Little change to Isaac, but intensification coming; Joyce forms

By: Dr. Jeff Masters , 3:13 PM GMT on August 23, 2012

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Tropical Storm Isaac is a large and impressive-looking storm on satellite images, but data from the Hurricane Hunters reveal that Isaac remains a minimal-strength tropical storm with 40 mph winds, as it heads westward across the Eastern Caribbean. An Air Force Reserve hurricane hunter aircraft which completed its mission into Isaac at 8 am EDT found top winds at the surface near 40 mph, and highest winds at their 5,000 foot flight level of 47 mph. A NOAA hurricane hunter aircraft flying at 10,000 feet has found top winds of 47 mph at that altitude. The Hurricane Hunters found a broad area of light winds with a central pressure of 1003 mb. The aircraft did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. There does not appear to be much in the way of dry air near the core of Isaac, as seen on water vapor satellite loops, which is a big switch from what we've seen previously. Visible satellite loops show that Isaac has a much more symmetric circular cloud pattern, and has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of an intensifying storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that an upper-level pattern of outflow supportive of significant strengthening has developed this morning, with an upper-level outflow channel now well-established to the north, and a new outflow channel opening to the south. Radar imagery from Puerto Rico shows some weak low-level spiral bands that are slowing intensifying and becoming more organized.


Figure 1. Morning radar image of Isaac taken from the Puerto Rico radar. Isaac's rain bands are weak, but are starting to take on a more spiraling shape.

Intensity forecast for Isaac
Isaac has consistently confounded predictions that it would intensify, but all the potential factors inhibiting intensification seem to have diminished to the point where intensification has to occur. Wind shear is low, 5 - 10 knots, and ocean temperatures are warm, 29°C, and these warm waters extend to great depth, giving the storm a high total heat content to work with. These factors, combined with the favorable upper-level outflow pattern and more symmetric cloud pattern, support intensification, and all of the intensity models except the HWRF model predict intensification of Isaac to a strong tropical storm or Category 1 hurricane by Friday afternoon. The latest 8 am EDT run of the SHIPS model predicts that wind shear will be light to moderate, 5 - 15 knots, for the next five days. The 11 am EDT wind probability forecast from NHC predicted a 34% chance that Isaac will become a hurricane by Friday afternoon, and a 6% chance it will be a Category 2 or stronger hurricane then. By Friday afternoon, Isaac will likely be close enough to Southwest Haiti that the inner core will be disrupted, and the storm will likely be a 45 - 55 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate, and the upper-level wind pattern favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane.

Impact of Isaac on the Islands
The south coast of Puerto Rico should see Isaac's heaviest rains and strongest winds beginning near 8 pm EDT tonight, with tropical storm-force winds of 40 - 45 mph potentially affecting the southwest portion of the island. The San Juan airport may be able to stay open during Isaac's passage, but with delays when spiral bands move overhead.

Heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic late tonight, and the Santo Domingo airport will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the Bahamas
Florida, Cuba, Jamaica, and the Bahamas are all at risk of receiving flooding rains and high winds from Isaac. The latest set of 00Z (8 pm EDT) and 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a west-northwestward track over Southwest Haiti and into Western Cuba. At the 4 - 5 day forecast period for Sunday and Monday, the models have come into better agreement, and have shifted west some. Our best-performing model, the ECMWF, has now shifted Isaac's path more to the east, but still is the westernmost of the models, predicting a landfall for Isaac near the Alabama/Florida border on Wednesday. While we do still have some models predicting a path up the east coast of Florida, model consensus now favors a path up the west coast of Florida through the Gulf of Mexico. The recent reformation of Isaac's center more to the south supports the idea that Isaac will take a track more to the west through the Gulf of Mexico. Since this now means a final landfall for Isaac in the Florida Panhandle is likely, the storm will probably have an extra day over water, increasing the odds that it will become a Category 2 or stronger hurricane before this final landfall. The NOAA jet is scheduled to fly into the storm this afternoon, to do a large-scale dropsonde mission to aid model forecasts. These missions can improve model forecasts by 10 - 20%, so the model runs that will be available early Friday morning should have increased reliability.

Impact on Tampa, Florida
The Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 15% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. The latest model tracks for Isaac suggests that the trough of low pressure pulling the storm to the north will not be strong enough to give Isaac a northeastward component of motion when it crosses Tampa's latitude. Thus, Isaac will have difficulty making a direct hit on Tampa without passing over a considerable amount of land first, making a multi-billion dollar hurricane disaster in Tampa very unlikely. I put the odds of a mass evacuation occurring during the convention at 1%; a limited evacuation of people in the Tampa Bay area living in mobile homes in low-lying areas is probably about 5 - 10 % likely. I have detailed information on Tampa's storm surge vulnerability in a post from last week.


Figure 2. Morning satellite image of Tropical Storm Joyce.

Tropical Storm Joyce forms in the Central Atlantic
The season's tenth named storm of the year, Tropical Storm Joyce, has formed in the Central Atlantic. Joyce's formation on August 23 puts 2012 in a tie for second place with 1995 for earliest formation date of the season's tenth storm. Only 2005 had an earlier appearance of the season's tenth storm, when Tropical Storm Jose formed at 2 pm EDT on August 22. None of the models show that Joyce will be a threat to the Lesser Antilles Islands, but it may be a storm that will affect Bermuda. It is possible that Joyce will complicate the track forecast for Isaac 4 - 5 days from now, when the storms may be close enough together to interact.

Jeff Masters

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Quoting louisianaweatherguy:
WOW.... A MAJOR model shift to the west... come on guys after that LONNGGGG WSW jog we should have expected that.... If Isaac starts more NW I believe they'll shift back to the east... I HOPE LOL
The jog or center reformation had nothing to do with the EURO shift, since the 48 hour forecast is pretty much the same as before and in line with most/all of the other models. 
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Recon is finding 45-47mph winds, I bet they up him to 45 at 5PM.
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Quoting Chiggy:
12Z Euro at 168 hrs, 959mb, TX/LA border...!

Texas/Louisiana (Western Gulf) has certainly had their share of storms in the past 5 years.
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Quoting Dunkman:


Then again the Euro rather substantially outperformed NHC forecasts after 72h so... I agree that I think the Euro scenario is somewhat unlikely and would need to see it probably on the next 2 runs before I really bought in, but you just can't discard the model that not only beats the other models but also the team of our best forecasters.


Yeah but you can't just change the track completely you have either by 600 miles as that will send the wrong message, the NHC can't make knee jerk reactions to one run that is now almost 600 mile difference from the previous run.
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Quoting Levi32:
Deep-layer steering analysis at 12z shows a sharp-nosed ridge extending through southern Florida with westerlies in the Caribbean, but Isaac is now west of the anchoring subtropical Atlantic ridge, evidenced by southerly currents due north of the storm. A strengthening tropical cyclone is capable of shaving off thin-nosed ridges like this, and poleward movement is likely to ensue.

yeah but he isnt really that strong. large but not strong. the next 24 hours will be telling
Member Since: July 25, 2011 Posts: 0 Comments: 1102
I think we can all stop worrying about Isaac now. TWC has switched it's programming over to lifeguards watching surfers in San Diego. Phew, I was worried for a minute here in Mobile, Al!!!
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1268. Chiggy
Quoting Levi32:
Deep-layer steering analysis at 12z shows a sharp-nosed ridge extending through southern Florida with westerlies in the Caribbean, but Isaac is now west of the anchoring subtropical Atlantic ridge, evidenced by southerly currents due north of the storm. A strengthening tropical cyclone is capable of shaving off thin-nosed ridges like this, and poleward movement is likely to ensue.



Operative word is "Strengthening" - which Issac is NOT! Looking more ragged than now than few hours earlier. And I think this is why some folks are models are getting confused. NHC has this as a hurricane approaching Haiti and hence going more NW...
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting StormHype:


Here come the TX Euro fans of course! Deja-vu Debby all over again. Ya'll have amnesia.

Lol! I already been here the past few days watching this personally its still a pandhandle storm now!
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1266. Levi32
Quoting hercj:
Ok that seems to apply here doesn't it?


Isaac developed before the Caribbean, but he has still struggled in the eastern Caribbean. He will strengthen better farther north and west.
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ECMWF was wrong with Debbie but spot on with Ernesto and GFS vice versa. Isaac will be the tiebreaker.
Member Since: August 21, 2006 Posts: 0 Comments: 254
Quoting StormTracker2K:


Not at all! A track up the west coast of FL or just offshore in the Gulf would cause big problems for the whole FL Penisula as Issac's circulation is 600 miles wide. Flooding, tornadoes, high winds along the Gulf Coast, and storm surge flooding in Tampa as Issac slides by to the north. All spell trouble and that son you can take to the bank.


If it stays just offshore or on the West Coast of Florida, then that means it will almost certainly have to cross over a lot of Cuba and possibly Haiti/Dominican Republic first. There really is no other way that that a scenario which includes riding up the Florida West Coast can happen with this storm. And if so, it will be weakened significantly by the land interactions. Then it won't have enough time to redevelop into a strong hurricane while over the Florida Straits before reaching land again.

Also, going right up the coast means that half of the circulation will be over land, in Florida. And that means little chance for strengthening of the system unless or until it can pull away from the land mass.

If your description refers to effects similar to what Debby did in parts of Florida just two months ago or what TS Fay did to other parts of Florida in 2008, then I would agree, that is a good possibility. But under your scenario we are not talking Katrina-like impacts here. That can only happen if the storm stays well out over water throughout its journey, similar to what the ECMWF model is predicting, and strikes the Western Panhandle without coming close to the peninsula.
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Joyce, like Isaac, has become a rather large system.
Member Since: June 29, 2009 Posts: 13 Comments: 5315
Quoting 1900hurricane:
I wonder if the NHC will mention the 12Z Euro in Isaac's next forecast discussion. It would be interesting to see what they have to say about it.


Watch the GFS go back East again. NHC you are going to have to choose which path you follow.
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I think there is model manipulation going on here. BP is trying to get their 20 billion back on model manipulation.I want an investigation I tell ya.
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Quoting AtHomeInTX:


A lot Rita-ish


A little bit country, and a little bit rock and roll?
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Could we be talking about possibly tomorrow, about if the new Levees in New Orleans would be able to hold off a possible Cat 2-3 Isaac barreling down its throat?  Seems like anything is possible now.
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All of this moisture gathering north of Haiti is a sure sign to where he is heading. Once Issac hits the mtns he might get pulled into these thunderstorms gathering off Haiti's North Coast. Something to watch.

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Quoting MississippiWx:


Just trying to lighten the mood a little in here. Tough to sift through the BS right now. Not sure if I'm blogging on WU or in a sewer.

The WU sewer?
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1256. Levi32
Deep-layer steering analysis at 12z shows a sharp-nosed ridge extending through southern Florida with westerlies in the Caribbean, but Isaac is now west of the anchoring subtropical Atlantic ridge, evidenced by southerly currents due north of the storm. A strengthening tropical cyclone is capable of shaving off thin-nosed ridges like this, and poleward movement is likely to ensue.

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Quoting MississippiWx:
The only explanation to all of this west business is that Isaac must be humping the ridge. Duh.



Well done!
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Nice pull to the N....he roundin' the bend?

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1253. Dunkman
Quoting sporteguy03:


It also makes it very hard for the NHC to give the Euro too much credibility on this run when it is almost 500-600 miles away from the GFS when they appeared to almost meet. We'll see what happens overnight.


