Mary Gilbert; June 28, 2001
Since this is about the start of the prime hiking season,
and everyone seems to be concerned about thunderstorms, I
offer some basic tips on short-term forecasting as to how to
predict thunderstorm activity up to several hours ahead, or
even the day before, one sees the rising thunderheads. I
teach this in the FC Group BMS and is based on study and
personal mountaineering experience. Remember, s-t
forecasting (as well as professional) is an inexact science,
but is fairly accurate, very specific to locale, and works
best in summertime.
Here are the steps:
- Identify the FIRST cloud that shows up the the morning
sky. If already present, go with it. If both upper and
lower level clouds are present, go with the high cloud.
- Determine what that cloud means. Low level cumulus might
mean a fair dry day ahead. Higher level cumulus looks like
a marching herd of little puffy 'sheep', ripples, or a fish
scale pattern ('mackerel sky'). That means uncertain
weather, because such clouds usually represent the leading
edge of an arriving cold front. Stratus can also mean
- Determine lower level wind direction--winds that revolve
around high and low pressure systems. (Prevailing upper
level winds in our region are westerly, which blows in our
major storms.) The MOST RELIABLE indicator of lower level
wind direction is how low level clouds (cumulus or stratus)
move. The wind you feel is often distorted by topography.
- Make a short-term forecast. If the lower wind's anywhere
NE thru S, you're probably in for falling weather. If the
lower wind is generally westery, chances are, it'll stay
fair. Monitor any wind shifts.
Storms can happen without fronts moving through. That's just
one mechanism that lifts air up to cool and condensate into
clouds, causing precipitation. Our abundant mountains also
facilitate rising air. One thing to look for on a day with
no high clouds present is if it's HAZY. That means there
may be enough moisture to fuel those innocent thin cumulus
clouds into thunderstorms. A classic case of this happened
last week when DIA got hammered by big hail. That morning
looked like Louisiana--very hazy, small thin cumulus. Lots
of fuel waiting for an explosion.
Yes, you can predict what is yet unseen. Doesn't work all
the time, but that's weather forecasting. Watch out for the
nasty 'sheep' in the sky!
Bill Lhotta asks:
In your description of wind directions I assume when you say NE you mean
that the wind is coming from the NE and not going towards the NE?
Another way of saying what I mean is if you turn so the prevailing wind is
in your face, you would be facing NE in the above example.
Mary Gilbert replies:
A NE wind means it's coming out of the northeast, a S wind
is coming from the south, etc. So, if you face NE into the
wind, you have a NE wind--the direction it's coming from.
If low clouds are moving from SW to NE, it's a SW wind.