ATL : TROPICAL DEPRESSION EIGHT (08L)

[ozonepete]

Big northerly component to its track...

[leanne_uk]

Maybe not time to go asleep on this one just yet



Still alot of dust to the west of the system but not as much as yesterday??

[cycloneye]

000
WTNT43 KNHC 260848
TCDAT3
TROPICAL DEPRESSION EIGHT DISCUSSION NUMBER 3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082009
500 AM AST SAT SEP 26 2009

WHILE DEEP CONVECTION HAS INCREASED OVER THE PAST FEW HOURS...
GEOSTATIONARY IMAGERY AND AN AMSR-E PASS FROM AROUND 0400 UTC
SUGGEST THAT THE LOW-LEVEL CENTER IS LOCATED ON THE SOUTHWESTERN
EDGE OF THE CONVECTION. THIS STRUCTURE IS LIKELY DUE TO THE 10-15
KT OF SOUTHWESTERLY SHEAR ANALYZED OVER THE CYCLONE BY UW-CIMSS AND
THE SHIPS MODEL. DVORAK CI-NUMBERS WERE 1.5 AND 2.0 FROM SAB AND
TAFB...RESPECTIVELY...AND THE INITIAL INTENSITY IS HELD AT 30 KT
FOR THIS ADVISORY. CONDITIONS WILL BE ONLY MARGINALLY FAVORABLE FOR
INTENSIFICATION OVER THE NEXT 2-3 DAYS. SSTS WILL BE 26-27C...AND
THE SHIPS MODEL SHOWS THE SHEAR DECREASING BELOW 15 KT ONLY FOR A
BRIEF PERIOD FROM 12 TO 24 HOURS. GIVEN THIS...THE OFFICIAL
FORECAST HOLDS THE INTENSITY AT 30 KT THROUGH 24 HOURS AND THEN
SHOWS A GRADUAL WEAKENING. HOWEVER...IT IS POSSIBLE THAT THE SYSTEM
COULD BRIEFLY REACH TROPICAL STORM STATUS DURING THE NEXT DAY OR
SO.

THE INITIAL MOTION ESTIMATE IS 300/12...A LITTLE TO THE LEFT OF THE
PREVIOUS ADVISORY. MOST OF THE TRACK GUIDANCE INDICATES THAT THE
CYCLONE WILL ROUND THE WESTERN EDGE OF THE SUBTROPICAL RIDGE AND
TURN TOWARD THE NORTHWEST OVER THE NEXT COUPLE OF DAYS. BEYOND THAT
TIME...THE GFS...NOGAPS AND HWRF SHOW THE DEPRESSION OR ITS REMNANTS
TURNING NORTHWARD AHEAD OF A FRONT...WHILE THE UKMET...ECMWF...AND
THE MEDIUM BAM SUGGEST THAT WHAT IS LEFT OF THE CYCLONE WILL TURN
WESTWARD. THE OFFICIAL FORECAST IS SOUTH OF THE PREVIOUS ONE FOR
THE FIRST 24 HOURS DUE TO THE INITIAL MOTION...AND IS AN UPDATE OF
THE PREVIOUS FORECAST BEYOND THAT TIME. AT DAY 3 ONLY A SLOW
NORTHWESTWARD MOTION IS INDICATED DUE TO THE LARGE SPREAD IN THE
GUIDANCE.

FORECAST POSITIONS AND MAX WINDS

INITIAL 26/0900Z 17.6N 33.8W 30 KT
12HR VT 26/1800Z 18.7N 35.3W 30 KT
24HR VT 27/0600Z 20.2N 36.9W 30 KT
36HR VT 27/1800Z 21.6N 38.1W 25 KT
48HR VT 28/0600Z 23.1N 39.3W 25 KT
72HR VT 29/0600Z 24.5N 40.5W 20 KT...REMNANT LOW
96HR VT 30/0600Z...DISSIPATED

$$
FORECASTER BRENNAN

[breeze]

From Dr. Jeff Master's Blog at Weather Underground:

Tropical Depression Eight no threat to land

Posted by: JeffMasters, 12:24 PM GMT on September 26, 2009
Tropical Depression Eight formed last night near the coast of Africa, but is not destined
to threaten land, and has only about a 50/50 chance of becoming Tropical Storm Grace.
While wind shear is a moderate 10 - 20 knots, Sea Surface Temperatures (SSTs) are right
at the threshold for tropical storm formation--26°C. There is only a small amount of dry
air that is interfering with the storm, but the combination of cool SSTs and moderate
wind shear will make TD 8 slow to organize. By Monday, shear is expected to reach a
very high 25 - 30 knots, and this should tear the storm apart.

