Thread: Tropical Depression Hilda - ESE of Hawaii (was 92E)

wtpa45 phfo 250911
tcdcp5

tropical storm hilda discussion number 12
nws central pacific hurricane center honolulu hi ep112009
1100 pm hst mon aug 24 2009

hilda is still in a sheared environment with good upper level
outflow to the southwest and much poorer outflow to the northeast.
The 15 kt vertical shear from the east is about 5 kt less than six
hours ago...which probably explains the continued slow north and
northeastward spreading of the high level clouds. Deep convection
remains limited to a single cluster just south of the estimated
circulation center. Both sab and jtwc came in with current
intensities of 3.5 while phfo increased the current intensity to
4.0. Considering that hilda is still battling vertical shear...will
stick with the majority and keep the initial intensity at 50 kt.

Hilda is moving to the west or 270 degrees at a slightly slower 8
kt. Over the next 24 hours...hilda will be in deep easterly flow
south of a mid and upper-level ridge extending eastward from the
hawaiian islands and the low-level subtropical ridge farther to the
north. Global models shift the ridge aloft to over and southwest of
hawaii...which should turn hilda on a more west-southwestward
course from 24 to 72 hours. Beyond 72 hours...the global models
have different solutions but in general dig an upper level
southward close to the hawaiian islands. The trough aloft should
weaken the surface ridge northwest of hilda...allowing the system
to assume a west-northwest course. The forecast track which follows
the above scenario is almost in lock step with the model consensus
guna tcon and tvcn. It should be pointed out that nogaps continues
to take hilda on a more northern track after 48 hours. It is still
too early to tell if this is a correct move.

The ships intensity forecast shows a gradual relaxation of the
vertical shear presently over hilda...so there remains a chance
that hilda will strengthen slightly over the next 24 hours. The
intensity forecast follows ghmi...hwrf and ships which strengthen
hilda to a hurricane after 48 hours.


Forecast positions and max winds

initial 25/0900z 14.8n 147.9w 50 kt
12hr vt 25/1800z 14.7n 149.1w 55 kt
24hr vt 26/0600z 14.6n 150.6w 55 kt
36hr vt 26/1800z 14.4n 152.0w 55 kt
48hr vt 27/0600z 14.2n 153.4w 60 kt
72hr vt 28/0600z 14.3n 156.2w 65 kt
96hr vt 29/0600z 15.0n 159.6w 70 kt
120hr vt 30/0600z 17.0n 163.0w 75 kt

$$
forecaster craig

tropical storm hilda discussion number 13
nws central pacific hurricane center honolulu hi ep112009
500 am hst tue aug 25 2009

the shear across hilda has diminished to less than 10 kt and outflow
of high clouds looks pretty good in all quadrants. Also with the
shear almost gone...deep convection is no longer relegated to the
southwest semicircle but rather appears to be over the estimated
low level center. Sab and jtwc again had a ci number of 3.5 while
phfo stayed at 4.0. The initial intensity of 50 kt is based on the
first two and an adt ci of 3.6.

Hilda will continue to move to the west over the next 24 hours as it
remains in deep layer easterlies located south of the subtropical
ridge. From 24 to about 72 hours...a mid and upper level ridge is
forecast to build northwest and west of the main hawaiian islands
which should steer hilda on a west-southwest course. Beyond 72
hours...the global models differ in solutions but in general dig an
upper level trough southward close to the hawaiian islands. The
trough should weaken the surface ridge northwest of hilda...
Allowing the system to assume a west-northwest course. The forecast
track follows this scenario and is just north of a tightly bunched
guna tcon and tvcn through 120 hours. The track is south of and
slightly faster than the previous track.

With weak shear and ssts from 27c to 28c...hilda is forecast to
gradually intensify. The intensity forecast in fact strengthens
hilda to a hurricane after 48 hours and is in close agreement with
icon and ivcn.


Forecast positions and max winds

initial 25/1500z 14.7n 148.9w 50 kt
12hr vt 26/0000z 14.5n 150.2w 50 kt
24hr vt 26/1200z 14.3n 151.9w 55 kt
36hr vt 27/0000z 14.1n 153.3w 55 kt
48hr vt 27/1200z 14.0n 154.9w 60 kt
72hr vt 28/1200z 14.2n 158.0w 65 kt
96hr vt 29/1200z 15.0n 162.0w 70 kt
120hr vt 30/1200z 16.8n 166.3w 70 kt

$$
forecaster craig

wtpa45 phfo 252051
tcdcp5

tropical storm hilda discussion number 14
nws central pacific hurricane center honolulu hi ep112009
1100 am hst tue aug 25 2009

hilda has been struggling this morning and the most recent infrared
images show warming cloud tops over the center and the deepest
convection confined to the system/s southern quadrant. Hilda/s
problems may be due to the ingestion of drier air from the
surrounding airmass. This is suggested by water vapor
images...satellite sounder precipitable water data and the presence
of outflow boundaries in the early morning visible images pushing
out from beneath the high level clouds. Ssmi 37 ghz data from 1632
utc showed the center to be displaced farther south than previous
positions. This is supported by low clouds lines in the visible
images. As a result...the initial position has been adjusted south.
Dvorak intensity estimates ranged from 65 kt to 45 kt. Cimss 1600
utc satcon came in at 35 kt. A quikscat pass from around 1600 utc
showed 40 kt winds in the northwest quadrant. Based on a blend of
the subjective and objective estimates...will go with an initial
intensity of 45 kt though this may be a bit high.

