When is a center an eye?
Posted by Chris in Tampa on 8/8/2017, 6:11 am
The 4am CDT Tuesday discussion regarding Franklin at/after landfall is interesting.

"Franklin's
inner-core circulation has tightened up considerably since the
previous advisory, likely due to frictional convergence, and that a
10-15-nmi-diameter closed eye has developed."

We see the core getting better organized sometimes with landfalling tropical cyclones, but I didn't think the NHC would actually say it has an eye. And a closed eye too. But then I looked up the definition of "eye":

"Eye:
The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud."

From: http://www.nhc.noaa.gov/aboutgloss.shtml#e

Doesn't mention it has to be a "hurricane", just a severe tropical cyclone. I just haven't thought about a tropical storm having an eye, but I guess it can. Interesting. I would have normally described a tropical storm with a well defined center like that as being "eye-like".

I guess this isn't new, I was looking it up and saw this example in 2005 of a 45 knot tropical storm Beta:

"WITH THE SMALL EYEWALL AND GOOD OUTFLOW IN THE WESTERN SEMICIRCLE...
BETA APPEARS SET UP FOR RAPID INTENSIFICATION."



Full discussion of Franklin at 4am CDT on Tuesday:



"Tropical Storm Franklin Discussion Number   7
NWS National Hurricane Center Miami FL       AL072017
400 AM CDT Tue Aug 08 2017

The Belize Doppler weather radar indicates that Franklin's
inner-core circulation has tightened up considerably since the
previous advisory, likely due to frictional convergence, and that a
10-15-nmi-diameter closed eye has developed. Radar reflectivity
values have been steadily increasing in the eyewall, and this
development trend is supported by infrared satellite imagery, which
shows deep convection with cloud tops of -75C to -80C now completely
covering the radar eye. The initial intensity is lowered to 45 kt
for this advisory based on a typical decay rate for landfalling
tropical cyclones.

The initial motion estimate is 290/12 kt, based primarily on radar
data. A deep-layer subtropical ridge that currently extends across
the northern and central Gulf of Mexico is expected to remain intact
through the forecast period, building slightly southward by 48 h and
beyond. This entrenched steering pattern is expected to keep
Franklin moving west-northwestward across the Yucatan peninsula the
next 24 h or so, followed by westward motion across the Bay of
Campeche until landfall occurs in 48-60 h. NHC model guidance
remains in good agreement on this track scenario, and only a slight
nudge to the south of the previous forecast track was required due
to the more southward initial position of Franklin.

Additional weakening is expected while Franklin moves across the
Yucatan peninsula during the next 15 h or so. The recent inner-core
structural improvements are expected to remain in place by the time
the cyclone emerges over the warm waters of the Bay of Campeche.
That, combined with low vertical shear values and a well-established
outflow pattern, will allow for restrengthening to occur by
Wednesday, possibly resulting in Franklin achieving hurricane status
around 36-42 h. However, the GFS and ECMWF models are still
forecasting significant northerly vertical shear near 20 kt to
develop by 36 h and beyond, which could cap the intensification
process or possibly even induce some weakening since mid-level dry
air entrainment will accompany the northerly shear. However, since
Franklin is expected to be near hurricane strength at landfall, a
Hurricane Watch for mainland Mexico is warranted. The new NHC
intensity forecast is similar to the previous advisory, and closely
follows a blend of the HCCA and IVCN intensity consensus models.

FORECAST POSITIONS AND MAX WINDS

INIT  08/0900Z 19.3N  88.5W   45 KT  50 MPH...INLAND
12H  08/1800Z 19.7N  90.2W   35 KT  40 MPH...INLAND
24H  09/0600Z 20.1N  92.4W   50 KT  60 MPH...OVER WATER
36H  09/1800Z 20.3N  94.4W   60 KT  70 MPH
48H  10/0600Z 20.2N  96.6W   60 KT  70 MPH
72H  11/0600Z 19.7N 101.8W   20 KT  25 MPH...INLAND
96H  12/0600Z...DISSIPATED

$$
Forecaster Stewart"

Franklin NHC discussions:
http://www.nhc.noaa.gov/text/refresh/MIATCDAT2+shtml/
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Hurricane watch issued for Franklin along eastern Yucatan coast - Chris in Tampa, 8/7/2017, 5:05 am
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