Official weather notice
Official notice, KTTV Trung Bộ, 16:12, Oct 27: «TIN DỰ BÁO MƯA DÔNG, LỐC XOÁY VÀ GIÓ GIẬT MẠNH TRÊN BIỂN TỈNH KHÁNH HÒA» (quoted in Vietnamese exactly as published; we do not translate official warnings) Source
This notice is for boats in the open sea off Khanh Hoa (including the Spratlys), not a swimming ban. Nha Trang Bay is sheltered by islands: whether you can swim depends on each beach's light, based on the waves and wind at that beach. Going to the islands by boat? Check with your tour first.
Beach map
Tue Oct 27 21:00NowLight wind, 7 km/h
White chevrons are waves, faint streaks are wind, dot colour is the light. The number by each dot is the wave at the shore. Diamonds are drain outfalls or river mouths (approximate positions).
Open-Meteo Marine model, updated 21:30 (Nha Trang time)
Tran Phu, north (near the Cai river mouth) caution
Swell from the east, 3 s period
- Wave at shore
- 0.4 m
- Period
- 3 s
- Wind
- 7 km/hnorth
- Gusts
- 16 km/h
- Rain
- 0.0 mm
- UV
- 0low
- Tide
- 1.7 mrising
- Water
- 28.9°C
Line = wave at the shore, bars = rain. Dashes mark 0.6 m (yellow) and 1.2 m (red).
Rain today 3.5 mm, no effect
Best tomorrow: 06:00 - 08:00
Island trip window: 7 days
Next good day for the islands: Tuesday Oct 27 (7/10). Also good: Sun Nov 1
- TueOct 277/10goodTuesday Oct 27: 7/10 - good conditions; offshore waves up to 0.6 m, gusts up to 33 km/h, rain 3.8 mm
- WedOct 286/10so-soWednesday Oct 28: 6/10 - caution; offshore waves up to 0.5 m, gusts up to 44 km/h, rain 1.1 mm
- ThuOct 294/10poorThursday Oct 29: 4/10 - difficult conditions; offshore waves up to 0.6 m, gusts up to 36 km/h, rain 30.3 mm
- FriOct 307/10so-soFriday Oct 30: 7/10 - caution; offshore waves up to 0.7 m, gusts up to 35 km/h, rain 5.8 mm
- SatOct 314/10so-soSaturday Oct 31: 4/10 - caution; offshore waves up to 0.6 m, gusts up to 44 km/h, rain 12.1 mm
- SunNov 18/10goodSunday Nov 1: 8/10 - good conditions; offshore waves up to 0.7 m, gusts up to 28 km/h, rain 2.4 mm
- MonNov 26/10so-soMonday Nov 2: 6/10 - caution; offshore waves up to 0.6 m, gusts up to 35 km/h, rain 10.9 mm
Whether boats sail is decided by the port and the tour operator - ask them before you book. A low score means a rough sea: tours are more likely to be cancelled or to cause seasickness.
A reference index, not an official forecast. Official warnings: Vietnam National Centre for Hydro-Meteorological Forecasting (nchmf.gov.vn).
Open-Meteo model (Marine + Forecast), updated 19:00 (Nha Trang time)
How it is worked out
- Wave at the shore = wave height × √cos(angle between the wave direction and the shore normal) (refraction) × shelter, then converted to breaking height with Komar & Gaughan (1972): H = 0.39·g^0.2·(T·H²)^0.4 - the longer the period, the higher the wave breaks. The shore normal comes from the OSM coastline; one model grid cell is about 8 km wide.
- Swell and wind waves are worked out separately, each with its own direction factor, then added by energy: H = √(H_swell² + H_wind²). Both parts are scaled so their total energy matches the model's total wave height.
- Shelter: for each wave direction, three rays run from the beach out to sea; a ray that crosses the OSM coastline (an island or a headland) within 12 km marks that direction as blocked, and the wave at the shore drops to 35%. This is a geometric estimate and does not include waves bending around islands.
- Hourly light: a wave at the shore of 0.6 m or more is yellow and 1.2 m or more is red; gusts from 50 km/h are yellow and from 60 red; 10 mm of rain in 3 hours is yellow and 25 mm red (about 20 and 50 mm over a day). Swell with a period of 9 s or more hitting the shore at 0.4 m or more is also yellow because rip currents are likely.
- From 18:00 to 05:00 there is no green: it is dark and Nha Trang beach lifeguards work in daytime. Wave height is shown for reference only; swimming at these hours is not recommended.
- Rain, wind, gusts and UV are taken at each beach's own coordinates (Open-Meteo Forecast). The wind streaks on the map use the city centre.
- Water temperature comes from the sea point of the beach; if that point has no data, the nearest point with data is used and the page says it was measured offshore.
- The best time to swim is the 2-hour slot between 6 am and 5 pm with the lowest waves at the shore, the least rain, low UV and light gusts. After 5 pm the page shows tomorrow's slot.
A reference index, not an official forecast. Official warnings: Vietnam National Centre for Hydro-Meteorological Forecasting (nchmf.gov.vn). Check the flags on the beach: a red flag or a danger sign means do not swim.
Frequently asked questions
Where is it safe to swim in Nha Trang today?
According to the reference traffic light, 5 of 9 beaches are rated “OK to swim” today; take more care at: Tran Phu north, Tran Phu centre, Tran Phu south and Bai Dai. Waves are coming from the east. Decide whether to swim by the flags and the lifeguards' instructions. This is a reference index, not an official forecast. Official warnings: Vietnam National Centre for Hydro-Meteorological Forecasting (nchmf.gov.vn).
What do the green, yellow and red lights on the beaches mean?
Green - calm conditions. Yellow - caution: waves of 0.6 m or more at the shore, wind gusts of 50 km/h or more, 20 mm or more of rain in a day, or a long swell (period of 9 s or more) that can cause rip currents. Red - dangerous: waves of 1.2 m or more at the shore, gusts of 60 km/h or more, or 50 mm or more of rain. A red flag on the beach itself always beats any index.
Why avoid swimming near the river mouth after rain?
After heavy rain (30 mm or more in a day), the water near the Cai river mouth and the Dang Tat drain is often murky and dirty for about 48 hours: litter and runoff. For the beaches next to them we raise an “after rain” flag - choose another beach for the next 1-2 days.