ATL: ISAAC - Post-Tropical - Discussion

[CrazyC83]

They need to send Recon out right at 50W if it develops since they would need watches by then for the Windward Islands - that would be within 48 hours from Barbados

[SouthDadeFish]

ASCAT caught the eastern side of the circulation a few hours ago. I see a few very weak west winds but they may be rain contaminated:

[cycloneye]

Here is this morning's Dr Jeff Masters discussion of 94L.

Posted by: Dr. Jeff Masters, 11:24 AM AST on August 18, 2012

A large tropical wave that emerged from the coast of Africa Thursday night (Invest 94L) is located a few hundred miles southwest of the Cape Verde Islands off the coast of Africa, and is headed west at 15 - 20 mph. This storm is a threat to develop into a tropical storm that will affect the Lesser Antilles Islands as early as Wednesday. The storm is under moderate wind shear of 10 - 20 knots, and is over waters of 27.5°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis. This morning's 8:15 am EDT ASCAT pass caught the east side of 94L, and showed a partial surface circulation. Satellite images show just a modest amount of heavy thunderstorm activity, and I expect the earliest that 94L could develop into a tropical depression would be Sunday.

Forecast for 94L

The latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, and ocean temperatures will gradually warm from 27.5°C to 28.5°C over the next four days, as 94L tracks westwards towards the Lesser Antilles. As is typical with storms making the crossing from Africa to the Antilles, dry air to the north will likely interfere with development. However, with shear expected to be low, dry air may be less of an issue for 94L than it was for Ernesto or TD 7. The storm should maintain a nearly due west track through Monday night, to a point near 50°W, about 700 miles east of the Lesser Antilles. At that point, a trough of low pressure passing to the north of 94L may be able to pull the storm more to the northwest, as suggested by the latest 06Z (2 am EDT) run of the NOGAPS model, and by three members of the GFS model ensemble forecast (Figure 2.) However, the models have been trending more towards a solution where this trough is not strong enough to influence 94L's path. This scenario will be more likely if 94L takes its time to develop, since a weaker storm will be smaller and shallower, and less likely to respond to the trough passing to the north. Our two best performing models, the GFS and ECMFW, both take 94L through the Lesser Antilles. The ECMWF, which predicts that 94L will stay weak and not develop, is faster, bringing the storm through the Lesser Antilles on Wednesday. The GFS model is slower, bringing 94L to the Lesser Antilles on Thursday as a strong tropical storm. The models have shown poor run-to-run consistency in both the timing and the track of 94L, so it is difficult to assess which land areas might be most at risk, and when. A database of historical probabilities of storms in the same location as 94L maintained by Dr. Bob Hart of Florida State University reveals that historically, 45% of storms in this location have eventually hit land, with Canada (13% chance) and North Carolina (15% chance) the most likely targets. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 40% chance of developing into a tropical depression by Monday morning.

[jinftl]

Great analysis as always by Dr. Masters...this system, like all to some degree, will have obstacles to overcome to develop, but it seems like that is more likely than we saw with Ernesto and Td 7. The longer the storm takes to develop, as Dr. M points out, the more likely he will be not to be turned out to sea by any troughs that approach.

Here is this morning's Dr Jeff Masters discussion of 94L.

Posted by: Dr. Jeff Masters, 11:24 AM AST on August 18, 2012

A large tropical wave that emerged from the coast of Africa Thursday night (Invest 94L) is located a few hundred miles southwest of the Cape Verde Islands off the coast of Africa, and is headed west at 15 - 20 mph. This storm is a threat to develop into a tropical storm that will affect the Lesser Antilles Islands as early as Wednesday. The storm is under moderate wind shear of 10 - 20 knots, and is over waters of 27.5°C. A large area of dry air lies just to the north of 94L, as seen on the latest Saharan Air Layer (SAL) analysis. This morning's 8:15 am EDT ASCAT pass caught the east side of 94L, and showed a partial surface circulation. Satellite images show just a modest amount of heavy thunderstorm activity, and I expect the earliest that 94L could develop into a tropical depression would be Sunday.

