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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 02:19 PM
A sunny good morning to you. A look at the infrared/surface combo map below shows clear skies and a nice High Pressure system sitting to our southwest, almost in the same location it was in yesterday. Remember that air circulates clockwise and slightly away from the center of a high pressure system, giving us west or west-southwest winds. This offshore flow of air will keep the sea breeze out to sea and bring us dry warm air.
A look at the dew point map is next. The dew point is a statement of how much water there is in the atmosphere (at the surface in this case). The higher the dew point (say 60's to 80's as below) the less you need to cool the air down to get moisture to condense out of the air. It is dry in the mountain west, and very very moist from Texas and Louisiana northward. This big blob of moisture is slowly spreading (along with heat) eastward across the country. You can also make out a tongue of moisture coming up from the south of us on the east side of the Appalachians. This moisture will help fuel the rain Friday/Saturday (see below).
The 500mb map (showing winds as they flow basically west to east half way up in the atmosphere) shows the ridge line just about on our heads. We are getting the maximum amount of sinking and heating air here now. Nice.
Even looking out 48hours into the future (Wednesday night 8pm) we see the high still dominates our skies even though the next system is getting it's act together for the weekend on on the plains.
Looking into the future, we see that the next rain event for Friday/Saturday (I'm sorry about another wet weekend) begins with a chance of showers (not seen in other computer models, incidentally) creeping in from the east with a bunch of moisture flowing onshore by Thursday evening. These will be scattered and not too heavy. Look to the west for the front itself!
By Friday night (8pm) the front is here and some places will get very wet again!
But, nicely, by Saturday night, the front is moving on and cooler/drier air is flowing in.
And though something seems to be forming to our west again, Sunday looks nice as well!
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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