Invest 98L spinning up; outlook for remainder of hurricane season
A tropical wave midway between Africa and the Lesser Antilles (Invest 98L) continues to look well-organized on satellite imagery, with a modest amount of heavy thunderstorm activity and spin. An ASCAT pass from 8:08 pm EDT last night showed 98L was close to closing off a well-defined surface circulation. Wind shear as diagnosed by the SHIPS model is light, less than 10 knots, and is predicted to stay light to moderate through Friday. Ocean temperatures are 28 - 28.5°C, well above the threshold typically needed for a tropical storm to spin up. Water vapor satellite images show 98L is embedded in a moist environment, but there is dry air to the system's northwest. However, given the light wind shear, this dry air may not pose a hindrance to development at this time. An analysis of upper level winds from the University of Wisconsin CIMMS group shows a pattern favorable for development, with an outflow channel open to both the north and south available to ventilate the storm and allow 98L to efficiently lift plenty of moisture to high levels.
Figure 1. Morning satellite image of 98L.
The models are not very aggressive about developing 98L into a tropical depression, but most of them do show some weak development. NHC gave the disturbance a 60% chance of developing into a tropical depression by Wednesday in their 8 am Tropical Weather Outlook. Given the recent increase in spin on visible satellite images and favorable environment for development, I'd bump these odds up to 70%. 98L is currently moving little, but is expected to begin a westward motion at 10 mph today. This motion would take 98L into the Lesser Antilles Islands by Friday or Saturday. The northern Lesser Antilles would be most likely to see the core of the storm, as has been the case for all of this year's disturbances. However, a more southerly path across Barbados, as predicted by the GFS model, cannot be ruled out. Once 98L does reach the Lesser Antilles, all of the models indicate the storm will see a sharp increase in vertical wind shear due to strong upper-level winds out of the west. This shear should make it difficult for 98L to intensify as it moves though the islands.
Atlantic hurricane outlook for the rest of September
Ocean temperatures are starting to decline in the North Atlantic, though remain much above average in the Main Development Region (MDR) for hurricanes, from the coast of Africa to the coast of Central America, between 10°N and 20°N latitude. The latest departure of sea surface temperature (SST) from average plot (Figure 2) shows a large area of ocean temperatures near 1°C above average. The water temperatures were 0.8°C above average in this region during August, which is the 4th highest such reading on record. These warm waters will allow for an above-average chance of African tropical waves developing through early October. By early October, the African Monsoon typically begins to wane, spawning fewer tropical waves that tend to be weaker, and we should stop seeing development of newly-emerged tropical waves off the coast of Africa.
Figure 2. Departure of sea surface temperature (SST) from average for September 19, 2011. Ocean temperatures were about 1°C above average over much of the Main Development Region (MDR) for hurricanes, from the coast of Africa to the coast of Central America, between 10°N and 20°N latitude. In the Pacific off the coast of South America, we can see the tell-tale signature of a La Niña event, with cooler than average waters along the Equator. Also note the cooler than average waters between Bermuda and Puerto Rico, due to the passage of Hurricane Maria and Hurricane Katia. Image credit: NOAA/NESDIS.
Wind shear has been near average over the tropical Atlantic this hurricane season, and is currently at its climatological low point, which occurs in mid-September. The latest 2-week run of the GFS model shows wind shear will remain at the sort of typical low levels we usually see this time of year. With ocean temperatures at near-record warm levels, this combination would tend to favor formation of at least two tropical storms between now and the beginning of October. One inhibiting factor, though, may be the continued presence of dry, stable air over the tropical Atlantic. Hurricanes like to have an unstable atmosphere, with moist, warm air near the surface, and cold, dryer air aloft. This situation helps the updrafts in the storm grow stronger. This year, we've had unusually stable air (Figure 3.) This has really put the brakes on intensification of most of the tropical storms that have formed. The current ratio of 14 named storms but only 3 hurricanes is unprecedented in the historical record, going back to 1851. Usually, just over half of all Atlantic tropical storms intensify into hurricanes. One other factor to consider, the 30-60 day pattern of increased thunderstorm activity known as the Madden-Jullian Oscillation (MJO), looks like it will have little influence over the coming week. The MJO has been weak all month, and is predicted to stay weak for the remainder of this week.
Figure 3. Vertical instability, as measured by the difference in temperature near the surface to the bottom of the stratosphere. The atmosphere in the Caribbean (left) and tropical Atlantic between Africa and the Lesser Antilles Islands (right) has been much more stable than average this year (average is the thick black line). Image credit: NOAA/CIRA.
