Posted: Mon Sep 29, 2008 8:35 pm Post subject: Laura becoming Tropical?
Snip from SUBTROPICAL STORM LAURA DISCUSSION NUMBER 3
...THE ASSOCIATED CONVECTION IS NOT ALL
THAT DEEP AND THE 75 NM RADIUS OF MAXIMUM WINDS IS IN THE GRAY ZONE
BETWEEN TROPICAL AND SUBTROPICAL. LAURA WILL CONTINUE TO BE
CLASSIFIED AS A SUBTROPICAL STORM FOR NOW ALTHOUGH THE TRANSITION
TO A TROPICAL CYCLONE IS LIKELY OCCURRING
I thought a subtropical storm was subtropical primarily becasuse of it's location. What would cause it to take on characteristics of and name change to a tropical storm
Posted: Mon Sep 29, 2008 9:42 pm Post subject: Re: Laura becoming Tropical?
Sharlevi wrote:
Snip from SUBTROPICAL STORM LAURA DISCUSSION NUMBER 3
...THE ASSOCIATED CONVECTION IS NOT ALL
THAT DEEP AND THE 75 NM RADIUS OF MAXIMUM WINDS IS IN THE GRAY ZONE
BETWEEN TROPICAL AND SUBTROPICAL. LAURA WILL CONTINUE TO BE
CLASSIFIED AS A SUBTROPICAL STORM FOR NOW ALTHOUGH THE TRANSITION
TO A TROPICAL CYCLONE IS LIKELY OCCURRING
I thought a subtropical storm was subtropical primarily becasuse of it's location. What would cause it to take on characteristics of and name change to a tropical storm
It really has nothing to do with location as sub-tropical storms have formed in the Gulf of Mexico and in the Bahamas, as well as the tropical and sub-tropical Atlantic. In most cases sub-tropical systems develop from an upper level low or frontal low which are cold core systems. It's usually a very slow process as the low pressure systems circulation slowly works it's way down to the surface creating a surface circulation. As the surface circulation intensifies the expanded wind field which is common to non-tropical and sub-tropical systems begins to contract inwards closer towards the surface circulation. As the convection develops near and over the center it creates a CDO (central dense overcast) which in turn causes pressure falls in the center. As the pressure in the center continues to fall the convection also continues to increase and as the system spins up it morphs into a warm core system, feeding off the water for it's energy as opposed to a cold core system that feeds off upper air energy. When the process reaches a point that has the lowest pressure in the center and strongest convection and winds close to the center it is reclassified as tropical cyclone. When the process does not quite complete the transition it is classified as a sub-tropical cyclone that has both the characteristics of a tropical and sub-tropical system. It may have a surface circulation but could have the strongest winds well away from the center and the pressure gradient would be flatter over a long distance as opposed to the lowest pressure at the center in a pure tropical cyclone. _________________ This post is for discussion purposes only. Please refer to the National Hurricane Center for official information.
Posted: Tue Sep 30, 2008 12:00 pm Post subject: Re: Laura becoming Tropical?
WTNT42 KNHC 301433
TCDAT2
TROPICAL STORM LAURA DISCUSSION NUMBER 6
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL122008
1100 AM EDT TUE SEP 30 2008
WHILE SOMEWHAT OF A PARADOX...LAURA CAN NOW BE DESIGNATED AS A
TROPICAL CYCLONE DESPITE THE FACT THAT IT CONTINUES TO MOVE
NORTHWARD OVER PROGRESSIVELY COLDER WATERS. THE ONE ASPECT WHICH
WAS HOLDING US BACK FROM DECLARING IT TROPICAL WAS THE UPPER LOW
COLLOCATED WITH THE SYSTEM. WATER VAPOR IMAGERY INDICATES THAT
THIS FEATURE HAS BECOME OFFSET AND STRETCHED TO THE SOUTHWEST OF
LAURA...AND UPPER LEVEL OUTFLOW IS NOW EXPANDING TO THE NORTH OF
THE CYCLONE. IN ADDITION...SCATTEROMETER DATA INDICATE THAT THE
RADIUS OF MAXIMUM WINDS HAS CONTINUED TO CONTRACT TO ABOUT 60 NM.
CLOUD TOP TEMPERATURES ARE NOT ALL THAT COLD...BUT THAT IS QUITE
NORMAL FOR HIGH-LATITUDE TROPICAL CYCLONES DUE TO THE LOWER
TROPOPAUSE. FRANKLY...LAURA NOW LOOKS LIKE A TROPICAL STORM.
THE INITIAL INTENSITY IS HELD AT 50 KT BASED ON AN 0827 UTC QUIKSCAT
PASS ALTHOUGH THE AREA OF STRONGEST WINDS HAS BEEN SHRINKING.
LAURA SHOULD BE ABLE TO HOLD THAT INTENSITY FOR ONLY ANOTHER 12
HOURS OR SO BEFORE MOVING OVER SEA SURFACE TEMPERATURES LESS THAN
18C. GLOBAL MODEL FIELDS INDICATE THAT THERMAL ADVECTION AROUND
THE SYSTEM BECOMES MORE PROMINENT LATER TONIGHT...AND LAURA IS
EXPECTED TO BE EXTRATROPICAL IN 24 HOURS OR SO AS IT MIGRATES INTO
THE COLDER AIR MASS. SOME GRADUAL WEAKENING THROUGH 48 HOURS IS
EXPECTED DUE TO THE COLDER OCEAN WATERS AND MARGINAL BAROCLINIC
FORCING. THEREAFTER...SOME RE-STRENGTHENING AND AN EXPANSION OF
THE WIND FIELD COULD OCCUR AS THE EXTRATROPICAL LOW RACES EAST
WITHIN THE MID-LATITUDE WESTERLIES TOWARDS THE NORTHERN BRITISH
ISLES. EXCEPT FOR A SLIGHT WESTWARD SHIFT IN THE TRACK GUIDANCE AT
36-48 HOURS...THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE GLOBAL
MODELS FOR THIS CYCLE AND THE OFFICIAL FORECAST LIES VERY CLOSE TO
THE MODEL CONSENSUS. _________________ This post is for discussion purposes only. Please refer to the National Hurricane Center for official information.
Posted: Tue Sep 30, 2008 2:28 pm Post subject: Re: Laura becoming Tropical?
Lou,
Thanks for the great explanation and for indulging my random questions. Now after re-reading the NHC discussions, the forecasters comments are beginning to make more sense...for now.
Posted: Tue Sep 30, 2008 3:26 pm Post subject: Re: Laura becoming Tropical?
Sharlevi wrote:
Lou,
Thanks for the great explanation and for indulging my random questions. Now after re-reading the NHC discussions, the forecasters comments are beginning to make more sense...for now.
Your welcome. That was just the tip of the iceberg as many other factors come into play such as temperature and dew point profiles sampled at different altitudes by recon both outside and inside the core, satellite presentation regarding storm structure using the Dvorak interpretations, microwave and infrared satellite images, and Doppler Eyewall profiles. _________________ This post is for discussion purposes only. Please refer to the National Hurricane Center for official information.
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