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Subtropical Storm Wanda Discussion Number 4
NWS National Hurricane Center Miami FL AL212021
900 PM GMT Sun Oct 31 2021
Wanda has struggled a bit this afternoon, as it appears that some
dry air has entrained into its circulation. Deep convection is now
mainly confined to a band wrapping around the eastern semicircle of
the cyclone. Remarkably, the 3 ASCAT overpasses today missed
the core of Wanda, however, the latest Dvorak intensity estimates
from TAFB, and the UW-CIMMS ADT and SATCON remain consistent, and
indicate an initial advisory intensity of 45 kt.
The storm made a rather abrupt halt in its eastward motion this
morning, and for much of the day has been drifting westward, with an
initial motion of 90/1 kt. Wanda is expected to make a slow, nearly
180 degree turn to the south, east, then northeast through Tuesday
as it pivots around a mid-tropospheric trough. A faster northeast to
north motion is expected through Wednesday as ridging builds to the
east of the cyclone. The model guidance is in reasonable agreement
on this scenario, and there were only minor tweaks to this portion
of the NHC track forecast. Thereafter, the guidance has made quite a
startling change from the previous runs, and is now indicating that
a blocking ridge may build to the north of Wanda by late this week.
This is a major shift in the guidance from previous runs that showed
the cyclone accelerating northeastward into the mid-latitudes. The
latest NHC track forecast tries to adjust for this change by
indicating a slower forward speed and less poleward motion late in
the 5-day forecast period, but this remains well north and east of
the bulk of the guidance. There is also a large amount of spread now
in the guidance beyond 72 h, and therefore, that portion of the
track forecast is of low confidence.
The upper-trough supporting the subtropical nature of Wanda is
forecast to lift out to the northeast by Monday. In addition, over
the past day or so, with each passing advisory cycle the cyclone has
taken on a little more of the appearance of a tropical cyclone
versus a subtropical cyclone. Model guidance is in good agreement on
this transition occurring soon, as per the latest FSU phase space
forecasts. Therefore, Wanda is expected to transition to a tropical
cyclone by Monday. The recent ingestion of dry air may have ended
the chance for further strengthening of the storm, and the latest
NHC intensity forecast shows no change in strength over the next few
days. Beyond 72 h, the intensity forecast becomes murky as there
is now a high level of uncertainty as to where Wanda may be headed
later this week. The previous NHC forecast had the cyclone moving
over much cooler waters late this week. However, the latest shift in
the track guidance suggests the cyclone could remain over the
current 24 degrees C waters throughout the next 5 days. Regardless
of the differing tracks and resulting sea surface temperatures
below, the cold pool of air in the upper troposphere that has been
giving Wanda a chance to maintain its deep convection will begin to
vanish by midweek, therefore, weakening is indicated in the
official NHC intensity forecast. The latest NHC intensity forecast
was decreased slightly from the previous one, and is close to the
various intensity consensus solutions.
FORECAST POSITIONS AND MAX WINDS
INIT 31/2100Z 36.3N 43.2W 45 KT 50 MPH
12H 01/0600Z 35.7N 43.0W 45 KT 50 MPH
24H 01/1800Z 35.2N 42.2W 45 KT 50 MPH...TROPICAL CYCLONE
36H 02/0600Z 35.6N 41.1W 45 KT 50 MPH
48H 02/1800Z 36.8N 40.2W 45 KT 50 MPH
60H 03/0600Z 38.6N 39.7W 45 KT 50 MPH
72H 03/1800Z 40.7N 39.4W 45 KT 50 MPH
96H 04/1800Z 44.1N 37.0W 45 KT 50 MPH
120H 05/1800Z 45.4N 32.0W 40 KT 45 MPH...POST-TROPICAL
$$
Forecaster Latto
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