Hurricane Gustav Discussion Number 20
Statement as of 5:00 PM EDT on August 29, 2008
Visible imagery has occasionally revealed an eye trying to
form...but that feature has not been persistent.
Nevertheless...the cyclone continues to increase in
organization...with solid convective bands and upper-level outflow
that is well-developed in all quadrants. The underlying waters of
the northwestern Caribbean Sea are quite warm...and to some depth
beneath the surface...so there are no apparent impediments to
additional strengthening before Gustav reaches western Cuba. Even
rapid intensification is possible...as supported by the latest
SHIPS-based rapid intensification index that shows a 35 percent
chance of an intensity increase of 30 kt or more during the next 24
hours...so it is possible that Gustav could reach major hurricane
status prior to crossing western Cuba. Passage over Cuba will not
likely have much impact on the storm’s strength...and all guidance
forecasts a strengthening trend over the southern Gulf...with
slight weakening possible in the northern Gulf. The official
forecast again calls for a major hurricane over the Gulf...and
despite the weakening implied by the lesser intensity over land at
96 hours...Gustav could make final landfall along some portion of
the northern Gulf Coast as a major Hurricane.
Gustav has turned more to the right and sped up a little...now
moving at about 305/10...as it heads for a break in the subtropical
ridge over the eastern Gulf of Mexico. The portion of that ridge
that is intact over the Southern Plains of the United States is
forecast by all models to evolve into a deep-layer high that will
reach the northeastern U.S. In a few days. There are important
differences among the models...however...in how much ridging will
extend southwestward from that high toward Texas...and in how strong
the upper-level trough currently over the central Gulf will be over
the western Gulf in a few days. These varying solutions lead to
different tracks for Gustav over the northern Gulf. Models with
the ridge extension and a stronger upper-level trough west of
Gustav...such as the NOGAPS and UKMET...forecast Gustav to turn
westward toward Texas. Others including the GFS...GFDL...and
HWRF...do not show the ridge extension nor a strong upper-level
trough...and forecast Gustav to be pulled into the northern Gulf
Coast farther east. The new official forecast leans toward the
latter solutions and is east of the consensus...and represents no
significant change to the previous advisory. Due to the notable
model spread late in the forecast period...it is once again
important to restate that it is simply not yet possible to
determine exactly where and when Gustav will make final landfall.
Forecast positions and Max winds
initial 29/2100z 19.2n 79.7w 65 kt
12hr VT 30/0600z 20.1n 80.9w 75 kt
24hr VT 30/1800z 21.7n 82.8w 90 kt...over Isle of Youth
36hr VT 31/0600z 23.4n 84.8w 100 kt
48hr VT 31/1800z 25.2n 86.8w 110 kt
72hr VT 01/1800z 28.0n 90.0w 105 kt
96hr VT 02/1800z 30.5n 92.5w 85 kt...inland
120hr VT 03/1800z 32.0n 94.0w 55 kt...inland
$$
forecaster Knabb
Tropical Storm Hanna Discussion Number 7
Statement as of 5:00 PM EDT on August 29, 2008
The center of Hanna remains near the western edge of the rather
shapeless area of deep convection seen in satellite imagery. A
1412 UTC ascat pass found winds of 40 kt over the northeastern
semicircle. Given the low bias typical of ascat winds in this
range...it supports maintaining the initial intensity at 45 kt.
This intensity is also supported by satellite estimates from
TAFB and SAB which remain t3.0 or 45 kt.
Water vapor imagery shows that the upper-level low located a
couple hundred miles west-northwest of Hanna has not budged
today and it continues to impart southwesterly shear on the
tropical cyclone. The dynamical models remain emphatic
suggesting that the upper-low will move west or west-
southwestward and weaken...allowing Hanna to gradually
strengthen. The new NHC intensity forecast continues to allow for
some strengthening during the next couple days but shows a lower
peak intensity than was forecast in the previous advisory. In
about 3 days...the models continue to forecast strong northwest to
north upper-level winds over the western Atlantic. These winds are
partially the result of outflow from Gustav. Despite the hostile
upper-level wind forecast...the intensity guidance does not predict
weakening. Our intensity forecast remains on the low side of the
guidance and still assumes that the northerly shear will have a
negative impact on Hanna’s intensity.
Hanna continues moving west-northwestward or 295/9. In
general...the forecast guidance has come into a little better
agreement on the future track of Hanna. The tropical storm is
forecast to continue west-northwestward during the next 2
days...followed by a turn toward the west-southwest. The HWRF
continues to be an outlier showing a cyclonic loop north of the
southeastern Bahamas. The new track forecast is very similar to
the previous forecast through 48 hours...but is a little faster
and farther west thereafter.
Forecast positions and Max winds
initial 29/2100z 21.8n 63.8w 45 kt
12hr VT 30/0600z 22.8n 65.4w 50 kt
24hr VT 30/1800z 24.0n 67.7w 55 kt
36hr VT 31/0600z 24.8n 69.8w 60 kt
48hr VT 31/1800z 25.0n 71.1w 65 kt
72hr VT 01/1800z 24.5n 72.5w 65 kt
96hr VT 02/1800z 23.7n 73.7w 60 kt
120hr VT 03/1800z 23.0n 75.0w 60 kt
$$
forecaster Brown