tropical storm lana discussion number 10
nws central pacific hurricane center honolulu hi ep062009
1100 am hst sat aug 01 2009
the cloud pattern of tropical storm lana has weakened rapidly since
late last night. Most of the active convection has dissipated...the
higher cloud tops are shifting off to the north northeast...and the
low level center is becoming exposed. These are good indications
that southwest shear is weakening the storm.
Last night locating the center of lana was problematical...but a
series of microwave and visible satellite images have allowed us to
locate the center with high confidence. These images have also given
us good confidence that the storm is being sheared as expected.
The forecast track follows the previous package closely with a
slight shift to the south. Lana continues to move westward to the
south of a low and mid-level subtropical ridge. The mid level ridge
is expected to continue building westward in phase with lana over
the next several days...keeping the tropical cyclone moving on a
slightly north of west track through the forecast period. The
forecast track is near the middle of the envelope of the dynamic
models. I have shifted the track slightly further south since lana
is now expected to weaken a bit faster. The forecast track has lana
passing over 250 miles south of the main hawaiian islands tonight
and sunday.
Over the past 24 hours lana has been moving over an area of slightly
warmer sea surface temperatures...around 27.5c that may have helped
produce the burst of convection last evening. Lana should now be
moving over slightly cooler water for the next 24 hours...around 1c
cooler. These ssts would still be adequate to maintain the storm but
increasing vertical shear is likely to weaken the storm. Since the
latest satellite imagery has already shown lana being decapitated by
the vertical shear we have forecast weakening to occur a bit faster
than the previous forecast.
tropical storm lana discussion number 11
nws central pacific hurricane center honolulu hi ep062009
500 pm hst sat aug 01 2009
convection around lana continues to pulse. At 1000 am hst...nearly
all of the convection around lana had dissipated but more convection
has now developed north of the center. The low level circulation
center remains mostly exposed so we still have high confidence in
the center location. The presence of the exposed low level center
and the small amount of deep convection near the center are
indications that lana is struggling to survive.
The forecast track follows the previous package closely with a very
slight shift to the south. Lana continues to move westward to the
south of a low to mid-level subtropical ridge. The mid level ridge
is expected to build west southwest over the next several
days...keeping the tropical cyclone moving on a slightly north of
west track through the forecast period. Lana has been tracking along
the south side of the envelope of the dynamic models. I have kept
the track nearly the same...with just a slight shift farther south
of the previous track. The forecast track has lana passing about 300
miles south of south point...the southernmost point in the hawaiian
islands...around 800 pm tonight.
As usual...the intensity forecast is much less straightforward than
the track forecast. Over the past 24 hours lana has been moving over
an area of slightly warmer sea surface temperatures...around
27.5c...and higher ocean heat content...that may be helping lana
survive. Lana should now start moving over slightly cooler water for
the next 24 hours. These ssts would still be adequate to maintain
the storm if the environment was otherwise favorable.so probably the
main factor that will determine lanas intensity will be vertical
shear.
There is a northeast to southwest trough aloft just northwest of
kauai. Southwest winds on the southeast side of this trough are
producing strong vertical shear. Lana is just on the edge of this
strong shear. As lana moves west over the next few days...the trough
aloft and the shear zone will also move west. I expect the shear
will be strong enough to kill lana over the next 96 hours or
so...but it is not a sure thing. Lana will start moving over warmer
water again west of 160w and that could give the cyclone a new lease
on life.
tropical storm lana discussion number 12
nws central pacific hurricane center honolulu hi ep062009
1100 pm hst sat aug 01 2009
at 800 pm hst...0600z...the low level circulation center of lana was
due south of the big island and moving to the west at a steady 16
kt. Deep convection continues to pulse...with thunderstorms just
northeast of the cyclone center at this time. Confidence in the
center position is good as is the cyclone movement. Jtwc...sab and
hfo all came in with final t-numbers of 3.0. The official intensity
therefore remains at 45 kt.
