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: 11/17/2003 09:57 AM
 

Good Afternoon Philadelphia!   

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

In short: Mild temperatures (very close to normal, actually) and rain every other day. Yes, rain is increasing.

We'll start today with a look at the past.  The last 7 days shows under 1" amounts for areas in Pennsylvania, but it has been a wet year. 

The Palmer Drought Index looks at the soil moisture and long term effects of rainfall.  We are in the Extremely Moist area.  Yes, this is a reversal of the past many years (it's hard to remember back- right?), but it can be said we don't need rain.  But we are going to get a bunch!

Just because I found this map this morning, we can look at the average date of the first freeze (but we've already been in the 20's in Philadelphia).  Normally it hits by November 1st... that means we were pretty normal this year for the onset of the first freeze. 

Ok, jumping to today.  We have some wind shift/surface trough thingy out in western Pennsylvania, but nothing very organized.  There are a few spots of morning low laying fog around, but nothing worth getting overly excited about. 

According to a different meteorologist (this time a weatherunderground.com ) we are just north of a boring stationary front with the nearest rainfall happening in upstate New York.  Boring. 

Winds, likewise, are very light and variable over the region (so no mentionable wind chill - how nice!). 

And temperatures are still quite mild for this time of year with morning temperatures in the upper 40's. 

We do have a lot of moisture in the atmosphere and wandering patches of clouds over much of the region.  You can see the 'bubbling up' of the clouds in northern New York state with those thunderstorms up there.  They are lit from the east-southeast by the rising sun. 

The upper level pattern, as always, is the 'man behind the curtain' in the weather (yes, I saw the "Wizard of Oz" on TV last night). This map (the 500mb map) shows winds at about 18,000feet above sea level as they blow from mainly west to east across the continent parallel to the dark black lines.  

There is an overall Ridge pattern in the east, and a Trough pattern in the west.   You must always look for storms on the right side of a trough, and good weather on the right side of a ridge. So you would expect good weather for us (and it is tranquil) because of this pattern, but the small trough embedded in the ridge (marked with red text) is enough to kick up clouds and storms (the clouds over the region and the storms in New York state). 

Neat- no?!

So let's look at this upcoming rain!  Monday 1pm and the embedded trough above will keep a chance of rain around. No organized front is responsible for this rain, and no heavy rain should fall, but showers are possible.  (Especially in the afternoon when the surface temperatures warm.  Another way to picture what a trough is - big or small - is a pool of cooler air aloft.  If you have cold air above, and warm air below, air will rise and clouds/storms will form). 

Come Tuesday morning, 7am, something big and scary is brewing out in the Great Plains (boom!). We are still sitting with a slight chance of showers.

By Tuesday night, 7pm, the big trough out west is moving towards us (like the shark fin in "Jaws").   Look for storms on the uphill/right side of the trough. 

And indeed, a big trough = big storms.  The leading edge of the rain from this trough and it's associated surface cold front dominates the US map!

Then Wednesday, 7pm, arrives (remember, subtract 5 hours from the UTC - Universal Time stamp on the maps for the Eastern Time Zone) and the trough is tremendously deep and narrow and HERE!

And the storms (at this same time) are heavy and dense up the east coast. 

Then the trough pinches off (it got too deep and narrow) and a pool of cool air aloft sits over us (that spells rain!). This is now Thursday 7pm.

And though the heaviest rain is off in the ocean by Thursday 7pm, we have showers wrapping around the surface low to our northest. 

And into Friday night, 7pm, the showers keep wrapping around the low and sliding over Philadelphia. Since the trough is cut-off from the main flow now, the severity of storms and rain (even over the ocean) is way down. 

Try to keep from drowning just walking to your car.  I'll see you back here Tuesday morning!

Meteorologist  John Ensworth

Surface Station sky cover color key: