[Tropical] Tropical Cyclone Discussion

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WTNT44 KNHC 171458
TCDAT4

Hurricane Humberto Discussion Number  20
NWS National Hurricane Center Miami FL       AL092019
1100 AM EDT Tue Sep 17 2019

An Air Force Reserve reconnaissance aircraft that investigated
Humberto earlier this morning found 700-mb flight-level winds of 96
kt and 82-kt surface winds on a dropsonde in the southwestern
quadrant, and 81-kt SFMR surface winds in the northwestern quadrant.
More importantly, the wind data indicate that Humberto's wind field
has expanded significantly and is becoming asymmetric with the
largest wind radii now located in the southern semicircle. The
aircraft also measured a central pressure of 961 mb, down 18 mb
since this time yesterday. The central pressure typically
corresponds to an intensity of around 100 kt, but the expansion of
the wind field has resulted in little increase in the eyewall wind
field for now. Based on these data, the initial intensity has been
increased to 85 kt.

Humberto has maintained an east-northeastward motion of 075/07 kt.
The hurricane is forecast to remain embedded within the mid-latitude
westerlies though the forecast period. A deep-layer trough is
forecast to dig southward to the west of Humberto in the 24-48 hour
period, causing the hurricane to gradually gain latitude and pass
just to the northwest of Bermuda in around 36 hours. The models are
in excellent agreement on this developing track scenario and are
tightly clustered around the previous advisory track. However, the
recent expansion of the wind field and the expected additional
increase in the wind on the southeast side of Humberto's circulation
will result in the island being in near-hurricane-force conditions.
By 72 hours and beyond, Humberto will accelerate northeastward
over the cold waters of the north Atlantic and interact with a
strong frontal system, resulting in the transition into a large
extratropical low. The official track forecast is similar to the
previous advisory track through 48 hours, and then is a little
faster in the 72-120 hour period, close to a blend of the various
consensus models.

Due to the expansion of Humberto's wind field and large 35-40-n mi
wide eye, only modest strengthening is expected during the next 36
hours or so. Although not explicitly shown, Humberto could still
become a major hurricane between 24-36 hours when the cyclone will
be located over the warmest water of about 29C. Thereafter,
gradual wakening is forecast due to cooler waters and strong
southwesterly vertical wind shear in excess of 40 kt. However, the
typical rate of weakening due to such hostile shear conditions will
be tempered owing to the increasing baroclinic forcing in the
right-rear entrance region of a polar jet maximum. The new intensity
forecast is nearly identical to the previous advisory, and follows a
blend of the consensus models HCCA and FSSE.

Key Messages:

1. Tropical-storm-force winds are expected to reach Bermuda by
Wednesday afternoon, with hurricane conditions possible Wednesday
night and Thursday morning. Residents there should follow advice
given by local officials.

2. Large swells will increase along the coast of Bermuda by
Wednesday. Dangerous breaking waves, especially along south-facing
beaches, could cause coastal flooding Wednesday night and Thursday.

3. Swells will continue to affect the northwestern Bahamas and the
southeastern coast of the United States from east-central Florida to
North Carolina during the next couple of days, creating life-
threatening surf and rip current conditions.

FORECAST POSITIONS AND MAX WINDS

INIT  17/1500Z 30.7N  73.5W   85 KT 100 MPH
 12H  18/0000Z 31.1N  72.0W   90 KT 105 MPH
 24H  18/1200Z 31.8N  69.4W   95 KT 110 MPH
 36H  19/0000Z 33.1N  66.1W   95 KT 110 MPH
 48H  19/1200Z 35.3N  62.6W   90 KT 105 MPH
 72H  20/1200Z 39.6N  58.4W   70 KT  80 MPH
 96H  21/1200Z 43.0N  52.2W   60 KT  70 MPH...POST-TROP/EXTRATROP
120H  22/1200Z 44.5N  40.0W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart
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