We alert you first: up to 72 hours in advance when there is a chance for severe weather. That is determined by finding something in the atmosphere that causes convergence.
The dryline separates warm moist air from warm dry air. Looking at satellite imagery there will be a line separating clouds from no clouds. Dew point is a very important tool we rely on when forecasting. If we see a sharp difference between the two air masses, this is a sign that convergence is possible and storms could form.
Clouds will form on the side of the dryline with moisture, but that doesn't always mean storm clouds. Another important ingredient is CAPE which I explain here.