8/27/04 FCST: Frances (Atlantic)

[David Wolfson]

To my eyes the late vis and IR shots indicate that whatever caused Frances to cough seems to be lessening and the gulf stream is coming into play. The outflow is again becoming more symmetric and expanding and rather impressive convection bands are starting to wrap around the west side. Next stop -- Abaco Islands.

... and in the 15:25Z shot just in, you can see some overshooting convection in the north inner eyewall. Haven't seen that for awhile.

 

[Anthony Petito]

To my eyes the late vis and IR shots indicate that whatever caused Frances to cough seems to be lessening and the gulf stream is coming into play. The outflow is again becoming more symmetric and expanding and rather impressive convection bands are starting to wrap around the west side. Next stop -- Abaco Islands.

... and in the 15:25Z shot just in, you can see some overshooting convection in the north inner eyewall. Haven't seen that for awhile.

Is anyone quite sure what caused Frances to cough like it did? I can't find anything that would cause it to weaken as it did. Although I am seeing the same thing as you David. Unfortunately, it looks like it might get a last wind before making landfall in Florida.

 

[David Wolfson]

Well, Andy, after it was happening, the experts were explaining it as an injection of dry mid- and upper-level air.

 

[B Ozanne]

At the time I had no idea what was happening. Dry air and a little sheer probably came just as an eyewall replacement cycle was taking place and everything sort of fell apart.

 

[Jeff Snyder]

I think it was mainly shear. If you watched the western edge of the outflow through the day, you could see the it wasn't really advancing much. Additionally, you could see that there was little or no anticyclone curvature on that side. Meanwhile, the eastern side had incredible outflow (probably 1000 miles north-to-south)...

 

[Glen Romine]

Well, Andy, after it was happening, the experts were explaining it as an injection of dry mid- and upper-level air.

Which experts? I didn't see any mention of dry air entrainment in the NHC forecast disussions. Where did you see it, because I'm curious. If you look at the soundings in FL from this morning, you'll see some WSW to SW flow up around 200 - 150 mb. If you look at the shear and upper level divergence, you'll note the current shear near Frances is ~ 20 knots, not horrible but more than ideal. Also, note the upper level anti-cyclone over Cuba, which has a local minima in shear, and is now much further from Frances than it was yesterday.

Current:
http://cimss.ssec.wisc.edu/tropic/real-tim...inds/wg8shr.GIF

24 hours ago:
http://cimss.ssec.wisc.edu/tropic/real-tim...ve/wg8shr-8.GIF

This change in environment was apparently not well forecasted - can't tell you why, sorry. maybe if I have time later I'll look into it.

Glen

 

[David Wolfson]

Glen, that was the consensus on TWC before I went to bed last night.

 

[Glen Romine]

Glen, that was the consensus on TWC before I went to bed last night.

Ok thanks, that explains a lot. Guess I'd have referred to them as "experts" instead of experts.... looking at the water vapor imagery - I don't see any indications of severe dry air entrainment- though it's hard to know otherwise as there isn't a sounding location upwind of the hurricane. Maybe there is some dropsonde data out there somewhere that I don't know whre to find.

As for the flareup you mentioned earlier - I see it - but it will need to wrap all the way around the hurricane to do much for intensification. Latest recon still showing no change in intensity. I guess time will tell.

Glen

 

[Mike Hollingshead]

I checked out the spc meso page last night after someone mentioned it was getting sheared some and did not get that impression at all. 500-300mb flow was very weak and flowing WEST ahead of the cane. The dry air on yesterday's water vapor loop was however VERY evident and to me looked to cleary be the problem. It made it look sheared as it encountered that small area of drying showing up on wv most all of yesterday. The twc cane expert(Lions?) explained and showed it very well over and over yesterday.

 

[Mike Peregrine]

I also pulled up a hodo last night from Miami just out of curiosity ... wind was almost completely unidirectional ahead of the storm at the time with extremely weak flow aloft.

 

[Jeff Snyder]

I also didn't see much dry air entrainment... Yes, there was dry out west of the cane, but that's likely self-induced! Most intense hurricanes induce strong subsidence (and thus drying) on the periphery of the hurricane. Given most Cat 4s, there's almost always going to be dry air near it, simply because it is Cat 4... Now, when you see entrainment of this year, it'll often look like a stream of dry air getting wrapped around and spiraling into the eye. I have not seen this in the past couple of days. The eye/center has almost always been surrounded by deep convection, and there hasn't been any "spiralling of dry air" towards the center that I could see on water-vapor loops... I'll try to find a good example of this and will post it on here if I can find one.

As for the weak 300-500mb winds Mike mentioned... He is correct.... HOWEVER, the shear was/is likely occurring ABOVE that, in the 100-200mb layer that Glen mentioned. The tropopause above many strong hurricanes in the tropics is anywhere from 75-150mb, considerably higher than the height of the tropopause over the continental US. Therefore, there is plenty of space for shearing above the 300mb case, which indeed likely was the case. Those higher plots (100-250mb) won't be on the SPC since that level trypically isn't involved in continental convection for which the SPC forecasts.

 

[Glen Romine]

I checked out the spc meso page last night after someone mentioned it was getting sheared some and did not get that impression at all. 500-300mb flow was very weak and flowing WEST ahead of the cane. The dry air on yesterday's water vapor loop was however VERY evident and to me looked to cleary be the problem. It made it look sheared as it encountered that small area of drying showing up on wv most all of yesterday. The twc cane expert(Lions?) explained and showed it very well over and over yesterday.

As I mentioned above, the wind shear is noticed comparing near surface winds with those well above 300 mb - the tropopause in the tropics is much higher up - as is the subtropical jet. Look up around 150 mb.

Thanks for the info on the dry entrainment. Can't argue against Lyons, he is a bright guy, but I'll have to get some archive WV imagery to see if I can convince myself. Regardless, dry air can only be entrained if there is shear to bring it into the hurricane core region.

Glen

 

[Glen Romine]

Dry entrainment looks like this on WV imagery:

http://www.atmos.uiuc.edu/~romine/opal/opva0917.gif

Here is a concept model from Opal, a classic entrainment example:

http://pampa.ncsa.uiuc.edu/~romine/hvr/rel...h_top_smpl2.jpg

The blue mess is the dry air, see how streams of it feeds toward the center.

Glen

 

[Mike Peregrine]

Interesting links -

Here are the current obs out of MFL - there is definitely shearing present above 300mb now:

http://www.rap.ucar.edu/weather/upper/mfl.gif

 

[Mike Hollingshead]

I wonder what the longest a cane has been able to sustain cat 4 or higher for. I would think it could only build up that strong sinking ahead of it for so long or so much before that effect begins to have a strong negative effect on the cane itself...which however it's all explained certainly seems to have happened. It really did appear to have entrained enough to 'knock it off balance' just enough that it could hardly come back from, and possibly allowing it to entrain even more. What ying did what to what yang?