Area Forecast Discussion
Issued by NWS Key West, FL
Issued by NWS Key West, FL
822 FXUS62 KKEY 201456 AFDKEY Area Forecast Discussion National Weather Service Key West FL 1056 AM EDT Sun Sep 20 2026 ...New DISCUSSION, MARINE, AVIATION... .KEY MESSAGES... - Well above normal rain chances (around 60-70%) will persist across the Florida Keys through the remainder of 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. && .NEW DISCUSSION... Issued at 1022 AM EDT Sun Sep 20 2026 Water vapor imagery highlights a compact mid to upper-level low just southwest of Tampa. Tampa`s 12Z RAOB sampled -9 degC at 500 mb, with MFL at -8 degC, and KEY at -7 degC. Our vertical profile featured southerlies through 850 mb, giving way to deep westerlies. The ML parcel method suggests weak instability and strong inhibition. This is fairly typical for the past several days. The match gets lit, sparking widespread convection and the subsequent outflow emissaries spreading outward from the dying thunderstorms. Eventually, much of the CWA is robbed of CAPE and the Keys go through several hours of quiet conditions and slowly thinning anvils. Lower tropospheric prognostic tools highlight a turn to the southwest in the steering this afternoon. Meanwhile, the island chain is now under potent late-September insolation. The first signs of cumulus development are noted on visible imagery, as are some orphaned anvils west of Key West that may temporarily halt diabatic heating. The latest HRRR derived ACARS soundings measure moderate ML instability and diminishing CIN. So, it shouldn`t be along before the island chain erupts, possibly from the thickening cu-field spreading north out of the Straits. We will maintain the likely POP chances. It should be noted, the HRRR Corfidi upshear vectors are 5 knots or less, so heavy rainfall will remain a threat for any slow moving thunderstorms. LAV P01 trends continue to indicate another dry period (overnight) after the afternoon/evening convection wanes. The digital forecast was modified earlier this morning to temporarily decrease weather coverage, otherwise no additional changes are needed or planned. && .DISCUSSION... Issued at 351 AM EDT Sun Sep 20 2026 GOES-19 satellite imagery shows broad cyclonic flow across the entire state of Florida as an upper level low continues to remain stalled in the eastern Gulf waters off the SWFL coast. This feature has failed to make much westward progress as ridging aloft near the Ark-La-Tex region remains in place so this disturbance will continue to linger in the eastern Gulf waters for the remainder of the weekend and into the start of the new week. Despite the close proximity of this feature, the core of cold air aloft has remained to the north of our area so far this weekend and the 20/00Z KEY RAOB sampled a 500 mb temperature of -5.7 C, which was considerably warmer than the sounding from Saturday morning which had a value of -8.3 C, near the 10th percentile for this time of the year. Thus, this warming trend aloft has likely played a significant role in why convective development has been more suppressed than what the official forecast and model guidance had suggested so far this weekend. That said, will continue to maintain elevated rain chances for the remainder of the weekend as the upper level low to the north continues to still favor increased forcing for ascent across the region with divergence aloft and an increase in vorticity advection. Since this feature remains in an environment of weak steering, it may be able to drift just far enough southward today to possibly allow mid level temps to cool a touch further once again and bring a slightly more favorable thermodynamic environment today with better mid-level lapse rates compared to what was observed yesterday. Additionally, the low level flow will become more southerly today so this should bring a slight uptick in moisture quality with PWATs in excess of 2 inches area-wide, though the southerly flow will also of course be more susceptible to Cuban shadow influence. Regardless, there should be enough ingredients in place to support scattered to occasional numerous showers and thunderstorms across the Florida Keys and surrounding coastal waters today, which could also be enhanced further if a cloud line is able to take advantage of a light/VRB wind flow developing in the late afternoon hours. While the cloud line scenario remains to be seen, activity should have no issues developing late evening and overnight as there should be plenty of mesoscale boundaries during the overnight hours to provide additional initiation sources. The upper level low is forecast to open into a trough late Monday and into Tuesday as it becomes absorbed in a mid-latitude trough into mid week. Lingering moisture in the wake of this feature should still support elevated rain chances on Monday with PoPs peaking from 50% to 70%. As the weakening upper low lifts off of Northeast FL on Tuesday, it will aid it in the development of coastal low off in Southwest Atlantic with this system expected to lift to the north-northeast through mid to late week, deepening as it does so. As this occurs, a frontal boundary associated with this system will drop southward across the central Florida peninsula mid week and potentially push through south Florida by the end of the week. Whether or not this frontal boundary can push all the way through south Florida and the Keys remains in question at the moment, but a deepening upper level trough across the eastern CONUS may provide enough push to allow the boundary to sweep through. If this frontal passage occurs, the air mass change in the wake of the boundary this early in the season may fall short of bringing a significant noticeable change in temperatures with a decrease in dewpoints and humidity values being the more likely outcome. In addition to this frontal boundary, there will also be a surge of tropical moisture being funneled northward from the Caribbean potentially associated with an inverted trough feature that may move across the Yucatan Channel. While amplifying ridging across east Texas in the middle of the week and the aforementioned frontal boundary pushing southward across the peninsula would suggest that most of the moisture associated with this increase in tropical moisture should get directed to the west of the Florida Keys, there should be enough increase in moisture to keep rain chances elevated through the middle of the week before moisture levels decrease towards the end of the week as the frontal boundary drags a drier air mass into the region. && .MARINE... Issued at 1022 AM EDT Sun Sep 20 2026 There are no watches, warnings, or advisories currently in effect for the coastal waters of the Florida Keys. Light to gentle mainly southerly breezes will be in place through the remainder of the weekend, though a period of variable breezes are expected late this afternoon and evening. In addition, a wet weather pattern will persist across the Florida Keys coastal waters into the new week as an upper- level low lingers in the eastern Gulf waters. As a result, scattered to numerous showers and thunderstorms will be a daily occurence with gusty winds, lightning strikes, blinding downpours, and locally hazardous seas the main hazards. Breezes will generally remain light to gentle into the new week but brief periods of moderate breezes may occur in the evening hours. && .AVIATION... (15Z TAFS) Issued at 1022 AM EDT Sun Sep 20 2026 VFR conditions will prevail at EYW and MTH. Near surface winds are on track to remain light with periods of variable conditions. VCSH to VCTS conditions are expected this afternoon, though the timing and extent is uncertain. MVFR to LIFR conditions are possible near showers, and EYW is more likely to see SHRA conditions than MTH. VCSH conditions are expected to taper off after sunset. && .PRELIMINARY POINT TEMPS/POPS... Key West 89 78 90 79 / 70 60 50 70 Marathon 88 80 88 80 / 70 60 60 70 && .KEY WATCHES/WARNINGS/ADVISORIES... FL...None. GM...None. && $$ Public/Marine/Fire...CLR Aviation/Nowcasts....AJP Data Acquisition.....AJP Visit us on the web at weather.gov/key Follow us on Facebook and X at: www.facebook.com/nwskeywest www.x.com/nwskeywest