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Tropical Storm Darby Forecast Discussion

WTPZ45 KNHC 102037

Tropical Storm Darby Discussion Number   5
NWS National Hurricane Center Miami FL       EP052022
1100 AM HST Sun Jul 10 2022

Darby is a small but fairly well-organized tropical cyclone. 
1-minute GOES-17 imagery has been quite helpful to diagnose the 
system's structure, which consists of a handful of curved bands 
around the periphery of the circulation, with pulsing convection 
near the center that has, at times, wrapped nearly all the way 
around. Earlier microwave imagery showed hints that Darby was 
starting to mix-out some of the dry-air entrainment that was noticed 
this morning, with even a hint of a mid-level eye becoming evident 
on an SSMIS 91-GHz microwave channel at 1421 UTC. The latest 
subjective Dvorak intensity estimates were a consensus CI 3.5/55-kt, 
which is the basis for Darby's current intensity this advisory.

Over the next 36-48 hours, the biggest question related to Darby's 
future intensity is just how effective the tropical cyclone will be 
at preventing additional dry-air intrusions into its inner core. 
Because the system has a very small core, it could be prone to rapid 
intensity changes, both up and down, even due to seemingly small 
fluctuations to its larger-scale environment. While SHIPS guidance 
indicates that the deep-layer 200-850 mb shear should remain under 
10 kt for the next 48 hours, the direction of the shear vector is 
from the north, where there is drier mid-level air. In addition, 
there is some sneaky higher northerly mid-level shear that may have 
played a role in dry air disrupting the convective structure of 
Darby last night and this morning. In any event, assuming the TC is 
able to close off its tiny inner core, at least steady 
intensification is still anticipated in the short-term. The latest 
NHC intensity forecast still peaks Darby as an upper-end Category 1 
hurricane by 48 hours, which is on the high end of the guidance, but 
closest to the latest SHIPS forecast. Afterwards, northwesterly 
shear increases, helping to import even drier mid-level air, and 
sea-surface temperatures decrease below 26 C, which will likely lead 
to rapid weakening of the small system towards the end of the 
forecast period. The latest forecast now indicates Darby becoming a 
post-tropical remnant low in 120 hours, not long after it crosses 
the 140-degree longitude. 

Darby remains on a due westward heading at 270/14 kt. The cyclone 
continues to be steered westward by a prominent mid-level ridge to 
its north. By 36 hours though, Darby will begin to approach a 
weakness in this mid-level ridge, which should allow the system to 
begin gaining latitude. While the track guidance continues to 
exhibit relatively low across-track spread, there is substantially 
more along-track spread between the various deterministic members. 
Notably, the UKMET continues to be on the fast-leftward end of the 
guidance envelope, which does not seem realistic since Darby is more 
vertically deep than that model currently depicts. The latest track 
forecast continues to place more emphasis on the slower and stronger 
guidance, notably the GFS and ECMWF forecasts, which results in the 
track forecast being a little slower and a bit further north than 
the reliable guidance aids HCCA and TVCE.


INIT  10/2100Z 14.4N 119.1W   55 KT  65 MPH
 12H  11/0600Z 14.5N 121.1W   65 KT  75 MPH
 24H  11/1800Z 14.6N 123.7W   75 KT  85 MPH
 36H  12/0600Z 14.8N 126.2W   80 KT  90 MPH
 48H  12/1800Z 15.0N 128.7W   80 KT  90 MPH
 60H  13/0600Z 15.5N 131.4W   70 KT  80 MPH
 72H  13/1800Z 16.2N 133.8W   60 KT  70 MPH
 96H  14/1800Z 17.8N 138.7W   40 KT  45 MPH
120H  15/1800Z 18.5N 144.0W   30 KT  35 MPH...POST-TROP/REMNT LOW

Forecaster Papin

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