The Story Behind the Weather - John Ensworth - forecaster 
  last updated: 09/14/2003

Goodmorning!  We'll start today with another comparison of the satellite views over Pennsylvania this morning.  The left picture is the visible satellite view.  White things would look white to your eyes (that is, clouds) , and dark things would look dark to your eyes (the dark waters of the Great Lakes).    The visible satellite view shows the low (see the big red L on the third image below- the surface weather map) off the east coast swirling over the Atlantic.  Thunderstorms (marked with a dark circle) are starting to form out over the ocean near the low's center. 

We have mostly clear skies (depending on where you live) over the eastern half of the state due to sinking air that is moving out of the way of the rising air in the low to our east.  This sinking air will give us an unusually (this spring) warm and mostly clear break from the gloom.  Western Pennsylvania has low clouds that will hang on longer today since they are at a higher elevation and further from the region around the ocean low that is sinking.

The clouds in the western part of the state are low clouds and some fog.  We can see this by looking at the right hand satellite picture.  This is an infrared picture where reds are warm surfaces (usually the earth) and oranges are cool (clouds near the ground).  The whites and blues are very cold clouds (thunderstorm tops for example) or cold snow (there isn't any snow around to look at though).   The clouds that are bright in the visible satellite picture are only a bit cooler than the ground in the infrared picture.    The thunderstorms circled in the visible picture have (predictably) blue colored (very cold) tops. 

    Visible Satellite Picture                                                    Infrared Satellite Picture

The current surface map shows our friendly low (again) off the east coast and a cold front extending down from it and back onto the continental US in southern Georgia.  North of this front lies our almost ever-present cool air.  Out on the plains we can see the next weather maker for Philadelphia. The Low over Minnesota and the front stretching to the south is going to slide to the east south east over the next 36 hours and bring a rapid increase in the chance of rain tonight and through Saturday.   It should slide out to sea on Sunday bringing the cool air in Canada (behind the cold front to the north of the Great Lakes) down over the area.

The future track of the low will approximately follow the height lines on the 500mb map (below) and the winds at that height (which parallel the lines most of the time).  If the next system doesn't go as far south as it appears it will now, then severe weather will be a problem for the southeastern half of the state on Saturday.  Keep an eye on that!

This graphic shows the regions of slight, moderate and high risk for severe weather over the United States. As the low now over the northern Great Plains

Keep your umbrellas at the ready and watch for the stronger thunderstorms tonight and over the day and evening Saturday. 

I'll see you back here tomorrow!

John Ensworth

(This page sometimes contains sources from AccuWeather.com   www.accuweather.com ).