Marine Weather Net

Altamaha Sound, GA to Fernandina Beach, FL 20 - 60 NM Marine Forecast


TONIGHT

SW
WINDS
10 - 15
KNOTS

WEDNESDAY

W
WINDS
5 - 10
KNOTS

WEDNESDAY NIGHT

W
WINDS
5 - 10
KNOTS

THURSDAY

NW
WINDS
5 KNOTS

The Marine Weather Forecast In Detail:
AMZ470 Forecast Issued: 330 PM EDT Tue Jul 27 2021

Tonight...Southwest Winds 10 To 15 Knots, Becoming West After Midnight. Seas 3 To 5 Feet With A Dominant Period Of 9 Seconds. Chance Of Showers And Thunderstorms.
Wednesday...West Winds 5 To 10 Knots. Seas 3 To 4 Feet With A Dominant Period Of 9 Seconds. Chance Of Showers And Thunderstorms.
Wednesday Night...West Winds 5 To 10 Knots. Seas Around 3 Feet With A Dominant Period Of 8 Seconds. Chance Of Showers And Thunderstorms In The Evening, Then Slight Chance Of Showers And Thunderstorms After Midnight.
Thursday...Northwest Winds Around 5 Knots, Becoming Southeast 5 To 10 Knots In The Afternoon. Seas 2 To 3 Feet. Slight Chance Of Showers And Thunderstorms In The Morning. Chance Of Showers And Thunderstorms In The Afternoon.
Thursday Night...Southwest Winds 10 To 15 Knots. Seas 2 To 3 Feet. Chance Of Showers And Thunderstorms In The Evening.
Friday...Southwest Winds 5 To 10 Knots, Becoming South 10 To 15 Knots In The Afternoon. Seas 2 To 3 Feet. Slight Chance Of Showers And Thunderstorms.
Friday Night...Southwest Winds 10 To 15 Knots. Seas 3 To 4 Feet.
Saturday And Saturday Night...Southwest Winds 10 To 15 Knots. Seas 3 To 4 Feet. Slight Chance Of Showers And Thunderstorms.
Sunday...Southwest Winds 10 To 15 Knots. Seas 3 To 4 Feet. Chance Of Showers And Thunderstorms.
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AMZ400: Synopsis for Altamaha Sound GA to Flagler Beach FL out to 60 nm
330 PM EDT Tue Jul 27 2021

A weak trough of low pressure positioned near the Florida and Georgia border will shift southward over the northeast Florida waters by Wednesday night and will then dissipate by Thursday night. Southwesterly winds overnight will shift to northwesterly by sunrise on Wednesday as this trough moves across our area. Numerous thunderstorms developing over land on Wednesday afternoon will move across our local waters during the late afternoon and evening hours, with stronger activity containing frequent lightning strikes and briefly gusty winds. Thunderstorm coverage will decrease over the Georgia waters by Thursday afternoon and then for all local waters on Friday. A weak cold front will then dive into the southeastern states during the weekend, resulting in gradually strengthening southwesterly winds and increasing chances for afternoon and evening thunderstorms on Sunday.

Gulf Stream
The approximate location of the west wall of the Gulf Stream as of Jul 27, 2021 at 1200 UTC...
- 56 nautical miles east northeast of Flagler Beach.
- 57 nautical miles east of Saint Augustine Beach.
- 61 nautical miles east of Jacksonville Beach.
- 67 nautical miles east of St Simons Island.
Area Forecast Discussion
National Weather Service Jacksonville FL
448pm EDT Tuesday July 27 2021

