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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 02:41 PM
Good Afternoon Philadelphia!
Here is a short discussion handling quiet weather with a hint of tropical trouble for the future...
We'll start with the 500mb upper air map (the air flows here, about 18,000 feet above sea level). The winds blow west to east parallel to the dark black lines. The big northward 'hill' in the air flow is our ridge. To the right of a ridge you get sinking air and clear skies. That is the theme for our weather in the near future.
The west coast/mountain trough is bringing increasing storminess to a dry western US... but isn't effecting us much today.
On the surface map, we have a high to the north and the remains of T.D. Henri (as a Low) down in the ocean to our distant south. It 'probably' won't move up here...but we STILL are not out of the possibility that the moisture will pull in here later in the week.
Right now, we have a strong onshore flow with the circulation around these two pressure systems as shown below in this graphic from www.accuweather.com.
This flow is keeping it windy here and moderately moist (the dew point map below shows dew points near 55F - which is a bit above average moisture content).
The infrared satellite view this evening, shows the high clouds (therefore very cold and colored blue) from the storms around the remains of Henri. We are mostly clear across the state- especially around Philadelphia.
With clear skies and onshore northeast winds (we label winds in meteorology by where the wind is blowing FROM), we will drop to the mid 50's tonight (near the dew point). It will be cool, but feel damp.
The highs tomorrow, even with lots of sun, the onshore flow will keep us in the mid-70's. It will just feel GREAT.
Looking beyond even what the slight chance that moisture from Henri will effect us, we do have to worry about Hurricane Isabel. This is a monster of a storm that will probably make it near or to the US. It is a Category 4 out of 5 storm far out to sea but heading toward the US in general.
It's projected window of movemne includes paths that could bring a strike close to us. We will have to watch this for mid-week next week!
In the near future, the high to our north will sink southward (and possibly repelling Henri moisture) and the onshore winds will decrease a bit.
Thursday morning, the high is almost on our heads, and we'll switch to a light north wind with the moisture from Henri still waiting a chance to 'maybe' come this way after the high moves off.
Enjoy this great weather. It just doesn't get much better than this...ever!
Meteorologist John Ensworth
Surface Station sky cover color key:
Flight category definitions:| Category | Ceiling | Visibility | |
|---|---|---|---|
| Low Instrument Flight Rules LIFR* (magenta circle) |
below 500 feet AGL | and/or | less than 1 mile |
| Instrument Flight Rules IFR (red circle) |
500 to below 1,000 feet AGL | and/or | 1 mile to less than 3 miles |
| Marginal Visual Flight Rules MVFR (blue circle) |
1,000 to 3,000 feet AGL | and/or | 3 to 5 miles |
| Visual Flight Rules VFR+ (green circle) |
greater than 3,000 feet AGL | and | greater than 5 miles |
| *By definition, IFR is ceiling less than 1,000 feet
AGL and/or visibility less than 3 miles while LIFR
is a sub-category of IFR. +By definition, VFR is ceiling greater than or equal to 1,000 feet AGL and visibility greater than or equal to 3 miles while MVFR is a sub-category of VFR. |
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