Forecast for 94L

The latest 8 am EDT run of the SHIPS model predicts that wind shear will be low, 5 - 10 knots, and ocean temperatures will gradually warm from 27.5°C to 28.5°C over the next four days, as 94L tracks westwards towards the Lesser Antilles. As is typical with storms making the crossing from Africa to the Antilles, dry air to the north will likely interfere with development. However, with shear expected to be low, dry air may be less of an issue for 94L than it was for Ernesto or TD 7. The storm should maintain a nearly due west track through Monday night, to a point near 50°W, about 700 miles east of the Lesser Antilles. At that point, a trough of low pressure passing to the north of 94L may be able to pull the storm more to the northwest, as suggested by the latest 06Z (2 am EDT) run of the NOGAPS model, and by three members of the GFS model ensemble forecast (Figure 2.) However, the models have been trending more towards a solution where this trough is not strong enough to influence 94L's path. This scenario will be more likely if 94L takes its time to develop, since a weaker storm will be smaller and shallower, and less likely to respond to the trough passing to the north. Our two best performing models, the GFS and ECMFW, both take 94L through the Lesser Antilles. The ECMWF, which predicts that 94L will stay weak and not develop, is faster, bringing the storm through the Lesser Antilles on Wednesday. The GFS model is slower, bringing 94L to the Lesser Antilles on Thursday as a strong tropical storm. The models have shown poor run-to-run consistency in both the timing and the track of 94L, so it is difficult to assess which land areas might be most at risk, and when. A database of historical probabilities of storms in the same location as 94L maintained by Dr. Bob Hart of Florida State University reveals that historically, 45% of storms in this location have eventually hit land, with Canada (13% chance) and North Carolina (15% chance) the most likely targets. In their 8 am EDT Tropical Weather Outlook, NHC gave 94L a 40% chance of developing into a tropical depression by Monday morning.

[CrazyC83]

That is a potential nightmare situation for the islands, since the rapid development would be just before them...

Personal Forecast Disclaimer:
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or storm2k.org. For official information, please refer to the NHC and NWS products.

[weatherwindow]

Already the squadron is preparing for next Tuesday afternoon. Look at those coordinates.

REPRPD
WEATHER RECONNAISSANCE FLIGHTS
CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
1030 AM EDT SAT 18 AUGUST 2012
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 19/1100Z TO 20/1100Z AUGUST 2012
TCPOD NUMBER.....12-091

I. ATLANTIC REQUIREMENTS
1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
2. OUTLOOK FOR SUCCEEDING DAY.....NEGATIVE.
3. REMARKS: ADDITIONAL DAY OUTLOOK--PSBL LOW LEVEL
INVEST NEAR 14N AND 50W ON 21/1800Z.

luis.....as i recall the yardstick for northeastern caribbean hits is remaining under 15deg Lat prior to reaching 50deg Long...soooo this would appear to be a potential candidate for a visit to the "isla del encanto"...rich

[gatorcane]

The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or storm2k.org. For official information, please refer to the NHC and NWS products.

My prediction is that the system develops gradually and gains strength somewhere between what the GFS and ECMWF is showing. I think the GFS is too bullish on some of those scary intensity runs into the Leewards we saw yesterday. When I look at the VIS loops I still see alot of SAL in the MDR, though not as bad as it was a couple of weeks ago. That will likely not allow this to organize and deepen very quickly. Shear won't be an issue it looks like. I don't think the trough to the north Master's mentions will be enough to turn it NW because I think we will only see gradual development with 94L keeping it shallow enough to be steered more by the lower-level flow. Leeward islands need to pay close attention, especially the Northern Leewards, where I think there is a good chance of seeing a first landfall. I don't think we will see a major hurricane into the Leewards at this point. I am thinking more like a 75-90mph hurricane.

[SouthDadeFish]

I see no reason why this won't be a code red at 2pm today. 60% seems like a very reasonable % considering model support at recent satellite trends.

