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Dryline Storms
As a chaser I love chasing dryline storms more than any other type of storm. What makes dryline storms so great is they are usually easily visible, rise quickly, and can provide a clear view of the tornado from beginning to end. If you have ever chased in dew points in the middle 70's with moisture streaming in continually, usually you can't see more than 1/4 a mile ahead of you. Trying to spot a storm in low visibility is often not worth the effort. You can be 1/2 mile away from a tornado and can't see it because the thick humid air diffuses your view. Whereas Dryline storms can pop up from clear blue skies at a rapid rate, and often leave you with a full 360 degree view while they build giving you a spectacular show.
A dryline can be picked out on a weather map such as this one below. 
Typical Dryline
Here I drew a red line to mark the dryline. In this case we go from dew points in the twenties to dew points into the upper sixties in a short distance. You look for this rapid change in dew points in a short distance. The other factor you would want to know is how thick the moisture layer is above, not just at the surface.  Looking at upper air maps see how fast the dew points drop off. If for instance the dew points drop slowly up to 2500 feet, but then dropped off quickly, the dryline may move rapidly once it begins it's march eastward. I have been left in bright sunlight when one dryline I was chasing moved 100 miles in less than an hour. Do the math, you can't drive that fast in the US legally. On the other hand I have seen drylines sit in one place for days, moving very little. A look at the upper air maps would most likely show the moisture levels in this case almost twice that of the fast moving dryline. So remember, look at moisture both at the surface and upper levels to get a better picture of what should happen and how fast the dryline might move.

Now, the dryline you see here still needs a trigger for severe storms to develop. One of the more pronounced drylines I chased had a jet stream run right over the top of it while a cold front marched into the dryline at the same time. To top off the storms rapid development a low pressure sat just north and west of the dryline pulling the moisture towards it, slamming the moisture into the dryline at 30 to 35 miles per hour.  The result, many tornadoes right near and along the dryline, barely over into the higher dew points, but not much further into the moist air.

This part is very difficult to forecast.  Often you will find yourself on the west side of the storms as they race east, faster than you can catch them or the opposite, you are sitting 50 miles east of the dryline and it sits there or even backs up to the west leaving you in clouds and rain 50 miles from the tornadoes. With all the predicting and reading of the data, this is the one part that often leaves chasers with nothing.  Careful reading of the latest upper air maps helps, but you will quickly find that storms often build 30 to 50 miles from where you predicted they would. Weather has many unpredictable twists.  With all the knowledge meteorologists have, and I am talking about the best of them, the weather doesn't much care about their predictions, it likes to surprise even the most educated.

So when chasing look for distinct drylines as this map shows, but remember, without a trigger effecting that dryline you may just go home with a bad sunburn rather than that trophy tornado video you dreamed about.  But when a dryline fires, get ready for the ride of your life, it can get wild fast.    Tornado Tim
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