...WIDESPREAD AFTERNOON STORMS CONTINUE ON WEDNESDAY... ...HEAT ADVISORY CONDITIONS LIKELY LATE THIS WEEK AND THIS WEEKEND... Late afternoon surface analysis depicts low level troughing extending along the FL/GA border in the wake of the weak tropical disturbance that was pivoting slowly north-northeastward along coastal portions of South Carolina. Otherwise, a weak pressure pattern prevails locally as the axis of Atlantic high pressure remains displaced across the Bahamas. Aloft...deep-layered ridging situated off the southeast U.S. coast was drifting eastward as a shortwave trough digs southward from the western Great Lakes region. Meanwhile, stout ridging remains centered over the front range of the Rockies in CO/WY. Latest GOES-East derived Total Precipitable Water imagery indicates that a deep tropical air mass remains entrenched over our area, with latest LAPS/RAP analyses displaying PWAT (Precipitable Water) values generally in the 2 - 2.3 inch range across our region. This tropical air mass in concert with the low level trough lagging in the wake of disturbance Invest 90L resulted in an early start to convection across our area, especially along the Interstate 10, 75 and 95 corridors. Convection has decreased in coverage late this afternoon for most of our area, as rainfall and cloud cover has cooled temperatures back to the upper 70s and lower 80s. However, portions of inland southeast GA have yet to be affected by convection, and temperatures from Waycross to Jesup remain in the 85-90 degree range as of 21Z.

.NEAR TERM [Through Wednesday]... Focus for convective development through the late evening hours will shift to southeast GA, where plenty of surface based CAPE remains per latest LAPS/RAP analyses, especially along the inland corridor from Waycross to Jesup. Stronger convection this evening will be capable of producing localized flooding due to slow storm motion, along with frequent lightning strikes and briefly gusty winds. Latest short-term, high resolution guidance indicates that mesoscale boundary collisions could keep convection firing through around or just after midnight, while activity mostly winds down across northeast and north central FL, where debris cloud cover should thin out during the late evening hours, while mid and high altitude cloudiness could linger through the predawn hours across southeast GA. Lows will mostly fall to the mid 70s.

A shortwave trough will dive into the southeastern states on Wednesday afternoon, which will result in another active afternoon and evening of convection as the weak surface trough tied to Invest 90L progresses slowly southward across the Altamaha River. Deep tropical moisture will remain in place to the south of this trough, and we expect convection to initially be triggered by the inland moving Gulf coast sea breeze by the late morning hours in western portions of the Suwannee Valley. Mesoscale boundary collisions will then increase coverage and intensity of convection by late afternoon as activity generally moves slowly eastward, with a few storms possibly pulsing towards late afternoon as convection encounters the pinned Atlantic sea breeze boundary at coastal locations. Localized flooding will be possible due to slow storm motion, particularly at urban and normally flood prone locations. Storms that pulse will produce frequent lightning strikes and briefly gusty downburst winds up to 40 mph. Highs will climb to the upper 80s to near 90 before convection and cloud cover increase during the early to mid afternoon hours, with maximum heat index values around 100.

.SHORT TERM [Wednesday Night Through Friday Night]... A massive "heat dome" ridge currently centered over the Rockies will nose into the southeastern states late this week as shortwave troughing departs our region. Low level troughing will be slow to depart northeast and north central FL on Thursday. Scattered to numerous afternoon and early evening thunderstorms will be focused to the south of Interstate 10, with mesoscale boundary collisions possibly developing widely scattered convection over southeast GA. Rising heigheights aloft and less convective coverage over southeast GA will allow highs to soar to the mid and upper 90s inland, with an earlier developing Atlantic sea breeze boundary keeping coastal highs closer to 90. Highs elsewhere will likely reach the lower 90s before convective coverage increases. The bigger story will be soaring heat index values, with maximum values reaching the 105-110 range for locations along and north of I-10 and 100-105 for locations south of I-10. Convection will mostly dissipate before midnight on Wednesday and Thursday evenings, with lows generally in the mid 70s inland and the upper 70s at coastal locations.