(Just my opinion, not a professional)

[abajan]

They need to send Recon out right at 50W if it develops since they would need watches by then for the Windward Islands - that would be within 48 hours from Barbados

IMHO, that's the longitude they should always send aircraft when powerful systems threaten (not that I'm suggesting that 94L will turn out to be powerful). It seems to me that more often than not they send planes out to investigate when systems are practically on our doorsteps. Although, I'm aware of budget constraints and such.

[gatorcane]

I see no reason why this won't be a code red at 2pm today. 60% seems like a very reasonable % considering model support at recent satellite trends.

(Just my opinion, not a professional)

Agreed we should see code red 60% chance for the next TWO and 80%+ by the advisory later today.

[CrazyC83]

They need to send Recon out right at 50W if it develops since they would need watches by then for the Windward Islands - that would be within 48 hours from Barbados

IMHO, that's the longitude they should always send aircraft when powerful systems threaten (not that I'm suggesting that 94L will turn out to be powerful). It seems to me that more often than not they send planes out to investigate when systems are practically on our doorsteps. Although, I'm aware of budget constraints and such.

They should extend it out to 45W when the islands are in line, but via a multi-lateral agreement with the countries involved.

[gatorcane]

It looks like 94L is gaining a little bit of lattitude and is moving WNW at times. How much it gains will certainly determine where it ends up downstream when it gets further west.

[SouthDadeFish]

It looks like 94L is gaining a little bit of lattitude and is moving WNW at times. How much it gains will certainly determine where it ends up downstream when it gets further west.

The 12Z GFS actually showed a hint of southerly component in its west motion over the next five days. The ridge to the north is forecast to be pretty legit.

[HurricaneWarning92]

I agree, it looks like it is moving just north of west. If it continues to do so, my guess would be that it would brush the northern Leeward islands, maybe head towards puerto rico. but this is just my opinion.

[MGC]

94L is getting that tumbleweed look....I expect the disturbance to continue to become better organized. Could become a TD this weekend. Ridge to the north should drive the system westward....could be an issue to the islands down the road.....MGC

The above is the opinion of MGC and not an official forecast.

[Aric Dunn]

its quite evident we have a llc defined to be classified. just need a little more persistent convection. sign of low level banding now. should go to 60 to 70 at 2 but they will be conservative since its out in the middle of the atlantic and will probably just go with 50. Also very little attachment to the itcz anymore as it is starting to wrap it in and break it down.

[TropicalAnalystwx13]

Up to 50%.

A TROPICAL WAVE LOCATED ABOUT 300 MILES WEST OF THE CAPE VERDE
ISLANDS CONTINUES TO PRODUCE DISORGANIZED SHOWER AND THUNDERSTORM
ACTIVITY. ENVIRONMENTAL CONDITIONS APPEAR CONDUCIVE FOR GRADUAL
DEVELOPMENT DURING THE NEXT FEW DAYS...AND THIS SYSTEM HAS A MEDIUM
CHANCE...50 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS AS IT MOVES WESTWARD AT ABOUT 20 MPH.

[Aric Dunn]

yep conservative.. they always do that out in the far east atlantic.

[HurricaneMaster_PR]

its quite evident we have a llc defined to be classified. just need a little more persistent convection. sign of low level banding now. should go to 60 to 70 at 2 but they will be conservative since its out in the middle of the atlantic and will probably just go with 50. Also very little attachment to the itcz anymore as it is starting to wrap it in and break it down.

Nice call Aric!

[gatorcane]

Looking at the wide-vide Atlantic WV loop, we see a ULL that is slowly dropping SSW. That could induce some SW shear into the system when it gets further west in 3-4 days...that combined with lingering SAL could keep this from getting very strong before the Leewards. But shear forecasts are tough to forecast in the medium to long-range. I wonder if the ECMWF is seeing that ULL and not developing the system as a result?



You can see the SAL in this VIS image. Notice all of the very hazy light gray covering the MDR out ahead of the system and just to the north of that system. 94L will need to deal with that SAL: