EPAC: IRWIN - Post-Tropical - Discussion

[Sciencerocks]

[cycloneye]

EP, 10, 2023082718, , BEST, 0, 170N, 1209W, 35, 998, TS

[Sciencerocks]

[cycloneye]

Tropical Storm Irwin Discussion Number 4
NWS National Hurricane Center Miami FL EP102023
1100 AM HST Sun Aug 27 2023

The cloud pattern has changed little in appearance this morning. The
center of circulation remains displaced to the north of a shapeless
deep convective mass, likely due to a combination of modest
northeasterly shear and dry mid-tropospheric air intruding from the
northwest. A scatterometer pass over the eastern half of the
cyclone indicated southerly winds of 35-37 kt well removed from the
center, and the initial intensity is held at 35 kt for this
advisory.

Irwin should be able to produce sufficient deep convection,
particularly in the eastern semicircle, along with intermittent
bursts near the center, thereby maintaining its tropical storm
status in the short term. Subsequently, the relatively large
circulation should take some time to spin down similar to what the
global and regional guidance suggests. Decreasing SSTs and a dry,
stable marine layer air mass is expected to cause the cyclone to
weaken to a depression in a few days and degenerate into a remnant
low toward the end of the forecast period.

Using surface center fixes from visible and recent microwave
imagery, along with the previously mentioned partial METOP-B/C
scatterometer wind overpasses, gives an estimated initial motion of
west-northwestward, or 295/9 kt. A subtropical high pressure
anchored to the north of the cyclone should maintain a generally
west-northwestward to westward heading through day 5. Along-track
inconsistency in the models continues to complicate the forecast
track solution. The best approach in this case, however, is to side
with a compromise of the best-performing HCCA and TVCE consensus
guidance, and is the basis of the official forecast.


FORECAST POSITIONS AND MAX WINDS

INIT 27/2100Z 17.2N 121.3W 35 KT 40 MPH
12H 28/0600Z 17.7N 122.5W 40 KT 45 MPH
24H 28/1800Z 18.4N 124.6W 40 KT 45 MPH
36H 29/0600Z 18.6N 127.2W 35 KT 40 MPH
48H 29/1800Z 18.6N 130.0W 35 KT 40 MPH
60H 30/0600Z 18.4N 132.7W 35 KT 40 MPH
72H 30/1800Z 18.4N 135.2W 30 KT 35 MPH
96H 31/1800Z 18.8N 138.9W 30 KT 35 MPH...POST-TROP/REMNT LOW
120H 01/1800Z 19.1N 141.4W 25 KT 30 MPH...POST-TROP/REMNT LOW

$$
Forecaster Roberts

[Sciencerocks]

[cycloneye]

Tropical Storm Irwin Discussion Number 5
NWS National Hurricane Center Miami FL EP102023
500 PM HST Sun Aug 27 2023

Cloud tops associated with Irwin's deep convection have warmed
during the past few hours, possibly due to the tropical cyclone
moving over cooler waters. The intensity estimate is still 35 kt,
based primarily on the latest Dvorak Final-T analysis from TAFB.

No substantial changes were made to the NHC track or intensity
forecast with this advisory. Along the forecast track, the sea
surface temperatures are in the 25-26 deg C range for the next 2-3
days. This could be warm enough for Irwin to sustain just enough
deep convection to retain its status as a tropical cyclone, as shown
in simulated satellite imagery from several dynamical models.
However that should also prevent Irwin from strengthening. After
that time, a combination of cool waters and a dry atmsopheric
environment should cause Irwin to become a post-tropical remnant
low. It's possible that transition could occur much sooner than
currently forecast. Irwin should turn westward tonight and continue
generally westward until it becomes a remnant low later this week.
The NHC track forecast is a little south of the various consensus
models for the first 48 hours based on recent satellite imagery
indicating that Irwin's westward turn has already begun, but is
close to them after that. The intensity forecast is directly in line
with the consensus aids.


FORECAST POSITIONS AND MAX WINDS

INIT 28/0300Z 17.4N 122.5W 35 KT 40 MPH
12H 28/1200Z 17.9N 123.9W 35 KT 40 MPH
24H 29/0000Z 18.3N 126.0W 35 KT 40 MPH
36H 29/1200Z 18.4N 128.7W 35 KT 40 MPH
48H 30/0000Z 18.4N 131.6W 35 KT 40 MPH
60H 30/1200Z 18.3N 133.9W 35 KT 40 MPH
72H 31/0000Z 18.5N 136.1W 30 KT 35 MPH
96H 01/0000Z 19.0N 140.0W 30 KT 35 MPH...POST-TROP/REMNT LOW
120H 02/0000Z 19.0N 142.0W 30 KT 35 MPH...POST-TROP/REMNT LOW

$$
Forecaster D. Zelinsky

[SconnieCane]

A tale of two "I" storms on trajectories you'd expect to be completely flipped between the basins in an El Nino.

Well played, 2023...

[Sciencerocks]

[Sciencerocks]

[Sciencerocks]

[Extratropical94]

Higher pressure, but also higher winds.

EP, 10, 2023082906, , BEST, 0, 195N, 1264W, 40, 1001, TS

[cycloneye]

Post-Tropical Cyclone Irwin Discussion Number 12
NWS National Hurricane Center Miami FL EP102023
1100 AM HST Tue Aug 29 2023

Irwin has failed to produce any convection near its center during
the last 15-18 h. Since it no longer satisfies the criteria of a
tropical cyclone, Irwin is being designated as a post-tropical
cyclone with this advisory. A recent scatterometer pass showed a
broad area of winds at or slightly above 30 kt in the northwestern
quadrant, and so the initial intensity remains 35 kt.

The post-tropical cyclone is moving slightly south of due west
(260/15 kt). A general westward motion at a gradually slower forward
speed is expected over the next several days while Irwin is steered
by a low-level ridge over the eastern Pacific. Gradual weakening is
forecast as the shallow cyclone spins down over cooler waters and in
a drier, more stable environment. While some intermittent bursts of
convection could occur during the next couple of days, the overall
environment does not appear conducive for Irwin to regenerate to a
tropical cyclone.

This is the last NHC advisory on Irwin. For additional information
on the post-tropical cyclone, please see High Seas Forecasts issued
by the National Weather Service, under AWIPS header NFDHSFEPI, WMO
header FZPN02 KWBC, and on the web at
ocean.weather.gov/shtml/NFDHSFEPI.php


FORECAST POSITIONS AND MAX WINDS

INIT 29/2100Z 19.1N 130.3W 35 KT 40 MPH...POST-TROPICAL
12H 30/0600Z 18.9N 132.4W 30 KT 35 MPH...POST-TROP/REMNT LOW
24H 30/1800Z 18.8N 134.7W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 31/0600Z 18.8N 136.5W 30 KT 35 MPH...POST-TROP/REMNT LOW
48H 31/1800Z 18.9N 138.0W 30 KT 35 MPH...POST-TROP/REMNT LOW
60H 01/0600Z 19.0N 139.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 01/1800Z 19.0N 140.3W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 02/1800Z 19.0N 141.5W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 03/1800Z 19.5N 143.5W 20 KT 25 MPH...POST-TROP/REMNT LOW

$$
Forecaster Reinhart