Friday, July 27, 2012

What is an outflow boundary or gust front?

The question has come up numerous times the past few times we've had storms in the Mid-South: "What is a gust front / outflow boundary?"  On our social media channels, we often refer to these features during nowcasting stints as they are an important part of the life cycle of summertime (especially) storms and can affect you in ways that are noticeable.  

In the course of a thunderstorm's lifetime, there is birth and death (or generation and collapse).  Thunderstorms contain updrafts of wind that push miles into the sky, creating billowing clouds, and downdrafts often associated with falling precipitation.  The updrafts also eventually become downdrafts, meaning the air is now rushing back towards the ground. When the rush of air hits the ground, it is commonly called a downburst or microburst and spreads out along the ground (as it can't go through the earth).

This creates an outflow of cooler air from the storm (remember the air originates miles up in the sky where it is much cooler).  As the outflow propagates away from the parent storm, it creates a boundary between the cooler more stable "outflow" wind and the surrounding air that it is moving into.  This is the outflow boundary or gust front (or in storm spotting/chasing lingo, the forward flank downdraft).

Because the outflow typically has gusty and turbulent wind conditions, it picks up dust from fields, insects, small children (ok, not really) and any other light objects that can be pushed along with the wind.  Doppler radar is able to detect the objects (even dust in large enough quantities) caught up in the outflow boundary, which becomes visible to radar-watchers as a narrow line of light returns (or thin line).  Below is a screenshot from StormView Radar taken Thursday night as a large outflow boundary moved across the Memphis metro.  Note the green worm-like line labeled with red arrows. Voila - an outflow boundary!

The other thing typically detectable by radar along an outflow boundary is low-level clouds.  Because two air masses are colliding (the cooler outflow-generated airmass and the warmer surrounding air ahead of the outflow), clouds are generated as the warmer air rises over the cooler outflow air. Rising air promotes cooling and condensation, thereby creating clouds.  With the strongest outflows, the type of cloud created is the shelf cloud, aptly named as it typically resembles a shelf in the sky.  Below is an approaching shelf cloud along an outflow boundary.  This picture was taken in Paragould, AR late yesterday afternoon before it made the long trek to the Memphis metro (same storm system). 

Shelf cloud, photographed by Pam Whitaker. Paragould, AR.

It's not unusual to see variations of this cloud type along outflow boundaries.  Sometimes they look very sinister, as they are often dark, low, and stretch for miles.  However, the clouds themselves usually pose no threat (in fact, many times there is not even rain in them).  But if one of these is coming your way, be prepared for gusty wind behind it, blowing from the direction in which the shelf cloud is moving from.

You'll also typically notice drops in the temperature, somewhat drier (less humid) air, and often rising pressure. Often the storms that generated the outflow may not be far behind the shelf, so also be prepared for stormy weather!  In the case of the outflow depicted on radar above, temperatures dropped about 10 degrees, dewpoints fell 5-6 degrees (hence, drier air), and the wind shifted to the northwest gusting to 30 mph or more.

The outflow boundary passed the MWN sensors in Bartlett about 9:30pm. Note the red arrow, indicating the passage of the outflow. Temps fell from the upper 80s to upper 70s and dewpoint fell from 75 to about 68 degrees.
Even if the outflow has moved well away from the storms that created it, the rising air along the leading edge of the boundary can often create new storms in the wake of the boundary, hence re-starting the thunderstorm life cycle! 

Watch the video below to see a time-lapse of an outflow boundary and associated shelf cloud move over Bartlett, TN in June.

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