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Subtropical Storm Andrea Discussion Number   3
NWS National Hurricane Center Miami FL       AL012019
500 AM AST Tue May 21 2019

During the past few hours, deep convection has waned considerably,
likely due to the entrainment of dry mid-level from the south and
modest southerly vertical wind shear as indciated by GOES-16 mid-
and upper-level water vapor imagery. As mentioned in the previous
advisory discussion, an earlier ASCAT overpass just barely caught
35-kt winds about 50-55 nmi northeast of the low-level center, which
was likely just outside the radius of maximum winds. Since slightly
stronger winds could have existed closer to the center, the initial
intensity is being maintained at 35 kt for this advisory despite the
aforementioned degradation in the convective pattern.

The initial motion estimate is 360/05 kt. It appears that Andrea is
slowing down, implying that a turn toward the northeast should occur
within the next 6-12 h as a deep-layer trough and cold front
steadily approach the cyclone from the northwest. The global and
regional models remain in good agreement that Andrea will turn
northeastward later today, and then move eastward by tonight and
Wednesday when the cyclone becomes embedded within the approaching
deep-layer mid-latitude westerly flow. The NHC track forecast is
essentially unchanged from the previous advisory and lies a little
south of the middle of the track guidance envelope.

Although deep convection has decreased markedly, thunderstorm
activity should redevelop some by this afternoon and tonight when
Andrea will be moving eastward over an SST thermal ridge and into
a region of convective instability that exist between 65W-68W
longitude. which will also be coincident with the vertical wind
shear decreasing to less than 5 kt as per SHIPS model output.
However, some slight spin down of the vortex will likely occur
before convection can re-fire and re-strengthens the system. Thus,
the intensity forecast essentially calls for little change in
strength for the next 24 h or so, followed by weakening due to
increasing northwesterly vertical wind shear, and then merger with
a cold front on Wednesday.  The official intensity forecast closely
follows the IVCN consensus model and the weakening trend noted in
the global models.

FORECAST POSITIONS AND MAX WINDS

INIT  21/0900Z 30.0N  69.0W   35 KT  40 MPH
 12H  21/1800Z 30.7N  68.3W   35 KT  40 MPH
 24H  22/0600Z 31.2N  66.7W   35 KT  40 MPH
 36H  22/1800Z 31.3N  65.0W   30 KT  35 MPH
 48H  23/0600Z...DISSIPATED

$$
Forecaster Stewart

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