Hurricane Humberto Forecast Discussion

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754 
WTNT44 KNHC 172046
TCDAT4

Hurricane Humberto Discussion Number  21
NWS National Hurricane Center Miami FL       AL092019
500 PM EDT Tue Sep 17 2019

Humberto's satellite appearance has improved somewhat since the
previous advisory, with the eye clearing out and becoming more
distinct. This has resulted in satellite subjective intensity
estimates increasing to T5.5/102 kt from TAFB and SAB, with
objective ADT estimates from UW-CIMSS increasing to T5.9/112 kt.
However, during the past 36 h or so, the satellite estimates have
been running higher than the actual surface winds by about 10-15
percent. A 1425Z ASCAT-C overpass indicated surface winds of 72 kt
in the southeastern eyewall, which is significant given that the
scatterometer is well undersampling the actual peak winds due to the
25-km footprint of the instrument. The intensity has been increased
to 90 kt based on the clearing and warming of the eye and allowing
for some overestimation by the satellite agencies. The scatterometer
wind data also showed that Humberto's wind field has expanded more
since the earlier recon wind data, thus some additional adjustments
were made to all of the wind radii in this advisory.

Humberto continues to move east-northeastward or 075/10 kt. The
large hurricane remains on track, and the latest NHC model guidance
remains tightly packed around the previous advisory track. Thus, no
significant changes were made to the previous forecast track through
48 hours. By 72 hours and beyond, however, the models are now in
better agreement on Humberto accelerating and moving faster toward
the northeast through 96 hours as an extratropical cyclone, followed
by a turn toward the east-northeast on day 5. The official forecast
track is based on a blend of the consensus models TVCN, HCCA, and
FSSE, and shows the center of Humberto passing just to the northwest
and north of Bermuda between 24-36 hours or late Wednesday night.

The latest SHIPS intensity guidance shows near 30 kt of deep-layer
vertical shear allegedly affecting Humberto, which obviously is a
significant overestimate based on the presence of a well-defined eye
and smooth CDO feature in visible satellite imagery. Given the
likely overestimation of the shear values, Humberto is expected to
strengthen to major hurricane status in the next 24 h. Thereafter,
Humberto is forecast to steadily weaken due to cold upwelling as the
hurricane moves over cooler waters, and into an environment
consisting of very strong shear exceeding 40-50 kt and more stable,
drier air. However, the usual rate of weakening due to the strong
shear conditions is expected to be tempered by the increasing
baroclinic forcing in the right-rear entrance region of a strong
jetstream wind maximum. The NHC intensity closely follows a blend of
the intensity consensus models IVCN, HCCA, and FSSE.

Key Messages:

1. Hurricane conditions are expected in Bermuda Wednesday night and
Thursday morning, with tropical-storm-force winds expected by
Wednesday afternoon. Residents there should follow advice given by
local officials.

2.  Storm surge and dangerous breaking waves could cause coastal
flooding Wednesday night and Thursday along the southern coast of
Bermuda.

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  17/2100Z 31.0N  72.3W   90 KT 105 MPH
 12H  18/0600Z 31.5N  70.5W   95 KT 110 MPH
 24H  18/1800Z 32.6N  67.6W  100 KT 115 MPH
 36H  19/0600Z 34.6N  63.9W   95 KT 110 MPH
 48H  19/1800Z 37.4N  61.0W   85 KT 100 MPH
 72H  20/1800Z 41.5N  57.2W   65 KT  75 MPH
 96H  21/1800Z 43.5N  48.5W   55 KT  65 MPH...POST-TROP/EXTRATROP
120H  22/1800Z 46.0N  34.0W   50 KT  60 MPH...POST-TROP/EXTRATROP

$$
Forecaster Stewart




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