Twin States Weather Blog

Forecasting Headache

* Even the seemingly "easy" forecast days can offer up some difficult and interesting challenges. Tonight's big issue: will we see low clouds and fog early tomorrow morning? The implications can be rather significant; if the clouds hang tough through much of the day, our high temperature forecast will likely be 10 degrees too warm.

* Step 1. Evaluate the low level moisture around the state. To best do that, look at the difference between the temperature and dewpoint. We call this number the "dewpoint depression." For example, if the temperature is 60 and the dewpoint is 57, the dewpoint depression is 3 degrees. If the dewpoint depression reaches 0 with light or calm winds, fog formation is certainly possible. Our current temperature as of this writing is 60, with a dewpoint of 57 and a dewpoint depression of 3 degrees. Most computer model guidance suggests that the dewpoint depression will fall to 0, which would support the formation of low clouds and fog...

* But, there isn't a lot of soil moisture. Soil moisture can be rather important in formation of fog and low stratus clouds. Also, the computer model guidance suggests that winds will not go completely calm, but a very light breeze will be in place. This breeze helps mix the air up, allowing it to hold more moisture.

* Another piece of evidence we'll evaluate is the latest upper air sounding from Jackson. We look for an "inversion", or an area in the atmosphere where the air is warming as it rises. Normally, air cools as it rises. When we have an inversion, that can trap moisture underneath it and lead to the formation of clouds. There is an inversion present on the latest sounding from Jackson. But, there is also a fairly dry vertical profile in the sounding - which argues against low clouds and fog.

* Bottom line...I believe low clouds and fog will develop as far south as I-20. The best chance will come along the MS/AL state line and points east. Also, low lying areas and places near water will stand a better chance at seeing some fog. However, I think the low clouds and fog will mix (evaporate) out by 9-10 AM, leaving us with ample opportunity for sunshine and subsequent warming into the lower to middle 80's. We'll see how it all works out...

posted by WTOK Weather Staff @ 10:23 PM