Hilda has been moving south of the subtropical ridge. The
orientation of the mid-level ridge has resulted in a more northerly
component in the steering flow thus the motion has been toward
the west-southwest. Global models indicate this pattern should
continue over the next 48 hours. Beyond 48 hours...a mid- and
upper level trough is expected to develop northwest of hilda which
should result in a turn toward the west-northwest. Objective aids
are in better agreement than yesterday with most of the key
dynamical models clustered relatively tight. Nogaps is the northern
outlier but has a less drastic northwest turn than before and keeps
hilda southwest of the hawaiian islands. The official forecast
stays close to the dynamical consensus and has been shifted south
from the previous forecast...mainly due to the shift in initial
position.

The intensity forecast is problematic. Aside from possible dry air
impacts...hilda is in a mild shear environment with more than
adequate sst/s. If the dry air influence eases...strengthening
should occur at least through 72 hours. Beyond 72 hours...global
models indicate an increase in vertical shear which should weaken
hilda. Ships...hwrf and ghmi all indicate additional strengthening.
Thus...the key uncertainties include the continued influence of dry
air and the strength of vertical shear beyond day 3. For now...the
official forecast calls for slight weakening over the next 24 hours
followed by strengthening to 72 hours. Weakening is forecast beyond
72 hours due to shear effects. Needless to say...there is not a
lot of confidence in this intensity forecast.

Forecast positions and max winds

initial 25/2100z 14.1n 149.1w 45 kt
12hr vt 26/0600z 13.8n 150.3w 40 kt
24hr vt 26/1800z 13.3n 151.9w 40 kt
36hr vt 27/0600z 13.0n 153.5w 45 kt
48hr vt 27/1800z 13.0n 155.1w 50 kt
72hr vt 28/1800z 13.6n 158.8w 60 kt
96hr vt 29/1800z 14.5n 163.0w 60 kt
120hr vt 30/1800z 16.1n 167.3w 55 kt

$$
forecaster kodama

wtpa45 phfo 260900
tcdcp5

tropical storm hilda discussion number 16
nws central pacific hurricane center honolulu hi ep112009
1100 pm hst tue aug 25 2009

hilda...in spite of minimal shear and warm sst/s...is barely hanging
on. The possible reason for the ongoing struggle is the intake of
dry mid-level air that surrounds hilda to the north...as depicted
by water vapor satellite imagery. The fix agencies and an adt all
report a 24 hour weakening trend with subjective dvorak estimates
ranging from 35 to 55 kt...the latter kept high due to dvorak
constraints. Deep convection has persisted near the storm center
over the last several hours...so rather than dropping the intensity
with this forecast...will keep it at 40 kt.

Hilda continues to move to the west-southwest...the result of
northeasterly winds flowing around a high centered northwest of
hawaii. There is good agreement amongst the objective aids that
hilda will remain on this course for 24 hours whilst under the
influence of the high to its northwest. Beyond 24 hours...global
models retreat the high to the north...allowing hilda to assume a
westerly course through 48 hours then a west-northwesterly track
thereafter. Most of the objective aides indicate to a degree that
this will occur. The official forecast is down the middle of a
somewhat tight aid envelop and is closest to consensus tvcn and the
regional hwrf model.

The intensity forecast remains a work in progress. It maintains the
slow weakening trend through 12 hours...followed by an equally slow
strengthening trend through 72 hours. Beyond 72 hours... Increasing
vertical shear is expected to slowly weaken hilda. About half of
the intensity aids maintain or slowly weaken hilda...while the
other half slowly strengthen the cyclone. All of the aids as does
the official forecast keep hilda below the 65 knot hurricane
threshold. The official forecast follows lgem through 72 hours then
ends up near hwrf icon and ivcn at 120 hours.

Forecast positions and max winds

initial 26/0900z 13.6n 150.6w 40 kt
12hr vt 26/1800z 13.2n 151.7w 35 kt
24hr vt 27/0600z 12.8n 153.2w 40 kt
36hr vt 27/1800z 12.7n 154.8w 45 kt
48hr vt 28/0600z 12.8n 156.7w 45 kt
72hr vt 29/0600z 13.5n 161.3w 50 kt
96hr vt 30/0600z 14.5n 165.7w 50 kt
120hr vt 31/0600z 16.3n 170.0w 45 kt

$$
forecaster craig

wtpa35 phfo 261442
tcpcp5

bulletin
tropical storm hilda advisory number 17
nws central pacific hurricane center honolulu hi ep112009
500 am hst wed aug 26 2009

...hilda moving to the west-southwest on a track far south of
hawaii...

