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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 02:23 PM

Good morning again!  Our next mega- water producer is slowly, slowly, slowly crawling past the state.  This mornings surface map shows the low center to our southwest still and the warm front stretching to the east across W. Virginia,  northern Virginia, and off the coast.  The heaviest rain is now to our east, but there are still wandering bands of showers over Pennsylvania.  The rain cooled air in the higher elevations in south central PA has resulted in some fog this morning that won't be quick to burn off (with the shielding from above that the storm system is giving it with it's cloudiness).   The rain also has some small streams flowing quite strongly in the mountains, but no wide scale flooding has occurred.   If we get too many of this inch+ rains though, that could change.

The rainfall totals from Doppler Radar show a broad swath of half inch to inch (and locally 2") rainfall totals.  Being north of the low this time, we saw a more smooth rainfall distribution than we did before.  Convective precitation (as is often found ahead and south of a low pressure system) tends to be more spotty with small areas of intense rainfall totals and areas almost entirely skipped. 

Looking into the future (this forecast map is valid Thursday at 8pm  (00Z Friday - 4 hours = 8pm CDT)) the chance of rain wrapping around the low as it pulls up the coast doesn't completely drop off until then.  This IS a slow mover.  But winds after this will eventually begin to come in from a more westerly direction.  The yellow lines are lines of equal surface pressure (adjusted to sea level) and the wind is blowing nearly along those lines (and a bit to the left of the direction the lines are flowing) counter-clockwise around lows.  This gives Pennsylvania a northwest wind at present, but you can see the more westerly winds (parallel to the yellow lines) over the Great Lakes.  This westerly air will be approaching the area and warming us up through the end of the week. 

The next system, due here on Saturday, can be seen in the northern Great Plains and Southern Canada on the surface map and forecast surface maps above, and it can be seen on the current Visible and  Infrared Satellite pictures.  Look for the tight swirl in the clouds north of Montana.  Pretty nice- no?  Remember the visible satellite picture is the view (in black and white) you would see if you were sitting up on the satellite looking down (so the sun has to be up)...

And the infrared satellite is looking at Infrared light emitted by everything on the earth (so it works all day and night).  So we are looking at the temperature of every surface below the satellite.  Dark reds are the hottest surfaces (usually bare ground), the blues are the coldest (usually the icy tops of convective clouds, thunderstorms, and clouds raining - not always, but usually).  

So you might want to move your Saturday outside events to Sunday to try to avoid the heaviest rains.  Until tomorrow, keep looking up!

Meteorologist John Ensworth, M.S. 

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