Friday, July 04, 2014

July 3: Hurricane Arthur, Update A

I think I'll launch right into the thick of things today...

Hurricane Arthur just made landfall between Beaufort, NC and Cape Lookout. Officially as a cat 2 storm with winds of 100mph.

Hurricane Arthur's official wind speed is 100mph, which makes him a weak-to-mid level category 2 hurricane (cat 2 range: 96-110mph). Central pressure is 976mb. I *almost* agree with this! I think he was this strong earlier today, but now he has started to deteriorate. It looks like he maxed out as a weak cat 2 storm a few hours ago, with winds in the 96-100 mph range, but is now a cat 1 again (cat 1 range: 73-95mph). But I'm glad that at least the NHC did finally call him a hurricane by this morning and then upped the intensity to catch up with the real world! Phew. No rant today!

You can see why I think he is deteriorating in the visible and infrared (IR) satellite images. Here is the latest visible satellite image:

You can clearly see a solid eye, which indicates winds of at least 85-90mph. But as he is interacting with land, the convection is decreasing. From yesterday's <Technical Alert!> you can more easily see what is happening with the convection from the infrared (IR) satellite images. Here's one I saved about 9 hours ago:

There was very strong convection in Arthur then - lots of thunderstorms and possible tornadoes as well as an eye. All signs that he was a robust cat 1/weak cat 2 earlier today. Now here is the latest IR image:

A marked difference! Dry air as well as his interaction with land has taken a bit of a toll. There is still some heavy convection, but it has greatly diminished in structure (not so 'solid' any more), which indicates dry air in the system.

He is definitely still a cat 1 storm, but I doubt very much that when he made landfall he was a cat 2 with 100mph winds! The NOAA station at Cape Lookout (the closest station to the eye) has so far recorded maximum sustained winds of about 77mph (sustained winds are the defining number to find in defining a storm, not gusts) as the eye-wall passed overhead, as you can see here (the eye passing over it is why the winds suddenly dropped):

(Wind speed is recorded in knots, and 1 knot = 1.15mph)

The circulation structure (vorticity) in the troposphere also shows the earlier increase in strength today and weakening now, again suggesting a stronger cat 1/weak cat 2 storm some hours ago, but now it is back to where it was yesterday evening... a cat 1 storm.

His center is at 34.7N, 76.6W and he is heading NNE at brisk 18mph. He passed to the southeast of Wilmington, NC earlier today and just made landfall north of Beaufort, NC as he continues to on his way up the US East Coast. It looks like the track shifted slightly to the west since yesterday, but definitely remained in the cone of tracks. Not much to add on the track front.

One of the impacts from this storm is, of course, the storm surge!  It had reached a maximum of about 1.5 ft above normal in Wilmington, but it is now just under normal. This is because the eye has passed and water is being pushed offshore by the winds instead of onshore. Whereas the water level at Beaufort has just peaked at about 2ft above normal (as the eye went by) and is now decreasing as the winds have turned and are pushing water offshore.

<Technical Alert!> To look at the storm surge, I use NOAA’s National Ocean Service website, Tides Online, which is an excellent site: Click on the state you are interested in, and then the location within that state. For example, here is the data from Beaufort:
The top graph shows the water level. In this graph, the red line is the actual observed sea level, the blue line is what the predicted water level would be because of the tides, and the green line is the difference between the two. The height of the green line gives you the storm surge level.  The graph below that is wind speed (which you can see was increasing) and direction. Below that is air pressure (which you can see dropping), and below that is air and sea surface temperature. <End Technical Alert!>

So far the reports I have from North Carolina (Wilmington) are that things are ok. Good luck out there! Stay safe if you are in the stormy area.

Everyone else... Happy July 4th!

Ciao for now,

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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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