Isabel End Game Beginning

[donsutherland1]

With Hurricane Isabel roughly two days away from landfall, we have now moved from the middle game to the end game as far as Isabel’s evolution is concerned. Therefore, it is appropriate to begin to focus ever more closely on details pertaining to this storm.

As discussed previously, the most likely area for landfall stretches from Morehead City to Cape Hatteras. Somewhat less probable is the Virginia coast and also Morehead City south to Charleston. Far less likely, though perhaps still in the distant realm of possibility is the Maryland or Delaware shore.

In addition, based on a combination of the consistent model guidance and hurricane analogs, it is likely that Isabel will pass to the west of Washington, D.C., Baltimore, and Philadelphia and well to the west of New York City and Boston. Yesterday, I believe she would pass somewhat to the east of D.C., though her track through today, the model data, and consideration of analogs suggests otherwise. The hint that the north Atlantic high is nearing the start of its westward expansion as seen on water vapor loops also argues against this initial thought. Hence, I’ve made an adjustment as I felt appropriate.

Based on the latest model guidance, it still appears reasonably likely that Isabel will have a central pressure of between 950 MB and 955 MB upon landfall. This would imply winds of around 120 mph to 125 mph. Also, the hurricane analogs suggested winds of 110 mph to 120 mph. Therefore, conservatively, the 110 mph to 120 mph range looks good.

This means that Isabel will need to strengthen from its current 105 mph status. Several factors argue for such intensification:

• Isabel had struggled to escape an area of rising wind shear that stuck with it longer than anticipated. It appears to have broken free of the wind shear earlier today (see animation of for 6, 12, and 18 hours ago).

<img src="http://www5.wright-weather.com/bb/attachment.php?s=&postid=109899">

It appears until close to landfall she will likely be passing through a corridor of low wind shear. By the time she encounters moderate wind shear, she should be on the verge of landfall, hence any weakening prior to landfall should be small.

• SSTs are sufficiently warm for intensification until just before the coast. Hence, as Isabel will be starting to accelerate prior to landfall, I do not believe that the narrow wedge of cooler waters will hurt her in the end. In addition, she has moved into an area with good heat potential for further intensification.

• Occassionally, as hurricanes approach landfall their eye tightens. Given her angle and possible location of landfall, it is difficult to argue one way or another as to whether her eye will tighten and further strengthen her winds.

In fact, it appears that Isabel is already regaining strength. The latest Recon report states:

URNT12 KNHC 162347
VORTEX DATA MESSAGE
A. 16/2347Z
B. 28 DEG 08 MIN N
71 DEG 28 MIN W
C. 700 MB 2696 M
D. NA
E. NA
F. 144 DEG 100 KT
G. 042 DEG 053 NM
H. 957 MB
I. 9 C/ 3106 M
J. 16 C/ 3106 M
K. 13 C/ NA
L. CLOSED WALL
M. E09/25/18
N. 12345/7
O. 0.1/1 NM
P. AF963 1713A ISABEL OB 31
MAX FL WIND 109 KT E QUAD 2303Z. MAX TEMP 18C AND LOWEST EXTRAP
SLP 950MB 220/11NM FROM FL CENTER.

This report indicates that Isabel’s weakening has ended and, in fact, that she may be strengthening for three reasons:

• Her eye wall is now closed.

• Her central pressure has fallen to 957 MB from 959 MB earlier in the day. This central pressure would typically translate into maximum sustained winds of 110 mph to 115 mph. The 5 p.m. report had estimated maximum sustained winds at 105 mph.

• The maximum 700 MB wind found was 109 knots. Given the general rule for extrapolating surface winds, this would translate into a 95 knot to 100 knot surface wind or, roughly 110 mph to 115 mph surface winds.

Consequently, I believe the 11 p.m. TPC report will note an increase in Isabel’s winds and a recovery that is now ongoing.

Given the estimated winds and track of Isabel, Washington, D.C. and Baltimore will likely feel tropical storm-force sustained winds with some gusts to or above hurricane force likely. Philadelphia will see gusts to near hurricane force. New York City will likely see gusts around 50 mph. Boston’s winds will probably be less than those in NYC, probably on the order of 20 mph to 40 mph.

Finally, Isabel has continued her turn more to the north:

• 24 hours ended 5 p.m., September 15: 296 degrees
• 24 hours ended 5 p.m., September 16: 331 degrees

For the next 24 hours, I anticipate that she will continue to track north-northwest. However, as landfall approaches, she should begin to shift somewhat more west of north than will be the case over the next 24 hours. Thus, she will be tracking between north-northwest and northwest at the time of landfall while accelerating her forward motion.

[Josephine96]

I'm looking at my computer program that I track hurricanes on.. and it looks like the Isabel icon is pointed right at North Carolina...

Scary thought... Hopefully everyone will stay safe...

[debbiet]

Based on all of that, do you think that she will track as far inland as current models are showing or a little closer to the coast as models yesterday suggested?

[wxman57]

You forgot to mention one thing. Take a look at the latest SFC analysis along the east coast:

http://adds.aviationweather.noaa.gov/da ... fc_bwi.gif

Now look at a water vapor loop. See that giant mass of very dry air that has already moved off the east U.S. coast? Look across the NE Gulf of Mexico. See the short wave rotating eastward (big southward dip in the dry air)? That's heading into Isabel as well. Generally, an approaching short wave like that can help enhance outflow (it's happening now). However, what about the effects of dry air entrainment? I've seen hurricanes lose most of their convection in under 6 hours due to rapid dry air entrainment. Gordon in 2000 across the eastern Gulf was a good example. From hurricane to nothing in 4-6 hrs. If that dry air gets drawn into Isabel's center, it won't matter how favorable upper winds, SSTs, or anything else is. She'll weaken very quickly. That's the only hope residents of NC have now.

[debbiet]

So...is that the most likely thing that will happen? She gets a bunch of dry air and weakens dramatically or totally falls apart? Just a layperson trying to get a handle on it all

[wxman57]

So...is that the most likely thing that will happen? She gets a bunch of dry air and weakens dramatically or totally falls apart? Just a layperson trying to get a handle on it all

I wouldn't say it is the most likely thing that will happen - yet. It's just a good possibility at this point. If the dry air doesn't move in, then Isabel could well have 115-125 mph winds at landfall.

[donsutherland1]

Good points.

At this point, I'm not sure to what extent the dry air will interact with Isabel. I am fairly confident that her ability to intensify will be limited so as to preclude explosive intensification to, let's say, Category 4 or above status.

[Rainband]

You forgot to mention one thing. Take a look at the latest SFC analysis along the east coast:

http://adds.aviationweather.noaa.gov/da ... fc_bwi.gif

Now look at a water vapor loop. See that giant mass of very dry air that has already moved off the east U.S. coast? Look across the NE Gulf of Mexico. See the short wave rotating eastward (big southward dip in the dry air)? That's heading into Isabel as well. Generally, an approaching short wave like that can help enhance outflow (it's happening now). However, what about the effects of dry air entrainment? I've seen hurricanes lose most of their convection in under 6 hours due to rapid dry air entrainment. Gordon in 2000 across the eastern Gulf was a good example. From hurricane to nothing in 4-6 hrs. If that dry air gets drawn into Isabel's center, it won't matter how favorable upper winds, SSTs, or anything else is. She'll weaken very quickly. That's the only hope residents of NC have now.

I remember Wxman57 Gordon crossed the Ft Myers area as a cane and by the time it got here..nothing

[donsutherland1]

Just a quick note:

TPC has reported that Isabel's maximum sustained winds are now up to 110 mph in its 11 pm report.