[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT44 KNHC 022047
TCDAT4

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.


FORECAST POSITIONS AND MAX WINDS

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
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