Then again the Euro rather substantially outperformed NHC forecasts after 72h so... I agree that I think the Euro scenario is somewhat unlikely and would need to see it probably on the next 2 runs before I really bought in, but you just can't discard the model that not only beats the other models but also the team of our best forecasters.
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It's so funny how some people dwell on the negative aspects of the storms structure and then the folks who point out all the positive stuff.
Member Since: September 18, 2010 Posts: 0 Comments: 5296
Still should be TS Isaac at 5pm, NE quadrent is where they are right now.
Time: 18:46:30Z
Coordinates: 16.0N 65.3167W
Acft. Static Air Press: 843.1 mb (~ 24.90 inHg)
Acft. Geopotential Hgt: 1,541 meters (~ 5,056 feet)
Extrap. Sfc. Press: 1007.6 mb (~ 29.75 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 177° at 44 knots (From the S at ~ 50.6 mph)
Air Temp: 15.0°C* (~ 59.0°F*)
Dew Pt: 15.0°C* (~ 59.0°F*)
Peak (10s) Flt. Lvl. Wind: 45 knots (~ 51.7 mph)
SFMR Peak (10s) Sfc. Wind: 37 knots (~ 42.5 mph)
SFMR Rain Rate: 0 mm/hr (~ 0 in/hr)
(*) Denotes suspect data
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Fundamentals, folks. Isaac is still a weak, but broad system, not even a strong TS yet. It's still trying to organize, uncertain real centers (likely 15n or maybe just below), very low in latitude and far from ANY weakness of any kind. It's motion has been and still will be mostly westwards underneath the high pressure above it. It had slowed, allowing some fraction of consolidation, and then, got faster again, not able to get vertical. There is still some dry air mixing in - has to be because it is present on two sides of the system.

Fundamentally, it is still a weak system and the general motion will continue until it is able to consolidate, get vertical and get the tropical engine running. Right now that engine is mis-firing. When he gets symmetrical, vertical, really pumping the low-level, lifting moisture and ventilating properly, then he will strengthen, but not until. And, hence, there will be NO dramatic turn northwards.

I don't look at models, not interested too much in them, although they've improved significantly. It is FAR south in latitude and it will take some time to climb. Meanwhile, the upper atmosphere of any tugging weakness north of the system will be of little consequence and likely missing by the time Isaac gains any real strength. It is a very broad system, huge - that takes time to spin up.

Minimum of 24 hours, probably more like 48 hours before we get any real hint of what he's going to do eventually. In the meantime, I'll venture to say Hispaniola will not be a direct target and Jamaica, central and western Cuba should be more concerned.

PEACE OUT! :) Y'all play nice!
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Yet again, Isaac is in no fit state to 'feel' the weakness far to his north.

A few hours ago (at work, tsk) I posted Isaac had to go straight NW to hit the eastern tip of Hispaniola.

Now he is due south - still 200 miles or so - of that point, so now its straight NW to the WESTERN tip of Hispaniola and WNW, soon WNW/NW to Cuba.

No way he is touching Hispaniola, and even Cuba may be missed (just) to the south, barring a more pronounced turn later in the frame for a crossing west of Havana.

This storm is MASSIVE in extent, a factor hugely under represented by windspeed at the core - giving a clue that any north component in track will be very gradual. He's also still moving at a fair clip, and the flattened east-west presentation of the storm is symptomatic of his (for now) weak condition.

The Euro, as I stated earlier, could really see its star shine with this one...



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Isaac has slowed down alot so it will give the ridge more time to build back i'm starting to fall for the euro model or we might see where the ridge builds to where Isaac is forced into Norther Mexico
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1246. Grothar
Quoting louisianaweatherguy:
WOW.... A MAJOR model shift to the west... come on guys after that LONNGGGG WSW jog we should have expected that.... If Isaac starts more NW I believe they'll shift back to the east... I HOPE LOL


That makes sense.
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1245. A4Guy
Wow...latest runs take almost all of the FL peninsula out of the center path. Big shift! Won't feel comfortable until we see tomorrow's runs that include the air data from today. If Isaac runs across the length of Cuba as many models are showing, that could completely destroy the circulation, perhaps beyond regeneration? (just a guess).
I remember Ernesto several years ago was forecast to hit SoFla as a potentially dangerous hurricane after spinning up over the FL straits, but traveled right along the coast of Cuba, then came ashore in FL with less wind than a summer t-storm.
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Quoting Abacosurf:
little gustavish


A lot Rita-ish
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Quoting STXHurricanes2012:
Don't think its breaking through a ridge I think the EURO has it right but I don't think the ridge is going to be that strong!


Here come the TX Euro fans of course! Deja-vu Debby all over again. Ya'll have amnesia.
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1242. TXCWC
Quoting 1900hurricane:

You and me both. The models have been trending westward ever since Isaac developed, and it sure will be nice to have all that data and see if that bucks the trend or not.


Think we will have a much greater confidence by tomorrow morning were this is really going - should be a very interesting 12-24hr model watching coming up :)
Member Since: May 18, 2012 Posts: 0 Comments: 529
1241. Grothar
Quoting MAweatherboy1:
Good afternoon everyone. Isaac sure looks good on satellite:



He just has a very disorganized central core. If he ever sorts that out then there could be trouble.


Is N S a proper abbreviation?
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I wonder if the NHC will mention the 12Z Euro in Isaac's next forecast discussion. It would be interesting to see what they have to say about it.
Member Since: August 2, 2006 Posts: 47 Comments: 11847
Quoting STXHurricanes2012:
Or the EURO playing like the GFS did against Debby when the GFS was the only one showing FL and the EURO showing TX! Could the other way around this time if the EURO is trying to do! LOL!!


It also makes it very hard for the NHC to give the Euro too much credibility on this run when it is almost 500-600 miles away from the GFS when they appeared to almost meet. We'll see what happens overnight.
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Quoting sdswwwe:
The anti-cyclone moved ahead of the system, therefore creating wind shear for the system in the north-to-south direction. This is why all of the convection is blown off to the south.

I believe people are looking at this blob of convection and assuming that the CoC is near the center of it as would be the case in a hurricane. But considering this is a weak tropical storm being sheared, the true CoC is well above the mass of convection. The satellite photos can be very deceiving...


Good Information. Thank You!
Member Since: September 18, 2010 Posts: 0 Comments: 5296
Quoting presslord:
These missions can improve model forecasts by 10 - 20%, so the model runs that will be available early Friday morning should have increased reliability.
I don't think this has sunk in yet. Can you please post it again but this time bold and caps.
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WOW.... A MAJOR model shift to the west... come on guys after that LONNGGGG WSW jog we should have expected that.... If Isaac starts more NW I believe they'll shift back to the east... I HOPE LOL
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1235. CJ5
Quoting TallyWeather15:
What is the chance that Tallahassee could get hit with TS winds and heavy rain? Also is it likely schools will be closed next week?


There is some chance that could happen. A far as schools go, I don't know, as your teacher what there plan is.
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1234. wn1995


My 18z track for Isaac.. Shifted track even further west along with the TVCN and still call for it to become a cat 3 before landfall in the western FL panhandle.
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Good afternoon everyone. Isaac sure looks good on satellite:



He just has a very disorganized central core. If he ever sorts that out then there could be trouble.
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1232. Matt74
Quoting WPBHurricane05:
So the EURO sticks Isaac under the Texas ridge. Starting to have Debby flashbacks...
Not saying its gonna happen, but my local met last night said they were starting to see signs that the ridge would lift north and east which might cause Issac to slow as he comes north and cause him to go wnw.I'm no forecaster i'm just an observer. I come here for the info you guys provide.
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Quoting AtHomeInTX:
This can't be good.



It's not good. That is why it will be discounted this run at least.
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Quoting cchsweatherman:
I cannot buy into the 12z ECMWF at all given how persistent and strong ridging has proven itself to be over the Southern Plains, Texas, and Western Louisiana all summer long.


Most of Louisiana has received record rains this year, other than NW Louisiana.

Ridging has been mainly concentrated over the SW and Plains.
Member Since: July 15, 2006 Posts: 17 Comments: 10284
1227. Grothar
Quoting Levi32:


Yes to all of the above, especially that last one.



LOL.
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1226. Chiggy
12Z Global Models, Euro, GFS, UKMET, HWRF, GFDL, even CMC are ALL now on WEST side of FL with varying degree,, Euro being the farthest!! WEST of FL consensus just established!!
Member Since: June 26, 2012 Posts: 0 Comments: 297
Don't think its breaking through a ridge I think the EURO has it right but I don't think the ridge is going to be that strong!
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1224. RJT185
Quoting MississippiWx:
The only explanation to all of this west business is that Isaac must be humping the ridge. Duh.


...the first real funny post in a while, thanks for the LOL.
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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