Fog risk forecast returning

Posted: 2017/09/11 in Weather
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As the colder weather months approaches, the fog risk forecast is returning as the prime time for river fog and eventually inversion type fog will return.  In recent weeks there have been posts on Facebook on nights where there were favorable conditions for such development, and almost every morning those posts were confirmed.

The reason why risk forecasts are posted from September through May starting last season was a good reason: river fog during the fall, winter, and spring months has the potential for a AM peak disaster.  The risk falls under a few categories and other weather conditions may play in a role in the risk.

Low – the conditions for fog to develop are not favorable

Medium – favorable conditions exist but not likely to cause the NWS to release Dense Fog Advisories

High – favorable conditions exist AND the NWS is likely to release Dense Fog Advisories

 

Fog risk forecasts are only posted via the Facebook page, likely on Sunday evenings.  The forecasts for early week are more likely going to be accurate than late in the week so there will be time that Thursday/Friday risk will be deferred.  Unless otherwise noted the risk will only be valid from 05:00 to 09:00.

Why is River Fog so dangerous?

Some of the biggest epic fails during the fall and winter months came on mornings that conditions for river fog to develop were favorable – which is warm days, cool nights, and a lack of a wind.  River fog only develops late at night or first thing in the morning around or near sunrise, and is much more likely to develop over or near a body of water.  It tends to dissipate late in the morning. From October through April, the temperatures in the morning can flirt with the freezing point and that can turn fog into ice.

What is Inversion Fog?

Inversion fog is more likely to occur during this time of the year and it can occur at any time of the day.  It is essentially warm air trapped over cold ground with a lack of wind.  The main difference is that it can last 24/7 and it is not limited to the river valleys.  It too can occur when the temperatures are at or below freezing.  Unlike River Fog that can happen on a sunny day, inversion fog usually occurs on a cloudy or rainy day, but there are times it can occur on a clear day when there is snow on the ground.

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