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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: 09/17/2003 09:52 AM

Good Morning Philadelphia!

There is much to talk about today, and little to talk about. Isabel is the story right now.

To cut through all the confusion, let's look at the expected hour by hour forecast from www.accuweather.com :

This Sequence begins THURSDAY 2pm. Isabel has already made landfall in the early morning probably in North Carolina and is being torn up by land and mountains as it goes into Virginia and W. Virginia.

There is a lot of information here, but the most important bit (aside from the unbelievable amounts of rain that will be falling for hours and hours) are the winds at the bottom of the column. Here winds are expected to stay in the upper 20mph region through 10pm with gusts into the 40mph realm.

TIME	2pm	3pm	4pm	5pm	6pm	7pm	8pm	9pm	10pm

Weather Conditions

Cloud Cover (%)	90%	90%	90%	90%	90%	92%	95%	97%	98%

Ceiling Height (ft.x100)	20	20	20	20	15	11	7	7	7

Visibility (mi.)	10.0	5.0	5.0	5.0	5.0	5.0	4.0	4.0	4.0

UV Index (1-10+)	1	1	0	0	0	0	0	0	0



Temperature (°F)	72	73	72	70	68	68	67	66	66

Realfeel Temp(tm) (°F)	69	69	68	66	64	64	63	62	62

Dewpoint (° F)	63	64	64	64	64	66	66	65	65

Relative Humidity (%)	73	75	77	81	87	93	97	97	97



Precipitation Type	Rain	Rain	Rain	Rain	Rain	Rain	Rain	Rain	Rain

1 hr Snowfall (in)	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0

1 hr Icefall (in)	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00

1 hr Rainfall (in)	0.05	0.05	0.05	0.05	0.05	0.07	0.07	0.07	0.07

1 hr Liquid Equiv. (in)	0.05	0.05	0.05	0.05	0.05	0.07	0.07	0.07	0.07



Wind Direction	ENE	ENE	ENE	ENE	ENE	ENE	ENE	ENE	ENE

Wind Speed (mph)	21	21	22	23	23	23	27	25	24

Wind Gust (mph)	35	37	39	40	40	39	42	38	33

The next jump goes from 10pm Thursday night to 6 am Friday morning. The storm will be largely past us by then. Winds remain steady in the 20mph realm with gusts into the 30mph area. These are not damaging winds and it will be the 3-6" or more of rain we'll need to worry about. YESTERDAY this same forecast showed 100mph winds in the Philadelphia and since the storm has not YET hit land, it is possible that it will curve a bit to the east at the last moment, head up the Chesapeake Bay and roar into our city with a LOT of force, and, as I will show below, there is one computer model showing that STILL happening.

TIME	10pm	11pm	12am	1am	2am	3am	4am	5am	6am

Weather Conditions

Cloud Cover (%)	98%	99%	98%	97%	95%	93%	91%	89%	88%

Ceiling Height (ft.x100)	7	7	5	4	3	4	5	7	7

Visibility (mi.)	4.0	4.0	5.0	5.0	5.0	8.0	6.0	4.0	4.0

UV Index (1-10+)	0	0	0	0	0	0	0	0	0



Temperature (°F)	66	67	67	68	68	68	67	67	65

Realfeel Temp(tm) (°F)	62	63	64	64	64	65	64	63	62

Dewpoint (° F)	65	66	66	67	67	67	67	66	65

Relative Humidity (%)	97	97	97	97	98	98	98	98	98



Precipitation Type	Rain	Rain	Rain	Rain	--	--	--	--	--

1 hr Snowfall (in)	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0

1 hr Icefall (in)	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00

1 hr Rainfall (in)	0.07	0.07	0.07	0.07	0.00	0.00	0.00	0.00	0.00

1 hr Liquid Equiv. (in)	0.07	0.07	0.07	0.07	0.00	0.00	0.00	0.00	0.00



Wind Direction	ENE	ENE	ENE	ENE	ENE	ENE	ENE	ENE	E

Wind Speed (mph)	24	22	23	24	24	23	22	21	19

Wind Gust (mph)	33	29	30	31	32	31	29	27	25

A quick look at the surface map shows the front we had pass a few days ago has (as advertised) vanished in the sinking air and new air flow around approaching Isabel. Isabel is easily visible churning away east of S. Carolina this morning. Something interesting to note is the big High pressure system over New York State behind that front. This High will actually matter as much as the hurricane to us IF the hurricane doesn't hit us straight on but goes to the west as is now expected. More on this High at the bottom.

Here is Isabel this morning. It has a hint of an eye and a very classic structure.

The strike probability map shows the National Weather Service thinking. It is expecting the most likely path to remain to the west of Philadelphia proper, but we will still get some serious weather out of this... and a more direct strike is not impossible ... yet.

Here are the expected path's of the eye from a number of different computer models. The BAM Medium (yellow) path could bring 100mph winds to our city... every other model is further west... but it's not over for us yet for a direct hit.

Historically we have NEVER had a hurricane strike from this location in the ocean (since we've been making observations). We don't really have a precedent to go by EXCEPT the storms 'like' to curve to the east when on this kind of path (which would be bad for us).

The wind field around the hurricane shows a greater 100 mile radius of hurricane force winds. This is a BROAD area under damaging winds. Landfall will not be pretty (probably in N. Carolina).

Looking closer to the NWS forecast, they have the eye almost going directly over College Station (making meteorologists there happy). But note, we are still in the envelope of possibilities.

Just for fun, you can see what it is expected to do after sliding through our state... it may skirt Hudson Bay. Canada and hurricanes. It almost seems like an oxymoron.

The 500mb map (which shows the air flow at about 18,000 feet above sea level) shows us the winds most likely to steer the hurricane. I've included many blue arrows showing the weak and variable flow in the SE US ahead of Isabel (giving forecasters a headache). The red path is the most likely track. You can see the winds do, sort of, get their act together and begin pushing the storm to the east the further north it goes. After that (north of us) it is really getting a push to the east.

Now to the winds. This graphic shows the past life of the storm. The Orange envelope is where tropical force storm winds happened (winds between about 35mph and 74mph). The Red areas show hurricane force winds (greater than 74mph). The storm was Cat 5 for much of the middle of this path. At that time the wind swath was smaller and tighter. It is expanding in coverage now even though the winds have dropped to Cat 2 strength this morning (about 110mph). There still is as much energy in that sense, weaker winds but over a MUCH larger area. This storm has a punch.

This graph from the National Hurricane Center tries to predict the maximum winds over time (with a big envelope for error). The blue line down the middle is their most confident forecast. The storm will be on a level with Philadelphia at about 48 hours (or a bit earlier). That means winds could be as low as 35mph and as high as 100mph with the most likely wind strength being about 65mph. IF it hits us head on, we will see some widespread damage and downed trees and power lines across the city.

Even if it stays on track and moves toward west central Pennsylvania, that High pressure system to our north will make a very tight "Pressure Gradient" over the northern east coast. I've drawn in red arrows showing the wind direction around the two systems - and it is STRONG and onshore. On the map, where the white lines are the tightest together, the wind is the strongest (see the center of Isabel). We (in the purple circle) still get a very strong wind over the area with high seas and beach erosion out at the ocean.

So to wrap it up, Thursday and Thursday night we get landfall of Isabel (thankfully and hopefully) to our south, and rain and winds rapidly pick up in Philadelphia.

Overnight, the bulk of the storm passes and by Friday morning, the storm is pulling out and we return to some very nice weather quite quickly.

Watch the news and listen to the radio and be ready to take a direct hit. Again, we are not QUITE out of the woods until the storm hits land.

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.