Marine Weather Net

Gulf of Mexico from Seven Mile Bridge to Halfmoon Shoal Marine Forecast


TONIGHT

E
WINDS
10 - 15
KNOTS

MONDAY

SE
WINDS
20
KNOTS

MONDAY NIGHT

E
WINDS
15
KNOTS

TUESDAY

SE
WINDS
10 - 15
KNOTS

The Marine Weather Forecast In Detail:
GMZ035 Forecast Issued: 423 PM EDT Sun May 24 2020

SMALL CRAFT ADVISORY IN EFFECT
Tonight...East Winds 10 To 15 Knots, Becoming Southeast And Increasing To Near 20 Knots Late This Evening And Overnight. Seas 1 To 3 Feet. Nearshore Waters A Light To Moderate Chop, Becoming Rough. Winds And Seas Higher Near Widespread Showers And Scattered Thunderstorms.
Monday...Southeast Winds Near 20 Knots, Decreasing To Near 15 Knots. Seas 1 To 3 Feet, Except 2 To 4 Feet West Of The Marquesas Keys. Nearshore Waters Rough, Becoming A Moderate Chop. Widespread Showers And Scattered Thunderstorms.
Monday Night...Southeast Winds Near 15 Knots, Becoming South. Seas 1 To 2 Feet. Nearshore Waters A Moderate Chop. Numerous Showers And Scattered Thunderstorms.
Tuesday...Southeast Winds 10 To 15 Knots. Seas 1 To 2 Feet. Nearshore Waters A Light To Moderate Chop. Scattered Showers And Thunderstorms.
Tuesday Night...East To Southeast Winds Near 10 Knots. Seas 1 Foot Or Less. Nearshore Waters A Light Chop. Scattered Showers And Thunderstorms.
Wednesday...East To Southeast Winds Near 10 Knots. Seas 1 Foot Or Less. Nearshore Waters A Light Chop. Scattered Showers And Thunderstorms.
Wednesday Night...East Winds Near 10 Knots. Seas 1 Foot Or Less. Nearshore Waters A Light Chop. Isolated Showers And Thunderstorms.
Thursday...East Winds Near 10 Knots. Seas 1 Foot Or Less. Nearshore Waters A Light Chop. Isolated Showers And Thunderstorms.
Friday...East Winds 10 To 15 Knots. Seas 1 To 2 Feet. Nearshore Waters A Light To Moderate Chop. Isolated Showers And Thunderstorms.
Friday Night...East Winds Near 15 Knots. Seas 1 To 2 Feet. Nearshore Waters A Moderate Chop. Scattered Showers And Thunderstorms.
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Synopsis for Keys coastal waters from Ocean Reef to Dry Tortugas: GMZ005
423 PM EDT Sun May 24 2020

Weak low pressure near the western tip of Cuba will move northward through the eastern Gulf tonight, Monday, and Monday night. As the weak low moves through the southeast Gulf tonight, fresh to occasionally strong breezes will develop over Keys waters. From Wednesday through Friday, a large Bermuda high will expand westward toward the U.S. Southeast coast.

Gulf Stream
The Approximate Shoreward Edge of the Gulf Stream as of May 23...
- 37 NM South of Dry Tortugas Light...on Loggerhead Key.
- 28 NM South of Cosgrove Shoal Light...off the Marquesas Keys.
- 17 NM South of Sand Key Light...off Key West.
- 13 NM South of Looe Key...off Big Pine Key.
- 11 NM South of Sombrero Key Light...off Marathon.
- 3 NM Southeast of Alligator Reef Light...off Islamorada.
- 4 NM Southeast of Molasses Reef Light...off Key Largo.
- 4 NM East of Carysfort Reef Light...off Ocean Reef.
Area Forecast Discussion
National Weather Service Key West FL
356pm EDT Sunday May 24 2020

Short Term
(Tonight through Wednesday) Heavy rainfall will be the main concern through Monday. WPC guidance forecasts widespread heavy rainfall amounts of 3 to 4 inches tonight through Monday, in addition to what has already occurred.

The air mass is extremely moist right now. The 12z MFL sounding observed a daily record-moist Precipitable Water (PW) value of 2.21", and KEY's value of 2.12" exceeded the 95th percentile.

Besides being extremely moist, the air mass is characterized as having weak convective instability but an increasing amount of low-level wind shear for organizing convection. Global models point to falling pressures over the southeast Gulf this evening, with the rowdier model solutions closing of a weak low center. Either way, it will increase our E-SE surface winds tonight beneath southerly flow in the 850-700 mb layer. Within the last 2-3 hours, radar has detected an increasing number of rotating convective cells over the waters, especially the Gulf waters. Storm Prediction Center has the Keys under a Marginal Risk of severe weather tonight and Monday morning, with low probabilities showing up for tornadoes and severe wind gusts. It is also possible that this evening's pressure falls over the southeast Gulf will organize some curved bands of heavy rainfall. If this happens, then the repeated training of cells over the same locations could lead to our heavier rainfall amounts exceeding WPC guidance.

This soupy air mass exists to the north of a negatively tilted 500 mb trough extending from the upper Texas coast to the Yucatan Channel to near Grand Cayman. This placement puts the Keys under difluent flow aloft, and this will continue until the slow-moving trough axis lifts north of the Keys on Monday PM. The mere presence of large-scale lift in a record-moist air mass would support heavy rain.

Once the trough axis lifts north of the Keys on Monday night, large-scale lift will exit, and the atmosphere will dry out above 700 mb. Rain chances will taper off back closer to June normals of 25-30 percent in a 12-hour period by Wednesday. (Yes, I know it is not yet June.)

Long Term
(Thursday through next Sunday) A weak and diffuse upper ridge will build over Florida and the eastern Gulf between cutoff upper lows over northeast Texas and south of Bermuda. It will not be enough to completely suppress convection, especially with PW values (Precipitable Water values) remaining at or above normal. Nonetheless, the return of more common east-southeast surface winds and pockets of convective inhibition will amount to near- or slight above-normal rain chances. In order to focus any convection, we will have to rely on diurnal influences from nearby land masses such as Florida, Andros Island and Cuba.

Marine
Weak low pressure over the Yucatan Channel or over far western Cuba will move northward through the eastern Gulf from this afternoon through Monday night. As the weak low moves northward near our far western Gulf waters tonight, fresh to occasionally strong southeasterly breezes are expected over the Keys waters. This will likely require the eventual issuance of a Small Craft Advisory. In addition, numerous showers and a few thunderstorms are expected through Monday. These will contain particularly heavy rainfall, therefore reducing visibility accordingly. Gusty convective winds are also a concern. Though any shower could produce gusty winds, keep an ear out for thunder, since that will come with the strongest gusts.

From Wednesday through Friday, a large Bermuda high will expand westward toward the U.S. Southeast coast, bringing gentle to occasionally moderate easterly breezes.

Climate
On this date in 2001, Marathon set the daily record rainfall 2.30 inches. On this date in 1937, Key West set its daily record rainfall of 3.15".

NOAA Key West FL Office: Watches - Warnings - Advisories
FL...None. GM...Small Craft Advisory for GMZ031>035-042>044-052>055-072>075.