How accurate is FloodRoute?

A day before each high tide, FloodRoute is usually off by 2 inches. Tide tables are off by 5.5.

We predict how high every tide that could flood will get, then check it against NOAA's real measurement at 8 gauges. Misses included.

Past 12 months · 1,533 high tides

Question	FloodRoute	Tide tables
Usually off by	2 in	5.5 in
How often within 3 inches	68%	26%

Question	FloodRoute	Tide tables
Usually off by	2.5 in	7.5 in
How often within 3 inches	57%	14%

Green means within 3 inches of the real water. Red means more than 3 inches off.

San FranciscoOct 30, 2:39 PM✗ 4.4 in too lowPredicted 7.39 ft · Actual 7.76 ft
BostonOct 30, 2:52 PM✓ 2.4 in lowPredicted 11.57 ft · Actual 11.77 ft
Virginia KeyOct 30, 1:07 PM✓ 1.0 in lowPredicted 4.16 ft · Actual 4.24 ft
Sewells PointOct 30, 12:56 PM✗ 7.3 in too highPredicted 4.12 ft · Actual 3.51 ft
South Port EvergladesOct 30, 12:19 PM✓ 0.7 in lowPredicted 4.75 ft · Actual 4.81 ft
The BatteryOct 30, 11:54 AM✓ 2.6 in highPredicted 6.01 ft · Actual 5.79 ft
CharlestonOct 30, 11:53 AM✓ 1.7 in highPredicted 7.44 ft · Actual 7.30 ft
St. PetersburgOct 30, 3:56 AM✗ 4.3 in too lowPredicted 3.40 ft · Actual 3.76 ft
Virginia KeyOct 30, 12:16 AM✓ 1.7 in lowPredicted 4.15 ft · Actual 4.29 ft
South Port EvergladesOct 29, 11:30 PM✓ 0.4 in lowPredicted 4.49 ft · Actual 4.52 ft
San FranciscoOct 29, 1:43 PM✗ 5.9 in too lowPredicted 7.56 ft · Actual 8.05 ft
BostonOct 29, 1:58 PM✓ 2.0 in highPredicted 11.92 ft · Actual 11.75 ft

How often each was within 3 inches of the real water, past 12 months.

Boston and New York are the hardest, because wind drives the water there. A surge that hasn't started yet can't show up a day ahead.

How often each was within 3 inches of the real water, past 12 months.

24 hours before68%

12 hours before78%

3 hours before89%

Tide tables26%

Tide tables are printed in advance, so they never improve.

St. PetersburgOct 18, 6:21 AM✗ 17.8 in too highPredicted 2.97 ft · Actual 1.49 ft
CharlestonOct 12, 9:46 AM✗ 16.1 in too highPredicted 7.34 ft · Actual 6.00 ft
CharlestonOct 11, 9:12 PM✗ 14.0 in too highPredicted 6.76 ft · Actual 5.59 ft
The BatteryOct 5, 4:34 PM✗ 11.5 in too highPredicted 6.04 ft · Actual 5.08 ft
Sewells PointOct 7, 7:15 AM✗ 11.3 in too highPredicted 4.06 ft · Actual 3.12 ft

Which tides. Every high tide NOAA predicts above the usual high-water mark at that gauge. Those are the ones that can flood.
Our prediction. NOAA's tide table plus how far the water is running above or below it, using only readings from at least 24 hours before the tide.
The real water. The highest reading at the NOAA gauge within an hour of the tide.
“Usually off by.” The middle result: half of the predictions were closer than this, and half were further off.
Why tide tables miss. They only count the moon and sun, not wind, storms or the ocean running high.
Too high or too low. On average our predictions run 0.3 inches low.
Locked results. A result marked Locked was saved before the tide and can never change. The rest were worked out afterwards from what was known a day ahead.
What this doesn't cover. This is the water height at the NOAA gauge, not whether your street flooded. That also depends on road height, drains, wind and rain. Marin and San Francisco share the San Francisco gauge, the nearest live one.
Fine print. Recent readings are NOAA's preliminary data, which NOAA can revise.

Think you can beat it? Guess the next high tide in the FloodRoute app, free.