Post-Tropical Cyclone Arlene Discussion Number 9
NWS National Hurricane Center Miami FL AL012017
1100 AM AST Fri Apr 21 2017
Arlene has become embedded within the circulation of a large
extratropical cyclone, and lost most of its deep convection while
surrounded by cold air. The post-tropical cyclone will move toward
the southwest and south at about 20 kt until dissipation later
today.
This is the last advisory on Arlene. Additional information on this
system can be found in High Seas Forecasts issued by the National
Weather Service...under AWIPS header NFDHSFAT1 and WMO header FZNT01
KWBC.
Tropical Storm Arlene Discussion Number 6
NWS National Hurricane Center Miami FL AL012017
500 PM AST Thu Apr 20 2017
I have to add one more surprise to my long hurricane forecasting
career. Unexpectedly, the subtropical cyclone became a tropical
depression this morning, and then it intensified to a tropical
storm. This intensity estimate is based on the cloud pattern
presentation on satellite imagery which shows moderate thunderstorm
activity surrounding an eye-type feature, and a convective ring in
microwave imagery. Initial intensity is set at 40 kt, although
estimates from TAFB suggest that the winds could have reached 45 kt
around 1800 UTC. Since that time, the cloud pattern has deteriorated
somewhat and winds probably have diminished. Despite the
intensification, Arlene is still forecast by all global models to
become absorbed by a nearby developing extratropical cyclone on
Friday.
Arlene is moving toward the west-northwest at 22 kt, while well
embedded in the fast flow surrounding the extratropical low. This
general motion around the low is expected until dissipation on
Friday.
Tropical storms in April are rare and Arlene is only the second
one observed in this month during the satellite era. It should be
noted, however, that this type of storm was practically impossible
to detect prior to the weather satellite era.
Tropical Depression One Discussion Number 5
NWS National Hurricane Center Miami FL AL012017
1100 AM AST Thu Apr 20 2017
Conventional satellite imagery indicate that the convection,
although not very deep, has become more symmetric around the center,
suggesting that the subtropical cyclone has transitioned into a
tropical depression. This is supported by AMSU data this morning
that shows that the system has developed a weak, but warm core. This
transition is a common process, and does not change the previous
intensity or track forecasts that call for the depression to
become a remnant low later today, and become absorbed by a larger
low tonight or early Friday. It is estimated that the depression is
producing winds of 30 kt within some convective bands. Since the
circulation is already becoming elongated, a weakening trend is
likely to begin soon.
The depression is moving toward the northwest at 12 kt, and this
general motion with a gradual increase in forward speed is expected
to continue until dissipation.
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