The rip current risk at every beach in St. Johns County is High today and High again tomorrow, and the National Weather Service expects both days to be sunny.

High is a defined category and the forecast prints the definition underneath itself. Low risk means rip currents often occur near groins, jetties, reefs and piers. Moderate means life-threatening rip currents are possible. High means they are likely.

What makes that worth reading twice is the rest of the Saturday block. Surf four to six feet. Thunderstorm potential none. Weather sunny. Water temperature 77 to 82. High in the upper 70s, north wind around 10. It is a good beach day by every measure on the page except the first one.

Two hazards, and they are on opposite sides of the county

The Coastal Hazard Message the Jacksonville office issued at 12:14 Saturday morning splits this county in half.

Northeast Coastal St. Johns and Southeast Coastal St. Johns, which between them cover this county’s ocean shore, carry the rip current risk and nothing else. The High Surf Advisory that had been running there was cancelled in that same message.

Western St. Johns is in a Coastal Flood Advisory until 5 a.m. Sunday, along with Eastern Clay, Eastern Putnam, Trout River and South Central Duval. The office heads that group the St Johns River Basin. The advisory is for minor coastal flooding, and the impact it lists is flooding of lots, parks and roads with only isolated road closures expected.

So the flooding this weekend is on the river side of the county, and the water that is dangerous is on the ocean side. Nothing is raining.

What is holding the water in

The moon is full today, and the forecasters call it the Harvest Moon in the discussion they file at half past midnight. Astronomical tides have been climbing into the weekend on the back of it.

Then the wind. North to northeasterly and onshore, which pushes against water trying to leave the St. Johns River and keeps it in. The discussion says water will be slow to drain Sunday into early next week, on lingering high astronomical tides and light winds.

The numbers the office puts on that are peaks of a foot and a half to two feet above mean higher high water. That happened on Friday evening’s high tide and it is forecast to happen again on both of Saturday’s.

At St. Augustine Beach the prediction for this morning’s high is 5.589 feet above mean lower low water at 8:20, and for tonight’s 5.442 at 8:42. On the station’s own datum sheet, epoch 1983 to 2001, mean higher high water sits about 5.1 feet above mean lower low water, so the prediction alone runs roughly half a foot over it before any wind is counted. NAVD88, which is the datum on the elevation certificate in a homeowner’s file, sits 3.13 feet above mean lower low water there, putting this morning’s predicted high at about 2.5 feet NAVD88.

Three measurements of the same ocean

They will not agree, and they are not supposed to.

The surf zone forecast gives four to six feet, which is the wave breaking on the sand where a person stands.

The buoy twelve miles east northeast of the St. Augustine Inlet published seas of 5.6 feet at 12:56 this morning, with a peak period of 14 seconds. NDBC prints those in feet and seconds itself. Underneath that number the station separates the water into a swell of 4.3 feet at 14.3 seconds out of the east northeast and a wind wave of 3.6 feet at 9.1 seconds from the same direction.

The marine zone forecast for the water from 20 to 60 miles out says seas seven to nine feet, occasionally to 11, with a small craft advisory through late tonight. Inside 20 miles it is six to eight, occasionally 10, and that advisory ends at 10 this morning.

The 14 second swell is the part that matters on the beach. Long period swell persisting through Sunday is what the discussion names as the reason the risk stays High to the end of the weekend. The forecast has the surf at three to four feet Monday and one to two by Wednesday. The risk category is not forecast that far out.

The buoy put the water at 81.9 degrees this morning. The Mayport gauge, thirty-five miles north at the mouth of the river, published 79.5 at 1:54. The pier gauge at St. Augustine Beach reported wind, gust and pressure, and the missing value code where the water temperature goes.

If you go

The weather service prints its own instructions in the hazard message and they are worth having in the same words it uses. Swim near a lifeguard. If caught in a rip current, relax and float. Do not swim against the current. If able, swim in a direction following the shoreline. If unable to escape, face the shore and call or wave for help.

High water at St. Augustine Beach comes at 8:20 this morning and again at 8:42 tonight. The advisory on the river runs out at 5 Sunday morning and the rip current risk runs to Sunday evening.