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Tropical Storm Henri Discussion Number  13
NWS National Hurricane Center Miami FL       AL082021
1100 PM AST Wed Aug 18 2021

Henri’s appearance this evening is one of a tropical cyclone that is 
battling vertical wind shear. The satellite structure has evolved 
into a large convective plume with overshooting cold -70 to -75 C 
cloud top temperatures near the center of the convective mass. 
Underneath the cirrus, Henri’s structure has remained steady-state, 
with a SSMIS pass at 2243Z showing a ragged mid-level eye on the 91 
GHz channel that remains displaced a bit southeast of the low-level 
signature on 37 GHz. The latest satellite intensity estimates have a 
large spread in values ranging from 55-kt up to 80-kt. I have 
elected to maintain Henri’s intensity at 60 kt for this advisory, 
but there is a larger than normal uncertainty in this estimate. 

The initial motion continues to be just south of due west at 265/8 
kt. The short-term track guidance is in general agreement that Henri 
will continue a general westward motion for the next day or so, as 
the cyclone is caught to the south of a pronounced mid- to 
upper-level ridge. However, the models disagree on how far westward 
Henri is able to advance in the short-term, and these track 
differences appear to be related to Henri's intensity and vertical 
depth of the circulation. This track dependency is nicely 
illustrated by the most recent ECMWF ensemble guidance, where 
stronger members move further south and west in the short-term, 
ending up on the left-side of the ensemble mean, while weaker 
members move slower and more poleward. The deterministic guidance is 
similar, with the weaker ECMWF and Canadian runs off to the 
east/right while the stronger GFS and UKMET runs are further 
left/west. These short-term track differences appear pivotal, 
because the ridge over Henri is then expected to quickly erode as a 
mid-latitude trough digs in over the Ohio Valley and becomes 
negatively tilted over the Mid-Atlantic. The leftward track guidance 
has Henri interacting with this trough leading to a track forecast 
much closer to the northeastern US, while the rightward track 
guidance is not captured by this feature, and stay further offshore 
escaping out to sea. In general, the mean of the track guidance is 
very similar, or just a shade to the right of the previous cycle. I 
have elected to maintain a very similar track to the previous 
advisory, splitting the difference between the previous forecast 
just left, and the reliable HCCA consensus aid just right. As 
mentioned previously, a NOAA Gulfstream IV aircraft has been 
scheduled to investigate the synoptic environment of Henri tomorrow 
and will hopefully provide more data ingested into the model 
guidance to reduce the large track spread seen in the current cycle.

Henri is now under moderate northerly shear that is forecast to 
increase to 25-30 knots over the next 24 hours. While this would 
ordinarily weaken the tropical cyclone, the system also remains 
under very warm 29-30 C sea-surface temperatures that are somewhat 
above normal for this time of year. Due to these offsetting factors, 
the latest NHC intensity forecast maintains Henri at 60-kt for the 
next 24 hours. Afterwards, the shear is forecast to decrease as the 
storm turns more poleward directly under the upper-level ridge axis. 
This favorable synoptic pattern should allow Henri to intensify and 
gradual intensification is shown up to a 80-kt peak in 72-h. The NHC 
intensity forecast is more or less in the mean of the large guidance 
spread, though it should be noted that the regional hurricane models 
(HWRF, HMON, COAMPS-TC) have somewhat stronger peak intensities 
later in the forecast period. 
 
Key Messages:
 
1. The forecast track of Henri remains near the northeast coast of 
the U.S. this weekend and early next week, and the risks of storm 
surge, wind, and rain impacts in portions of the northeastern U.S. 
and Atlantic Canada remains a distinct possibility. Interests in 
these areas should closely follow the progress of Henri and check 
for updates to the forecast.
 
2. Swells from Henri will begin to reach much of the east coast of
the U.S. and Atlantic Canada by the end of the week and continue
through the weekend. These swells could cause life-threatening surf
and rip currents.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  19/0300Z 29.8N  68.5W   60 KT  70 MPH
 12H  19/1200Z 29.8N  69.9W   60 KT  70 MPH
 24H  20/0000Z 30.0N  71.8W   60 KT  70 MPH
 36H  20/1200Z 30.7N  72.7W   65 KT  75 MPH
 48H  21/0000Z 32.3N  72.8W   70 KT  80 MPH
 60H  21/1200Z 34.4N  71.8W   75 KT  85 MPH
 72H  22/0000Z 36.9N  70.7W   80 KT  90 MPH
 96H  23/0000Z 40.4N  69.4W   70 KT  80 MPH
120H  24/0000Z 41.8N  68.3W   60 KT  70 MPH
 
$$
Forecaster Papin/Beven
 
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