ATL: ISAAC - Post-Tropical - Discussion

[wxman57]

4hr movement the same - 24.4kts. Latest satellite indicates the LLC is opening up and falling apart. It just can't handle the westward acceleration, as couldn't Seven. Development chances east of the Caribbean are looking less likely. Less than 50%, maybe.

[tolakram]

Latest Visible

[Steve H.]

Yeah I have to agree. It appears the LLC is very weak and may be collapsing. May degrade into an open wave. Convection just can't keep up.

[RL3AO]

Clouds on the NW side of the LLC are accelerating NW instead of towards the center. Thats not good.

[San Felipe II]

Clouds on the NW side of the LLC are accelerating NW instead of towards the center. Thats not good.

Good Morning!!!

Why is not good??

[RL3AO]

Clouds on the NW side of the LLC are accelerating NW instead of towards the center. Thats not good.

Good Morning!!!

Why is not good??

If the system was intensifying the clouds NW of the center should be blowing towards the center not away.

EDIT: By "not good" I meant not good for the systems development.

[otowntiger]

4hr movement the same - 24.4kts. Latest satellite indicates the LLC is opening up and falling apart. It just can't handle the westward acceleration, as couldn't Seven. Development chances east of the Caribbean are looking less likely. Less than 50%, maybe.

Very cool! Hope it only brings some refreshing rain and nothing more to the islands and anyone else for that matter! Good news wxman57!

[otowntiger]

Clouds on the NW side of the LLC are accelerating NW instead of towards the center. Thats not good.

Good Morning!!!

Why is not good??

If the system was intensifying the clouds NW of the center should be blowing towards the center not away.

EDIT: By "not good" I meant not good for the systems development.

So really its good!

[canes04]

It's a large developing system. Give it some time people.

[HurrMark]

4hr movement the same - 24.4kts. Latest satellite indicates the LLC is opening up and falling apart. It just can't handle the westward acceleration, as couldn't Seven. Development chances east of the Caribbean are looking less likely. Less than 50%, maybe.

I think this could be classified as a TD, but it could definitely meet the same fate as TD Seven. Again, the GFS seems to know what's going on.

[Gustywind]

Posted by: Dr. Jeff Masters, 14:43 GMT le 20 août 2012 +4

http://www.wunderground.com/blog/JeffMa ... rynum=2194


A large tropical wave (Invest 94L) located about 1100 miles east of the Lesser Antilles Islands is headed west at 20 - 25 mph, and is showing increasing organization today. The storm is under light wind shear of 5 - 10 knots, and is over waters of 27°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis and water vapor satellite loops. This dry air is interfering with development, and this morning's visible satellite loop shows that 94L's heavy thunderstorm activity is sparse. However, the satellite loops do show that 94L has now separated from the clumps of heavy thunderstorms to its south, and a pretty well-defined surface circulation has developed. Heavy thunderstorms are now attempting to fire up around this circulation center, but are being hampered by dry air. The center of 94L was about 80 miles to the north of buoy 41041 at 10 am Monday morning, and the buoy recorded SW winds of 10 mph, confirming that 94L probably does have a closed surface circulation. The disturbance will have to build and maintain more heavy thunderstorms than it has now to be considered a tropical depression, though. The first hurricane hunter mission into 94L is scheduled for Tuesday afternoon.


Figure 1. Morning satellite image of Invest 94L.

Forecast for 94L
The latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, for the next five days. Ocean temperatures will warm from 27°C this morning to 28.5°C by Wednesday morning, and the total heat content of the ocean will increase sharply during that period, as well. The main impediment to development will be dry air to the north, and the SHIPS model predicts the amount of dry air will change little over the next five days. I expect that 94L will continue to struggle with dry air through Wednesday, when it will probably have had enough time to moisten the surrounding atmosphere and protect itself against the dry air. The models have shown increasing unity in taking 94L through the Lesser Antilles on Wednesday, and I expect the storm will be a tropical depression or weak tropical storm with 40 - 50 mph winds at that time. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 80% chance of developing into a tropical depression by Wednesday morning. None of the reliable models predict that 94L will reach hurricane strength over the next five days, and it is unlikely that 94L will be able to organize quickly enough to become anything stronger than a 60 mph tropical storm before reaching the Lesser Antilles, given the storm's current struggles with dry air, and the lack of model support for intensification. However, once 94L enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to occur. The main factor that might prevent intensification into a hurricane late this week would be a close pass by the island of Hispaniola. Our top models for long-range 4 - 5 days forecasts all show a path for 94L very close to the island.

