Tropical Storm Isaac, Part 2: Isaac Moving Into The Gulf Of Mexico-Hurricane Warnings For Morgan City, LA/MS/AL & FL Panhandle To Destin, FL

[dan11295]

000

URNT15 KNHC 252013

AF303 1609A ISAAC HDOB 22 20120825

200500 2121N 07547W 8430 01478 9981 +195 +156 131013 014 023 010 03

200530 2121N 07545W 8429 01482 9981 +192 +156 132016 017 021 009 03

200600 2122N 07544W 8425 01483 9977 +196 +155 130018 019 026 009 00

200630 2123N 07542W 8429 01479 9981 +188 +160 123021 021 028 009 03

200700 2125N 07541W 8431 01477 9985 +178 +174 124024 025 033 010 00

200730 2126N 07540W 8428 01480 9987 +177 +166 131029 031 035 010 03

200800 2128N 07540W 8433 01477 9989 +175 +165 137033 034 039 010 00

200830 2129N 07539W 8436 01473 9991 +172 +172 138037 040 043 011 03

200900 2131N 07538W 8432 01478 9996 +170 +170 137039 041 043 012 05

200930 2132N 07537W 8424 01490 0003 +160 +160 138045 048 044 012 05

201000 2134N 07536W 8431 01485 //// +157 //// 136052 055 046 012 05

201030 2135N 07535W 8447 01466 0007 +160 +160 138049 052 /// /// 05

201100 2135N 07533W 8433 01483 //// +155 //// 138046 051 047 013 05

201130 2135N 07532W 8435 01484 0009 +160 +160 138045 048 047 014 01

201200 2134N 07531W 8432 01488 //// +152 //// 139053 056 051 015 01

201230 2134N 07529W 8439 01479 //// +152 //// 144047 052 051 015 01

201300 2134N 07528W 8421 01494 //// +150 //// 147050 052 051 014 05

201330 2134N 07526W 8431 01485 0008 +160 +160 147049 051 051 014 01

201400 2133N 07525W 8420 01502 0015 +160 +160 149047 049 053 011 05

201430 2133N 07524W 8431 01485 0013 +160 +160 142045 048 051 011 01

Should hold it at 50 kt at 5pm I think. Actually held together fairly well pressure/wind wise. Looks to be trying to emerge off the N coast now.

[Superstorm93]

I have the circulation right over Banes, Cuba

Pressure is down to 997 with a ton of 50 knot SFMR reports (some contaminated)

[gulfcane]

There is a cold OHC pool just to the west of where Isaac is expected to track, but this shouldn't be a main inhibiting factor since the storm will be moving between 10-15 mph. OHC becomes a much bigger deal when you have a large slowly moving TC which is what Isaac will do as it approaches landfall.

I know people have made the argument that near the shoreline the water is very shallow and therefore OHC products are not useful. However, I'd argue that you can still get upwelling in these regions, since instead of the water being replaced directly from underneath the TC, the water is transported from a deeper part of the Gulf of Mexico when it is overturned. Think of it this way, winds mix the water to a certain depth. If that depth is deeper than the depth of the water in that particular region, then it simply is drawn from another region. Remember that TCs are large entities, and the larger a TC the more upwelling you receive. Isaac is already a fairly large tropical cyclone, so it needs latent heat from a large portion of the Gulf of Mexico to sustain itself. Thus, while the water directly underneath the system may not be cooled all that much directly below the central core, it almost certainly will be in other regions. Mixing is not just in the vertical, but also the horizontal, so the regions with cooler water will mix with the regions of warmer water, still cooling the warm SSTs near the shoreline.

Thus, upwelling may occur as Isaac slows down for its final landfall.

Until the storm clears Cuba, I remain cautious, but I generally agree.

To tho contrary, there is a warm pool of OHC to the west, thanks to the Gulf Loop Current. Need to be careful on those maps as that "cold pool" is simply a data gap. The loop current has extremely high OHC and it is not depicted correctly on that map your referring to. And OHC is plentiful even to landfall and will not be an inhibiting factor. I'm more concerned with dry air and EWRC than oceanic factors. People seem to be spooked by the weakening of the katrinas, ritas, dennis's etc in the N GOM but that was due to EWRC and other factors, not OHC.

[Alfoman]

[PSUBlizzicane2007]

Just realised that the first TCHP map posted was from August 22.... today is August 25... gulfcane's TCHP map is more accurate.

Let's try to pay better attention to the date/time of images, guys.

