At Mayport the water stood above the predicted tide at every reading from midnight at the start of Monday to two o’clock Tuesday morning. That is 262 six-minute readings, and not one of them came in under the chart.

The gap was never smaller than about nine tenths of a foot and never larger than about two and a tenth. Those are subtractions this paper did, timestamp by timestamp, off NOAA’s own observed series and its own prediction for the same station.

Mayport sits at the mouth of the St. Johns River, up in Duval County, and it is the nearest gauge to this county that publishes an observed water level at all. St. Augustine Beach publishes a prediction and nothing to check it against.

The National Weather Service office in Jacksonville put three products out at 9:56 Monday morning for Coastal Flagler, Northeast Coastal St. Johns and Southeast Coastal St. Johns. A Coastal Flood Advisory for minor coastal flooding, until 4 a.m. Thursday. A High Surf Advisory for “large breaking waves of 7 to 10 feet,” until 8 p.m. Tuesday. And a High Rip Current Risk for dangerous rip currents, through late Wednesday night. Coastal Duval carries all three. Nassau carries the surf and the rip currents and no flood advisory.

The chart is not the high part

The afternoon high at St. Augustine Beach is predicted for 4:24 Tuesday at 4.78 feet above mean lower low water, which works out to 1.65 feet NAVD88, the datum a surveyor writes on an elevation certificate. Measured against that station’s published datums, epoch 1983 to 2001, it is roughly four tenths of a foot below mean higher high water, the higher of each day’s two highs averaged across the epoch.

So the astronomy this week is ordinary. The water is not.

What the forecast has instead is wind and a long swell. The coastal waters forecast issued at 1:47 Tuesday morning gives north northeast winds of 15 to 20 knots, seas of 7 to 10 feet occasionally to 13, and a wave detail line reading “Northeast 9 feet at 12 seconds.” Buoy 41117, offshore of St. Augustine, reported at 1:56 a.m. with seas of 6.6 feet and a peak period of 13 seconds, which NDBC splits into 3.6 feet of swell at 12.5 seconds out of the east northeast and 5.2 feet of wind wave at 8.3 seconds out of the same quarter.

Where the water actually got to

The highest reading the Mayport gauge took in that window was 5.827 feet above mean lower low water, at 4:18 Monday afternoon. The prediction for that same minute was 4.782.

Off that station’s datums, 5.827 feet is about nine tenths of a foot over mean higher high water and about seven tenths of a foot under the highest astronomical tide the gauge has on file, which it recorded in October 2011. In NAVD88 the reading works out to about 2.8 feet. That is the number to hold a finished floor elevation against, because a slab certified at two and a half feet NAVD88 is below it and one certified at three and a half is not.

The advisory text is specific about what the word minor is doing there.

“Flooding of lots, parks, and roads with only isolated road closures expected,” it reads, and for the beach, “dangerous swimming and surfing conditions and localized beach erosion.”

The instruction under it is the usual one and it is the only one worth repeating. “Do not drive around barricades or through water of unknown depth.”

The water itself

Water temperature off St. Augustine was 77.9 degrees at the buoy at 1:56 Tuesday morning. At the Mayport gauge, where the river water is, it read 72.3 at 2:06. The surf zone forecast puts the nearshore water at 75 to 80.

Bacteria sampling is a separate question and it came back clean. All seven of the county’s Florida Healthy Beaches points were sampled on Oct. 13 and all seven read Good, which the state defines as 0 to 35.4 enterococcus per 100 milliliters, with no advisory at any of them. Those samples predate this swell by six days.

The high surf advisory ends at 8 o’clock Tuesday evening. The rip current risk runs through late Wednesday night, and the flood advisory until 4 a.m. Thursday.