How do I know whether a storm day is good for chasing?

Not all storm days are good for chasing. In fact, as early spring arrives, many chasers get excited about very deep and potent systems emerging from the Rockies. But more often than not, these systems do produce severe weather, but in the form of marginal hail and winds buried within rainy lines of storms. Why is this?

Tornadoes and isolated storms depend on a delicate balance of ingredients. The presence of a very sharp boundary, such as a cold front, can cause convergence to spread out over a very broad area. This results in many thunderstorm cells that have to compete with one another. Likewise if upper-level forcing is too strong, than any cap (lid) will be eroded quickly over a broad area and multiple storm cells will occur. These multiple storm cells typically manifest theirselves as the dreaded squall line. Though tornadoes can and do occur in squall lines, they're usually the death knell for photogenic opportunities.

Some things to look for:

Patterns detrimental to tornadic storms
1. A strong front, particularly a fast-moving cold front with frigid air behind.
2. A weak or nonexistent cap (check the sounding).
3. A very strong cap (check the sounding). Sometimes called a steel cap or nuclear cap.
4. Clear skies or high stratocumulus in the early morning (indicates weak moisture).
5. Winds are veering during the day (shifting to southwesterly).

Patterns supportive of tornadic storms
1. Target area is along warm front or ahead of surface low (better shear).
2. The presence of weak boundaries that depend on good analysis to keep track of.
3. Winds are backing during the day (becoming more easterly).
4. Cumulus clouds show strong development, even past midafternoon.
5. High dewpoints (60s and 70s) which remain high well into afternoon.

Of course, every weather situation is different, and there is no cookie-cutter approach. This is why forecasting is so tough, and why some veteran chasers get caught at home on the "big day" when they know they should have been chasing.

Also note that an SPC moderate- or high-risk day does NOT necessarily mean a jumbo tornado outbreak. Quite often this means high confidence that a line of severe storms will occur, and may imply numerous cells that can throw chase strategy into chaos. Many of the better photogenic storms occur on slight-risk days, where weak patterns favor more isolated storms.

Good luck!