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Tropical Storm Aletta Discussion Number  21
NWS National Hurricane Center Miami FL       EP022018
200 PM PDT Sun Jun 10 2018

Despite southerly vertical wind shear of about 15 kt, Aletta has
been able to maintain a small cluster of strong convection in the
northwestern quadrant. Recent ASCAT passes missed the now partially
exposed low-level center and much of the inner-core wind field, but
they did manage to capture tropical-storm-force winds in the eastern
semicircle. Also, a Mexican Navy automated weather station on
Clarion Island reported a sustained wind of 33 kt and a gust to 47
kt a few hours ago as an outer band passed over the island. Based on
that observation, along with a TAFB Dvorak estimate of T3.0/45 kt,
the intensity is being maintained at 40 kt for this advisory.

Microwave satellite position fixes indicate a motion of about 280/06
kt since the previous advisory. No significant changes to the
previous forecast track were required. The new model guidance
remains in fair agreement on Aletta moving slowly westward for the
next 48-72 h along the southern periphery of a weak low- to
mid-level ridge to the north of the cyclone.  After that time,
steering currents are forecast to collapse, along with Aletta coming
under the influence of rapidly strengthening Hurricane Bud currently
located to the east-southeast, which could result in the shallow
cyclone stalling. The official forecast track is similar to the
previous advisory and remains in the middle of the tightly packed
track model guidance.

Although the wind shear is forecast to weaken to less than 10 kt
during the next 24-36 hours, which would generally favor
re-strengthening, Aletta will also be moving over sub-26C SSTs and
into a drier and more stable airmass. The poor thermodynamics, along
with cold-air stratocumulus clouds already being ingested into the
western and southern quadrants of the cyclone, argues for gradual
weakening to occur over the next day or so, with Aletta degenerating
into a remnant low pressure system by Tuesday. A steady spin down of
the low is expected during the 72-120 hour period, with the shallow
cyclone possibly even dissipating on day 5. The official intensity
forecast is similar to the previous advisory, and closely follows
a blend of the HCCA and IVCN consensus intensity models.

FORECAST POSITIONS AND MAX WINDS

INIT  10/2100Z 16.6N 115.7W   40 KT  45 MPH
 12H  11/0600Z 16.6N 116.6W   35 KT  40 MPH
 24H  11/1800Z 16.6N 117.6W   30 KT  35 MPH
 36H  12/0600Z 16.5N 118.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  12/1800Z 16.4N 119.6W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  13/1800Z 16.2N 120.6W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  14/1800Z 16.1N 120.9W   20 KT  25 MPH...POST-TROP/REMNT LOW
120H  15/1800Z 15.9N 121.1W   15 KT  15 MPH...POST-TROP/REMNT LOW

$$
Forecaster Stewart

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