New tropical depression could form off the coast of Africa

By: Dr. Jeff Masters , 1:12 PM GMT on September 25, 2009

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A tropical wave (99L) is located near 15N, 30W, about 300 miles west of the Cape Verdes Islands. This wave has seen an increase in heavy thunderstorm activity this morning, and low-level spiral bands have formed. The system can already be classified as a tropical depression using the satellite-based Dvorak technique. This morning's QuickSCAT pass showed a loose but closed circulation, with top winds of 30 mph in the heaviest thunderstorms. Wind shear is moderate, about 10 - 20 knots. There is a modest amount of dry air to 99L's west that does not appear to be interfering with the storm's organization.


Figure 1. Morning visible satellite image of Invest 99L (left side of image) and a new tropical wave moving off the coast of Africa.

As 99L moves west-northwest over the next two days, sea surface temperatures will cool by 1°C and winds shear will remain in the moderate range. Some dry air may also affect the storm. These conditions give 99L a good chance of forming into a tropical depression, and NHC has given the system a moderate (30 - 50%) chance of developing into a tropical depression by Sunday. By Monday, wind shear will increase to the high range, 20 - 30 knots, and 99L will probably weaken or be torn apart. None of the computer models forecast development of 99L or any other system in the Atlantic over the next seven days.

A new tropical wave with plenty of heavy thunderstorm activity and some spin is moving off the coast of Africa today. This wave is under a moderate amount of wind shear, 10 - 20 knots, and has some potential for development this week as it moves west or west-northwest past the Cape Verdes Islands.

The latest wind shear forecasts from our major computer models show high values of wind shear affecting most of the tropical Atlantic for the next ten days. This is typical for an El Niño year, and it will be difficult for any storms to get to hurricane strength over the next ten days because of the high shear. The latest 16-day GFS forecast predicts wind shear will decline some by the 2nd week of October, though.


Figure 2. Wind shear forecast for Thursday, October 2 at 00 UTC made by last night's 00Z run of the GFS model. Wind shear is expected to be high over most of the tropical Atlantic for the next ten days, including the Caribbean. Wind shear values below 8 m/s (about 15 knots, red colors), are typically needed to support tropical storm formation.

Jeff Masters

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The model that starts to loop td # 8 looks interesting
Quoting iceman55:



oh wow


The green model that starts to loop td# 8 looks interesting
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Hurricane frequency is up but not their strength, say Clemson researchers

The increasing frequency of storms in the last 50 years is to be expected, due to better reporting and improved technology like satellites, Hurricane Hunter planes, and Doppler Radar. NOAA agrees on the improved reporting issue in a study here.
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Quoting PcolaDan:
On History channel right now (10:00 central) That's Impossible: Weather Warfare

Using tornados, hurricanes and other natural phenomenon as weapons of war.


Thanks - on it
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On History channel right now (10:00 central) That's Impossible: Weather Warfare

Using tornados, hurricanes and other natural phenomenon as weapons of war.
Member Since: August 22, 2008 Posts: 12 Comments: 6010
Quoting mikatnight:


Sounds like a shampoo commercial...

forgot to wash the hair last night .... thanks
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511. JLPR
Quoting iceman55:



oh wow


what? XD
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I'm not buying. You probably need those.
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Quoting iceman55:
kaboom td8#
11:00 PM AST Fri Sep 25
Location: 17.4°N 32.3°W
Max sustained: 35 mph
Moving: NW at 14 mph
Min pressure: 1008 mb
Member Since: October 9, 2008 Posts: 0 Comments: 8436
Its good Iceman, my hurricane calendar runs out of days tomorrow.
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Goodnight guys.

I fully expect a TS in the AM.

NHC forecast seems spot on to me.
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Quoting iceman55:
looking at td8# fire up


We have ignition and lift off of TD#8!!! Yes, she's not through yet!!!
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Quoting mikatnight:


Sounds like a shampoo commercial...


