|
Convective Outflow Boundaries
Every three hours beginning at 0000 UTC the
Hydrometeorological Prediction Center (HPC) in Camp Springs, Maryland
issues a new mean sea level (MSL) surface analysis chart.
The MSL surface analysis chart depicts
surface weather features such as an isobaric analysis (including high
and low pressure centers), surface observations, fronts, squall lines,
troughs and outflow boundaries. Outflow boundaries
are depicted by a dashed tan line (two of which are shown on the right)
and are labeled "OUTFLOW
BNDRY" on the chart. Pressure troughs are also depicted
similarly, but do not carry any specific label. Click here to view a larger image
of the MSL surface analysis chart above.
Outflow boundaries are found in the vicinity of
deep, moist convection, better known to pilots as thunderstorms.
According to research meteorologist Dr. Charles Doswell, III,
"Cold, stable air is the 'exhaust' of deep, moist convection,
descending in downdrafts and then spreading outward like pancake batter
poured on a griddle." These outflow boundaries are frequently
depicted on the MSL surface analysis chart.
During the daylight hours, some outflow boundaries
can be seen on the visible satellite image. In particular,
the outflow boundary depicted on the MSL surface analysis chart in
southwest Missouri (shown above) is very clearly shown in the visible
satellite image (left) just to the north of the Missouri and Arkansas
border. Click here to view a larger image.
Deep, moist convection seen to the north produced downdrafts
containing cold, dense air that pushed outward away from the
convection. After spreading outward the leading edge of the
boundary often has ascent and may produce cumuliform clouds on
the boundary typically in the shape of an arc as seen in this image.
At
night or when the boundary is essentially invisible on the visible
satellite image, the outflow boundary may be detected on the NWS
WSR-88D NEXRAD Doppler radar. Outflow boundaries are low
level events and do not necessarily produce precipitation.
Instead, the radar is detecting the density discontinuity of
the boundary itself.  If
the outflow boundary is more than 100 miles from the radar
site, the lowest elevation scan of the radar may indeed overshoot the
boundary. In this
particular case, the outflow boundary shows up very well on the NWS
radar
image out of Springfield, Missouri as a bow-shaped line of low
reflectivity returns. Click here to view a larger image.
Outflow boundaries are a form of a
"gust front." However, they may not always move in advance of
a line or area of thunderstorms like a traditional gust front.
An outflow boundary that moves in advance of a line of
thunderstorms will often produce strong straight line winds and low
level convective wind shear. Often this kind of gust front
will be contoured similar to the line.
The area of convection in southwest
Missouri (shown above) is moving to the southeast, however,
the outflow boundary is moving to the south-southwest or nearly
perpendicular to the movement of the cells. Outflow
boundaries such as this can be associated with strong low level
convective turbulence and wind shear especially close to the
storm. Normally as they move further away from the convection
the turbulence assocated with the outflow boundary normally dissipates rapidly.
|
