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

The Discussion of Weather Events Daily for Philadelphia and Pennsylvania

 Last updated: 10/20/2003 09:32 AM
 

Good Evening Philadelphia!   

    I'm glad to announce that these discussions will continue until at least the end of November.  

Here comes a FANTASTIC week! Wow.

We start off with absolutely clear skies statewide (except come very spotty valley fog visible in the Visible satellite picture below as small linear clouds following the contours of the land in central and SW PA). 

We have the center of a High Pressure system sitting on our heads with our next front sweeping across the Plains from Minnesota to Colorado. 

North of the High, we have temperatures in the 30's and 40's, with upper 40's sitting in place over our city. 

But with the High on our heads, winds are light and there is no appreciable wind chill (unless you are jogging or bicycling!). 

The reason for the High being present and the good weather forecast is this Broad Ridge in the upper air flow (at about 18,000feet above sea level - moving west to east parallel to the dark black lines on the 500mb map below). The right side of a ridge is usually very nice weather. The Trough that brought our last storm (and the cool weather) is sitting off to our east now. 

The Jet Stream is seen on the map below as being strongest over northern Pennsylvania.  It happens where the lines on the map above are closest together.  The closer the lines, the greater the change in pressure over a small area.  The greater the change of pressure over an area, the harder the winds blow (think of a Hurricane!).  This isn't a hurricane though, this is the normal operation of the upper atmosphere in the region between the boundary marking cold polar air to the north, and warm tropical air to the south. 

The Great Plains front sweeps in here by Tuesday night (fast ride on that jet stream!), but the precipitation is expected to remain to the north of Philadelphia.  We'll just see the air cool and a period of clouds Tuesday night. 

The rest of the week will be so nice because we remain on the right side of the broad US ridge that has formed.  Tuesday night shows the trough a bit backed up over us (and we get the front at that time!), but the ridge is broad and huge to our west. 

Thursday night has the trough back to the east and we are still sitting on the right side of the ridge (even if the big ridge is getting a new dip in it over the northern Rockies). 

Saturday night next week and we have the ridge on our heads (expect a warm dry weekend next week!).  The ridge out west has flipped into a deep trough.  Once that moves eastward, we'll see storms and cold air return next week. 

Have fun enjoying this week (and watch the wind Tuesday afternoon)!

Meteorologist  John Ensworth

Surface Station sky cover color key: