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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 02:29 PM
Good Morning Philadelphia!
The clouds are breaking up already over most of Pennsylvania and even Philadelphia. The front is moving through.
The winds north of the blue dotted line are out of the north now and temperatures (the red numbers) are about 10 degrees below those south of the front. It is definitely south of us, and a bit further than expected in the Saturday discussion. This will decrease our chances of rain after today. Cooler, drier air and low chances of rain? Very nice!!
The surface map shows the front being analyzed over the southern tier of counties. But looking at the red dotted line I've added, it lies in the bottom of the trough marked in the isobars (lines of equal pressure) that are thin and white lines. The front us usually in that trough axis. I would guess that this map analysis this morning is flawed.
The National Weather Service has the front a bit further south (and this map was made an hour earlier) and in the bottom of the surface trough axis (the yellow lines = isobars in this map). This looks more reasonable.
The rainfall this morning is limited to very light showers in the southeast part of the state with heavier thunderstorms down in West Virginia moving to the east. This ripple of weather should be following the frontal boundary and should be sliding south as it goes to the east all day.
The rainfall around Philadelphia over the last day (as the front passed) shows only 0.10 to 0.3 inches fell in a spotty pattern. A nice dampening of the soil, not a show stopping rain.
Our chance of more rainfall will really depend on the nearness of the front and the passage of short waves, or ripples of cool upper atmospheric air, passing over the front. There is a big ripple in the flow marked in the orange arrow pattern. If you compare it to the upper air flow one picture further down, you can see that the area with the blue arrow is not an upper air feature (the lines do not move parallel to the moisture flow shown in this water vapor satellite picture). The upper air trough is marked in red and is drawing moisture up ahead of it.
When this approaches this afternoon, IF the front is near enough, we will get thunderstorms around here with the warming of the ground helping out.
This map shows winds running basically west to east across the continent parallel to the dark lines. This map is used in the paragraph above.
So looking into the future, we see the bulge above near us giving us a chance of showers tomorrow as it is still in the area, and the front is still fairly close to our south.
Wednesday has the front too far south and dry conditions for Philadelphia.
The next trough is deepening over the central US in the upper air flow on Wednesday, with the showers forming as seen above.
The showers move eastward on Thursday,
...with the trough over the western Great Lakes moving toward us.
On Saturday the trough is over the lakes and the rain formation area returns to Pennsylvania.
We'll handle those days as these days go by.
Until then, keep looking up!
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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