This series of Haby Hints investigates problems that cause a forecast to bust. A bust occurs when a certain weather parameter is expected but one or more factors cause the forecast to be wrong. This particular Haby Hint will focus on how the cap causes forecast problems.
The cap is a region of stable air in the troposphere that traps convective lifting that originates under the cap to remain under the cap. The cap will be present due to warm air aloft or warmer air that has been advected into the forecast area aloft. The cap when present will often be above the boundary layer and in the lower troposphere. The temperature lapse rate within the cap will be stable (inversion, isothermal layer, or weak temperature decrease with height). The cap is very important in that it can determine the timing that convection takes place if convection even occurs at all. It will also influence the strength of the convection indirectly.
The cap is famous for producing severe weather busts. This is because the difference between a major severe weather outbreak and no storms can depend on if the cap weakens enough to allow boundary layer based convective instability release to take place. The cap will weaken due to daytime heating and uplifting dynamic forcing mechanisms. If there is not enough daytime heating and/or not enough dynamic uplift the cap will not break. In some forecasting situations it will be easy to tell whether the cap will break or not but in other situations in is far from obvious. For information on assessing cap strength go to: http://www.theweatherprediction.com/habyhints/308/.
The timing on when the cap breaks is one factor that determines how strong storms will be. When a cap breaks in the presence of large instability the storms will likely be more severe than if the cap breaks in the presence of weak instability. Instability generally reaches a maximum in the afternoon hours due to daytime heating. A weak morning cap will often break before a strong morning cap. If the cap is strong then the forecaster must determine if it will break during the day. A delayed cap breakage will generally lead to stronger storms as compared to a cap breaking in the morning since again instability generally increases during the day. But if the cap is too strong then the situation could occur that there is large instability in the afternoon but no storms develop.
In a lake-effect snow situation the height of the cap is important. A layer of polar air will be capped with warmer air aloft. Unlike a thunderstorm situation, the precipitation occurs under the cap in a lake-effect snow storm. Thus, the higher in elevation the cap the greater the depth of the troposphere that convection can take place under the cap. If the cap is too close to the surface (generally closer to the surface than the 700 mb level), snow amounts will be greatly limited.
So far we have looked only at how the cap impacts precipitation. Temperatures can also be impacted since precipitation is a cooling agent. We looked at how precipitation impacts temperatures here.