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Tropical Storm Isaac, Part 1: Moving Through The Eastern Caribbean Sea


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New HWRF finally showing some love, GFDL remains quite unimpressed. HWRF unimpressed next couple of days. Not unsimilar to 6Z GFS in general trend or track. Vis imagery does suggest there are dry air issues, which surprises me a tad, despite MIMIC TPW, and I thought shear was light enough dry air would be modified before reaching the center. Still, only thing lacking for an upgrade, IMHO, is more concentrated convection near the center.

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New HWRF finally showing some love, GFDL remains quite unimpressed. HWRF unimpressed next couple of days. Not unsimilar to 6Z GFS in general trend or track. Vis imagery does suggest there are dry air issues, which surprises me a tad, despite MIMIC TPW, and I thought shear was light enough dry air would be modified before reaching the center. Still, only thing lacking for an upgrade, IMHO, is more concentrated convection near the center.

Taking a close look at the 00Z EC would tend to agree with what Phil said above. Looks like the low and mid level circulations do not really get aligned until the system is approaching the Islands, probably due to fast motion and easterly shear. Then after that it shows an aligning and strengthening vorticity maximum through the low and mid levels.

The low level circulation continues to develop and look good, but my guess it we don't see any significant increase in overall system organization until after it gets west of 50W, and then particularly as it approaches the Islands.

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Taking a close look at the 00Z EC would tend to agree with what Phil said above. Looks like the low and mid level circulations do not really get aligned until the system is approaching the Islands, probably due to fast motion and easterly shear. Then after that it shows an aligning and strengthening vorticity maximum through the low and mid levels.

The low level circulation continues to develop and look good, but my guess it we don't see any significant increase in overall system organization until after it gets west of 50W, and then particularly as it approaches the Islands.

I tend to agree, although I won't be surprised to see actual development further W in the Caribbean. Are tropical storm conditions likely in the Antilles? Probably. The EC ensembles are rather insistent that the NW Caribbean will be the area to watch for more aggressive development early next week.

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ed are u a bit concerned that this thing is movin at 25 mph, the LLC is not lookin that great in last couple hours either.

Ya know, since the main LLC does have convection with it, and is getting further away from the Eastern lobe of the stretched mid-level vort closer to 40ºW, almost wondering if it will completely seperate, and having low level vorticity and convection, develop new mid level vorticity independent of that Eastern lobe.

Also wondering if the trailing vort might tend to minimize latitude gain, or even cause a hint of a S component to the fast Westerly motion.

I could be completely wrong on all of this.

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I tend to agree, although I won't be surprised to see actual development further W in the Caribbean. Are tropical storm conditions likely in the Antilles? Probably. The EC ensembles are rather insistent that the NW Caribbean will be the area to watch for more aggressive development early next week.

Yep. I don't think we'll see anything truly significant until the weekend at the earliest.

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From what I can tell from the trends tonight, Invest 94L is not really getting better organized. If anything, it looks like the mid-level vortex and low-level vortex are still somewhat decoupled, with the low level circulation northwest (around 15N 40.5) ahead of the mid-level circulation center which has been moving wsw over the last few hours. The system still appears to be having some difficulty tapping into the moister air to the south in the ITCZ. While the storm is still likely to develop over the next 24-48 hours, I don't think we are going to see a very organized system before it reaches the Lesser Antilles.

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One other thing I'd like to discount. Some people were mentioning the ECMWF has been preforming better than the GFS with regards to the curtailment in development. I wouldn't really agree. Both the ECMWF and GFS were pretty agressive with the genesis of Invest 94L and robust development in this range. The ECMWF might have backed off a run before the GFS followed suit but its still markedly weaker with the forecast of 94L in comparison to the GFS, which I think is not going to turn out to be correct.

It may not be correct going forward, all that was mentioned is that so far I think it's been decent with this. Apples to oranges when talking about the forecast going forward. Today's 00z run weren't too far off seemingly with the interpretation of strength, but the timing was defintely different.

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Taking a close look at the 00Z EC would tend to agree with what Phil said above. Looks like the low and mid level circulations do not really get aligned until the system is approaching the Islands, probably due to fast motion and easterly shear. Then after that it shows an aligning and strengthening vorticity maximum through the low and mid levels.

The low level circulation continues to develop and look good, but my guess it we don't see any significant increase in overall system organization until after it gets west of 50W, and then particularly as it approaches the Islands.

Sounding over Guadaloupe shows quite a bit of dry air above 850 mb along with a belt of 20-30 knot easterlies (with slight improvement from the previous sounding 12 hrs before). It makes me wonder if this thing will do much until it is west of the Antilles.

post-6373-0-93705500-1345478929_thumb.jp

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Sounding over Guadaloupe shows quite a bit of dry air above 850 mb along with a belt of 20-30 knot easterlies (with slight improvement from the previous sounding 12 hrs before). It makes me wonder if this thing will do much until it is west of the Antilles.