Low level troughing will shift over central FL on Friday, with PWAT (Precipitable Water) values falling to around 1.75 inches and subsidence aloft strengthening as the heat dome becomes centered over the Southern Plains states and extends its axis southeastward over the Deep South. Dry weather is expected across most of southeast GA, while only widely scattered convection develops during the late afternoon and early evening hours across northeast and north central FL. Highs will soar to the upper 90s across most of inland southeast GA, with mid to upper 90s elsewhere at inland locations and low to mid 90s all the way to area beaches. Heat index values should reach Heat Advisory criteria, with most areas experiencing maximum values of 108-112 degrees by the afternoon hours. Lows Friday night will only fall to the mid to upper 70s inland and around 80 at coastal locations.

.LONG TERM [Friday Night Through Monday Night]... Troughing diving southeastward across the Great Lakes region this weekend will gradually suppress the heat dome ridge and will drive a cold front into the Deep South by Sunday. Heat index values on Saturday will likely reach Heat Advisory criteria once again, but thunderstorm coverage will begin to increase by the late afternoon and early evening hours as PWAT (Precipitable Water) values begin to climb back towards 2 inches across our region to the south of the approaching frontal boundary. Inland highs on Saturday will reach the mid to upper 90s, with low to mid 90s all the way to area beaches. Thunderstorm coverage will then become widespread by Sunday afternoons the front edges closet to our area. Deep west southwesterly flow will still bring highs up to the low and mid 90s before convection becomes widespread during the mid to late afternoon hours. Heat index values on Sunday will flirt with Heat Advisory criteria, with values generally in the 105-110 range before cloud cover and convection increases later in the afternoon. We expect widespread thunderstorms to occur on Monday afternoon as the front settles over southeast GA, and highs will fall back to the upper 80s to lower 90s. Lows will continue to only fall to the mid and upper 70s this weekend and early next week, except around 80 at coastal locations.

Marine
A weak trough of low pressure positioned near the FL/GA border will shift southward over the northeast FL waters by Wednesday night and will then dissipate by Thursday night. Southwesterly winds overnight will shift to northwesterly by sunrise on Wednesday as this trough moves across our area. Numerous thunderstorms developing over land on Wednesday afternoon will move across our local waters during the late afternoon and evening hours, with stronger activity containing frequent lightning strikes and briefly gusty winds. Seas of 3-5 feet will prevail through Wednesday both near shore and offshore. Thunderstorm coverage will then decrease over the Georgia waters by Thursday afternoon and then for all local waters on Friday. Seas will decrease to the 2-4 foot range from Thursday through the weekend. A weak cold front will then dive into the southeastern states during the weekend, resulting in gradually strengthening southwesterly winds and increasing chances for afternoon and evening thunderstorms as the weekend progresses, with widespread thunderstorm activity expected early next week as the frontal boundary stalls over the GA waters.

Rip Currents
Prevailing offshore winds should result in a low risk at most area beaches through midweek.

Fire Weather
Light westerly surface and transport winds will develop shortly after sunrise on Wednesday, resulting in marginally low daytime dispersion values. A drier air mass will then filter into southeast Georgia on Thursday and Friday, with scattered to numerous mainly afternoon and evening thunderstorms continuing across northeast and north central Florida on Thursday. Light westerly surface and transport winds on Thursday morning will become variable in direction during the afternoon hours, resulting in marginally low daytime dispersion values again. Thunderstorm coverage will become isolated for locations along and north of Interstate 10 on Friday, with scattered afternoon and evening thunderstorms possible for locations south of I-10. Temperatures on Friday will soar to the mid to upper 90s at most locations, with heat index values approaching 110 degrees.

Hydrology
Minor flooding will continue along lower portions of the Santa Fe River during the next several days, as gauges further upstream rise back to action stage due to recent runoff. Minor flooding will be possible tomorrow through early Thursday along portions of the St. Marys River near the Macclenny gauge due to recent runoff from heavy downpours.

NOAA Jacksonville FL Office: Watches - Warnings - Advisories
FL...None. GA...None. AM...None.