[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 080257
TCDAT4

Tropical Storm Michael Discussion Number   6
NWS National Hurricane Center Miami FL       AL142018
1000 PM CDT Sun Oct 07 2018

On the last outbound leg to the north, the Air Force Reserve
reconnaissance aircraft measured a peak 925-mb flight-level wind of
60 kt and an SFMR surface wind of 53 kt in very light rain. Since
that time, deep convection has developed in the same area where
those peak wind values were measured, so the initial intensity has
been increased to a conservative 50 kt. Another recon aircraft is
scheduled to investigate Michael around 0600 UTC.

The initial motion estimate is northward, or 005/03 kt based on the
recon and microwave satellite data. The bulk of the forecast
rationale remains unchanged, although there remains some notable
differences between the models that will ultimately determine where
and when Michael will make landfall in about 72 h or so. The GFS,
ECMWF, HWRF, and HMON models are tightly clustered along the western
side of the guidance envelope. In contrast, the UKMET model is the
farthest east of the dynamical models. Interestingly, the NOAA
corrected-consensus HCCA model and the simple consensus IVCN models
are close to each other and lie between the aforementioned model
track dichotomy. Given that modest westerly vertical wind shear is
expected to affect Michael throughout the forecast period until
landfall occurs, which should keep the strongest convection,
associated latent heat release, and pressure falls occurring in the
eastern semicircle, the official forecast track leans more toward
the HCCA/IVCN and UKMET model solutions. As a result, a slight
eastward shift to the previous forecast track was made on this
advisory through 48 hours, with little change made to the previous
advisory track on days 3-5.

Michael's overall cloud pattern remains unchanged, with the bulk of
the convection being displaced into the eastern semicircle due to
about 20 kt of westerly shear. However, the inner-core convective
pattern within about 60 nmi of the center has been improving over
the past few hours, and a recent SSMI/S microwave pass reveled a
tightly curved band wrapping about 75 percent around the center.
This better structure combined with very warm sea-surface
temperatures of at least 29 deg C supports at least gradual
strengthening until landfall despite the expected persistent
westerly shear conditions. The HWRF model is the most robust of the
intensity guidance and makes Michael a category 4 hurricane just
prior to landfall. However, all of global models and the HWRF and
HMON regional models are indicating westerly to west-northwesterly
shear keeping the upper-level outflow restricted to the eastern
semicircle, which is not a pattern conducive for the development of
an intense hurricane. Therefore, the HRWF intensity solution has
been discounted, and the new official intensity forecast remains
similar to the previous advisory, which is close to but a little
lower than the IVCN and ICON intensity consensus models. Although
the 72-h forecast shows a decrease to 75 kt, this is due to the
cyclone being inland, and should not be interpreted as being an
indication of a weakening trend prior to landfall.

Key Messages:

1.  Michael is expected to produce heavy rainfall and flash flooding
over portions of western Cuba and the northeastern Yucatan Peninsula
of Mexico during the next couple of days.

2.  Tropical storm conditions are expected tonight over portions of
western Cuba and the northeastern Yucatan Peninsula, where tropical
storm warnings are in effect.

3.  Michael is forecast to be a hurricane when it reaches the
northeastern Gulf Coast by mid-week, and the risk of dangerous storm
surge, rainfall, and wind impacts continues to increase. In
addition, Michael is expected to affect portions of the Florida Gulf
Coast that are especially vulnerable to storm surge, regardless of
the storm's exact track or intensity. Residents in these areas
should monitor the progress of this system and follow any advice
given by local officials.


FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 20.0N  85.4W   50 KT  60 MPH
 12H  08/1200Z 21.1N  85.3W   60 KT  70 MPH
 24H  09/0000Z 22.7N  85.6W   65 KT  75 MPH
 36H  09/1200Z 24.4N  86.0W   75 KT  85 MPH
 48H  10/0000Z 26.3N  86.1W   85 KT 100 MPH
 72H  11/0000Z 30.4N  84.5W   75 KT  85 MPH...INLAND ERN FL PANHNDL
 96H  12/0000Z 34.9N  78.4W   55 KT  65 MPH...INLAND
120H  13/0000Z 40.7N  64.4W   55 KT  65 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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