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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 02:20 PM
Good morning again! The skies are so shockingly clear again, that on the visible satellite picture below, you can see the folds in the mountain chains in the state. There are only some light cirrus coming from the west.
This is caused by the upper level (500mb level is about 17,000 feet above sea level) level winds that are flowing over a ridge (marked in purple dashes) pattern overhead. This allows the air to sink, warming it and keeping clouds from forming. And indeed, that is what is going on.
By 2pm this afternoon, we see clear skies continuing with the next front still out in the Ohio valley. Winds (now circulating counterclockwise around the low in S. Canada) are still from the southwest bringing Gulf moisture and interior US heat toward us.
This mornings surface observations show the winds reflecting the presence of the ridge over the area (which means winds flow north then back south in an upside down 'U' shape). Over Ohio and western PA the winds are coming from the south west, over eastern PA and NJ etc. the winds are flowing out of the northwest. Dew points in the south eastern part of the state (including the Philadelphia area) are very high now (73 or so is a LOT of moisture)...
... and since your body can't cool itself well via perspiration when the air is already nearly full of water has led to the enacting of a heat advisory for us. Take it easy outside from late morning into the early evening. It will be stressful being outside today!
The next front arrives in the western reaches of the state by the time of this forecast map - Friday 8am. But then the components in the atmosphere that allow rain to form weaken over the state and the next map shows...
...that the rain becomes much more spotty over the mid-Atlantic states and the rain becomes concentrated over the deep south and up in Canada. That's ok, we are wet enough. The rivers near flood stage across southern Pennsylvania have just returned to near normal.
Enjoy the sun and I'll see you here again tomorrow!
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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