Might get a little better before 94L arrives...

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Not looking too hot right now. Dry air coupled with a 20 knot forward speed isn't helping at all.

This is a very large system with a large cyclonic envelope, so it will take at least 24-36 hours to completely mix out dry air. Once this nears the islands, foward motion might slow a little to 10-15 knots, but that might only help a little.

Right now I could see a 50 mph tropical storm affecting the islands in 48 hours. Thereafter, I would look for more steady intensification in the eastern/central Caribbean. Anywhere from 50-70 knots seems reasonable right now. The IVCN has made a big adjustment up to C2 just south of Hispaniola by D5, so this does have the potential to strengthen into something formidable ONLY IF 94L can take advantages of small breaks in forward speed and dry air.

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Turning north at 162. South Florida is getting hit hard.

f162.gif

i say this gently and with all respect to weak storms, but S FL really only needs to be extremely concerned IMO if this misses hispanola to the north and finds a way to move back into SFL. i can't think of many storms that have gone over or south of hispanola and cuba and hit S FL as more than a formidable cat 1. and yes i undestand there is damage with a cat 1 but fl can handle it. and no this isn't pullng a wilma

does anyone know...have any storms shot the gap between cuba and hispanola and hit S FL as CAT 2 or greater?

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Not looking too hot right now. Dry air coupled with a 20 knot forward speed isn't helping at all.

This is a very large system with a large cyclonic envelope, so it will take at least 24-36 hours to completely mix out dry air. Once this nears the islands, foward motion might slow a little to 10-15 knots, but that might only help a little.

Right now I could see a 50 mph tropical storm affecting the islands in 48 hours. Thereafter, I would look for more steady intensification in the eastern/central Caribbean. Anywhere from 50-70 knots seems reasonable right now. The IVCN has made a big adjustment up to C2 just south of Hispaniola by D5, so this does have the potential to strengthen into something formidable ONLY IF 94L can take advantages of small breaks in forward speed and dry air.

I agree. :hug: The fact that the circulation has become so large means that dry air well to the north of the system is being ingested. Your forecast sounds reasonable at this point, although I wouldn't be surprised if 94L hits the islands as a TD.

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does anyone know...have any storms shot the gap between cuba and hispanola and hit S FL as CAT 2 or greater?

Absolutely.

For example, King 1950 bombed out in the short distance between Cuba and FL and hit Miami head-on as a very strong Cat 3 (110 kt). Cleo 1964 took a similar track and also hit Miami directly, probably as a strong Cat 2.

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The biggest hindrance going forward might be the landmasses in its way, once it does try to organize better. The shear looks relatively weak with that ridge to its north. There is a weakness way in the western Carb, but it looks pretty far west. I suppose the one thing to watch is the shear out in the nrn Antilles right now. That needs to get out of the way.

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In all honesty, this is probably already a TD or weak TS. Perhaps the convective organization isn't there, but the low-level circulation seems vigorous enough. The ironic thing is that if the storm were more vertically deep, it would probably be taking a further south track and not be in as much risk as skirting the greater Antilles as it is currently shown by the models. The mid-level ridge is actually oriented more ENE/WSW vs. the low-level ridge which is straight E/W.

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Gibbscast has Dennis (1981). Talk about weenie suicides.

FMI, but whats Gibbscast?

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In my honest opinion, the sheer size of the cyclonic envelope is very impressive. This may have trouble with easterly shear and dry air, but as long as it keeps that southern inflow open, it shouldn't die out.

GOES16452012233kc0LCh.jpg

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Absolutely.

For example, King 1950 bombed out in the short distance between Cuba and FL and hit Miami head-on as a very strong Cat 3 (110 kt). Cleo 1964 took a similar track and also hit Miami directly, probably as a strong Cat 2.

Also, the center of Georges of 1998, though never hitting the FL pen., traveled over much of PR, Hisp., and Cuba and still managed to restrengthen to a cat. 2 almost immediately after leaving Cuba.

I'd say based on King, Cleo, and Georges that the GFS track, which gives it ~24 hours over water between Cuba and the FL pen., could allow for a cat. 2 strength to hit the S FL pen.

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Also, the center of Georges of 1998, though never hitting the FL pen., traveled over much of PR, Hisp., and Cuba and still managed to restrengthen to a cat. 2 almost immediately after leaving Cuba.

I'd say based on King, Cleo, and Georges that the GFS track, which gives it ~24 hours over water between Cuba and the FL pen., could allow for a cat. 2 strength to hit the S FL pen.

Agreed.

I didn't include Georges because it didn't fit the King/Cleo model. King and Cleo rebuilt their inner cores after crossing Cuba and came ashore in FL as rapidly strengthening, tight, small-RMW cyclones. Georges' core never rebuilt-- it actually never really had an inner core after the Greater Antilles-- which is why its winds didn't pick up much during its trek across the Gulf, despite a significant pressure drop (20 mb).

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