ATL: DEBBY - Post-Tropical

[cycloneye]

Subtropical Storm Debby Discussion Number 3
NWS National Hurricane Center Miami FL AL042018
1100 PM AST Tue Aug 07 2018

A little bit of deep convection has developed near the center of
Debby, but the overall cloud pattern still resembles more of a
subtropical cyclone. Most of the convection is in a ragged band
well to the northeast of the center. The initial intensity is held
at 35 kt based on a subtropical ST2.5 classification from TAFB.
The global models are in good agreement that the system will open
up into a trough on the southern side of a large low pressure
system within 48 hours.

The initial motion estimate is a slightly slower 360/11 kt. Debby
is likely to turn northeastward in the flow on the eastern side of
a mid-level trough. The official forecast is somewhat faster than
the latest model consensus to maintain some continuity with the
previous NHC track.

FORECAST POSITIONS AND MAX WINDS

INIT 08/0300Z 40.3N 49.1W 35 KT 40 MPH
12H 08/1200Z 41.2N 48.9W 35 KT 40 MPH
24H 09/0000Z 42.5N 47.5W 35 KT 40 MPH
36H 09/1200Z 43.9N 44.7W 30 KT 35 MPH
48H 10/0000Z...DISSIPATED

$$
Forecaster Pasch

[NDG]

Not bad on latest ASCAT.

[AJC3]

BULLETIN
Tropical Storm Debby Advisory Number 4
NWS National Hurricane Center Miami FL AL042018
500 AM AST Wed Aug 08 2018

...DEBBY STRENGTHENS SLIGHTLY WHILE MAKING THE TRANSITION TO A
TROPICAL STORM OVER THE FAR NORTH ATLANTIC...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...40.8N 48.8W
ABOUT 1175 MI...1890 KM WNW OF THE AZORES
MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H
PRESENT MOVEMENT...NNE OR 15 DEGREES AT 9 MPH...15 KM/H
MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES

Tropical Storm Debby Discussion Number 4
NWS National Hurricane Center Miami FL AL042018
500 AM AST Wed Aug 08 2018

Over the past several hours, deep convection with cloud tops of -55
to -60 deg C has developed in the southeastern semicircle, with
some of the convective tops covering the previously exposed
low-level circulation center. In addition, outer banding
features have dissipated, and an elongated upper-level anticyclone
has developed over the cyclone. These convective- and synoptic-scale
features indicate that Debby has made the transition from a
subtropical to a tropical cyclone. The initial intensity of 40 kt
is based on a blend of UW-CIMSS ADT and SATCON intensity estimates
of 39 kt and 42 kt, respectively. Furthermore, the 34-kt wind radii
and radius of maximum winds (RMW) were decreased significantly on
this advisory based on ASCAT wind data.

The initial motion estimate is now 015/08 kt. Debby has made the
forecast turn toward the north-northeast, and a further turn toward
the northeast is expected later today as the cyclone moves around
the northwestern periphery of a deep-layer ridge and ahead of an
approaching mid-level trough. The latest model guidance remains in
good agreement on this developing track scenario, and the new NHC
forecast track is similar to the previous advisory, and lies close
to a blend of the track consensus models HCCA and FSSE.

Debby will be moving along a tight sea-surface temperature (SST)
gradient for the next 24-36 hours or so, with the northwestern half
the circulation being over sub-25C SSTs and the southeastern
semicircle being over warmer waters where convection could continue
to develop. Given the relatively low vertical wind shear regime
that the cyclone will be moving through, along with the possibility
of deep convection persisting near the center, the intensity
forecast calls for little change in strength today, followed by
only slight weakening tonight and early Thursday. By Thursday night
or early Friday, SSTs beneath the cyclone are expected to decrease
to near 20 deg C and the shear is forecast to increase to more than
20 kt, an unfavorable combination that should result in Debby
dissipating over the far north Atlantic by 48 hours.

FORECAST POSITIONS AND MAX WINDS

INIT 08/0900Z 40.8N 48.8W 40 KT 45 MPH
12H 08/1800Z 41.8N 48.2W 40 KT 45 MPH
24H 09/0600Z 43.2N 46.1W 35 KT 40 MPH
36H 09/1800Z 44.5N 43.4W 30 KT 35 MPH
48H 10/0600Z...DISSIPATED

[NotoSans]

The only basin in the world to recognize such systems regularly and is good at it. I know of a few in the WPAC this year that were likely *Subtropical*.

The MFR also classifies subtropical storms.

[NDG]

No question that Debby has now transitioned into a fully tropical system this morning.

[AnnularCane]

Anyone still thinking she shouldn't have been named?

[CyclonicFury]

Debby has transitioned into a more traditional tropical cyclone this morning with the center now located under a ball of moderate convection.

[HurricaneEnzo]

Debby has transitioned into a more traditional tropical cyclone this morning with the center now located under a ball of moderate convection.

Agree, has definitely earned a name now although impacts to any landmass will be minimal to none.

