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Tropical Depression Grace No Biggie For Florida, But Depression 8 Sure Is Odd

Tropical depression 8

National Hurricane Center Now Watching Three Systems.

Tropical Depression Eight. Unlikely to affect Florida. (National Hurricane Center).


BOCA RATON, FL ( (Copyright © 2021 MetroDesk Media, LLC) — Tropical Storm Grace is unlikely to be much of an issue for Florida, according to the latest forecast track from the National Hurricane Center. But Tropical Depression 8, while also unlikely to directly impact South Florida weather, is certainly one to watch. It’s moving south. That’s rare.

We are publishing the National Hurricane Center updates for both “eight” and “Grace,” below.


Tropical Depression Eight Discussion Number 2
NWS National Hurricane Center Miami FL AL082021
500 AM AST Mon Aug 16 2021

Satellite imagery and radar data from Bermuda indicate that the
tropical depression located about 100 nmi east of Bermuda has been
moving southward and also remains a sheared tropical cyclone — the
ongoing saga of the Atlantic basin thus far this hurricane season.
Convection has increased and developed closer to the center and a
late-arriving ASCAT-C overpass indicated surface winds of 26-27 kt
to the south of the well-defined circulation center. Based on those
data, the slightly improved convective structure noted in both
satellite and radar data, and satellite classifications of T2.0/30
kt, the initial intensity has been increased to 30 kt.

The initial motion estimate is 195/04 kt. The small cyclone is
starting to make the clockwise turn toward the south-southwest and
eventually southwest that the global and regional models have been
advertising the past couple of days. The latest NHC model guidance
now shows less of a turn to the northwest after 72 hours, with the
exception of the GFS, GFS-ensemble, and HMON models, which continue
to take the cyclone northward and northeastward around Bermuda on
day 4 and 5. The latest NHC track forecast is similar to the
previous advisory track through 60 hours, and then widens the track
slightly thereafter, but still shows a complete clockwise turn. This
track scenario is to the right of the consensus models at all
forecast periods, and is of low confidence on days 4 and 5.

The vertical wind shear is forecast to remain at or below 10 kt for
the next 36 hours or so, and that is most likely the time when the
cyclone will strengthen. Thereafter, an increase in the shear to 20
kt or more is expected to cap the intensification process. The HWRF
model turn the cyclone into a major hurricane by 72 hours, but this
scenario is considered to be an extreme outlier given the amount of
shear forecast to affect the system on days 3-5. So far this season,
the HWRF model has been very shear resistant and has had a high
intensity bias. The official intensity forecast closely follows a
blend of the IVCN and HCCA consensus models through 60 hours, and
then is below those models due to the high-bias contribution of the
HWRF model.


INIT 16/0900Z 32.4N 62.9W 30 KT 35 MPH
12H 16/1800Z 31.6N 63.1W 35 KT 40 MPH
24H 17/0600Z 31.0N 63.7W 40 KT 45 MPH
36H 17/1800Z 30.7N 64.5W 45 KT 50 MPH
48H 18/0600Z 30.7N 65.5W 50 KT 60 MPH
60H 18/1800Z 30.8N 66.7W 55 KT 65 MPH
72H 19/0600Z 31.1N 68.0W 55 KT 65 MPH
96H 20/0600Z 32.0N 69.0W 55 KT 65 MPH
120H 21/0600Z 34.0N 68.5W 55 KT 65 MPH

Tropical Depression Grace as of Monday morning, August 16, 2021.


Tropical Depression Grace Discussion Number 12
NWS National Hurricane Center Miami FL AL072021
500 AM EDT Mon Aug 16 2021

The structure and organization of Tropical Depression Grace has
changed little over the past several hours. An earlier ASCAT-B
overpass showed that the circulation is elongated from the NE-SW,
with peak winds of 30 kt. An Air Force Reserve Hurricane Hunter
Aircraft investigated Grace a few hours ago, and indicated that the
cyclone refuses to make the decrease in forward speed and turn to
the west-northwest that most of the guidance has been calling for.
The aircraft also measured peak flight level winds of 37 kt at 925
mb, which equates to about 28 kt at the surface. Although there were
some SFMR vectors of tropical storm force, those winds were sampled
in heavy rainfall and were likely not representative of the actual
surface winds. Based on this data, the initial intensity is kept at
30 kt for this advisory.

Grace is moving toward the west, or 275/13 kt. The forecast models
continue to make a shift to the south, as they are coming into very
good agreement on maintaining a strong mid-level ridge to the north
of the cyclone throughout the forecast period. The latest track
forecast was adjusted a little south of the previous one through the
first couple of days of the forecast, with a larger shift to the
south farther out in time. This forecast track remains to the north
of the track consensus, so if the track models remain consistent,
additional adjustments to the south will likely be made in the next
forecast cycle.

The evolution of the forecast track of Grace is having major
implications on the future intensity of the cyclone. It is becoming
more likely that the system will track over only the southern
portion of Hispaniola, which would result in a lesser disruption
of Grace’s circulation. Furthermore, the more southern track would
bring the cyclone over the warm waters of the northwestern Caribbean
beginning on Tuesday. The environment over that portion of the
Caribbean is not all that hostile, with about 10-15 kt of northerly
shear being the main inhibiting factor for intensification. By
midweek, Grace may interact with the Yucatan Peninsula, which
should limit strengthening or perhaps weaken the system depending on
the track. Once over the western Gulf of Mexico, late in the
forecast period, some additional strengthening is expected,
although the wind shear is forecast to increase slightly during
that time frame. The latest NHC intensity forecast was raised
slightly to reflect the additional time expected over water.
However, this forecast is a little lower than most of the guidance.

Key Messages:

  1. Heavy rainfall across the Dominican Republic, Haiti, Cuba,
    Jamaica, and the Cayman Islands may lead to flash, urban, and small
    stream flooding, with the potential for mudslides highest in Haiti
    and the Dominican Republic.
  2. Tropical storm conditions are possible over portions of
    Hispaniola today and tonight.
  3. There is a risk of some wind and rainfall impacts across
    southeastern Cuba and Jamaica beginning Tuesday morning, and the
    Cayman Islands by Tuesday night. Interests there areas should
    monitor the progress of Grace and updates to the forecast.


INIT 16/0900Z 17.4N 70.2W 30 KT 35 MPH
12H 16/1800Z 17.9N 72.2W 30 KT 35 MPH
24H 17/0600Z 18.6N 74.8W 30 KT 35 MPH
36H 17/1800Z 19.3N 77.4W 35 KT 40 MPH
48H 18/0600Z 20.1N 80.2W 35 KT 40 MPH
60H 18/1800Z 20.8N 83.3W 40 KT 45 MPH
72H 19/0600Z 21.6N 86.3W 45 KT 50 MPH
96H 20/0600Z 22.6N 91.8W 45 KT 50 MPH
120H 21/0600Z 23.4N 96.2W 50 KT 60 MPH

The National Hurricane Center is watching three systems as of August 16, 2021.



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