The forecast track follows the previous track with only a small
shift to the south. Lana lies in easterly flow south of a
subtropical high and ridge. The low- and mid-level ridge is located
north of hawaii and is forecast to build westward over the next 120
hours...keeping lana well within the trade wind belt. The forecast
therefore keeps lana on a westward track close to the previous
track and just south of most of the tightly grouped objective
aides.
An upper-level low centered southwest of kauai near 18n 166w has
been stationary for the last 24 hours. Southwest winds aloft east
of the low are riding up over the low- and mid-level easterlies
resulting in substantial vertical shearing. A consensus of global
models keep the upper low near its present position over the next
24 to 30 hours. They then weaken and lift the upper low out to the
northwest...resulting in a slackening of the shear. The persistent
shear should take its toll on lana...forcing it to slowly spin down
through the forecast period. The intensity forecast is in good
agreement with ships and icon.
tropical storm lana discussion number 14
nws central pacific hurricane center honolulu hi ep062009
1100 am hst sun aug 02 2009
at 800 am hst...1800 utc...lana showed signs of weakening with deep
convection deteriorating in the northeast quadrant of the system.
The low level cloud center is gradually becoming more removed from
this area of deep convection. Phfo and sab came up with final t
numbers of 3.0 while jtwc went with 2.5. Deep convection has had a
history of pulsing with this system. The official intensity
estimate remains at 45 kt but the current intensity may be a bit
generous if deep convection does not reform near the systems center
over the next several hours.
Lana lies in deep easterly flow south of a subtropical ridge located
north of hawaii. Global models are in good agreement in building
the low and mid level ridge to the west...keeping lana well within
the broad easterly trade wind belt. The official track remains
nearly the same as the previous forecast and lies just south of the
vast majority of model consensus which is closely grouped overall.
Forward motion remains in good agreement with most of the objective
aides and is an extension of the old track.
An elongated upper level trough lies to the west and southwest of
kauai. The uw-cimss output shows south to southeasterly shear
estimated to be about 25 kt in the vicinity of lana. This should
continue to inhibit any chance of re-intensification. This trough
is expected to move westward over the next few days and gradually
weaken with time. With lana expected to remain in a hostile shear
environment...gradual weakening is expected through the forecast
period which is in general agreement with intensity guidance...with
only the ukmet and shifor5 keeping lana as a weak tropical storm
through 120 hours. This may be due to warmer sea surface
temperatures along the forecast track well southwest of the main
hawaiian islands.
tropical depression lana discussion number 16
nws central pacific hurricane center honolulu hi ep062009
1100 pm hst sun aug 02 2009
lana is devoid of deep convection within 60 nm of center as of 800
pm hst...0600 utc monday...keeping this system no stronger than
depression status for this forecast package. Cphc had to rely on
met to derive a current intensity of 2.0...while jtwc judged this
system too weak to classify. Satellite loops show that lana is
marked by a swirl of low clouds...while scattered to spotty broken
high clouds fly across the center towards the northeast. The
official initial intensity is set at 30 kt with these winds likely
confined to the northern semicircle.
Lana remains in deep easterly flow south of the subtropical ridge.
Global models continue to build this ridge as lana moves
westward...keeping lana well within the broad easterly trade wind
belt. The official track remains the same as the previous
package...lying just south of the majority of closely grouped model
consensus. System motion remains in good agreement with most of the
objective aides.
High clouds moving rapidly across this system towards the northeast
betray the presence of debilitating shear. Uw-cimms output shows
southwesterly shear at about 26 kt across lana...while ships
initializes at 34 kt from the same direction. Ships forecasts shear
to remain chiefly in the 30 to 40 kt range through the next 120
hours. Individual thunderstorms love shear...so convection may
occasionally flare near the center in the near future...but with
shear this strong lana is unlikely to redevelop into a stronger
tropical system as a whole. Therefore...the intensity forecast
calls for lana to weaken into a remnant low by 36 hours.
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