Forecast of the rest of hurricane season
We are past the half-way point of the Atlantic hurricane season, which typically peaks on September 10. On average, about 60% of the activity has occurred by this point in the season. Since we've already had 14 named storms and 3 hurricanes, at the current rate, we would expect to see another 8 or 9 named storms, with 1 or 2 of them reaching hurricane strength. It's pretty tough to maintain the sort of activity levels we've seen so far this year, so I am forecasting we'll see 7 more named storms during the remainder of this season, taking us all the way to "W" in the alphabet. With the unusually stable air over the Atlantic showing no signs of abating, I predict that we'll see just 2 of these storms reach hurricane strength. As far as steering currents go, the latest 2-week forecast from the GFS model doesn't show any significant changes to the jet stream pattern we've seen all summer. There will continue to be a parade of troughs of low pressure moving off the U.S. East Coast that will tend to curve any storms northwards and then northeastwards out to sea, once they penetrate north of the Lesser Antilles Islands. This pattern favors strikes on North Carolina and New England, and discourages strikes on Texas. I doubt Texas will see a tropical storm this year given this steering pattern, and considering that Texas' tropical cyclone season tends to peak in late August and early September. It is quite unusual for Texas to have a tropical storm or hurricane this late in the season, so they will probably have to look elsewhere for drought-busting rains.
Jeff Masters
Reader Comments
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As a teacher who taught students who were not born in the USA, and English was their second language, I will say this: English is VERY difficult to learn because we break most of our own rules!!!! Good luck, Woody and don't get discouraged by inconsiderate and/or ill-tempered people. The rest of us will gladly help you along.
as long as the order was correct, that's all that matters....
850MB:
700MB:
500MB:
Still has some dependency on the Monsoonal TROF @ 700MB/500MB levels but once it gets wrapped up some more it should be able to break off.
I give it 80% by 8PM.
That's a great site, thanks Pat.
I hadn't saved that one before.
This year we are having so many sever TS and rain in PR.
There all on the wunderground tropical & hurricane page chicklit.
I use it for 90% of what I post.
On the menu bar at the top of the page, under "severe weather".
Have a good afternoon too.
I only came on here to learn of the weather and for some enjoyment and play with others. If you do not want me here then you can play with yourself.
Thank you. You are one of the kind ones. I do not think I am returning here. This was a wonderful web place, but no longer, if it is allowed for people to insult each other. A shame.
This is one of those more obvious anomalies that is likely reconciled by the fact that with technology today, we can name every spinning cloud we see, and we see them all. Could not do that even 50-years ago much less 150 years ago. These historical comparisons are meaningless, similarly to the MMGW comparisons. It is just not effective for use in decision making or forecasting.
We have one invest -- 98L.
Everyone deserves respect, and bobbutts apparently doesn't share that opinion. I've been flamed by people before and I generally just don't answer at all. Why perpetuate someone's bad manners and ill temper?
This is still a good, informative blog where I get sound weather news interspersed sometimes with people posting nutty stuff. Take the good stuff and ignore the rest. There are lots of really good people on this blog.
JMO.
TAZ!!!!
Actually, Dr. Masters is the blogger. Everyone else just responds in their own way......
I definitely agree. Have to look past the written word and catch the meaning. My sister-in-law, who is deaf, has a terrible time with English. Even though she born and raised in Louisiana. She types like she signs. Everything is abbreviated with joining words like and, but, or, a completely missing. Just have to learn to read between the lines.
I suspect that a current investigation will show the immediate influx of poor behaviours are being supported by a single, off-site source. To react to them, or to encourage them, is to be a party to the attack.
I do apologise to the blog. I just posted some models and pictures. I was not prepared for such a response. I do think it best if I just not post. I think this place has had enough of this over weeks past as I have seen. I do not want to add. Thank you all and I shall look but not post. Some of you are good at these models and pictures.
Back to the kiddy pool for you...
The only way to get good is to post. Do not let other people decide for you what you should do.
Ok buffalo and cantgetit whatever. You win, I bite the bait. Is there any way you two guys could be the first loonies here to wake up, smell the roses, notice that you are interfering with a serious discussion and either mend your ways or return to we playwithbarbies.com or wherever you came from?
No?
Didn't think so.
To everyone else, over four years I have rarely if ever responded to trolls, just felt the need. Forgive me one time.
Oh, and i am an englishman, living in the US. I do know the english language, and an awful lot of the people in here who think it correct to criticize others for their grammar and spelling need to take a little peek in the mirror! ;-) Have a nice day
TropicalAnalystwx13
LOL
Agree 100%
"Also does anybody know what the last 98L was?."
Invest list rounds, please :)
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