|MORNING CAP AND THUNDERSTORMS
METEOROLOGIST JEFF HABY
A common severe weather spring situation in the Great Plains is to have a morning cap.
The cap is one
reason why storms are more common in the afternoon than they are in the morning. When the cap is
in place, surface based convection is prevented even if there is
CAPE. Daytime heating will
increase CAPE and weaken the cap. If the cap is too strong then storms may not occur even
when daytime heating occurs. If the cap does get eroded by daytime heating then storms can
occur at the same time of the day CAPE is highest. This can produce explosive
severe thunderstorms in the afternoon. The cap can act to hold the potential energy until
it is released later in the day.
Storms that occur in the morning tend to be weaker since CAPE is weaker. In situations where
the cap is weak in the morning, thunderstorms will occur much earlier in the day. These
storms tend to produce less severe weather than the explosive afternoon storms. Thus, the
cap can act to prevent the premature release of CAPE until CAPE increases further.
If the cap stays in place all day then no surface based storms will occur. This can lead to
severe weather "bust" days. The CAPE and
wind shear may all look great for severe weather
but if the cap is too strong then all that potential energy will not be released.
Since the cap is a stable layer and it prevents surface based convection, the sun will not be
cirrus anvils and towering cumulus clouds. When there is a strong cap there will
tend to either be clear skies or low level stratus under the cap. The morning stratus
will often mix out through daytime heating. On a clear or clearing day the stable cap
will allow more daytime heating since the storm initiation is delayed until later in the day.