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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 02:25 PM

Good morning once again.   We have the long awaited storm system on our doorstep, and ... it's not raining as much as forecast.  See, you still need meteorological research!

The radar view shows a shocking LACK of any rain of any kind around us.  Amazing. 

We do have plenty of clouds with the warm front on our heads, copious amounts of water from the south and an approaching cool front from the west.  These clouds will help keep the daytime temperatures down a bit for us.  Nice. 

Looking at the infrared satellite picture we don't see the clouds as clearly.  Remember, this view shows the temperature of cloud tops, so they are warm and near the ground (visible in the clear areas above). The blues are the highest, coldest, iciest clouds and where the air is rising to make precipitation.  And this process is not going on anywhere near Philadelphia. 

The surface observation map shows the upper 70's across the southeast part of Pennsylvania (red numbers/upper left) and dew points (green numbers/lower left)  in the lower to middle 70's around us.   The temperatures are a bit cooler than they have been due to the cloud cover and they are very near the dew point.  The dew point is the temperature to which you need to cool the air to to get liquid water to form out of the water vapor in the atmosphere.  This is the cause for the fog and haze across the city this morning.  The double purple dashes between the temperature and the dew point means moderate fog is occurring.  [the key to sky conditions on this map is located at the very bottom of the page]. 

The surface map this morning shows the warm front just about to pass us with rainfall far to the northeast of the state.  The cold front is approaching from the west and only has a thin line of showers forming ahead of it. 

This unusual map has many many features to it that I won't get into today, but check back for more on it later. It is called a vertical sounding.  Twice a day, stations across the world release weather balloons (aka UFO's) that rise up through the atmosphere and take measurements.  The 500mb map (further down)  we look at is the plot of all those stations at just the 500 line on this chart.  This is the vertical plot of all temperatures, dew points, and wind speed and directions above the city where the balloon was released (Mount Holly, NJ).   

The temperatures are marked with a big thick red line, the dew points are the green line on or to the left of that line.  The values of temperature and dew point are light blue lines tilted to the upper right and the pressure (decreasing as you rise in the atmosphere) are the horizontal blue lines.  

Today's use for this map is the wind measurement on the right side. This reads like a clock.  Each place were the flag connects to the vertical black line is the height the wind measurement was made at.  The wind is (like the surface maps) is blowing from the direction the flags are drawn, down the stem to the point where the flag connects to the vertical black line. The wind direction is always read as the direction the wind is coming from.   On these maps, from the top of the page is a North wind, from the right side of the page in East wind, from the bottom of the page is a South wind, and from the left side of the page is a West wind.   (The wind speeds are like roman numerals, a half barb is 5knots, a full barb is 10 knots, and a triangle flag is 50 knots.)  An example:  The very top wind is at 100mb and is from the west-southwest at 15 knots.  The fifth from the top is from the same direction at 55 knots. 

After all that, what I want to show you is the warm front in side profile.  The winds at the very bottom is where it is all happening. The bottom two wind directions are from the south east, then the winds switch to the southwest from there on up. This shows that the warm front (which is sliding overhead) ... (jump down to the next graphic)

Here (from www.usatoday.com) is a graphic of how a warm front looks in side view.  We are almost at the point where the warm air is at the surface.   This is why the temperatures will be much warmer today than they were yesterday.  Here comes heat from the southwest!

The overall pattern of the upper level winds is making a big trough in the east.  This is the push behind the cold front that is approaching us.  Be glad you're not under the big ridge (hot hot hot air) in the southwest US!

So we have a front passing and warmer temperatures coming.  The forecast surface map shows, most definitely, too much rain, but at least the positions of the fronts can be trusted  (by Friday 8am).

The cold front is catching up with the warm front to our north and west by Friday 8pm (the purple occluded front) which means the entire system is dying. The rainfall is much lighter (and spottier) across the state and the cold front is about to pass us.  Some rain will fall, but it may not be as great as previously believed. 

For Saturday 8am, the rain is STILL overdone on these maps.  The front is past and only widely scattered showers should threaten western Pennsylvania. 

Then, for Saturday night 8pm, we are in the clear, relatively speaking, with drier but still quit warm air in place.  Cold fronts bring in drier air at these times of year, but with the sun high in the sky, the air quickly warms each morning to temperatures as high or higher than was found ahead of the front. 

Have a GREAT  (dry!) weekend and I'll see you here tomorrow.

Meteorologist  John Ensworth

Surface Station sky cover color key: