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Tropical Depression BLANCA


Tropical Depression Blanca Discussion Number  13...Retransmitted
NWS National Hurricane Center Miami FL       EP022021
300 PM MDT Wed Jun 02 2021

The satellite structure of Blanca has continued to deteriorate this 
afternoon. Visible satellite imagery shows an exposed low-level
swirl, and that circulation is becoming increasingly elongated. The
nearest deep convection is now more than 100 nm southeast of the
circulation center. A fortuitous 1519 UTC ASCAT-A pass over Blanca
revealed that the tropical cyclone may have been weaker than earlier
assessed with a peak wind retrieval of only 28 kt in the
northeastern quadrant. Even after accounting for instrument
undersampling, the lack of significant convection near the wind
field since that time has likely resulted in further spin down of
the vortex. The latest subjective Dvorak estimates were also down to
30 kt from SAB and 35 kt from TAFB. Given the deterioration of the
structure since then, Blanca is being downgraded to a 30-kt
tropical depression at this time.

As Blanca's low-level circulation has become increasingly detached 
from the deep convection, the motion of the storm has turned more
westward at 280/4 kt. Now that Blanca appears to be a shallow 
low-level feature, it will be primarily steered by the low-level
easterly trade wind flow. This will force Blanca westward over the
next several days as the circulation slowly spins down. The latest
NHC track forecast is largely an update from the previous one and
remains closest to the HCCA consensus model which is on the left
side of the guidance envelope.

Aside from the limited deep convection well separated from Blanca's
center, the storm already barely meets the criteria of a tropical
cyclone. It appears that dry, stable air that entered the
circulation from the northwest, aided by unfavorable vertical wind
shear, is winning the battle over sufficiently warm sea-surface
temperatures beneath the circulation. With the wind field spinning
down compared to 24 h ago, nocturnal re-formation of deep central
convection will find it more challenging to overcome the dry stable
airmass that the storm is embedded in. In addition, both the 12z
GFS- and ECMWF-model-forecast simulated IR brightness temperature
have trended towards less convective activity the last few cycles.
The current NHC forecast assumes occasional diurnal convective
bursts will help to maintain the status of the tropical cyclone for
another 24-36 h, but it is possible that Blanca could become a
remnant low as soon as tomorrow afternoon if convection does not
redevelop by then. 

INIT  02/2100Z 15.5N 112.8W   30 KT  35 MPH
 12H  03/0600Z 15.5N 113.4W   30 KT  35 MPH
 24H  03/1800Z 15.4N 114.2W   30 KT  35 MPH
 36H  04/0600Z 15.3N 115.5W   25 KT  30 MPH
 48H  04/1800Z 15.2N 117.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 60H  05/0600Z 15.2N 118.5W   20 KT  25 MPH...POST-TROP/REMNT LOW
 72H  05/1800Z 15.2N 119.8W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  06/1800Z...DISSIPATED
Forecaster Papin/Stewart