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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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