[wxsmwhrms]

Hard to see much in the broad mean that is on the ECMWF site but seem like at landfall at 96 hours the mean surface center in the ensemble looks a bit left of the op run, more toward the western FL panhandle. Key thing though seems to be a fair amount of spread west of the center (again assuming those are StDev I am looking at, I really don't look at this site much).

Edit: Now up on Allan's site and it is clearer, mean position at 96 hours is over PNS/DTS while the ops is over PAM/AQQ. Trough of low pressure indicative of more members to the west extends back toward SE LA, while another trough extends east toward the op run center. One thing to note though is that any centers east of the op run would probably be over land and starting to weaken to any signal to east could be muted by that.

[Superstorm93]

Hi-Res WW3 is weaker than most of the models, but still has some very impressive wave heights approaching Florida.

Due to the fact that this system will be decently large in the Gulf, it has the potential to fully acquire all the threats a 80-100 knot cyclone can have (damaging wind, flooding rains, waves, surge, and potential tornadoes)

Surge could be the dark horse with Isaac. It's a long ways out, but its something to keep in the back of the brain.

[Srain]

Hard to see much in the broad mean that is on the ECMWF site but seem like at landfall at 96 hours the mean surface center in the ensemble looks a bit left of the op run, more toward the western FL panhandle. Key thing though seems to be a fair amount of spread west of the center (again assuming those are StDev I am looking at, I really don't look at this site much).

[PSUBlizzicane2007]

Hard to see much in the broad mean that is on the ECMWF site but seem like at landfall at 96 hours the mean surface center in the ensemble looks a bit left of the op run, more toward the western FL panhandle. Key thing though seems to be a fair amount of spread west of the center (again assuming those are StDev I am looking at, I really don't look at this site much).

That doesn't seem surprising to me... but it seems like the models will likely oscillate between the two extremes they are at right now (GFS vs. Euro).

I now feel fairly confident in calling for a landfall in the western Florida panhandle somewhere between Gulf Shores (yes I know that is Alabama technically) east through Fort Walton Beach/Destin.

[Superstorm93]

..CENTER OF ISAAC MOVING ALONG THE NORTHERN COAST OF EASTERN CUBA WITH LITTLE CHANGE IN STRENGTH...

[wxsmwhrms]

That doesn't seem surprising to me... but it seems like the models will likely oscillate between the two extremes they are at right now (GFS vs. Euro).

I now feel fairly confident in calling for a landfall in the western Florida panhandle somewhere between Gulf Shores (yes I know that is Alabama technically) east through Fort Walton Beach/Destin.

That seems very reasonable to me.

[packbacker]

NHC definitely agrees with Phil....

5pm Update

IT SHOULD BE NOTED THAT THE GLOBAL MODELS ARE

FORECASTING A VERY FAVORABLE PATTERN OF UPPER-LEVEL WINDS OVER THE

NORTHERN GULF OF MEXICO BY 72 HR. SHOULD THIS VERIFY AND SHOULD

ISAAC ESTABLISH AN INNER CORE...THE CYCLONE COULD BE SIGNIFICANTLY

STRONGER THAN CURRENTLY FORECAST.

[Drz1111]

I have a paleooceanography masters degree, which means I know a fair bit of physicial oceanography. The below is factually incorrect. You will NOT get transport from a deeper part of the Gulf.

There is a cold OHC pool just to the west of where Isaac is expected to track, but this shouldn't be a main inhibiting factor since the storm will be moving between 10-15 mph. OHC becomes a much bigger deal when you have a large slowly moving TC which is what Isaac will do as it approaches landfall.

I know people have made the argument that near the shoreline the water is very shallow and therefore OHC products are not useful. However, I'd argue that you can still get upwelling in these regions, since instead of the water being replaced directly from underneath the TC, the water is transported from a deeper part of the Gulf of Mexico when it is overturned. Think of it this way, winds mix the water to a certain depth. If that depth is deeper than the depth of the water in that particular region, then it simply is drawn from another region. Remember that TCs are large entities, and the larger a TC the more upwelling you receive. Isaac is already a fairly large tropical cyclone, so it needs a surface heat flux from a large portion of the Gulf of Mexico to sustain itself. Thus, while the water directly underneath the system may not be cooled all that much directly below the central core, it almost certainly will be in other regions. Mixing is not just in the vertical, but also the horizontal, so the regions with cooler water will mix with the regions of warmer water, still cooling the warm SSTs near the shoreline.

