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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 02:29 PM

Good Afternoon Philadelphia!  The high pressure system that heralded the cooler dry air is now moving off to the southeast. Winds around a high move clockwise, so we have west winds increasing and bringing in warm moist air. 

The front that will be giving us unsettled weather for a good chunk of the work week is gathering itself together from Minnesota to Colorado.  But first... the heat...

Temperatures are climbing this afternoon.  We have low to mid 80's across the east coast except for a bubble of heat from Philadelphia southward where temperatures are already up to near 90 F. 

The overall heat is hot air being shuttled from the south central part of the US by the departing high (from above) toward us. The bubble of heat is caused by this air flowing over the Appalachian mountains.  It descends on the east side of the mountains.  As air sinks, it compresses (higher pressure near sea level).  As air compresses it warms up.  This is making things a bit more uncomfortable around home.

The dew points (and the discomfort from humidity) is increasing as well.  It won't last long, but a taste of summer is firmly here for a couple of days this weekend. 

When you combine temperature and relative humidity, you get what is called the heat index.  The human body can't cool itself as efficiently via the evaporation of sweat if the air is already full of moisture.  We express this stress on humans by giving you the effective 'felt' temperature if you are outside and exerting yourself slightly.  It is near 90 right now and the heat index is about the same.  As the dew point increases over the next 24 hours, the heat index will begin to exceed the temperature.  Be careful outside until the front arrives. 

We do see some rain showers out west, but (as we saw in the surface map up top) the front isn't here yet.  This is caused by the following map...

The red dashed lines show those famous short waves I've discussed many times on this discussion. The air likes to rise on the right side of the troughs (short waves) in the air flow at 500mb (or 18,000 feet). That is where we see the showers kicking off. 

Jumping into the future to Sunday night, that short wave has rotated off the east coast and a new short wave begins to amplify into a downright big trough over the eastern US. 

This brings in the next cool front and a chance of showers and thunderstorms for Sunday night.

By Monday night, the trough is deepening (with some interesting other short wave in the bottom of it) but is still tilted to the southwest.  This is not a pattern that moves out quickly. 

Because of that, the front stalls in our vicinity or just to our south.  The chance of showers remains in the forecast.

Tuesday night and the trough is deeper, but is still to our west and tilted to the southwest. 

So the front is not very far off and the chance of showers is still in the forecast Tuesday. 

It's Wednesday and the front is STILL (!!) with us.  Showers will kick off each time a short wave ripples around through the trough and over the front. 

If the long range forecast is right, then the front finally starts to slide south and east and give us more sun than clouds. But we'll be here together to follow the particulars as the week proceeds.

As always, stay dry, unless you want to get rained on!

Meteorologist  John Ensworth

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