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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 02:24 PM
Good morning Philadelphia! We had a sprinkle pass with the frontal passage yesterday, and some rain upstream (but not as much widespread heavy rain as we expected yesterday - still much more may form). You can se heavier rain out in Michigan and a line of moderate showers/thunderstorms just entering the western part of the state.
Looking at a more sensitive radar (view), we have light showers and sprinkles around Philadelphia this morning...
And in the Doppler Radar storm total view, we can see just about a tenth of an inch of rain has fallen near the city.
The visible satellite view shows a very distinct band of bright clouds orientated west to east over Pennsylvania...this is well correlated with the position of the front (see the surface map down below).
In an interesting contrast the infrared view (showing where the coldest clouds are) doesn't show this west to east band well, but instead shows colder clouds on that line east of us and west of us. This is where the atmosphere is rising and where rain is falling (check out the radar views up top). Now you can see the value of different takes on satellite images!
So the low is right on track heading into Wisconsin this morning and the current front has stalled near where it was forecast to (but more on top of us that originally thought, which will make it easier for us to get more rain as the system approaches).
The 500mb map, showing winds blowing basically from west to east across the nation (parallel to the black lines) at about 18,000 feet, shows a ridge (marked in purple) growing in the western US which is, effectively, making a trough appear in the eastern US. If you have followed these archives for the last month and longer - you know that a trough over our heads means ... yep... more rain. We'll see how long it lasts...
By 2pm today, the next system is still out west and the front is to our south. Cloudy, cooler and a tad drier. Nice!
By 2am Friday morning, the next system is HERE! The front to our south returns to the north as a warm front and the low and all it's moisture and instability is bearing down on us.
At 8am Friday, the warm front is north, we are hotter and more humid with rain breaking out all over.
Then the cold fronts hits by Friday 8pm with thunderstorms possible and heavy rain in places. But it will again feel nice behind the front! There is always a silver lining!
As I noted yesterday, the T.S. Claudette is visible down in the southern Gulf of Mexico, but is heading rapidly westward and is looking less and less like it will directly effect us. But we'll keep an eye on it (no pun intended, but hurricanes get eyes... ha!) through the weekend together.
Stay dry!
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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