Not like last year!! The "Hurricane Conveyor Belt" of 2008!! A CV wave pops off Africa, next day TS, then another, then another, then another!! We had 4 at a time Gustav, Ike, Hanna and Josephine!!!
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Quoting iceman55:
. Bordonaro that so true


Ana before she became a TD looked AWESOME then POOF! A day later TS Ana appeared, the next day, poof TD Ana! Then for a few days she played, "Where's Ana", or follow the "pulsating blob"! Then she dropped heavy rain over the Leeward Islands and poof, "Bye Ana"!!!
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Quoting Bordonaro:

You know, wild convection flare up then poof, repeat cycle!!!


Sounds like a shampoo commercial...
Member Since: October 18, 2005 Posts: 4 Comments: 3052
Quoting iceman55:
Bordonaro no.why !

You know, wild convection flare up then poof, repeat cycle again, convection flare up & convection wanes away!!!
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Dan yer still da man!
Member Since: October 18, 2005 Posts: 4 Comments: 3052
Quoting mikatnight:


Hey Dakster - didn't see your message til after I got back; next time I'll try to get some money for everyone (especially if I win the WU Million Dollar poll).


Member Since: August 22, 2008 Posts: 12 Comments: 6010
Quoting iceman55:
poor td8




Is it pulling an "Ana" or an "Erika" on us????
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Quoting Dakster:
I figured the blog would be busier with a system out there... I guess I was wrong.


Hey Dakster - didn't see your message til after I got back; next time I'll try to get some money for everyone (especially if I win the WU Million Dollar poll).
Member Since: October 18, 2005 Posts: 4 Comments: 3052

AOI

AOI

AOI

AOI
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For those who are wondering WHAT happened to Typhoon Choi-Wan see below:

NORTHERN ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FAIRBANKS AK
232 PM AKDT FRI SEP 25 2009
.DISCUSSION...

THE REMNANTS OF TYPHOON CHOI-WAN...NOW A 986 MB LOW
SOUTHEAST OF KODIAK ISLAND
...IS EXPECTED TO MOVE TO
NEAR PRINCE WILLIAM SOUND BY 00Z SAT...WEAKENING A FEW MB
WHILE A SECONDARY LOW DEVELOPS JUST EAST OF THE ZONE 226-YUKON
TERRITORY BORDER. BY 12Z SUN...THE REMAINING LOW CENTER
NEAR PRINCE WILLIAM SOUND IS EXPECTED TO FILL WHILE
THE SECONDARY LOW PROCEEDS EASTWARD INTO CANADA...LEAVING
BEHIND A BROAD TROUGH EXTENDING FROM THE PRINCE WILLIAM
SOUND AREA NORTHEASTWARD INTO THE SOUTHERN YUKON
TERRITORY.
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I don't know if I'd use the word stripped. Sat doesn't look like it is convection being sheared off.

Looks more like simply waning convection, to me.
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483. beell
Quoting tornadofan:


Easy come, easy go.

Hey Beell - nioe post man. We need more like that.


I guess I was over-due lol. There are a few here who try on a regular basis.
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Quoting iceman55:
poor td8




Easy come, easy go.

Hey Beell - nioe post man. We need more like that.
Member Since: April 5, 2007 Posts: 83 Comments: 12345


08L/TD/XX
mark
16n/32w

pretty well stripped clean lets see if it will rebuilt itself
Member Since: July 15, 2006 Posts: 178 Comments: 56141
478. beell
850, I probably spent way too much time and space on a reply and I see msphar did respond to your question.

Quoting 850Realtor:
How does a newbie identify a "wave" on a map or satellite (other than waiting for a pro met to post a pic)? Does a wave always have convection associated with it and what is the best type of map/satellite to use to identify new waves/about to be waves?


Probably the fastest way to get a handle on tropical waves is to spend a little time reading the NHC Tropical Weather Discussion (TWD). Check out the "Tropical Waves" section.

An excerpt from the latest:

...TROPICAL WAVES...

