11pm Thursday Discussion
Posted by Chris in Tampa on 8/19/2021, 11:24 pm
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













The 18Z GFS had some dropsonde data in it. It had 15 in it. 12Z GFS had 0 in it. But, that could be some from Grace too. There were 37 dropsondes in Henri for the G-IV mission so there are more to come into the model. At the same time, there are about 28 sondes so far for Grace during the period in which some might be in the 18Z run and some in 0Z run. (or even later) I can't recall if the GFS gets dropsonde data from all the aircraft, not just the G-IV. I would assume so based on the count of dropsondes for some previous runs. There seem to be more than just from the G-IV.

Even if every one of the 15 were for Henri, they were likely mostly south and east-northeast of the storm based on the path of the plane. The last part of the mission was northeast, north and a closer track around the storm. That part definitely was not in it. That should be in either the 0Z GFS out in the next few hours or maybe even the 6Z. I don't know at what point the cut off is for getting into a run.

The 10th item in my Other Data & Information section on this page has information about how to tell if sonde data is in the GFS:
http://tropicalatlantic.com/reconnaissance/#other

Look for:
NOAA/NCEP Real Time Data Monitoring System (to determine if any dropsondes were added to the GFS)

There could be other dropsondes in there from other storms if there is sonde data from all the plane types, so you always have to look to see what other recon is out there too to determine where the dropsonde number might come from.

Here is a recon image of the G-IV mission. Again, they started with the southern route and then finished with the northern.



From the HurricaneCity recon system:
http://hurricanecity.com/recon/

Each little long icon is where they dropped a sonde. Some are hidden by the wind barbs along the path the plane took. The NHC mentions looking at the G-IV data in the discussion.
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Tony here from LI NY - MarkoNY, 8/18/2021, 5:01 pm
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