None of the computer models are forecasting tropical storm formation elsewhere in the
Atlantic over the next seven days.

[wxman57]

Just a sheared mess this morning. Visible satellite always tells the true story. Maybe a name won't be wasted on it...

[Dean4Storms]

Does not stand much of a chance at even becoming a TS! Next!

[cycloneye]

000
WTNT43 KNHC 261431
TCDAT3
TROPICAL DEPRESSION EIGHT DISCUSSION NUMBER 4
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082009
1100 AM AST SAT SEP 26 2009

SATELLITE IMAGERY THIS MORNING INDICATES THAT TROPICAL DEPRESSION
EIGHT HAS BECOME A LITTLE LESS ORGANIZED...WHICH IS LIKELY TO DUE
TO 20 KT OF WEST-SOUTHWESTERLY VERTICAL WIND SHEAR INDICATED BY
ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN. THE
CIRCULATION HAS BECOME ELONGATED ALONG A NORTHEAST-SOUTHWEST
AXIS...WITH TWO OR THREE EMBEDDED VORTICITY CENTERS NOTED. IN
ADDITION...THE CONVECTIVE BANDING IS FARTHER FROM THE CENTER THAN
24 HR AGO. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN
30 AND 25 KT...SO THE INITIAL INTENSITY REMAINS 30 KT.

THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 295/12. THE LARGE-SCALE
MODELS FORECAST A LARGE-DEEP LAYER TROUGH AND ASSOCIATED SURFACE
COLD FRONT CURRENTLY OVER THE WESTERN ATLANTIC TO MOVE EASTWARD
DURING THE NEXT 48-96 HR. THIS SHOULD CAUSE THE SUBTROPICAL RIDGE
OVER THE EASTERN ATLANTIC TO WEAKEN...AND SHOULD ALLOW THE TROPICAL
DEPRESSION TO TURN NORTHWARD AND THEN RECURVE INTO THE WESTERLIES
ON THE SOUTH SIDE OF THE TROUGH. MOST OF THE TRACK GUIDANCE CALLS
FOR THIS SCENARIO...AND THE NEW TRACK FORECAST IS FOR RECURVATURE
AFTER 72 HR. AN ALTERNATE SCENARIO IS SHOWN BY THE ECMWF...WHICH
FORECASTS THE DEPRESSION TO WEAKEN WITH THE REMNANT TROUGH MOVING
WESTWARD IN THE LOW-LEVEL FLOW.

THE INTENSITY FORECAST IS A LITTLE PROBLEMATIC. THE DEPRESSION IS
NOT ONLY ENCOUNTERING MODERATE SHEAR...BUT IS ALSO OVER A TONGUE OF
COLDER SEA SURFACE TEMPERATURES AND HAS AN AREA OF DRY AIR JUST TO
ITS WEST. WHILE THE SHEAR IS FORECAST TO CHANGE LITTLE DURING THE
NEXT 48-60 HR...THE SSTS INCREASE ALONG THE FORECAST TRACK. THE
MOST LIKELY SCENARIO IS THAT THE COMBINATION OF SHEAR AND DRY AIR
WILL CAUSE THE DEPRESSION TO GRADUALLY DECAY OVER THE NEXT
48-72 HR. ON THE OTHER HAND...WITH THE INCREASING SSTS...THE
CYCLONE COULD STRENGTHEN IF AND WHEN THE SHEAR DECREASES...AS
SUGGESTED AFTER 72 HR BY THE SHIPS...LGEM...AND GFDL MODELS. THE
NEW INTENSITY FORECAST GOES WITH THE WEAKENING SCENARIO...CALLING
FOR THE DEPRESSION TO DECAY INTO A REMNANT LOW IN ABOUT 72 HR.
HOWEVER... THERE IS LOWER CONFIDENCE IN THIS SCENARIO THAN THERE
WAS YESTERDAY.