Will 94L hit the U.S.?
This storm is a long-range threat to the U.S., as historically, 16% of storms in 94L's location have gone on to hit the U.S., with North Carolina the preferred target (10% chance.) A trough of low pressure capable of pulling 94L to the north enters Western Canada Thursday night, and the exact timing and amplitude of this trough will determine the ultimate landfall location of 94L. The long range 7 - 14 day runs of the GFS model over the past three day have all predicted an eventual landfall for 94L in the U.S., though these long-range runs are notoriously unreliable. The predicted landfall locations have ranged from New England to Texas--which isn't much help. The past three runs beginning on Sunday afternoon have all taken 94L over Florida during the August 27 - 29 time frame, which I'm sure is making organizers of the Republican National Convention uncomfortable, since the convention is in Tampa August 27 - 30. However, 94L could miss Florida completely, as the average error in model forecasts going out 7 days is in excess of 500 miles. We can't rule out a North Carolina landfall, but the pattern we've seen so far this year is for landfalls in Mexico's Yucatan Peninsula, so a more southwards path for 94L into the Yucatan is definitely a possibility. Also, we have that huge drought region in the Midwest, which tends to create its own high pressure bubble, which reduces the odds of storms making the turn and hitting the Central or Western Gulf Coast. If 94L makes it to the Western Caribbean, I see the two most likely options as a landfall in Mexico's Yucatan Peninsula (and then westwards into Mexico south of the Texas border), or recurvature into the Florida Gulf Coast.

[rainstorm]

another model monster falls prey to the bad conditions of 2012.

[cpdaman]

Wxman57

is there a time frame when u could see this system slowing down closer to 20 mph?

also rainstorm this thing has not fallen apart

[SoupBone]

Note that I just measured a 3-hr speed of that LLC at 24.4kts. From 15.6N/44.16W at 1015Z to 15.7N/45.4W at 1315Z. That's 73.3NM in 3 hrs, or 24.4kts.

No question is moving quite fast now. That's why it's out-running the convection. Seven sped up to over 30 kts in 3-6hr movements as it approached the Lesser Antilles. That's what destroyed the LLC.

Is the center really that far north? I thought I saw it hovering around 14.6N last night but people were discussing two areas where the center could potentially be located.

[ConvergenceZone]

Hmm, no wonder why most of the atlantic systems are really struggling this year. The wind patterns this year are just moving too darn fast.....I wonder what causes the difference from year to year?.

[cpdaman]

1445Z makes it looks like a jaunt Nw @ near 15.85/.9 or so and 45.7

the low level banding on visible would lead me to think the llc is still quite formidable for the time being, but i'm just making an amateur observation

[wxman57]

RAOBS (upper air soundings) across the Lesser Antilles indicate 25-30kt easterly flow just above the surface and very dry air in the mid levels. This indicates that 94L will continue to struggle with low-level speed shear (not upper-level shear) along with dry air. May be a less than 50% chance it will become a TS east of the islands. Will likely cause a few heavy squalls with wind gusts to 50-60 mph as it is now (assuming a few more squalls develop) when it passes the islands on Wednesday. Center should reach the islands Wednesday morning.

[Blown Away]

IMO, hasn't really changed much over the past few hours, except for slightly more convection scattered a little closer to the LLC. I see the LLC is still spinning, still a little naked. JHMO
http://www.ssd.noaa.gov/goes/east/tatl/loop-vis.html

[BatzVI]

So, based on what everyone is saying so far today, the shutters most likely will not have to go up. Not to worry, I'll still be watching closely. Been through enough of these things to know that they can change quickly.

[cycloneye]

So, based on what everyone is saying so far today, the shutters most likely will not have to go up. Not to worry, I'll still be watching closely. Been through enough of these things to know that they can change quickly.

Well said. We know the drill in the NE Caribbean.