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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 02:32 PM
Good Afternoon Philadelphia!
There STILL is a weak frontal boundary near (over) Philadelphia this afternoon and with plenty of moisture to the south and east of that boundary, and some day time heating... we are seeing isolated HEAVY storms popping up.
The hot moist air can be seen again in the map of heat index this afternoon. The heat index measures the stress of the combination of temperature and high relative humidity. If the atmosphere is already near saturation (100% RH) then your sweat won't cool you down very much. It is 'as if' the temperature were higher.
The storms are few and far between, but are dropping a LOT of water in them. They are also moving slowly so flash flooding will happen under them.
There are some cumulus clouds (the smaller puff balls) across the state, and the isolated storms above are visible here. If you look closer though, you'll see something interesting...
Around each storm (and at the location of earlier storms) there are semi-circles of clouds. These are pools of cool air the storms and rain have left behind once they died. I've marked many of the arcs with blue arrows. Those can, if conditions are right, kick off new storms.
The upper air map, again, shows the growing ridge (labeled in red). Our old trough is getting pinched off by the ridge to the west and east. The southern part of the trough is now an inverted trough over the deep south. This building ridge will bring sinking, hotter air (with humidity in place). Yuck.
Looking into the future, the ridge is even bigger by Thursday (and the old inverted trough is over west Texas).
At that same time, the surface map shows something interesting... no rain near Philadelphia!
For Friday morning (8am) the ridge still dominates us...
And the surface map is free of showers.
Amazingly, a trough pops back into place on the east coast Sunday morning, but the main region of lift and showers will be from Philadelphia eastward (see the forecast on the main page!).
It's Tuesday morning (8am) and we see the trough again in place. (We just can't shake it this year!).
Enjoy a hot humid break in the cool rainy/cloudy weather.
I'll be back tomorrow morning. Until then, keep looking up and stay dry and cool!
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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