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The Story Behind the Weather - By Forecaster John Ensworth M.S. - 

The Discussion of Weather Events Daily for Philadelphia and Pennsylvania

 Last updated: 09/14/2003 02:26 PM

A great good morning to you!  

Today's story will be how short waves in the upper atmosphere interact with fronts.  Sometimes they aren't enough to make rain and storms by themselves.  Let's jump right to it with the surface map.  The next front is creeping towards us, and though is has entered the NW side of the state, it will take some time to get down to us.  

In the visible satellite picture (showing thick, bright (highly reflective) clouds) we see a really significant band of clouds ahead of, on top of and behind the front (marked in blue).   The clouds will be steadily thickening over Philadelphia today, which will keep temperatures a bit down.

Looking at the regional radar, the most significant storms are on and behind the front. Very light rain is spreading out ahead of the front, but it shouldn't effect Philadelphia much right away. 

Looking now to the surface map, you can see the winds coming from the north, north of the front.  Winds are coming from the southwest ahead of the front.   Temperatures (red numbers) at 9:45am were in the 60's behind the front and in the mid to upper 70's ahead of it. The dew points (green numbers - the temperature you need to cool the air down to to get dew/fog to form - higher = moister air = less comfortable) are in the 50's way back behind the front.  This drier air is what we can look to for the weekend!

Now for the story of the short waves.   The 500mb map below is the picture of winds at the 1/2 way up point in the atmosphere (pressure-wise) and shows winds blowing basically from west to east across the US at about 18,000 feet.  The little ripples in the 500mb flow that matter to us have been marked with red dashes.  There are a lot of them! These are small bubbles of colder air aloft that (because warm air likes to rise into cold air above it) can increase the instability and kick off showers and thunderstorms ... sometimes. 

On the national radar, I've included the front and the short waves.  Except for the showers in N. Dakota, the only showers happening are to the right of the short waves and near the front.  For these storms to kick off, we need the combined effects of a bubble of cooler air up above and air colliding at the ground level (what happens at a front) to get air to rise and make showers and thunderstorms.

Similarly, the risk for severe weather is right along the front and ahead of the short waves. See? The weather CAN make sense!

Finally, looking into the future, you can see by 2pm today that the front is a bit closer and the chance of rain region has spread closer to Philadelphia.  Most of the US precipitation is associated with our front and those short waves. 

Between the map above and the map below (for 2am Saturday) the front hits and one of the short waves intersects the front at that time.  We get a good chance of showers and thunderstorms overnight. 

Saturday morning at 8am has the front off the shore to our east and just a bit of lingering clouds and shower activity near Philadelphia. 

Saturday night at 8pm, the front has stalled to our east and we are on the very edge of feeling the drier air coming from the northwest, and the remaining chance of showers and moisture to the east.  I'll look at it tomorrow morning with you to see which will win over our city. 

Watch for some nice lightning tonight!

Meteorologist  John Ensworth

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