THE DYNAMICAL MODELS SHOW THE DEPRESSION OR ITS REMNANTS SURVIVING
LONGER THAN THEY DID 24 HR AGO...SO A 96 HR FORECAST POINT HAS BEEN
ADDED. THE SYSTEM IS EXPECTED TO DISSIPATE OR BE ABSORBED INTO THE
BAROCLINIC TROUGH BY 120 HR.

FORECAST POSITIONS AND MAX WINDS

INITIAL 26/1500Z 18.0N 34.8W 30 KT
12HR VT 27/0000Z 19.1N 36.4W 30 KT
24HR VT 27/1200Z 20.5N 37.8W 30 KT
36HR VT 28/0000Z 21.9N 38.9W 25 KT
48HR VT 28/1200Z 23.0N 40.0W 25 KT
72HR VT 29/1200Z 24.5N 40.5W 20 KT...REMNANT LOW
96HR VT 30/1200Z 26.0N 37.0W 20 KT...REMNANT LOW
120HR VT 01/1200Z...DISSIPATED

$$
FORECASTER BEVEN

[cycloneye]

...SUMMARY OF 1100 AM AST INFORMATION...
LOCATION...18.0N 34.8W
MAXIMUM SUSTAINED WINDS...35 MPH
PRESENT MOVEMENT...WEST-NORTHWEST OR 295 DEGREES AT 14 MPH
MINIMUM CENTRAL PRESSURE...1008 MB

[leanne_uk]

Anyone got the latest models for this system?

[HURAKAN]

Looking better

[HURAKAN]

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

----- Current Analysis -----
Date : 26 SEP 2009 Time : 154500 UTC
Lat : 18:04:48 N Lon : 34:54:37 W

CI# /Pressure/ Vmax
2.4 /1005.8mb/ 34.0kt

Final T# Adj T# Raw T#
(3hr avg)
1.4 1.2 1.0

Latitude bias adjustment to MSLP : +0.0mb

Center Temp : +16.8C Cloud Region Temp : -5.9C

Scene Type : SHEAR (>1.25^ TO DG)

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : 0.5T/hour
Weakening Flag : ON
Rapid Dissipation Flag : FLAG

****************************************************

[brunota2003]

It looks worse, the LLC is way below and to the left of the convection (just to the left of the 35* line down near the bottom.

[HURAKAN]

Loop

[HURAKAN]

It looks worse, the LLC is way below and to the left of the convection (just to the left of the 35* line down near the bottom.

Look at this loop, convection appears to be developing over the center.

Loop: http://rammb.cira.colostate.edu/product ... 251830.GIF

[ncweatherwizard]

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

----- Current Analysis -----
Date : 26 SEP 2009 Time : 154500 UTC
Lat : 18:04:48 N Lon : 34:54:37 W

CI# /Pressure/ Vmax
2.4 /1005.8mb/ 34.0kt

Final T# Adj T# Raw T#
(3hr avg)
1.4 1.2 1.0

Latitude bias adjustment to MSLP : +0.0mb

Center Temp : +16.8C Cloud Region Temp : -5.9C

Scene Type : SHEAR (>1.25^ TO DG)

Positioning Method : FORECAST INTERPOLATION

Ocean Basin : ATLANTIC
Dvorak CI > MSLP Conversion Used : ATLANTIC

Tno/CI Rules : Constraint Limits : 0.5T/hour
Weakening Flag : ON
Rapid Dissipation Flag : FLAG

****************************************************

That algorithm has been stuck on 2.4 for a few hours now. Their adjusted T-number has been erratic but is showing a weakening trend this morning. We'll have to see if a circulation can sustain itself near the convection.

[brunota2003]

How does one get 2.4 from 1.4, 1.2, and 1.0?

