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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: 02/13/2004 04:34 PM
Good Afternoon Philadelphia!
I'm glad to announce that these
discussions will continue until at least the end of February
(it's a short month)!
I
have the necessary server space (especially with missed days!).
In short: The quiet and not too cold weather holds on. A cold snap hits Sunday (see yesterday's discussion for more into on that)
In the Long: With nothing but the coming cold snap to worry much about, I'll take us through the 500mb progression through the start of next week to see what the big picture will look like.
Currently it is even a bit warmer than it has been (with strong southwest winds) and the really cold air for Sunday isn't visible yet.
The 500mb map (this is the air flow aloft at about 18,000 feet above sea level with winds traveling west to east over the continent parallel to the dark black lines- this steers storm systems and is caused by areas of cold air (troughs) and warm air (ridges) throughout the atmosphere) shows a big trough very tilted and laid back over into the southwest states. The region you'd expect storms is on the right side of the trough (in the purple circle). This region is FAR from us presently.
The surface map shows the storminess in the deep south. It also shows the arctic front pressing out of Canada that will chill us down on Sunday/Monday.
The surface map for 1pm Saturday shows the arctic front roaring down the plains, but also sinking southeast.
By Saturday night 7pm, the front is not yet into the state but Lake Effect snows are breaking out as a powerful cold dry northwest flow starts up behind the front.
The 500mb map (see above) for Saturday 7am shows the trough across the US breaking into southern and northern pieces. The southern part will kick up some snow in Texas if you can believe that! You can see the northern trough sliding past us to the north.
Sunday morning sees the arctic front pressing past us and the deepest juiciest air stays to the south with the system moving straight east. We'll see colder air flooding in and Lake Effect snows way out west.
Sunday night 7pm, the rains down south are ahead and with the southern trough (now separate from the northern trough).
Monday AM sees the northern trough past us (as well as the southern trough). A new trough is moving into the central US.
Monday night, the trough in the central US is pretty deep, and it's active region is to our south again. The 'mostly quiet' continues. With the northern trough is past us, a ridge moves in and starts a new warm up.
I'll see you here again Saturday and see how cold we may expect it by Monday AM.
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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