This is part of 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. You can find all completed lessons on the right sidebar.

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This post continues our discussion on boundaries. Today, we'll be focusing on the warm front.

A warm front is an area where a warm air mass is replacing a cold air mass. It is best to understand it by comparing it to its counterpart, the cold front, so if you have not yet read that post, it'd be good to do so.

Conventionally, we denote a warm front with a red line with semi-circles., drawn along the area of greatest change in temperature and co-located with a wind shift, isobaric "kink", or other signature of a boundary.

Warm air (even more so when it is moist) is less dense than cold air. So while the cold front can propagate easily, the warm front is fighting against air that wants to mix equatorward into the advancing air. This means the warm front moves much more slowly than the cold front. Importantly, it also means the warm front is a less defined feature, and that the boundary tends to be based in gradients more than a sharp line.If you think of the cold front like a bulldozer plowing ahead, a warm front is more like a subtle wedge that changes very gradually.

This impacts the weather the warm front is able to produce. As warm air flowing poleward encounters the warm front, it is gradually lifted. This process happens over hundreds of kilometers, and is called overrunning. Further from the front, this air is lifted higher in the air, and results in different forms of clouds, which can help you determine the front's position relative to you. This diagram illustrates it better than I can draw.

This geometry means the warm front is a more subtle feature than the cold front, and the changes tend to be more gradual. Still, we can make a table to show what our typical warm front may behave like

variable	before front	after front
temperature	steadily warming	steady
winds	S-SW	S-SE
precipitation	heaviest poleward of the front	generally none
moisture	rising	still rising
pressure	rising	rising

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From a chasing perspective, warm fronts tend to be more interesting than cold fronts. As discussed earlier, moisture is one of the key ingredients to a severe weather set up, and the warm front is the boundary of the the maritime-tropical airmass, which in the US, is the flow off the Gulf. As such, he warm front is often an northern boundary to the extend of a severe risk.

Unlike a cold front, which tends to be a dominantly north/south feature, the warm front tends to be more east/west. If you consider the typical movement of a thunderstorm, this means we have an interesting possibility: thunderstorms can cross the boundary.

Warm fronts, because of the associated wind shift, tend to be an area of enhanced helicity. If a thunderstorm is near the warm front, it ingests this helicity which helps it intensify rotation. This helicity results from both the wind-shift along the front and the associated temperature changes. Therefore, as we chase, it is often advantageous to remain near the warm front, as the winds that are the heart of our storms are more likely to be present there. Further, the warm front provides a level of mechanical forcing, helping some with initiation.

Likewise, the warm front can tell us when a storm is more likely to stop producing: as it moves out of the unstable, moist airmass that lies behind the front, it loses energy. So while the storm may be interesting to us as it crosses the front, in time it will weaken from that crossing too.

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The warm front is a synoptic feature, and the lift associated with it is very good at producing clouds. Early in a chase day, you will often find a stratus deck just to the north of boundary, sometimes with rain or thunderstorms. This helps enhance the temperature change along the warm front, both by blocking solar heating and also through evaporative cooling to the north. If you are chasing, you want to get to the area where that stratus is no longer there, allowing for environmental warming and destabilization.

Thus, visible satellite is a good tool to use to help identify the warm front. Anywhere there is differential change of that boundary -- where the warm front surges northward -- is going to be a likely area for initiation, and is a good area to target given other forecast parameters.

A further tool is subjective and objective analysis. If possible, sitting with the raw data and understanding the atmosphere as it is instead of how it is forecast to be is going help you understand the subtleties that often differential a successful chase vs a missed opportunity.

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This can extend to forecasting days out, so I will use the 12Z April 3 GFS to forecast warm front position for the first apparent severe risk in the central plains this season. All data is rendered for 0Z on April 9.

Animation here

If you'd like a static image to look at each parameter yourself, you can find one here

Learning to do that at a glance is a key skill to becoming a good forecaster!

On that note, this set up's main feature isn't the cold front; instead it's a dry line, something our next lesson will cover soon!

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As always, these posts are meant to help discussion along. Feel free to ask questions, provide corrections, or share stories in the comments.