[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 102059
TCDAT2

Potential Tropical Cyclone Two Discussion Number   2
NWS National Hurricane Center Miami FL       AL022019
400 PM CDT Wed Jul 10 2019

Data from An Air Force Reserve reconnaissance aircraft, surface
observations, and satellite imagery indicate that the broad low
pressure system located over the northeastern Gulf of Mexico still
lacks a well-defined circulation center. Multiple low-level swirls
associated with individual convective cells were noted in the
aircraft wind data. However, shower and thunderstorm activity has
been increasing over the past couple of hours in the previously
convective-free northern semicircle, an indication that low-level
wind field is beginning to improve in that region of the cyclone.
Upper-level outflow has become well established except to the north
where modest northerly wind shear is inhibiting the outflow. The
initial intensity of 25 kt is based on earlier scatterometer wind
data and recent recon winds of 25-30 kt to the south and west of the
center.

The initial motion estimate remains 245/07 kt. The latest model
guidance continues in good agreement on the cyclone moving generally
toward the west-southwest or southwest for the next 24 hours or so,
followed by a westward motion on Friday. Afterwards, however, the
model guidance diverges significantly. The 12Z ECMWF, GFS, HWRF,
and HMON models have shifted farther east and turn the cyclone
northwestward to northward in 48-72 hours, moving it inland along
the south-central and southeastern coasts of Louisiana. In contrast,
the UKMET has shifted farther west and keeps the system on more of
westward track, taking it inland along the central Texas coast. The
main difference is how the models handle the ridge to the north,
with the ECMWF, GFS, HWRF, and HMON rapidly eroding the ridge as a
weak shortwave trough passes to the north of the cyclone, whereas
the UKMET shows the ridge not weakening as much due to the
shortwave trough weakening as it lifts out to the east, which
allows the ridge to remain intact. Due to this significant
bifurcation in NHC's most reliable track model guidance, the best
course of action is to slow down the forward speed and only make
minor adjustments to the overall tack, which has been shifted
slightly to the east, but not as far east as the simple consensus
and HCCA models.

Only slow strengthening is forecast for the next 24-36 hours due to
the lack of a well-defined center and inner-core wind field, along
with some modest northerly wind shear. By 48 hours and beyond,
however, the combination of very low vertical wind shear, an
impressive outflow pattern forecast by all of the global and
regional models, and anomalously warm sea-surface temperatures of
30-31C should allow for significant intensification to hurricane
strength before landfall occurs after 72 hours. Given that the
system is still in the formative stages, the official intensity
forecast remains a little below IVCN consensus through 48
hours and trends higher toward the ECMWF-based SHIPS guidance at
72 hours.

Key Messages:

1. A tropical depression is expected to form by Thursday over the
northern Gulf of Mexico. Conditions appear favorable for this system
to strengthen to a hurricane that will bring storm surge, rainfall,
and wind hazards to the central Gulf Coast.

2. A dangerous storm surge is possible in portions of southern and
southeastern Louisiana where a Storm Surge Watch is in effect.
Additional storm surge watches may be needed later tonight or
tomorrow. Residents in these areas should monitor the progress of
this system and listen to any advice given by local officials.

3. A Hurricane Watch has been issued for much of the Louisiana coast
and additional tropical storm or hurricane watches could be needed
later tonight or tomorrow. Residents in the watch area should ensure
they have their hurricane plan in place.

4. The slow movement of this system will result in a long duration
heavy rainfall threat along the central Gulf Coast and inland
through the lower Mississippi Valley through the weekend and
potentially into early next week. Flash flooding and river flooding
will become increasingly likely, some of which may be significant,
especially along and east of the track of the system.

FORECAST POSITIONS AND MAX WINDS

INIT  10/2100Z 28.1N  87.4W   25 KT  30 MPH...POTENTIAL TROP CYCLONE
 12H  11/0600Z 27.7N  88.1W   30 KT  35 MPH...POTENTIAL TROP CYCLONE
 24H  11/1800Z 27.5N  89.0W   35 KT  40 MPH...TROPICAL STORM
 36H  12/0600Z 27.6N  90.2W   40 KT  45 MPH
 48H  12/1800Z 28.2N  91.5W   55 KT  65 MPH
 72H  13/1800Z 29.3N  92.4W   75 KT  85 MPH
 96H  14/1800Z 31.9N  93.0W   55 KT  65 MPH...INLAND
120H  15/1800Z 34.8N  93.1W   25 KT  30 MPH...INLAND

$$
Forecaster Stewart
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