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Weathermuffin’s novel “WxHeadsUp” feature proactively monitors and alerts about upcoming weather conditions sailors care about. In this article we deep-dive into wave steepness, confused seas, calm winds, and more.
Calm Winds
Refers to winds that may be too light for sailing. This is important to know to avoid being becalmed, and is described in three categories:
- ≤ 10 knots
- ≤ 5 knots
- ≤ 3 knots
Source Data: NOAA (GFS)
Publisher: NOAA
Confused Seas
This attempts to capture when more than one swell or wave patterns coming from different directions meet. Could also be referred to as “lumpy seas.” Uncomfortable at best, dangerous at worst. This is described in three categories:
- moderate
- heavy
- extreme
Source Data: NOAA (GFS)
Publisher: Weathermuffin
Gustiness
A ratio of maximum gust speed to sustained wind speed. Thus it refers not to the gusts themselves, but how significant the gusts are relative to the sustained wind speed.
For instance, if sailing in 15 knots sustained, gusts of 20 knots are not out of the ordinary. But if the gusts are expected to be 40 knots, that would be useful to know..
“Gustiness” is described in three categories:
- gusts ≥ 1.5x sustained wind
- gusts ≥ 2.0x sustained wind
- gusts ≥ 3.0x sustained wind
Source Data: NOAA (GFS)
Publisher: Weathermuffin
High Current
Shows areas of high-speed current, which could lead to navigational challenges. Three levels of current may be alerted:
- ≥ 1 knot
- ≥ 2 knots
- ≥ 3 knots
Source Data: NOAA (RTOFS)
Publisher: NOAA
Important caveat: Ocean current forecasting models are not yet high enough resolution to capture smaller sources of currents such as inlets and channels. Therefore, an absence of this type of alert does not mean no chance of encountering a high-current situation. Be sure to also study and understand local geography as well as local current and weather behavior.
Lightning Strikes
Count of lightning strikes observed by satellite for the preceding half hour.
Currently these observations are limited to a rectangular geographic region spanning from 80°N, 110°E on the top left to 25°S, 0°E on the bottom right. Lightning strikes that occur outside of this region will not generate an alert.
There are three categories:
- > 0 strikes observed (1 or more)
- > 2 strikes observed (3 or more)
- > 5 strikes observed (6 or more)
Note that these are observations, not a forecast.
Source Data: NOAA
Publisher: NOAA
Precipitation
The rate of total precipitation for a time period, where
- “light” is 0.25 to 2.5 mm/hour
- “moderate” is 2.5 to 7.6 mm/hour, and
- “heavy” is more than 7.6 mm/hour.
Source Data: NOAA (GFS)
Publisher: NOAA
Satellite Infrared
Infrared imagery from geostationary satellites indirectly depicts cloud-top height, which can indicate or relate to squalls, thunderstorms, and other convective phenomena.
Generally speaking, the lower the temperature, the higher the altitude of the cloud tops. These are given in three categories, in order of colder temperature and higher cloud tops:
- ≤ -20°C
- ≤ -40°C
- ≤ -60°C
Source Data: NOAA
Publisher: Weathermuffin
Significant Wave Height
Refers to the average wave height (trough to crest) of the highest one-third of waves. This is described in three categories:
- ≥ 8 ft (2.4 m), representing Force 6, “strong breeze”
- ≥ 18 ft (5.5 m), representing Force 8, “gale”
- ≥ 29 ft (8.8 m), representing Force 10, “storm”
Source Data: NOAA (GFS)
Publisher: NOAA
Sustained Wind Speed
Refers to the average wind speed for a time period. This is described in three categories:
- ≥ 22 knots, representing Force 6, “strong breeze”
- ≥ 34 knots, representing Force 8, “gale”
- ≥ 48 knots, representing Force 10, “storm”
Source Data: NOAA (GFS)
Publisher: NOAA
Visibility
A measure of how far you can expect to be able to see due to various atmospheric conditions, including fog. See also https://www.weather.gov/safety/fog-boating.
This is described in three categories:
- ≤ 5 NM, “moderate”
- ≤ 2 NM, “poor”
- ≤ 0.5 NM, “very poor”
Source Data: NOAA (GFS)
Publisher: NOAA
Wave Steepness
“The steepness of the waves is often of far greater interest to the seaman than the mere heights of the waves.”
Graham P. Britton, Captain, Royal Navy (Retired) – source
The ratio between significant wave height and dominant wave length. From less to more steep, values are “swell,” “average,” “steep,” and “very steep.” This follows NOAA conventions that you’re already familiar with if you use their buoys to monitor sea state conditions.
Wave steepness is described in three categories:
- average
- steep
- very steep
Source Data: NOAA (GFS)
Publisher: Weathermuffin
See also: What is Wave Steepness?
Waves ≥ 5% Chance
Alert based on probabilistic wave height forecast showing a 5% or greater chance of occurring:
- ≥ 5% chance of significant wave height ≥ 3 m
- ≥ 5% chance of significant wave height ≥ 4 m
- ≥ 5% chance of significant wave height ≥ 7 m
Source Data: NOAA (GEFS)
Publisher: Weathermuffin
See also: What is a Probabilistic Forecast?
Wind ≥ 5% Chance
Alert based on probabilistic sustained wind speed forecast showing a 5% or greater chance of occurring:
- ≥ 5% chance of sustained wind speed ≥ 22 knots
- ≥ 5% chance of sustained wind speed ≥ 34 knots
- ≥ 5% chance of sustained wind speed ≥ 48 knots
Mariner’s Tropical Cyclone Guide
Source Data: NOAA (GEFS)
Publisher: Weathermuffin
See also: What is a Probabilistic Forecast?
Wind Opposing Current
Shows areas of opposing wind and current, which can result in steep and possibly breaking waves. An alert of this type should be considered potentially hazardous to small vessels.
Three intensities, determined by a proprietary formula, are provided:
- minor
- moderate
- severe
Important caveat: Ocean current forecasting models are not yet high enough resolution to capture smaller sources of currents such as inlets and channels. Therefore, an absence of this type of alert does not mean no chance of encountering a wind opposing current situation. Be sure to also study and understand local geography as well as local current and weather behavior.
Source Data: NOAA (GFS, RTOFS)
Publisher: Weathermuffin
See also: What is Wind Opposing Current?