EPAC Tropical Action 2011

[tmagan]

Hard to believe a tight, intense, small circulation center would only have a 940 mb central pressure (est.). I thought it was a mistake when I first read it.

[HurricaneJosh]

Hard to believe a tight, intense, small circulation center would only have a 940 mb central pressure (est.). I thought it was a mistake when I first read it.

How come? I'd say that seems about right for a deep-tropical microcane with winds of 125 kt.

[tmagan]

How come? I'd say that seems about right for a deep-tropical microcane with winds of 125 kt.

With the cold cloud tops indicating deep convection completely surrounding the eye, the difference in IR temps between they eyewall and the eye, the very small diameter of the eye, it just seems to me that the central pressure should be much lower than 940 mb.

[phil882]

Hard to believe a tight, intense, small circulation center would only have a 940 mb central pressure (est.). I thought it was a mistake when I first read it.

Small TCs with very tight pressure gradients often do have higher than typical pressures since it you don't need as low of a pressure if the radius of last closed isobar is very compact. Wind is not derived from pressure alone but the pressure gradient.

[phil882]

Don't know if anybody saw this earlier, but the TMI made a direct pass over Hilary. Not to take away from how amazing he presentation of this storm is, but it does appear the eyewall is now open to the northwest.

[tmagan]

Small TCs with very tight pressure gradients often do have higher than typical pressures since it you don't need as low of a pressure if the radius of last closed isobar is very compact. Wind is not derived from pressure alone but the pressure gradient.

Yes, but with the eye diameter being 10 nautical miles, per the NHC, I can't think of many tropical cyclones with that small of an eye, and that high of a pressure.

[HurricaneJosh]

With the cold cloud tops indicating deep convection completely surrounding the eye, the difference in IR temps between they eyewall and the eye, the very small diameter of the eye, it just seems to me that the central pressure should be much lower than 940 mb.

Yes, but with the eye diameter being 10 nautical miles, per the NHC, I can't think of many tropical cyclones with that small of an eye, and that high of a pressure.

These are actually fairly normal metrics for small, very strong (upper-end Cat-4) 'canes that are intensifying or peaking. Charley 2004 and Iris 2001 come to mind immediately. They had even higher pressures, with winds as high or higher than Hilary's (and even smaller eyes).

[HurricaneJosh]

The latest IR imagery suggests to me that the cyclone has peaked. Phil had pointed out some deterioration to the NW eyewall, and I notice that the latest IR imagery shows a "smushed" look to the NW quad. I think perhaps a little bit of shear is getting it:

[tmagan]

These are actually fairly normal metrics for small, very strong (upper-end Cat-4) 'canes that are intensifying or peaking. Charley 2004 and Iris 2001 come to mind immediately. They had even higher pressures, with winds as high or higher than Hilary's (and even smaller eyes).

Yes, Charley, had a 941 mb central pressure at landfall. Wilma had a 5 nm diameter and a 884 mb central pressure. I'm trying to remember if the eye wall cloud tops in Charley were as cold as Hilary?

[tmagan]

The latest IR imagery suggests to me that the cyclone has peaked. Phil had pointed out some deterioration to the NW eyewall, and I notice that the latest IR imagery shows a "smushed" look to the NW quad. I think perhaps a little bit of shear is getting it:

The symmetrical presentation for the last day and a half of the CDO is nearly gone with the eye not equidistant from all edges of the CDO.

[HurricaneJosh]

Yes, Charley, had a 941 mb central pressure at landfall. Wilma had a 5 nm diameter and a 884 mb central pressure. I'm trying to remember if the eye wall cloud tops in Charley were as cold as Hilary?

I can't remember exactly, but I think they were about the same-- and same with Iris. As great as Hilary has looked, it actually didn't sustain a solid ring of grey around the center for that long. And also remember Felix 2007, another microcane that was much stronger than Hilary (140 kt), with the pressure only a few mb lower.

[HurricaneJosh]

Yes, Charley, had a 941 mb central pressure at landfall. Wilma had a 5 nm diameter and a 884 mb central pressure. I'm trying to remember if the eye wall cloud tops in Charley were as cold as Hilary?

P.S. Not to nitpick, but Wilma's stats were even more crazy than that: it peaked at 882 mb, and at that time, the eye was only 2 nmi wide! It's the smallest known eye.

[HurricaneJosh]

Yep, as we suspected, it's weakened a little-- down to 946 mb/120 kt as of 2 am PDT.

Interestingly, the motion is a bit to the right of the previous forecast; it's gaining some latitude. The later end of the forecast track suggests an eventual threat to the Baja Peninsula, although the cyclone will be a shadow of its formal self by that point.

[HurricaneJosh]

Re: the models... Almost all of them suggest an eventual turn toward the S Baja Peninsula-- but, as I said above, the cyclone will be much weaker (maybe not even a 'cane or even a coherent system) by that point.

The 00Z Euro continues the trend shown in the 12Z-- bringing the cyclone waaaaaay W (to 120W) and then essentially killing it as it heads back ENE toward the coast.

[Ginx snewx]

Pretty cool evolution as the microcane absorbed the blob.

[David Reimer]

Re: the models... Almost all of them suggest an eventual turn toward the S Baja Peninsula-- but, as I said above, the cyclone will be much weaker (maybe not even a 'cane or even a coherent system) by that point.

The 00Z Euro continues the trend shown in the 12Z-- bringing the cyclone waaaaaay W (to 120W) and then essentially killing it as it heads back ENE toward the coast.

Hopefully Hilary's moisture will get pushed into Texas. We could really use the rain.

