|Prefrontal squall lines in Biloxi, Mississippi
“A squall line or instability line located in the warm sector of a wave cyclone, about 50 to
300 miles (80 to 480 kilometers) in advance of the cold front, usually oriented roughly
parallel to the cold front, and moving in about the same manner as the cold front. Also
called non-frontal squall line; pre-cold-frontal squall line (answers.com).” Basically
translated, a squall line is a very long, narrow band of showers and thunderstorms,
sometimes severe, but it’s not a very wide band of weather. It is usually several times
longer than it is wide based on the dynamics of its formation.
Then you may ask, what are the dynamics and under what conditions does it form? The
dynamics and conditions with which the pre-frontal squall line occurs are under a
generally accepted theory which follows: “… that as thunderstorms develop along the fast
moving cold front, large quantities of cold air from aloft descend in downdrafts along the
front and form a wedge of cold air ahead of the front. The wedge of cold air then serves
as a lifting mechanism for the warm, moist, unstable air; and a line of thunderstorms
develops several miles in advance of the front (tpub.com).” Squall lines typically occur in
the southern United States, east of the Rocky Mountains and most often occur in the
spring and summer months (tpub.com). This would make the most sense with the
smoother topography at play east of the Rockies enabling these types of cold fronts to
sweep over the fairly flat terrain and have the dense, cold air push down and out ahead
of the front to form a pre-frontal squall line. Within this line, there will be numerous perils
associated with this system.
The types of weather phenomenon associated with pre-frontal squall lines typically are
heavy rain showers and thunderstorms, hail, and tornadoes are possible if the warm air
mass is extremely unstable (tpub.com). These may be short lived in a thin squall line or
there could be a wider squall line and the weather would persist a bit longer. And in some
rare cases, more than one squall line may form. Each situation is going to be unique and
keeping a record or a journal of events can help a meteorologist determine the type and
severity of each event in the future.
The type of damage it can do is unprecedented: devastation to aircraft, crops, homes,
people, and live-stock are all susceptible to damage and destruction that severe weather
usually entails. This could be hail of a small to large size, strong gusty winds that blow
over your fences and take part of your roof off, lightning that strikes your home and
catches your abode on fire, flash flooding if the storms train over your area and the
surface was recently saturated, or the extraordinary occurrence of tornadic activity that
tears your home, possessions, and your lives to shreds. Severe weather is nothing to
sneeze at; we must always be on the lookout and take appropriate cover from the storm.
No matter the size, it is still a dangerous and powerful entity with no regard for life or
This leads us to our next inquiry: How does a squall line move? That is a good question.
These systems tend to move the same direction as the cold front is moving. The speed
can be faster than the cold front, at times. The speed at which they move is
approximately 40% of the 500mb wind field, as a general rule of thumb (tpub.com).
Based on this, a forecaster should be able to look at the latest 500mb chart and try to
determine wind speed and direction of the squall line. If one is not available, try your
area’s latest Skew-T sounding and strive to make your determination of the movement
and speed from that source.
Below is an example of how a prefrontal squall line may form in front of swiftly moving
Figure 4-34.—Prefrontal squall line development (tpub.com).
Here in south Mississippi, we are in a unique position and have experienced many
prefrontal squall lines. Primarily due to our latitude and topography we are prone to be in
the warm sector of a mid-latitude cyclone and have the low level jet set up in our vicinity
in advance of a strong cold front. Then, when the cold air begins to funnel into our area
ahead of the front, we see the clash of the air masses at its greatest right here.
Subsequently, we will see squall line forming to our west and move in an easterly
direction bringing with it showers and thunderstorms with heavy rain and frequent
lightning. It is good if the weather forecasters for our area stay on top of the situation
and alert the public via television and radio broadcasts of the possible weather threats,
so that they can take the appropriate actions regarding their personal property and
The majority of the time in our northern viewing areas, as far north as Jackson, MS, will
receive more precipitation than the southern viewing areas. I believe this is due to the
fact that the moist air that is transported north from the low level jet clashes more
dramatically than here along the beach front. Also a factor, that being this far south we
see the tail end of cold fronts most often and the majority of the inclement weather
usually rides further to the north and we will receive a lesser extent of the heavy rain
showers and thunderstorms provided by the squall line. Most of these systems do occur
during the spring and summer months, primarily spring here in the South Mississippi due
to the clash of the Continental Polar (cP) air masses moving into our typical Maritime
Tropical (mT) air mass that readily sits atop our area.
This event is going to be hard to forecast due the fact that the meteorologist is going to
have to determine the strength of the particular cold front and the properties of warm,
moist air mass out ahead of the front. Then monitor the surface observations out ahead
of the front and see if a line of thunderstorms appears to forming and determine the
500mb wind speed and direction to further aide in determining the strength and
movement of the squall line. Each system will have its own unique fingerprint and as I
mentioned above, keeping a journal may help the forecaster to keep track and then,
climatologically come up with a first forecast.
Below are a few examples of squall line images found from various websites.
A shelf cloud such as this one can be a sign that a squall is imminent (wikipedia.com)
Squall line enters Collin County (outlined in white) from the north on Sunday, August 25,
2002. Image courtesy of the National Weather Service (Yahoo Images Search).
Showing the typical pressure and precipitation field around a squall line. From the COMET
web site at http://meted.ucar.edu/ of the University Corporation for Atmospheric
Research (UCAR) funded by the National Weather Service. ©2000 UCAR (Yahoo Images
Works Cited Page
Integrated Publishing. "Instability & Squall Lines." Integrated Publishing: the Most
Informative Site on the Internet. 1998. Department of the Navy. 07 Mar. 2008
McGraw-Hill Companies, In. "Sci-Tech Dictonary: Prefrontal Squall Line." Answers.Com.
2003. McGraw Professionals. 07 Mar. 2008 http://www.answers.com/topic/prefrontal-
Riusabruce, Et Al. "Squall Line." Wikipedia: the Free Encyclopedia. 08 Mar.-Apr. 2008. 11
Apr. 2008 http://en.wikipedia.org/wiki/Squall_line.
Ted, Best. "August Squall Lines Visit Collin County." Yahoo Images Search. Ed. Goldblatt
Berry. 10 Sept. 2002. 11 Apr. 2008