A TROPICAL WAVE IS OVER THE E CARIBBEAN ALONG 61W SOUTH OF 18N
MOVING W AT 10-15 KT. AN INVERTED-V LOW CLOUD PATTERN IS NOTED
ON SATELLITE IMAGERY.

Stop by the Satellite Services Division (SSD) page and select an area of interest. In the snippet above, a wave is identified as being "ALONG 61W SOUTH OF 18N". The inverted v cloud pattern is just what it sounds like-an upside down V. Common convention is to split the V down the middle from N to S and use the closest longitude to locate this type of feature and is defined as the "wave axis".
Find 61W and 18N on the sat loop and look for the inverted V. Some waves are classic and hard to miss. Some are more of an inverted U shape and some are almost flat. The more we get away from the classic V, the more you read terms like "ill defined" or "broad".

For this post, check out the SSD Caribbean Visible Loop
or
Caribbean Infrared Loop

Take note of the direction of the ADDED: low level clouds on the right/east side of this wave axis. Moving from the SE towards the NW. Clouds on the left/west side of the axis are moving towards the SW. This change or break in the wind direction is where "cyclonic" (counterclockwise) turning usually begins.

Here's a link for Africa that is not too bad. Larger sizes, satellite type, and loops can be selected:

NexSat

One other tool that has come into wide use is the Morphed Integrated Microwave Imagery at CIMSS - Total Precipitable Water product. Thankfully, also known as "MIMIC-TPW" It is a look at moisture through a fairly thick chunk of the atmosphere. This product is commonly used by the NHC to evaluate waves and usually looks something like this:

TROPICAL WAVE EXTENDS ALONG 23N43W TO 15N46W MOVING WNW NEAR 10
KT. THE WAVE IS APPROACHING A 1012 MB SURFACE LOW NEAR 14N48W.
SATELLITE IMAGERY INDICATES BROAD LOW-LEVEL CYCLONIC TURNING
AROUND THE WAVE AXIS EXTENDING TO THE LOW. A BROAD DEEP LAYER
MOISTURE MAXIMUM IS ALSO OBSERVED IN TOTAL PRECIPITABLE WATER
IMAGERY
ALONG THE WAVE AND LOW
(from 09/22)

Happy waving!
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Quoting KoritheMan:


It doesn't matter, because it isn't threatening the United States.
T.D.8 is not strengthening either.
Member Since: September 27, 2007 Posts: 1 Comments: 22697
.
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Quoting Dakster:
I figured the blog would be busier with a system out there... I guess I was wrong.


It doesn't matter, because it isn't threatening the United States.
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Quoting msphar:
#423 850Realtor - I sent you an WU email message telling you how I do it. Hope it helps.


Thanks So Much!
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Quoting iceman55:
homelesswanderer hey .:(~!~!~


Hey Ice. :)
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I figured the blog would be busier with a system out there... I guess I was wrong.
Member Since: March 10, 2006 Posts: 0 Comments: 10804
469. Skyepony (Mod)
Harbor cam from the fray in the Philippines..

UW - CIMSS
ADVANCED DVORAK TECHNIQUE
ADT-Version 7.2.3
Tropical Cyclone Intensity Algorithm

----- Current Analysis -----
Date : 25 SEP 2009 Time : 233000 UTC
Lat : 15:05:00 N Lon : 124:03:28 E


CI# /Pressure/ Vmax
2.4 / 997.6mb/ 34.0kt


Final T# Adj T# Raw T#
(3hr avg)
2.3 2.5 2.7

Latitude bias adjustment to MSLP : +0.0mb

Center Temp : -10.3C Cloud Region Temp : -25.8C

Scene Type : CURVED BAND with 0.42 ARC in DK GRAY
Maximum CURVED BAND with 0.68 ARC in DK GRAY
at Lat: 15:40:48 N Lon: 123:26:59 E

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : WEST PACIFIC
Dvorak CI > MSLP Conversion Used : PACIFIC

Tno/CI Rules : Constraint Limits : 0.7T/6hr
Weakening Flag : ON
Rapid Dissipation Flag : OFF
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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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