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Tropical Storm Blanca Discussion Number   9
NWS National Hurricane Center Miami FL       EP022021
300 PM MDT Tue Jun 01 2021
 
Over the last 12 h, Blanca's convective structure has continued to 
degrade. The low-level circulation center is now fully exposed to 
the west of a small region of deep convection, though 
disorganized convective towers are currently trying to redevelop 
closer to the center. Even though deep-layer southwesterly 
200-850-mb vertical wind shear diagnosed by GFS-SHIPS is still only 
15-20 kt, stronger 25-30 kt mid-level shear appears to be 
undercutting Blanca's outflow layer. This shear may have resulted in 
Blanca ingesting dry, stable mid-level air from the west that has 
significantly disrupted the cyclone's convective structure today. 
Unfortunately all three scatterometer passes missed Blanca's center 
and maximum winds this afternoon. However, given the marked decrease 
in organization of Blanca's structure, plus recent subjective 
satellite estimates from SAB and TAFB at T3.0/45 kt, the initial 
intensity has been lowered to 45 kt for this advisory.

Blanca's exposed center has made it easier to determine its current 
position and motion, which over the past 12 h is estimated at 300/8 
kt, though the shorter-term motion has been more westward. As 
previously discussed, the mid-level ridge that had been steering 
Blanca to the west-northwest has been gradually weakening as a 
pronounced mid- to upper-level trough centered over the Baja 
California peninsula digs in. The end result is Blanca's forward 
motion toward the west-northwest is likely to slow further. 
Additional asymmetric convective bursts primarily occuring east of 
the low-level center may also act to slow down Blanca's forward 
motion. As the storm becomes more vertically shallow, it will 
increasingly be influenced by the low-level steering flow resulting 
in a gradual westward bend in the forecast track until Blanca 
dissipates. The latest NHC forecast track has shifted a bit more 
south and west this cycle, owing to the possibility that Blanca may 
become a shallow vortex sooner than expected, but still agrees 
closely with the HCCA corrected consensus with a little more weight 
placed on the leftward bending guidance.

The same mid- to upper-level trough slowing the steering currents 
have also resulted in a significant increase in mid-level shear over 
Blanca, halting any further intensification. Over the next 12-24 h, 
intermittent diurnal convective bursts as the cyclone remains over 
28-29 C sea surface temperatures should lead to only gradual 
weakening. However, even drier mid-level air and lower sea-surface 
temperatures exist along Blanca's forecast track and the cyclone is 
forecast to weaken to a tropical depression by 60 h and degenerate 
to a remnant low by 96 h, though this could occur sooner if 
organized convection dissipates faster than forecast.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  01/2100Z 14.7N 110.9W   45 KT  50 MPH
 12H  02/0600Z 15.0N 111.6W   40 KT  45 MPH
 24H  02/1800Z 15.4N 112.3W   40 KT  45 MPH
 36H  03/0600Z 15.6N 112.9W   35 KT  40 MPH
 48H  03/1800Z 15.7N 113.5W   35 KT  40 MPH
 60H  04/0600Z 15.7N 114.4W   30 KT  35 MPH
 72H  04/1800Z 15.8N 115.5W   30 KT  35 MPH
 96H  05/1800Z 15.9N 117.6W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  06/1800Z 15.9N 119.2W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Papin/Stewart
 
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