Official Discussion issued by the National Hurricane Center

Katia (AL122023) DATA RELEASED: 9/2/2023 9:00:00 PM UTC

Copy of official data

Tropical Storm Katia Discussion Number   6
NWS National Hurricane Center Miami FL       AL122023
500 PM AST Sat Sep 02 2023

Deep convection continues near and north of the center of Katia, 
with microwave data showing a tilted vortex due to the southerly 
shear.  Overall, the system is showing more internal structure than 
this morning, which is confirmed by increasing satellite estimates 
from many sources which mostly lie between 45-55 kt.  The initial 
wind speed is raised to 50 kt as a blend of this data.

Katia has turned northwestward at about 11 kt.  The storm should 
move between a large middle- to upper-level low to the west
and a narrow mid-latitude ridge over the eastern Atlantic for the 
next couple of days.  The biggest changes from this morning are 
that uncertainty is increasing in a few days when Katia, or the 
remnants, reaches a col point between a mid-latitude ridge building 
over the central Atlantic and deep trough over the northeastern 
Atlantic. The guidance suite literally has motions in all cardinal 
directions after that point, indicating a very low confidence 
situation, and the GEFS ensemble lows show the proverbial squashed 
spider pattern. Thus, the new forecast stalls the system before 
dissipating, but it will require some time to sort out the 
long-range details.

The earlier microwave data was showing a growing distance between 
the mid-level and low-level centers of the cyclone, suggesting that
southerly shear could be winning the battle soon (also indicated 
by the latest infrared satellite data).  A continuation of that 
shear plus a slow increase in dry air is likely to cause Katia to 
slowly weaken for the next few days.  The global models show no 
significant deep convection after 36 h, and remnant low status is 
indicated for 48 h.  Little change was made to the previous 
forecast, similar to the NOAA corrected consensus guidance.


INIT  02/2100Z 25.0N  30.5W   50 KT  60 MPH
12H  03/0600Z 25.9N  31.5W   45 KT  50 MPH
24H  03/1800Z 26.8N  32.8W   40 KT  45 MPH
36H  04/0600Z 27.5N  33.9W   35 KT  40 MPH
48H  04/1800Z 28.2N  35.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
60H  05/0600Z 28.5N  35.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
72H  05/1800Z 29.0N  35.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
96H  06/1800Z...DISSIPATED

Forecaster Blake