Written by cuweathernerd    Saturday, 16 March 2013 00:00   
Chasing 101: Section 1:9 Forecasting: Cold Fronts

This is part of the continuing series called chasing 101, a course to help people who are new to chasing learn the fundamental skills to chase productively and safely. They are meant as both information and as a forum for discussion.


A front is the interaction between two airmasses of different density forced to interact by a low pressure system. Each airmass has a distinct combination of temperature and moisture, and where that changes most rapidly can be a focusing area for thunderstorms.

As storm chasers, we care about fronts because they help to initiate our thunderstorms, as well as signal when a threat has passed. Fronts are a wonderful way to learn an important skill, called subjective analysis which involves you looking at raw data and quickly diagnosing what is going on.

A successful chase often hinges on successfully identifying and positioning yourself relative to these boundaries early in the day. Getting to that point requires that we understand the different types of front, the cold front, warm front, and dry line in a decent amount of depth, so that is what the next few posts will focus on, before we bring them together to understand how they work together in a classic severe weather set-up.


Today, we're going to focus on the cold front.

A cold front is the boundary between cold and warm air where the cold air is advancing, and the warm air is retreating. In the great plains, the cold air tends to come from Canada, so it tends to be dry. Meanwhile, the warm air tends to flow north from the Gulf of Mexico, making it moisture rich. Where these two meet, we denote the boundary with a blue line with blue triangles pointing in the direction the cold air is advancing. We don't draw that line anywhere the cold air is retreating or not moving, because there isn't a cold front there.

Cold fronts are the leading edge of a mass of dense air which wants to spread out. Since dense, cold air will flow to warm, less dense air (think of opening a freezer), the front doesn't encounter much resistance as it moves, allowing it to move more quickly than the warm front or dry line.

This motion means that the warm air piles up before the front. Since the air can't go into the ground, it is forced upwards along the leading edge of the more dense cold air. If there is ample moisture present, this leads to clouds, even thunderstorms. We call this form of rising motion forcing because the air is forced upwards by the front. Even if the atmosphere is capped (warm air aloft inhibiting air rising on its own), the forcing of the front can often be enough to overcome the inhibition.

Fronts aren't just a line on a map, but rather exist in three dimensions. They extend above the surface, and tend to arc back over the cold air. This geometry helps explain why cold fronts are excellent at producing thunderstorms in the right environments -- the relatively steep slope of the leading edge leads to a lot of forcing.

A cold front brings with it all kinds of changes that can happen very quickly. The diagram below is called a meteogram, which is a plot of different meteorological variables against time. It shows a very strong cold front passing through Friona, TX. As you can see -- the frontal passage changed every variable listed here, except for the dew point.

Generalizing, we might expect the front to produce the following impacts:

VariableBefore FrontAfter Front
Temperature Warm Significantly Cooler
Winds Predominately Southerly Predominately from the north
Precipitation Dry Thunderstorms near front, showers behind
Moisture High Lower
Pressure Steady Decrease Sharp Increase

You may look at that and say, but cuweathernerd, the meteogram doesn't show that! And you're right -- cold fronts don't all behave the same way, but this table is a good baseline of how to identify a cold front from a single observation point.


Of course, we are more interested in severe weather than just predicting changes in temperature and wind. So what can a cold front tell us about the chance of severe weather?

A cold front is a large feature: it crosses several states. The forcing is relatively constant along it, and so if conditions are supportive of thunderstorms along the front, it tends to create a line of thunderstorms, not discrete cells. Lines of thunderstorms do not tend to be the best place to find tornadoes, but, under the right conditions, can still produce severe thunderstorms. Days where the cap is uncertain to break, or the best target is out of reach can be a good day to play the cold front; while you largely forgo the chance at tornadoes, you position yourself with a high certainty of initiation.

There exist exceptions to every rule, and of course cold fronts can produce tornadoes under specific circumstances. Most often, that is caused by an embedded supercell, a thunderstorm that remains relatively discrete in a linear feature. An example is the 2008 tornado which destroyed the Little Souix Scout Camp.

In addition, because they tend to be located in the richest moisture and have the highest instability, with uninterrupted flow from the south, the furthest south storm on a front, called the tail end charlie tends to be the best location to find severe weather. Furthermore, these storms tend to be more cellular/discrete in nature, which aids in their ability to produce stronger convection.

What is something of a hard and fast rule is that the passage of a cold front signals the end of a severe weather set up for a location; the air befind the front is significantly more stable. Storms may develop a few tens of miles behind the front, but not much further.

What distinguishes a cold front and makes it more likely to produce severe weather is more complex than an introductory post, and involves how the winds change across the front (shear vector), the kind of convergence present, the ambient wind profile, and more.

The cold front is the least likely of the three boundaries to produce a tornado in a classic severe weather set up, so soon we will cover its brothers, the warm front and dry line, and discuss what they mean for us as chasers.


In the meantime, it so happens that there is a stark cold front about to pass over my house in Kansas City, so it's a wonderful opportunity to work with real data to show what a cold front looks like "in the wild". This front is currently "dry" meaning the environment isn't right for it to produce precipitation. Nevertheless, it is going to very much change the weather here soon. Please excuse my sketch-y subjective analysis, but then if you are doing this "in the field" your pictures are never very pretty either.


 

The above is an article written by Reddit user cuweathernerd and has been modified by Ryan Lehms. The original article and discussion can be found here:

http://www.reddit.com/r/stormchasing/comments/1af04m/chasing_101_cold_fronts/

Last Updated ( Wednesday, 18 September 2013 20:17 )
 

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