end of next week slowly approaching the Florida Keys. As a result, will maintain a consensus blend of models approach to PoPs to account for the uncertainty regarding this pattern, which still keeps PoPs in the high chance to likely range through the end of the forecast period.

MARINE

Issued at 1100 AM EDT Fri Sep 18 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, high pressure in the North Atlantic will weaken today, allowing winds to slacken across the Florida Keys coastal waters. Light to gentle breezes will continue through the weekend. 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 Florida Peninsula and eventually slides into the Gulf this 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 light to gentle. 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.

KEY WATCHES/WARNINGS/ADVISORIES

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