Thunderstorm Prediction

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:

  1. 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.
  2. 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 uncertain weather.
  3. 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.
  4. 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.