textproduct: Key West

KEY MESSAGES

- There is a marginal risk for excessive rainfall through Thursday morning as topical moisture spreads into the area over the next several days.

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

- Increasing northeasterly swell in the wake of the cold front will allow seas to build by the end of the week and into the weekend, especially in the Straits of Florida.

DISCUSSION

Issued at 1041 AM EDT Tue Sep 22 2026

It took some time but the atmosphere is coming this life this morning. The Florida Keys are in a prime position to receive quite a bit of rain today and through the next couple of days. What makes today interesting is all of the dynamics in play. 1) We have an upper level trough parked across the Gulf basin. 2) We have copious amounts of moisture streaming from the western Caribbean, northward across the Keys and South Florida. 3) On the western end of our CWA we lie at the exit region of a 500 mb jet streak going through. 4) Meanwhile, on the eastern end we will have a low level kinematic jet developing and pushing potentially 25 to 30 knots at 700 mb.

This all to say that we should see more widespread lift going on today with embedded pockets of stronger/heavier activity at times. I think the main wind concern with storms will be in the Upper Keys and the far eastern coastal waters. All other storms will carry the risk of potentially locally heavy rainfall today. This will be compounded by higher tides, especially as we get closer to the full month. This means that rainfall rates will not need to be as high as usual to see some flooding. This also means that any localized flooding we do see will carry the potential to be brackish. No changes were needed or made to the going forecast.

MARINE

Issued at 1041 AM EDT Tue Sep 22 2026

Shower and thunderstorm coverage is slowly increasing across the coastal waters this morning and mariners can expect coverage to further increase this afternoon. Outside of convection, expect gentle to occasionally moderate southerly breezes. Breezes are then expected to gradually slacken beginning this late afternoon and evening and persist into middle of the week. Abundant moisture will remain trapped across the region, allowing for well above normal rain/thunder chances to persist.

AVIATION

(15Z TAFS) Issued at 1041 AM EDT Tue Sep 22 2026

Shower and thunderstorm coverage is beginning to increase but is expected to remain out over mainly open waters for the next couple of hours. As coverage increases through the afternoon expect VCTS for both EYW and MTH terminals. Given the scattered nature of this convection, will refrain from any TEMPO reductions to categories until it becomes more evident.

PREV DISCUSSION

Issued at 409 AM EDT Tue Sep 22 2026 The upper level low that we have been tracking the past several days has now opened into a trough as it lifts away from the region off the northeast coast of Florida throughout the day. Remnant energy associated with this departing disturbance will aid in the development of a coastal low that will track in a general northeast direction through the end of the week, deepening as it gains latitude. A cold front attached to this system will push southward into central Florida by Thursday and south Florida into Friday as the low pressure system becomes increasing occluded and retrogrades westward towards coastal New England by the end of the week. Meanwhile, before this frontal boundary approaches our area, we have a wet and stormy weather pattern on tap across the Florida Keys over the next several days. 250 mb analysis is showing a positively tilted trough extending into the Yucatan Peninsula left in the wake of the departing aforementioned disturbance. To the east of this feature, a deep feed of tropical moisture with its source regions in the southwest Caribbean is directed northeastward across Cuba, The Bahamas, and the Florida Keys with PWATs possibly peaking as high as 2.5 inches either later today or Wednesday. The increasing moisture advection and PVA throughout the day supports rounds of scattered to numerous showers and thunderstorms that will generally drift in a northward direction across the region. Despite these favorable ingredients to support well above normal PoPs today (60-80%), the southerly steering flow in place does add some uncertainty to the convective evolution as a Cuban shadow influence could ultimately bring a decrease in overall coverage at times. Current thinking is that forcing for ascent and anomalous moisture should ultimately win the battle so our forecast reflects these likely/categorical PoPs but certainly something to mention.

The heavy rainfall threat continues on Wednesday as an upper trough axis sharpens across the eastern US, which will help to maintain a tropical moisture connection. If PWATs are able to reach 2.5 inches, this would be at daily maximum values for this time of the year but even if it is slightly lower, values should still be at or above the 90th percentile of 2.16 inches. Steering flow on Wednesday might also be able to have a slightly more westerly component and this may be just enough to limit the drying influence from Cuba. Additionally, forecast soundings show near saturated atmospheric columns with the classic tall and skinny CAPE appearance so this activity should be very sufficient rain producers with heavy rainfall rates and quick accumulations. Thus, PoPs will continue to remain well above normal throughout this time with localized hydro impacts a possibility, particularly during high tide cycles where drainage capacity is less. Overall, QPF amounts through Wednesday night could be around 1.5-3 inches across the Florida Keys, which aligns with HREF 50th percentile values. However, should this event exceed expectations, values of 4-6 inches with locally higher totals can't be ruled out, which is shown in several 90th percentile solutions. As a result, WPC highlights all of the Keys for a Marginal risk of excessive rainfall through Thursday morning.

Rain chances will drop a bit closer to climatological values later in the week as aforementioned frontal boundary makes southward progress across the Florida peninsula. Drier air aloft spreads southward in the wake of this frontal boundary on an exceptionally deep northerly flow across the entire eastern CONUS on the backside of the nor'easter in the north Atlantic. However, some PoPs may still linger late week and into the weekend as the frontal boundary is expected to stall just north of Cuba in the Straits so the deeper moisture may not be able to completely scour out. Regardless, this frontal passage will certainly still be noticeable despite a much less impressive air mass change compared to what many areas further north will experience. Dewpoints are still projected to drop into the low to mid 70s and coupled with highs in the mid/upper 80s, this will be noticeable after the typical prolonged duration of extreme humidity experienced during Florida summers. However, it is still late September and long range guidance suggests that the slight decrease in humidity levels will be short-lived as return flow setting up early next week allows moisture values to recover and rain chances continuing to persist through the remainder of the forecast period.

KEY WATCHES/WARNINGS/ADVISORIES

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