By David Wilkinson
Starpath Instructor
Strong winds come from a variety of weather patterns. Some are large like a mid-latitude Low, some mid-size like a tropical wave, and some quite small like those found in narrow gaps between islands. Some winds are transient like a downdraft from a passing thunderstorm while others are more persistent like the strong summer winds along the southern Oregon and northern California coast.
Strong winds from larger, longer lasting weather systems are generally well quantified by weather models and included in the official forecasts. But smaller scale, shorter duration winds can fall below the resolution, temporal and spatial, of weather models and can be better described by how likely they are to occur in generalized areas. Winds driven by Wake Lows fall into this latter category.
A Wake Low is defined by the American Meteorological Organization as:
a surface low pressure area or mesolow (or the envelope of several low pressure areas) to the rear of a squall line; most commonly found in squall lines with trailing stratiform precipitation regions, in which case the axis of the low is positioned near the back edge of the stratiform rain area.
Because squall lines are bands of thunderstorm, (a.k.a squalls when over water) typically ahead of cold fronts, it is useful to look at the structure of a single squall.
The squall has a life cycle that starts with a growing phase. In this initial phase, surface winds flow radially inward at the base. These we watch with a weather eye to see if they may eventually become towering cumulonimbus. If they grow to full maturity, there is a second phase that has a downdraft creating strong wind that comes with heavy rain, perhaps even hail.
Wind patterns with the two phases of the squall are shown in Figure 1. Notice the strong winds from the downdraft are in front of the squall while behind it the wind can be light or flukey. The difference in the winds fore and aft of the squall is because the speed of the movement of the squall adds to or subtracts from the wind circulating in the squall.
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| Figure 1 (from Modern Marine Weather by David Burch) |
In Figure 2, typically the squall movement would be from left to right. In the mid-latitudes that would be roughly west to east or in the trade winds from east to west. Because squalls are embedded in and move with the upper level winds, it is best to review the 500 mb maps or model data or even local soundings to get a sense of squall movement.
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| Figure 2 |
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| Figure 3 |
As the squall moves from left to right, the leading low pressure area experiences strong winds blowing from the Mesohigh and toward the low. This is the source of the common wind gusts commonly experienced on the leading edge of a squall.
On the aft side of the squall, the wind is again driven by the Mesohigh toward low pressure. Because this area of low pressure is on the aft side of the low or in its “wake”, the term Wake Low seems to fit. One key takeaway is that the wind direction will reverse or at least make a very large veer due to the reversal of the pressure gradient as the squall passes. How quickly the wind direction changes would be affected by the strength of the pressure gradient and speed of the squall.
Wind speeds can be estimated using the pressure gradients and scaling provided in Figure 3. With some unit conversions, the pressure gradient in millibars/degrees latitude would be about 4 mb/0.6*. From Figure 4, assuming 45* latitude, this gradient estimates a wind speed of 76 kt! Although this is just a graphic for demonstration purposes, the magnitude of the wind speed is worth noting.
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| Figure 4 |
Figure 5 is from a case study of a Wake Low that occurred on September 2, 2010. The National Weather Service analyzed the pressure drop over a 2 hour period (blue dashed contours) of up to -3 mb. The Duluth International Airport actually observed a 6.1 mb pressure drop in only 28 minutes resulting in a wind speed of 50 kt.
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| Figure 5 |
Because Wake Lows are a relatively small scale, short duration event, they are difficult to forecast in terms of wind direction and speed at a specific time and location. However, squall or thunderstorm potential is routinely forecast by the NOAA Storm Prediction Center in its Mesoscale Discussions and Convective Outlooks for CONUS and coastal waters, Figure 6.
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| Figure 6 |
While the case studies tend to be in the upper mid-west area of CONUS, they do not suggest that Wake Lows would be limited to those areas. It may just be that only over land is there enough observational data to support the analysis of this relatively small-scale, transient event. This leaves mariners to ask whether winds resulting from Wake Lows could happen more generally anywhere squalls are found. After all, strong winds that radically change direction are something to look out for!
Summary
· Wake Lows are atmospheric low pressure areas found on the aft side of squall lines
· Fluctuating pressure gradients caused by Wake Lows can cause dramatic changes in wind direction
· Strong winds are a potential both on the leading and trailing sides of squall lines
· Wake Lows are small scale, transient events that may be anticipated where squall lines are forecast
· For safety, anticipate strong and gusty winds, as well as heavy rain and lightning with cumulonimbus clouds
References:
· American Meteorological Society Glossary
· Modern Marine Weather, 3rd ed.
· https://www.weather.gov/meg/wakelowres
· https://www.weather.gov/fsd/20180511_wakelow_SDNEIA
· https://www.weather.gov/dlh/100902_wakelow
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