A coastal flood warning takes effect on the ocean side of St. Johns County at 5 p.m. on Saturday and runs until 8 p.m. on Monday. The National Weather Service issued it at 7:33 Friday evening for Northeast Coastal St. Johns, Southeast Coastal St. Johns and Coastal Flagler, and the words it uses are significant coastal flooding expected.

A warning is the step above an advisory. One has been running on this coast since Monday. The advisory that covers Saturday daytime, until the warning starts at 5 p.m., says minor coastal flooding. The warning that follows it says numerous roads may be closed, and that low lying property including homes, businesses, and some critical infrastructure will be inundated.

This is tidal flooding. Water arrives from the ocean side and stands on the ground until the tide leaves. It is not rainfall flooding and it is not storm surge, and the difference matters on a day with a 90 percent chance of rain in it, because the two kinds of water will be on the same streets and only one of them drains when the sky clears.

What the chart says before the wind is added

High tide at St. Augustine Beach was predicted for 7:06 Saturday morning at 5.707 feet above mean lower low water, and the evening high is predicted for 7:29 p.m. at 5.241 feet on the same datum. On the station’s own datums record, published for the 1983 to 2001 tidal epoch and accepted in December 2009, mean higher high water sits 5.14 feet above mean lower low water, which is 8.15 minus 3.01 on station datum.

So both of Saturday’s predicted highs are above the average of the higher high tides. The morning high is about half a foot above it and the evening high about a tenth of a foot, each figure being one printed number subtracted from another. In NAVD88, which is the datum an elevation certificate uses and therefore the one that means something at a doorstep, mean higher high water here is 2.01 feet and the 7:29 p.m. prediction is 2.11 feet.

That is the part of this weekend that is different from Tuesday. The advisory that ran on Tuesday sat under a predicted afternoon high that was below mean higher high water, so the chart was not the high part of it. This time the chart is already high. A harmonic prediction carries no wind, no rain and no surge. Everything the forecast is describing gets added on top.

Station 8720587 is a prediction station with no instrument in the water. Nothing observed at St. Augustine Beach can be set against any of this. The nearest gauge that reports is Mayport, about thirty miles north and in a different river.

The wind, inland as well

The wind advisory is the piece that reaches the parts of the county that never flood. It runs from noon Saturday to 1 a.m. Sunday and it covers Northern Inland St. Johns and Southern Inland St. Johns alongside the two coastal zones, for northeast winds of 15 to 25 mph with gusts up to 40 mph. The service’s own impact line is that gusty winds will blow around unsecured objects, that tree limbs could be blown down and that a few power outages may result.

The gridpoint forecast for St. Augustine, pulled at 2 a.m., puts Saturday’s daytime wind at north 18 to 26 mph with gusts as high as 39, a high near 74, and rain showers before 2 p.m. turning to showers and thunderstorms. The northeast wind then holds in that forecast every period through Thursday.

On the water, a small craft advisory runs until 8 p.m. on Saturday for northeast winds of 20 to 30 knots and seas of 7 to 10 feet. A gale warning replaces it at 8 p.m. for the coastal waters from St. Augustine to Flagler Beach out 20 nautical miles, northeast winds of 25 to 35 knots gusting to 40 and seas of 10 to 14 feet, and it runs to 8 p.m. Monday. The high surf advisory on the beach, from 8 a.m. Saturday, is for large breaking waves of 7 to 11 feet.

The high rip current risk runs through Monday evening.

The water at Mayport was 70.9 degrees at 1:54 Saturday morning. That is the custodian’s own figure. It is river water, not ocean water.

What the county said, and what it attributed

St. Johns County Emergency Management put out a release at 2:03 Friday afternoon urging residents to avoid flooded roadways, stay out of dangerous surf and secure outdoor items. It attributes to the weather service’s Friday morning briefing a forecast of peak gusts of 35 to 45 mph Sunday into Monday and moderate to locally major tidal flooding along the Atlantic coast, the Intracoastal Waterway and the St. Johns River basin.

The release also says the combination of strengthening northeast winds and the highest astronomical tides of the year is expected to worsen the flooding. That is the county’s characterization. This paper has not set Saturday’s predictions against a full year of them and is not adopting it.

The county’s instruction on a flooded road is the same as the weather service’s, and both are worth the space: do not drive around a barricade, and do not drive through water of unknown depth.

The next predicted high at St. Augustine Beach after the warning begins is 7:29 on Saturday evening. The predictions pulled for this story cover Saturday only. The warning has two more days to run.