[HURAKAN]

Latest

[HURAKAN]

AL, 08, 2009092618, , BEST, 0, 186N, 352W, 25, 1008, TD

Down to 30 mph

[ozonepete]

How does one get 2.4 from 1.4, 1.2, and 1.0?

After they come up with the Final, Adjusted and Raw T numbers, they apply their Rule 9 for weakening.

Here's the excerpt:
E.) Weakening Flag (Dvorak EIR Rule 9)
The ADT Weakening Flag is modeled after the Dvorak EIR Rule 9 (Dvorak, 1984), but also utilizes a modification that is applied at some operational TC forecasting centers. This rule is used in the determination of the CI# after a storm has reached its maximum intensity and is weakening. The EIR Rule 9 will hold the CI# at values 0.5 to 1.0 higher in value than the current Final T#. Subjective application of this rule (e.g. how and when to apply it) varies between forecasters and is the focal point of much debate. The value of 1.0 is utilized in the ADT since it provides the most statistically accurate estimates of storm intensity when compared with reconnaissance pressure measurements.

Here's the flowchart:

[cycloneye]

000
WTNT33 KNHC 262031
TCPAT3
BULLETIN
TROPICAL DEPRESSION EIGHT ADVISORY NUMBER 5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082009
500 PM AST SAT SEP 26 2009

...DEPRESSION WEAKENS TO A TROPICAL WAVE...

AT 500 PM AST...2100 UTC...THE CENTER OF THE REMNANTS OF TROPICAL
DEPRESSION EIGHT WERE LOCATED NEAR LATITUDE 18.9 NORTH...LONGITUDE
35.7 WEST OR ABOUT 795 MILES...1280 KM...WEST-NORTHWEST OF THE CAPE
VERDE ISLANDS.

THE REMNANTS OF THE DEPRESSION ARE MOVING TOWARD THE NORTHWEST NEAR
13 MPH...20 KM/HR...AND THIS GENERAL MOTION SHOULD CONTINUE WITH
SOME DECREASE IN FORWARD SPEED DURING THE NEXT COUPLE OF DAYS.

MAXIMUM SUSTAINED WINDS ARE NEAR 30 MPH...45 KM/HR...WITH HIGHER
GUSTS.

ESTIMATED MINIMUM CENTRAL PRESSURE IS 1008 MB...29.77 INCHES.

...SUMMARY OF 500 PM AST INFORMATION...
LOCATION...18.9N 35.7W
MAXIMUM SUSTAINED WINDS...30 MPH
PRESENT MOVEMENT...NORTHWEST OR 305 DEGREES AT 13 MPH
MINIMUM CENTRAL PRESSURE...1008 MB

THIS IS THE LAST PUBLIC ADVISORY ISSUED BY THE NATIONAL HURRICANE
CENTER ON THIS SYSTEM UNLESS REGENERATION OCCURS.

$$
FORECASTER BEVEN

TROPICAL DEPRESSION EIGHT DISCUSSION NUMBER 5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082009
500 PM AST SAT SEP 26 2009

SATELLITE IMAGERY THIS AFTERNOON SHOWS THAT THE CIRCULATION OF
TROPICAL DEPRESSION EIGHT HAS WEAKENED TO AN ELONGATED TROUGH WITH
SEVERAL EMBEDDED VORTICITY CENTERS. WHILE THE SYSTEM CONTINUES TO
PRODUCE CONVECTION THAT IS CLASSIFIABLE USING THE DVORAK
TECHNIQUE...IT NO LONGER HAS A WELL-ENOUGH DEFINED CIRCULATION TO
BE CONSIDERED A TROPICAL DEPRESSION. THE REMNANT TROUGH OR
TROPICAL WAVE IS EXPECTED TO CONTINUE MOVING NORTHWESTWARD FOR THE
NEXT COUPLE OF DAYS.

THIS IS THE LAST ADVISORY ISSUED ON THIS SYSTEM BY THE NATIONAL
HURRICANE CENTER UNLESS REGENERATION OCCUR.

FORECAST POSITIONS AND MAX WINDS

INITIAL 26/2100Z 18.9N 35.7W 25 KT
12HR VT 27/0600Z...DISSIPATED

$$
FORECASTER BEVEN