Thus, upwelling may occur as Isaac slows down for its final landfall.

Until the storm clears Cuba, I remain cautious, but I generally agree.

[dan88]

850mb center based on recon is roughly 21.5N, 76.3W

[dan88]

Got a fix, 998mb drop was with 11kt at the surface.

000

URNT12 KNHC 252125

VORTEX DATA MESSAGE AL092012

A. 25/21:14:30Z

B. 21 deg 30 min N

076 deg 18 min W

C. 850 mb 1403 m

D. 59 kt

E. 108 deg 90 nm

F. 139 deg 56 kt

G. 085 deg 44 nm

H. 998 mb

I. 17 C / 1528 m

J. 19 C / 1522 m

K. 15 C / NA

L. NA

M. NA

N. 1345 / 8

O. 0.05 / 7 nm

P. AF303 1609A ISAAC OB 07

MAX FL WIND 56 KT E QUAD 20:12:00Z

[Superstorm93]

Vortex message has it off of the coast.

Visible presentation looks pretty decent and we could see some reformation soon.

EDIT:

Wow...there's your center

[phil882]

I have a paleooceanography masters degree, which means I know a fair bit of physicial oceanography. The below is factually incorrect. You will NOT get transport from a deeper part of the Gulf.

Ok, and what about horizontal transportation near the surface or just below the subsurface. My argument is not upwelling from the bottom of the Gulf of Mexico. My argument is that mixing takes place in regions both near the coastline, and well away from the coastline, in a region that has a deeper column of water. If an area away from the shallow gulf coast upwells, this colder water is transported horizontally and vertically to the where the warmer water is located near the coastline and mixing takes place there.

Read a little bit more on Ekman_transport

http://en.wikipedia....Ekman_transport

Ekman transport is a factor in coastal upwelling regimes which provide the nutrient supply for some of the largest fishing markets on the planet.^[10] Wind in these regimes blows parallel to the coast (such as along the coast of Peru, where the wind blows North). From Ekman transport, surface water has a net movement of 90 degrees to the left in such a location. Because the surface water flows away from the coast, the water must be replaced with water from below.^[11] It is important to remember that in shallow coastal waters, the Ekman spiral is normally not fully formed and the wind events that cause upwelling episodes are typically rather short. This leads to many variations in the extent of upwelling, but the ideas are still generally applicable.^[12]

Using this argument, you easily see both vertical and horizontal transports of cooler water. Its one of the fundamental process that leads to La-Nina and the cold pool near Peru. While the Ekman spiral is not fully formed in shallow waters and may be disrupted in unpredictable ways when you start observing large waves and white caps, the general idea that there is a transport of water at some depth horizontally is not an invalid statement.

(1) is the surface wind vector, (2) is the force from the flow above (3) is the direction of the ocean current (4) is the Coriolis vector.

[Superstorm93]

18z GFS is even further west than the 12z.

Looks like a LA/MS landfall.

----

Time for Isaac to rebuild.

convection is already forming around the LLC

[icebreaker5221]

Ok, and what about horizontal transportation near the surface or just below the subsurface. My argument is not upwelling from the bottom of the Gulf of Mexico. My argument is that mixing takes place in regions both near the coastline, and well away from the coastline, in a region that has a deeper column of water. If an area away from the shallow gulf coast upwells, this colder water is transported horizontally to the where the warmer water is located near the coastline and mixing takes place there.

Read a little bit more on Ekman_transport

http://en.wikipedia....Ekman_transport

Using this argument, you easily see both vertical and horizontal transports of cooler water. Its one of the fundamental process that leads to La-Nina and the cold pool near Peru.

Sorry Phil, I kinda disagree. Horizontal mass transport at the bottom of the Ekman layer should still be along isopycnals. So if you have upwelling in a warm column down to only 200 m or so, and there is a need to pull in more water from the central Gulf to maintain mass continuity, you're still predominantly just advecting (relatively warm) 200 m water from the central-Gulf.

Also, I believe horizontal Ekman transport is usually only up to ~1 m/s even under strong winds, so it would probably only be relevant to a very slow moving storm. You're correct that it occurs during the development of La Nina, but that's over much longer time scales.

[Nikolai]

Isaac stalls just to the east of New Orleans on the 18z GFS--wouldn't such a scenario be a nightmare for the city? It literally moves under 100 miles in 12 hours, with winds pushing the surge into Lake Pontchartrain the entire time... such a scenario could be worse than Katrina, no?

