Loaded Gun Chasing
website of storm chaser Mikey Gribble
|Analyzing surface charts is key to locating boundaries and determining where storms are going to
develop and as a result are a critical part of targeting for storm chasers. I will cover the basics of
identifying surface boundaries on this page. I will use a typical plains spring time surface low scenario
to discuss this, where there is a dryline, warm front, and cold front.
Surface Low - There are two easy ways to locate a surface low on a surface chart. One way is by
finding the low pressure center and the other way is by analyzing the wind fields.
Finding the surface low by pressure fields is easy. Simply look at surface pressure numbers for each
station and the surface low is located in the area with the lowest pressure.
Finding the surface low by analyzing wind fields is pretty simple too. The wind will blow towards the
low pressure center. There are two forces at work here, the pressure gradient force and the Coriolis
force. The pressure gradient force causes the wind to blow from higher pressures to lower
pressures. The Coriolis force causes winds to deflect to the right in the northern hemisphere due to
the rotation of the earth. As a result, with a surface low you have the pressure gradient force pulling
the winds to left (towards the low pressure center) and the Coriolis force pulling the winds to the
right (away from the surface low). The strength of the Coriolis force is proportional to the speed of
the winds. The faster the wind is blowing, the stronger the Coriolis force is. When wind speeds are
lower, the Coriolis force isn't as strong. At the surface friction causes winds to blow slower than they
otherwise would, which upsets the balance between the Coriolis force and pressure gradient force.
Usually the two forces nearly balance each other out causing winds to blow parallel to the isobars
(lines connecting areas of equal pressure). At the surface when friction gets thrown into the
equation the pressure gradient force gains the advantage and winds blow across the isobars
towards the surface low as a result. So typically surface winds will be blowing towards the surface
low and around it counter clockwise.
Dryline - The dryline is the boundary that separates dry warm air to the West from warm moist air to
the East. Once again there are two easy ways to identify the location of this boundary (there are other
ways to identify the boundary, but these two are easiest for beginers IMO).
The first way to find the dryline is by analysing dewpoints on a surface chart. The whole purpose of
finding the dryline is to determine where storms are going to fire. Typically in the spring I watch the 50
degree dewpoint line. This seems to be pretty close to where storms fire on an average spring setup.
The other way to locate the dryline is through analyzing windfields. Behind (to the West) the dryline
winds will usually be out of the west to southwest. Ahead of the dryline over the warm sector (east of
the dryline in the warm humid air) winds are typically backing more (turning more southerly, backing
means turning more counter clockwise). So you should have winds out of the south or southeast over
the warm sector East of the dryline. To locate the dryline with wind fields you need to find the wind
shift line, where the surface winds turn from southwesterly to southerly or southeasterly. This is
usually a pretty well defined wind shift and not hard to find on a surface chart. The strongest
convergence and area where storms will develop may be slightly upstream of the wind shift line
(Doswell), so don't take these as exact markers of the boundary. The best way to find the dryline is to
apply both of these methods and to also use satellite once towers start to go up along the dryline.
Utilizing visible satellite images is by far the most effective way of determining exactly where storms will
develop along the dryline.