ZCZC MIATCDAT3 ALL
TTAA00 KNHC DDHHMM
 
Tropical Storm Henri Discussion Number  17
NWS National Hurricane Center Miami FL       AL082021
1100 PM EDT Thu Aug 19 2021

The satellite presentation on Henri this evening continues to 
exhibit a persistent bursting pattern, with the center estimated to 
be just to the north and west of the coldest cloud tops which 
occasionally have been below -80 C in the overshooting tops. This 
current satellite presentation is primarily due to continued 
moderate to strong northerly vertical wind shear, which is forcing 
the convection underneath the cirrus canopy down-shear of the 
low-level center, as seen on a 2230 UTC SSMIS microwave pass.  While 
the mid-level vortex with the convection also remains tilted 
down-shear of the low-level center, it has not completely separated 
due to the persistent convection, preventing the low-level center 
from escaping poleward in more shallow low-level steering. 
Tonight's subjective Dvorak estimates from SAB/TAFB are in 
agreement with T3.5/55 kt and given that this value is near what 
the earlier Air Force Reconnaissance mission found, the latest 
intensity is being maintained at 55 kt for this advisory.

There is a bit of uncertainty determining if Henri has begun a more 
poleward motion since the center remains under the convective cirrus 
plume, but my best guess is now 285/9 kt. Over the next 12-24 hours, 
the mid- to upper-level ridging that has dominated the synoptic 
steering pattern for Henri the last few days will quickly break 
down, as a shortwave trough drops in from the Great Lakes into the 
Mid-Atlantic and cuts off. This feature is now forecast to continue 
digging in to the west of Henri. To the east, a new mid-level ridge 
is also forecast to build in to the right of Henri. This synoptic 
pattern should draw the cyclone poleward with an acceleration to the 
north-northeast in the 24-48 h period. Afterwards, the 
aforementioned trough takes on a negative tilt to the southwest of 
Henri, helping to reorient the mid- to upper-level flow out of the 
south-southeast, and this flow could result in a slight leftward 
bend in the track between 48-72 h. The majority of guidance this 
cycle now is forecasting the mid-level ridge east of Henri to build 
poleward with the storm, blocking an easy path for the storm to stay 
on a more northeast heading out to sea. Consequently, the latest NHC 
forecast track now explicitly shows landfall in southeast 
Massachusetts at 72 h. The track guidance this cycle has come into 
better agreement, though there remain some leftward (UKMET) and 
rightward (ECMWF) outliers. The latest forecast track lies very 
close to the HFIP corrected consensus approach (HCCA) guidance, 
which is also very near the latest GFS forecast track.  

Data from the NOAA G-IV synoptic mission around Henri shows that 
just north of the tropical cyclone there remains some very dry 
mid-latitude air, which is being advected into the storm by 20-25 
kt of northerly vertical wind shear. Over the next 24-36 hours, 
this shear is forecast to gradual subside, as Henri moves near the 
center of an upper-level ridge axis. By 36-48 hours, the vertical 
wind shear is forecast to be under 10-kt by both the GFS- and 
ECWMF-based SHIPS guidance, while the storm is also traversing 
28-29 C sea-surface temperatures (SSTs). Thus, the latest NHC 
intensity forecast still calls for strengthening beginning after 12 
hours, and the rate of strengthening could be a bit quicker as the 
storm moves over the warm gulf stream waters between 36-48 hours. 
Thereafter, Henri will cross a very sharp SST gradient with 
sea-surface temperatures down below 23 C near the New England coast 
to the east of Long Island. Henri is forecast to begin weakening 
after 48 hours, but the storm could still be near hurricane 
intensity by the time Henri is forecast to be near the Northeast 
coastline. Transition to a post-tropical storm is expected to begin 
shortly thereafter which should be sometime in the 96-h to 120-h 
points as deep convection ceases over the storm over cold SSTs
 
As noted previously, the wind field of Henri is expected to expand, 
especially as it interacts with a mid-latitude trough located to its 
west. Therefore, users are reminded to not focus on the exact 
forecast points as impacts will extend far from the center.
 
Key Messages:
 
1. Henri is forecast to be near the northeast coast of the U.S.
on Sunday and Monday, and the risks of storm surge, wind, and rain
impacts in portions of southern New England and eastern Long Island
are increasing.  Watches will likely be required for portions of
this area early Friday.
 
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.
 
3. Heavy rainfall may lead to flash, urban, and small stream
flooding over portions of southeastern New England Sunday into
Monday.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  20/0300Z 29.8N  72.3W   55 KT  65 MPH
 12H  20/1200Z 30.3N  73.2W   55 KT  65 MPH
 24H  21/0000Z 31.8N  73.3W   60 KT  70 MPH
 36H  21/1200Z 34.4N  72.2W   65 KT  75 MPH
 48H  22/0000Z 37.6N  71.0W   80 KT  90 MPH
 60H  22/1200Z 40.1N  70.6W   75 KT  85 MPH
 72H  23/0000Z 41.7N  70.8W   60 KT  70 MPH...INLAND
 96H  24/0000Z 42.5N  70.1W   40 KT  45 MPH...OVER WATER
120H  25/0000Z 43.7N  65.4W   35 KT  40 MPH...POST-TROPICAL
 
$$
Forecaster Papin/Brown
 
NNNN