At 500 am hst...1500 utc...the center of tropical storm hilda was
located near latitude 13.4 north...longitude 151.3 west or about
505 miles south-southeast of hilo hawaii and about 700 miles
southeast of honolulu hawaii.

Hilda is moving toward the west-southwest near 9 mph and is
expected to maintain this course for the next 24 hours. The
tropical storm is then forecast to turn to the west through 48 hours.

Maximum sustained winds are near 45 mph with higher gusts. Little
change in strength is forecast during the next 24 hours...with a
small increase forecast through 48 hours.

Tropical storm force winds extend outward up to 70 miles from the
center.

Estimated minimum central pressure is 1007 mb...29.74 inches.

...summary of 500 am hst information...
Location...13.4n 151.3w
maximum sustained winds...45 mph
present movement...west-southwest or 255 degrees at 9 mph
minimum central pressure...1007 mb

the next advisory will be issued by the central pacific hurricane
center at 1100 am hst.

$$
forecaster craig

wtpa45 phfo 262109
tcdcp5

tropical storm hilda discussion number 18
nws central pacific hurricane center honolulu hi ep112009
1100 am hst wed aug 26 2009

the deep convection which flared up overnight has waned over the
past few hours. Early morning visible satellite shows isolated deep
convection mainly over the northern half of the system. T-numbers
have pretty much leveled off in the 2.5 to 3.0 range over the past
18 hours. The quikscat pass this morning barely caught the eastern
half of the hilda. While data reliability at the extreme edge of
pass is something to consider...the pass did show 40 knot winds just
east of the apparent center. Without overriding evidence to stray
from persistence...will keep the initial intensity constant at 40
kt.

Track positions over the past 18 hours were re-bested to the south
based on microwave imagery. This southward shifts falls better in
line with the current position. Hilda/s west-southwest motion is
expected to continue over the next 24 hours under the influence of a
surface high and upper level ridge northwest of the hawaiian
islands. The high is forecast to retreat over the weekend and be
replaced by a weak low at the surface and trough aloft...which will
allow hilda to gain some latitude in days 3 through 5. Given the
southward rebest of previous positions and track verification which
a north bias of nearly all the guidance thus far...have shifted the
track forecast slightly to the south. The forecast falls within the
southern half of the guidance envelope...south of most consensus
tracks with the notable exception of tvcc which lies along the
southern boundary of the guidance envelope. Very little change was
made in forward motion.

Majority intensity guidance suggests relatively little change in
intensity over the next 5 days. Ships...hwfi...and lgem are still
holding on to strengthening beyond 24 hours. Previous forecast
package lowered the strengthening throughout 5 days in response to
increased shear in the next few days. The current intensity
forecast reflects persistence and no change from the previous.

Forecast takes hilda just south of johnston island as a tropical
storm in days 4 through 5. Will continue to monitor whether any
watches are warranted for johnston island as we head into the
weekend.

Forecast positions and max winds

initial 26/2100z 13.1n 151.4w 40 kt
12hr vt 27/0600z 12.9n 152.5w 40 kt
24hr vt 27/1800z 12.7n 154.2w 40 kt
36hr vt 28/0600z 12.6n 156.0w 45 kt
48hr vt 28/1800z 12.8n 158.1w 45 kt
72hr vt 29/1800z 13.3n 163.1w 45 kt
96hr vt 30/1800z 14.6n 168.4w 40 kt
120hr vt 31/1800z 16.9n 172.6w 40 kt

$$
forecaster tanabe

Wtpa35 phfo 280232
tcpcp5

bulletin
tropical depression hilda advisory number 23
nws central pacific hurricane center honolulu hi ep112009
500 pm hst thu aug 27 2009

...hilda moving west and dissipating to the south of hawaii...

At 500 pm hst...0300 utc...the center of dissipating tropical
depression hilda was located near latitude 13.5 north...longitude
156.1 west...or about 435 miles south of hilo hawaii and about 550
miles south-southeast of honolulu hawaii.

The dissipating depression is moving toward the west near 14 mph...
And this general motion is expected to continue until hilda
dissipates in 24 to 36 hours.

Maximum sustained winds are near 30 mph with higher gusts. Hilda is
forecast to continue weakening...and dissipation is expected in 24
to 36 hours.

Estimated minimum central pressure is 1010 mb...29.83 inches.

...summary of 500 pm hst information...
Location...13.5n 156.1w
maximum sustained winds...30 mph
present movement...west or 260 degrees at 14 mph
minimum central pressure...1010 mb

this is the last public advisory issued by the central pacific
hurricane center on this system.

$$

forecaster birchard