5 Sep Wed 2018

FLORENCE FLOURISHING IN THE FACE OF SOUTHWESTERLY SHEAR.

Hurricane Florence Discussion Number 25
NWS National Hurricane Center Miami FL AL062018
1100 AM AST Wed Sep 05 2018

Remarkably, Florence has continued to strengthen. The hurricane
has a compact central dense overcast with cold cloud tops completely
encircling a clear, well-defined eye. With the improved structure,
subjective Dvorak estimates have increased to T5.5/102 kt from TAFB
and T6.0/115 kt from SAB, while the latest objective UW-CIMSS ADT
estimate is T5.8/110 kt. The initial intensity is therefore set at
110 kt as a blend of these numbers, with Florence having become a
major hurricane earlier this morning.

Given the estimated maximum winds, Florence has been rapidly
intensifying since yesterday, an event that was not foreseen by any
intensity models, nor forecasters. Diagnostics from the SHIPS
model and UW-CIMSS shear analyses have been consistently showing
southwesterly shear of 20-25 kt in the vicinity of Florence, but it
is possible that those schemes are averaging over a larger area
than might be reasonable given the hurricane’s small size. Florence
has apparently been able to find a small pocket of relatively low
shear, and with waters becoming progressively warmer, the hurricane
has strengthened significantly more than anticipated. This makes
the intensity forecast incredibly uncertain. SHIPS actually shows
the shear increasing over the next 24 hours, but global model
fields suggest that the hurricane may still be able to continue
within the protected pocket of lower shear for the next several
days. As a result, the NHC official forecast keeps Florence’s
intensity well above the available guidance, which all show the
hurricane weakening over the next day or two. The new NHC
prediction follows this trend and also shows some weakening, but
this is a low-confidence forecast. Either way, Florence is
expected to remain a hurricane throughout the 5-day period.

The track forecast also has its challenges. The initial motion
estimate is northwestward, or 305/11 kt. It now appears that
stronger upper-level ridging may take shape to the north of
Florence over the next few days, forcing the hurricane to turn back
toward the west-northwest from 36-72 hours. After 72 hours, a
break in the ridge should allow Florence to turn back toward the
northwest, but the bulk of the track models have trended westward
since yesterday. In light of these trends, the NHC official track
forecast has been shifted westward on days 4 and 5, but not quite
as far as the various model consensus aids. It is worth stressing
that there is still a significant amount of spread among the GFS
and ECMWF ensemble members by the end of the forecast period, and
just like the intensity forecast, the track forecast is of low
confidence.

FORECAST POSITIONS AND MAX WINDS

INIT 05/1500Z 22.0N 45.7W 110 KT 125 MPH
12H 06/0000Z 22.7N 47.2W 110 KT 125 MPH
24H 06/1200Z 23.8N 49.0W 100 KT 115 MPH
36H 07/0000Z 24.7N 50.6W 95 KT 110 MPH
48H 07/1200Z 25.1N 52.1W 90 KT 105 MPH
72H 08/1200Z 25.7N 54.7W 90 KT 105 MPH
96H 09/1200Z 27.0N 57.0W 95 KT 110 MPH
120H 10/1200Z 28.5N 59.0W 95 KT 110 MPH

$$
Forecaster Berg/Rhome

Tropical Storm Gordon Discussion Number 12
NWS National Hurricane Center Miami FL AL072018
400 AM CDT Wed Sep 05 2018

Gordon is moving farther inland and continues to weaken. Surface
synoptic observations suggest that the cyclone is now of, at most,
minimal tropical storm strength. These tropical-storm-force winds
may be occurring over a small inland area near the center.
Continued weakening is expected, and Gordon should become a
tropical depression later this morning.

Gordon continues moving northwestward, with an initial motion
estimate of 325/12. The cyclone should continue on a northwestward
heading with a decrease in forward speed, along the southwestern
periphery of a mid-level ridge, for the next couple of days. Later
in the period, Gordon’s remnant is forecast to turn northward
and northeastward with increasing forward speed as it approaches
the mid-latitude westerlies north of 40N. the official forecast
track is close to the latest dynamical model consensus, TVCA.

All coastal watches and warnings associated with Gordon are being
discontinued at this time.

Key Messages:

1. Heavy rainfall from Gordon will affect the western Florida
Panhandle, southwest Alabama, southern and central Mississippi,
northeastern Louisiana, Arkansas, Missouri, southern Iowa and
Illinois, with isolated maximum amounts of 12 inches through early
Saturday. This rainfall will cause flash flooding across portions
of these areas.

FORECAST POSITIONS AND MAX WINDS

INIT 05/0900Z 31.5N 89.5W 35 KT 40 MPH
12H 05/1800Z 32.4N 90.4W 25 KT 30 MPH…INLAND
24H 06/0600Z 33.4N 91.6W 25 KT 30 MPH…INLAND
36H 06/1800Z 34.1N 92.5W 25 KT 30 MPH…INLAND
48H 07/0600Z 34.8N 93.3W 20 KT 25 MPH…POST-TROP/REMNT LOW
72H 08/0600Z 36.7N 93.6W 20 KT 25 MPH…POST-TROP/REMNT LOW
96H 09/0600Z 39.0N 91.0W 20 KT 25 MPH…POST-TROP/EXTRATROP
120H 10/0600Z 42.0N 86.0W 20 KT 25 MPH…POST-TROP/EXTRATROP

$$
Forecaster Pasch