First tropical depression of the season may form from 92L
An unusually large and well-developed African tropical wave for so early in the season has developed midway between the coast of Africa and South America. The storm was designated Invest 92L by the National Hurricane Center yesterday, and has a good chance of becoming the first tropical depression of the Atlantic hurricane season. Surface winds measured by the 8:23am EDT pass of the European ASCAT satellite revealed that 92L already has a closed surface circulation, though the circulation is large and elongated. Top winds seen by ASCAT were about 25 mph. METEOSAT visible satellite loops show a large and impressive circulation that is steadily consolidating, with spiral bands building inward towards center, and upper-level outflow beginning to be established to the northwest and north.
Figure 1. Morning satellite image of Invest 92L.
Climatology argues against development of 92L, since only one named storm has ever formed between Africa and the Lesser Antilles Islands in the month of June--Tropical Storm Ana of 1979 (Figure 2). However, sea surface temperatures (SSTs) underneath 92L are an extremely high 28 - 30°C, which is warmer than the temperatures reached during the peak of hurricane season last year, in August - September. In fact, with summer not even here, and three more months of heating remaining until we reach peak SSTs in the Atlantic, ocean temperatures across the entire Caribbean and waters between Africa and the Lesser Antilles are about the same as they were during the peak week for water temperatures in 2009 (mid-September.) While 92L will cross over a 1°C cooler patch of water on Monday, the storm will encounter very warm SSTs of 28-29°C again by Tuesday.
The disturbance doesn't have to worry about dry air--Total Precipitable Water (TPW) loops show a very moist plume of air accompanies 92L, and water vapor satellite loops show that the center of 92L is at least 300 - 400 miles from any substantial areas of dry air. The 60-day cycle of enhanced thunderstorm activity called the Madden-Jullian Oscillation is currently favoring upward motion over eastern tropical Atlantic, and this enhanced upward motion helps create stronger updrafts and higher chances of tropical cyclone development.
Figure 2. Tropical Storm Ana of 1979 was the only June named storm on record to form between Africa and the Lesser Antilles Islands.
The forecast for 92L
A major issue for 92L, like it is for most June disturbances, is wind shear. The subtropical jet stream has a branch flowing through the Caribbean and tropical Atlantic north of 10° N that is bringing 20 - 40 knots of wind shear to the region. Our disturbance is currently located at 7°N, well south of this band of high shear, and is only experiencing 5 - 15 knots of shear. This moderate amount of shear should allow for some steady development of 92L over the next few days as it tracks west-northwest at 10 - 15 mph. The National Hurricane Center is giving 92L a medium (30% chance) of developing into a tropical depression by Tuesday morning. Based on visible satellite imagery over the past few hours, I believe this forecast is not aggressive enough, and that 92L has a 50% chance of developing into a tropical depression by Tuesday morning. Another factor holding 92L back is its proximity to the Equator. I would give 92L higher chances of developing if it were not so close to the Equator. The system is organizing at about 7°N latitude, which is so close to the Equator that it cannot leverage the Earth's spin much to help it get spinning. It is quite unusual for a tropical depression to form south of 8°N latitude.
The farther south 92L stays, the better chance it has at survival. With the system's steady west-northwest movement this week, 92L should begin encountering hostile wind shear in excess of 30 knots by Thursday, which should be able to greatly weaken or entirely destroy the storm before it gets to the Lesser Antilles Islands. However, residents of the islands--particularly the northern Lesser Antilles--should follow the progress of 92L closely, and anticipate heavy rains and high winds moving through the islands by Saturday or Sunday next weekend. The GFDL and HWRF models are predicting that 92L will develop into a moderate strength tropical storm that will then be weakened or destroyed by the end of the week, before it reaches the islands. This looks like a reasonable forecast.
Figure 3. The departure of sea surface temperature (SST) from average for June 10, 2010. Image credit: NOAA/NESDIS.
Oil spill wind forecast
There is little change to the oil spill wind forecast for the coming two weeks. Light winds of 5 - 10 knots mostly out of the south or southeast will blow in the northern Gulf of Mexico all week, according to the latest marine forecast from NOAA. These winds will keep oil near the coast of Louisiana, Alabama, Mississippi, and the extreme western Florida Panhandle, according to the latest trajectory forecasts from NOAA and the State of Louisiana. The long range 8 - 16 day forecast from the GFS model indicates a typical summertime light wind regime, with winds mostly blowing out of the south or southeast. This wind regime will likely keep oil close to the coastal areas that have already seen oil impacts over the past two weeks.
Jeff Masters
Reader Comments
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The NHC cant issue warnings for other countries.
The NHC likes t see a ball of deep convection on an organized, but ragged invest to name it. Just going by past storms like this. Most likely to happen in the morning.
Yes I back our men and women forecasters fully, there is a reason the government hires them.
Those are not Islands, my friend.
They are Countries in South America.
No problem lol....good test for us.
dude, I was agreeing with you 100% I think that our military and our postal service along with our air traffic controllers and our mets ALL do a fantastic job.
ok
But again, we don't need a ball. That's 50-knot+ TS-type pattern.
13/1745 UTC 6.7N 36.1W T1.0/1.0 92L -- Atlantic
true but its not 5 pm yet.
Atlantic Ocean Basin: Imagery
DATE/TIME LAT LON CLASSIFICATION STORM
13/1745 UTC 6.7N 36.1W T1.0/1.0 92L
Perhaps we'll see a 2nd update today.
13/1745 UTC 6.7N 36.1W T1.0/1.0 92L -- Atlantic
Finally some Dvorak. What is the TD threshold?
1.5
Normally 1.5 - 2.0
Thats all we can do
hmmm correct?
OK, I have to admit, that is elongated.
1.5 to 2.0
Dvorak huh? Hey Pat doesn't that qualify it for a Tropical cyclone formation alert.
No Prob :))
I know, I'm just saying thats what the NHC typically likes to see. Whats ur opinion on the 5-10 knot wind shear north of the islands IF this system survives the band of 30-40 knots.?
Well if its 1.5 then we arent far from it.
CI MWS MWS MSLP MSLP Saffir-Simpson
Number (Knots) (MPH) (Atlantic) (NW Pacific) Category
1 25 KTS 29 MPH (Approximate)
1.5 25 KTS 29 MPH
2 30 KTS 35 MPH 1009 mb 1000 mb
2.5 35 KTS 40 MPH 1005 mb 997 mb
3 45 KTS 52 MPH 1000 mb 991 mb
3.5 55 KTS 63 MPH 994 mb 984 mb
4 65 KTS 75 MPH 987 mb 976 mb 1 (64-83 KTS)
4.5 77 KTS 89 MPH 979 mb 966 mb 1 (64-83 KTS); 2 (84-96 KTS)
5 90 KTS 104 MPH 970 mb 954 mb 2 (84-96 KTS); 3 (97-113 KTS)
5.5 102 KTS 117 MPH 960 mb 941 mb 3 (97-113 KTS)
6 115 KTS 132 MPH 948 mb 927 mb 4 (114-135 KTS)
6.5 127 KTS 146 MPH 935 mb 914 mb 4 (114-135 KTS)
7 140 KTS 161 MPH 921 mb 898 mb 5 (136+ KTS)
7.5 155 KTS 178 MPH 906 mb 879 mb 5 (136+ KTS)
8 170 KTS 196 MPH 890 mb 858 mb 5 (136+ KTS)
CI -- Current Intensity
MWS -- Mean Wind Speed
MSLP -- Mean Sea Level Atmospheric Pressure in Millibars
Viewing: 901 - 951
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