Thunderstorms and line squalls comprised of numerous
individual cells often intrude on pleasant summer afternoons.
The severity of these localized bundles of bad weather is directly
proportionate to the vertical development of each cell. Severe
storms can reach 35,000 to 50,000 feet in height and their distinct, anvil-like cloud-tops signify up and down drafts that can
result in localized surface winds in excess of 50 knots. When the
underside of towering cumulonimbus clouds take on a rolling or
fragmented appearance, and the sky is obliterated by several cells
in close proximity, prudent sailors may douse the mainsail and let
the tempest pass. You can reach along with a scrap of unfurled
jib, a storm jib, or proceed under power until the worst is over.
Under such conditions, smart powerboaters may choose to slowly
head into the wind and developing seas, or run with the waves
downwind. Faster vessels may be able to avoid the worst of these
severe thunderstorms by taking evasive action. Radar can be a big
help because it clearly shows the rain-laden cells and indicates the
direction of their movement.
These tempests can be short-lived due to their fast rate of
advance and the relatively short lifespan of most individual cells.
However, when multiple cells are involved, such as with a cold
front, the bad-weather experience is quite prolonged.
There are several good indicators of thunderstorm severity
and they include a large amount of static on an AM-radio receiver,
a visible green tinge and/or lumpy texture to the cloud bottoms
(cumulonimbus mammatus). There may be a descending shelf cloud
on the cell’s leading edge, and another sure sign of an impending
front is a wisp of noticeably colder air on a hot, sultry day.
NOAA Warnings On The VHF
One of the least understood facets of a marine weather fore-
cast is the difference between a “weather watch” and a “warning.”
Severe weather watches are issued by NOAA when conditions are
ripe for thunderstorm development. This doesn’t mean that storms
have developed in the watch area; for boaters it’s a precautionary
advisory justifying extra vigilance, not an indicator that storms are
rolling over the local waters. When a warning is issued, however, it
means that severe thunderstorms have been sighted in the watch
area; this should be a “red flag” to those about to leave the dock
to stay put, and a game-plan changer to those already underway.
Avoiding an encounter with a severe thunderstorm is always better
than a confrontation with such volatile weather. But being caught
in shoal water or in a nasty inlet while attempting to get to safe
shelter can be far worse than encountering the same conditions
with sufficient sea room around you to keep a collision or running
aground from being part of the hazard. Attempting to outrace a squall
line with a mad dash toward a distant harbor can be a tough gamble.
Instead of risking getting caught in shallower water when the squall
hits, it may make more sense to head for deeper water, or where there’s
no traffic, fewer obstacles, and a fair distance from a hungry lee shore.
PHO TOS BY RALPH NARANJO