Regardless of specifics, 18z GFS looks like a catastrophe.

[PSUBlizzicane2007]

18Z GFS has strengthening Isaac headed towards MS gulf coast, while it juts westward and makes landfall near the Rigolets/Slidell/Chalmette, LA.

That's different. I'm glad we'll have even more plane data in at 00Z.

[Alfoman]

18Z GFS has landfall in LA with a minimum pressure of 959 mb with winds at 105 KT.

http://www.weatherbellmodels.com/weather/gfs/2012082518/tropical/gfs_mslp_uv900gulf_tropical_mouse.php

[dan88]

Another fix as recon went back in fairly quickly. Drop was with single digit wind values from flight level to surface, with 9kt at the surface

000

URNT12 KNHC 252203

VORTEX DATA MESSAGE AL092012

A. 25/21:45:50Z

B. 21 deg 34 min N

076 deg 19 min W

C. 850 mb 1408 m

D. 44 kt

E. 291 deg 21 nm

F. 082 deg 26 kt

G. 296 deg 34 nm

H. 999 mb

I. 17 C / 1526 m

J. 19 C / 1523 m

K. 16 C / NA

L. OPEN N-W

M. C25

N. 12345 / 8

O. 0.02 / 2 nm

P. AF303 1609A ISAAC OB 10

MAX FL WIND 56 KT E QUAD 20:12:00Z

[phil882]

Sorry Phil, I kinda disagree. Horizontal mass transport at the bottom of the Ekman layer should still be along isopycnals. So if you have upwelling in a warm column down to only 200 m or so, and there is a need to pull in more water from the central Gulf to maintain mass continuity, you're still predominantly just advecting (relatively warm) 200 m water from the central-Gulf.

Also, I believe horizontal Ekman transport is usually only up to ~1 m/s even under strong winds, so it would probably only be relevant to a very slow moving storm. You're correct that it occurs during the development of La Nina, but that's over much longer time scales.

Good arguments Will. However, if we have a large tropical cyclone, then a substancial part of the basin is being upwelled for a substancial period of time. Assuming we have a 300-400 mile radius of tropical storm force winds, there will be places that are upwelled quite a far away from the US coastline for possibly a 48 hour period or longer. The depth of the 26 degree isotherm is above 200 m across the majority of the Gulf (except for a few warm eddies) and is generally in the 75-100 m range. If some portion of the gulf is upwelled in this region at that depth, it could easily be transported to the coastline where it would then mix with the warmer waters along the coast.

[Superstorm93]

L. Eye Character: Open from the north to the west

M. Eye Shape & Diameter: Circular with a diameter of 25 nautical miles (29 statute miles)

Another fix as recon went back in fairly quickly. Drop was with single digit wind values from flight level to surface, with 9kt at the surface

It appears they are doing rapid fixes to try and get a heading.

[HKY_WX]

I wouldn't be surprised to see the center jump a little north tonight as the convection trys to wrap around the south side of the COC as it moves off of Cuba. Some of the models hinted at this yesterday.

[Eyewall2005]

Good arguments Will. However, if we have a large tropical cyclone, then a substancial part of the basin is being upwelled for a substancial period of time. Assuming we have a 300-400 mile radius of tropical storm force winds, there will be places that are upwelled quite a far away from the US coastline for possibly a 48 hour period or longer. The depth of the 26 degree isotherm is above 200 m across the majority of the Gulf (except for a few warm eddies) and is generally in the 75-100 m range. If some portion of the gulf is upwelled in this region at that depth, it could easily be transported to the coastline where it would then mix with the warmer waters along the coast.

Phil can you comment on south florida impacts which is first in line.

[Superstorm93]

18z GFS

[CoastalWx]

Sorry Phil, I kinda disagree. Horizontal mass transport at the bottom of the Ekman layer should still be along isopycnals. So if you have upwelling in a warm column down to only 200 m or so, and there is a need to pull in more water from the central Gulf to maintain mass continuity, you're still predominantly just advecting (relatively warm) 200 m water from the central-Gulf.

Also, I believe horizontal Ekman transport is usually only up to ~1 m/s even under strong winds, so it would probably only be relevant to a very slow moving storm. You're correct that it occurs during the development of La Nina, but that's over much longer time scales.

These are good points. That ekman example is really noticeable in areas with a shallow warm layer. The Peru example makes sense when you think about this.

[TUweathermanDD]

My map showing a little model info and some population figures for reference.