[phil882]

Much more classical presentation today. The outer bands are now encircling the small inner core leading to conditions favorable for EWRC to begin within the next 24 hours.

[hm8]

Couple of awesome loops here:

[HurricaneJosh]

Couple of awesome loops here:

Wow! Yeah-- really awesome loops. The details are incredible. Thanks for sharing these.

[thewxmann]

ERC

[phil882]

ERC

Yep this was expected given the more symmetrical presentation of the outer banding today.

[k***]

I'm surprised it took this long to get an ERC going...usually these pinholes collapse quickly and this one persisted forever.

[phil882]

I'm surprised it took this long to get an ERC going...usually these pinholes collapse quickly and this one persisted forever.

Yep it was an interesting case study, which is why it would have great to fly into this system rather than Ophelia.

My best hypothesis, for why we saw such a long duration EW was that the outer convection remained fairly asymmetric and had difficulty in wrapping all the way around the circulation. This could be partially because the system was initially close to the Mexican coast where the strong temperature gradient between the land and ocean led to multiple MCSs thanks to the copious amount of warm moist air that kept feeding into the thermal gradient between the land and ocean. That was leading towards large bursts of convection to the west of Hilary that weren't necessarily associated with the feeder bands of Hilary over the past 48 hours. Today as the system has pushed further away from the coastline we have seen the return of a more classical presentation with banding features encircling the storm. The symmetrical nature of the storm has allowed for the bands to start consolidating into a new eyewall, choking off the potential energy for the old eyewall.

[k***]

Yep it was an interesting case study, which is why it would have great to fly into this system rather than Ophelia.

My best hypothesis, for why we saw such a long duration EW was that the outer convection remained fairly asymmetric and had difficulty in wrapping all the way around the circulation. This could be partially because the system was initially close to the Mexican coast where the strong temperature gradient between the land and ocean led to multiple MCSs thanks to the copious amount of warm moist air that kept feeding into the thermal gradient between the land and ocean. That was leading towards large bursts of convection to the west of Hilary that weren't necessarily associated with the feeder bands of Hilary over the past 48 hours. Today as the system has pushed further away from the coastline we have seen the return of a more classical presentation with banding features encircling the storm. The symmetrical nature of the storm has allowed for the bands to start consolidating into a new eyewall, choking off the potential energy for the old eyewall.

That's some radical stuff...this one is definitely causing me to rethink my usual policy of rooting against these MCS/tumors. I had just always assumed that something like that would disrupt the flow of available moisture into the core of the system, throw off its balance, etc...since that's what those weird appendages seem to usually do. Your explanation makes total sense, though.

[calamity]

Hurricane Juliette (2001) seems like a fairly good analog for Hilary. Not to mention, they look the same.

[thewxmann]

Yep it was an interesting case study, which is why it would have great to fly into this system rather than Ophelia.

My best hypothesis, for why we saw such a long duration EW was that the outer convection remained fairly asymmetric and had difficulty in wrapping all the way around the circulation. This could be partially because the system was initially close to the Mexican coast where the strong temperature gradient between the land and ocean led to multiple MCSs thanks to the copious amount of warm moist air that kept feeding into the thermal gradient between the land and ocean. That was leading towards large bursts of convection to the west of Hilary that weren't necessarily associated with the feeder bands of Hilary over the past 48 hours. Today as the system has pushed further away from the coastline we have seen the return of a more classical presentation with banding features encircling the storm. The symmetrical nature of the storm has allowed for the bands to start consolidating into a new eyewall, choking off the potential energy for the old eyewall.

My hypothesis is that the local subsidence around the large "tumors" kept Hilary's core small, and therefore, no outer eyewall was able to consolidate.

[Ginx snewx]

Fantastic loops, upper one especially. Someone should name that radiation type symbol in the eye. Seen it in other canes.

[phil882]

That's some radical stuff...this one is definitely causing me to rethink my usual policy of rooting against these MCS/tumors. I had just always assumed that something like that would disrupt the flow of available moisture into the core of the system, throw off its balance, etc...since that's what those weird appendages seem to usually do. Your explanation makes total sense, though.

MCSs can definitely disrupt moisture inflow into a system. In this case though, the circulation was so small, the numerous MCS that were ongoing to the west of the circulation had little impact on the inner core circulation. Plus, I'd argue that southerly inflow from the ITCZ is far more important for convective maintenance than most other quadrants. When this ITCZ moisture advection is cutoff (through SAL/dry air/ or an MCS blocking the inflow) you normally see core convection struggle to become better organized. However, the MCSs likely did have an influence on convective development on the outer band structure of Hilda and that's what I am arguing may have prolonged its well defined small inner eyewall state.

Again I'm really disappointed the NHC decided to investigate Ophelia rather than Hilda, as I believe it would have revealed additional clues as to what was going on with the inner structure. My guess is that they feel the same was as well, but they didn't have enough time to fly all the Air Force and NOAA planes back to Biloxi, MS when Hilary blew up.

Hurricane Juliette (2001) seems like a fairly good analog for Hilary. Not to mention, they look the same.

Are you working on any loops of your own calamity?

Fantastic loops, upper one especially. Someone should name that radiation type symbol in the eye. Seen it in other canes.

They are called mesovorticies. Here is a nice paper for additional reading.

[Ginx snewx]

Phil I know what MVs are, great paper by the way, just looking to name that eye signature.

[phil882]

Phil I know what MVs are, great paper by the way, just looking to name that eye signature.

Oh well in that case, it looks like Hilary might have 4 mesovorticies as opposed to the 6 that were witnessed with Isabel. So instead of the star pattern seen with Isabel we have a triangular pattern.