Thursday, May 15, 2014

Hail Possible Today

Although temperatures are below average for this time of the year we are still expecting a few hail showers this afternoon.  Right now it does not look as though we will see much in the way of severe hail, quarter size or larger, but a few thundershowers will contain a lot of pea size hail, and in some places could cover the ground.  So why are we seeing a threat of hail when in May cooler temperatures usually mean no storms?

The satellite above shows what the Heartland looked like from space around 11:30 this morning.  We see lots of sunshine but we are also seeing cumulus clouds developing across our northwestern counties.  These are the clouds that will grow into thunderstorms.  The reason is because there is an area of very cold temperatures in the mid levels of the atmosphere, around -15 degrees, moving across much warmer air at the surface.  This causes the air to rise rapidly which leads to thunderstorm development.

Above is a profile of the atmosphere, a skew-T diagram.  The blue line represents the temperature of the air as it is lifted.  The red line is the actual air temperature.  The area between the blue and red lines where the blue line is on the right represents the amount of CAPE (convective available potential energy) for thunderstorms development.  At first glance it is not very impressive. We call this a short skinny CAPE which usually leads to a few scattered showers.  But a closer look shows that most of the CAPE lies in an area where temperatures are below -10°C or the hail growth region.  So hail will form very easily in these showers.  But since it is a skinny CAPE, the updraft of the storm will be weak meaning the hail will not be able to grow to a very large size.  As of this writing we have already seen pea size hail across parts of central Missouri and I expect that to increase as the afternoon progresses.   

The above image shows a forecast for where storms should be around 4PM.  Don't pay attention to the exact location, just note there will be scattered thunderstorms with small hail throughout the Heartland later this afternoon. So a few areas of the Heartland will experience hail today.  You may want to keep the car in the garage if possible even though damaging hail is not anticipated.  If the small hail is heavy enough it could chip some paint.

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