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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
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