[CyclonicFury]

190
WTNT44 KNHC 090241
TCDAT4

Tropical Storm Debby Discussion Number 7
NWS National Hurricane Center Miami FL AL042018
1100 PM AST Wed Aug 08 2018

A very recent ASCAT pass indicates that Debby is stronger than
previously estimated. The pass showed maximum winds of about 40 kt,
and since the entire circulation was not captured, the initial
intensity is set at 45 kt. This estimate is also in agreement with
a Dvorak CI-number from TAFB. Although the relatively compact
tropical storm is maintaining an area of moderate convection, the
thunderstorm activity is displaced to the south-southwest of the
center due to about 15 kt of wind shear. Debby is currently over
fairly cool 25 deg C SSTs, but it will be headed for much cooler
water during the next 24 to 36 hours. These unfavorable oceanic
conditions combined with an increasingly stable airmass should cause
weakening and extratropical transition. The NHC intensity forecast
shows Debby becoming extratropical by 24 hours and dissipating
shortly thereafter, following the GFS and ECMWF models.

The tropical storm is moving northeastward, or 040 degrees, at 13
kt. A faster northeastward motion is expected until the cyclone
opens into a trough as Debby becomes more embedded in the
mid-latitude flow. The NHC track forecast lies near the middle of
the guidance envelope.

FORECAST POSITIONS AND MAX WINDS

INIT 09/0300Z 42.8N 46.5W 45 KT 50 MPH
12H 09/1200Z 44.4N 43.7W 40 KT 45 MPH
24H 10/0000Z 46.8N 37.7W 35 KT 40 MPH...POST-TROP/EXTRATROP
36H 10/1200Z...DISSIPATED

$$
Forecaster Cangialosi

[Hammy]

Wonder if this could've been 50kt earlier in the day around 18z.

[CyclonicFury]

Debby is the perfect example of why you can't just assume some naked swirl in the subtropics won't develop because conditions don't appear favorable. Often times models struggle with these subtropics developments. Debby is no exception. Models had a weak low but not a bonafide TC.

[CyclonicFury]

214
WTNT44 KNHC 091434
TCDAT4

Tropical Storm Debby Discussion Number 9
NWS National Hurricane Center Miami FL AL042018
1100 AM AST Thu Aug 09 2018

Debby continues to produce a small area of deep convection just
south of its exposed low-level center. Overnight and early this
morning, the cyclone passed over a warm eddy within the Gulf Stream,
and this may have provided enough energy to allow the cyclone to
maintain its convection and intensity. A recent ASCAT overpass
around 1230 UTC showed that the maximum winds of Debby are still
near 40 kt, but the cyclone has nearly opened up into a trough of
low pressure. Furthermore, the wind field has broadened, typically
an indication that a system is losing its tropical characteristics.

No significant changes have been made to the track or intensity
forecasts. The tropical storm continues to move quickly
northeastward, with an initial motion of 055/15 kt. The cyclone, or
its remnants, should accelerate northeastward today ahead of a
fast-moving shortwave trough until dissipation occurs. Debby is now
crossing over the northern wall of the Gulf Stream, so weakening is
expected to resume later today. The forecast conservatively shows
Debby as a post-tropical cyclone at 12 hours, but it is very
possible that Debby will dissipate later today as its forward speed
increases.

FORECAST POSITIONS AND MAX WINDS

INIT 09/1500Z 44.5N 43.1W 40 KT 45 MPH
12H 10/0000Z 46.2N 39.2W 35 KT 40 MPH...POST-TROP/EXTRATROP
24H 10/1200Z...DISSIPATED

$$
Forecaster Zelinsky

[HurricaneRyan]

Debby lasted longer than I thought she would

[CyclonicFury]

Farewell, Debby!

388
WTNT44 KNHC 092036
TCDAT4

Post-Tropical Cyclone Debby Discussion Number 10
NWS National Hurricane Center Miami FL AL042018
500 PM AST Thu Aug 09 2018

Debby has become post-tropical. The circulation of the cyclone has
become ill defined, and it is no longer producing enough organized
deep convection to be classified as a tropical cyclone. Therefore,
Debby has become post-tropical, and this is the last advisory. The
cyclone is accelerating northeastward ahead of a shortwave trough,
and this should continue for the next few hours until Debby opens up
into a trough of low pressure and merges with a larger frontal
boundary later tonight.

Debby is likely still producing gale-force winds to the southeast
of its center, however all of the guidance indicates that these
winds will gradually decrease through early tomorrow. Additional
information on this system can be found in High Seas Forecasts
issued by the National Weather Service, under AWIPS header
NFDHSFAT1, WMO header FZNT01 KWBC, and available on the Web
at https://ocean.weather.gov/shtml/NFDHSFAT1.shtml.


FORECAST POSITIONS AND MAX WINDS

INIT 09/2100Z 45.5N 40.5W 35 KT 40 MPH...POST-TROPICAL
12H 10/0600Z...DISSIPATED

$$
Forecaster Zelinsky