Clouds and potential for freezing fog Monday

We are keeping a close eye on the atmosphere from the surface up to around 5,000 feet above the ground tonight and Monday.  Data suggests the potential for a low cloud layer developing overnight, which could lead to slick roads and exposed surfaces.

Road and bridge surface temperatures provided by INDOT Sunday evening.

We start with temperatures at the surface.  Not the usual thermometers that are approximately six feet above the ground, but instead temperatures on roads and bridges.

Thanks to INDOT we can see temperatures on interstates around central Indiana.  I've chosen three to show at I-65 and I-865 on the northwest side of the Indianapolis metro, I-65 and Market Street in downtown Indianapolis, and 465 and I-74 on the southeast side of Marion County.

At a little after 6pm Sunday evening, road temperatures were running anywhere from 28° to 33° - almost all below freezing.  Not surprisingly, bridge temperatures are mainly cooler - with the exception of 465 and I-74.  Nonetheless, they are all below freezing.

If the roads are not treated, any moisture that comes in contact with them will likely freeze and create ice.


This becomes the big question.  Is this just going to be low cloud cover or will it be fog.  It will also make a big difference if we see freezing fog, since temperatures will below freezing - forecasting lows in the teens Monday morning.  If it is mainly low clouds, there is less of a chance of any moisture freezing on the surface.

Several computer models suggest the visibility will drop to less than a mile at times in central Indiana, especially along and north of I-70.

Computer model projection of visibility in miles Monday.

Above is the forecast visibility in miles from our high-resolution RPM computer model.  You can see the visibility dropping to feet in Kokomo prior to sunrise Monday.

A drop in visibility can also be due to low clouds in the area.  At this time, I suspect we are mainly talking about low clouds with some areas of patchy fog.


Lets look a little closer at the data to see what's going on in the atmosphere.  The image below shows a vertical slice of the atmosphere over Indianapolis as forecast by the North American computer model (NAM) from Sunday afternoon.

A look at a vertical slice of the atmosphere over Indianapolis Sunday through Wednesday by a computer model projection.

To read the image correctly, you should know that it reads from right to left.  Times are listed along the bottom of the image.  Each horizontal dashed line is an additional 1,000 feet above the surface.  In this instance, we are look from the surface to 7,000 feet.  The yellow lines represent the air temperature at various levels.  The 0's with the line show that is the 0° Celsius line.  The areas shaded in white represent where the software believes clouds will develop.

Two things to notice when you first look at the data above.  The model is suggesting there will be a layer of clouds from roughly the surface to just over 1,000 feet starting overnight.  The second thing to notice is the area of 0°+ Celsius temperatures ABOVE the surface.  It begins at approximately 2,000 feet in the air.  Below that, the temperature is -5° Celsius (it is hard to read).  This indicates an inversion - when warm air resides above a layer of cold air.  This happens often in the winter and keeps clouds near the surface.

The model picking up on the inversion helps to strengthen my confidence in the low level cloud deck developing.  And in fact, this model suggests the low clouds still around until early Thursday morning.

It should be noted there are a couple computer models that do not pick up on the low clouds.  In fact, they suggest it will be a sunny day.  That's what makes the temperature forecast tricky.  If we don't have the clouds, temperatures will be in the upper 30°s to lower 40°s.  With the clouds, temperatures will only be in the low 30°s, and some may not get out of the 20°s.

At this point, it is best advised to check road conditions before leaving home for work in the morning.  And depending on how long the clouds/fog stick around, you'll want to check road conditions before leaving work for home in the afternoon.

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