Tropical Storm Isaac

[USCAPEWEATHERAF]

This is a future tropical cyclone, invest 92L has a 10% chance in the next 2-days to develop with a 40% chance in the next five days to develop.  It is counted as a surface low with 1005mb already, has a potential to be a Caribbean Sea threat, although not much model support for development even with better conditions then Florence is dealing with, with a warmer ocean and lower shear values.

[yoda]

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
Issued by the NWS Weather Prediction Center College Park MD
200 PM EDT Tue Sep 4 2018

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

[...]

1. A broad low pressure system located a few hundred miles south of
the Cabo Verde Islands is forecast to move slowly westward to
west-northwestward across the eastern tropical Atlantic for the next
several days. Environmental conditions are expected to gradually
become more conducive for development, and a tropical depression is
likely to form by late this week or this weekend.
* Formation chance through 48 hours...low...30 percent.
* Formation chance through 5 days...high...80 percent.

Forecaster Bann/Stewart

[bigtenfan]

Actually looking at the 12Z Euro it shows that Florence has no less than 3 siblings behind it. The first 2 looks to have gotten past 20N before hitting 50w. Using what I think is the Don Sutherland rule those 2 should not be a threat to the US Mainland. the 3rd one looks to be too early to tell.

[Kmlwx]

4 hours ago, bigtenfan said:

Actually looking at the 12Z Euro it shows that Florence has no less than 3 siblings behind it. The first 2 looks to have gotten past 20N before hitting 50w. Using what I think is the Don Sutherland rule those 2 should not be a threat to the US Mainland. the 3rd one looks to be too early to tell.

I mean...they're all too early to tell...even Florence. 

[yoda]

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Wed Sep 5 2018

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Hurricane
Florence, located over the east-central tropical Atlantic Ocean,
and has issued its last advisory on Tropical Depression Gordon,
located over west central Mississippi.  Future information on
Gordon can be found in Public Advisories issued by the Weather
Prediction Center.

1. Shower activity associated with a broad area of low pressure
centered a couple of hundred miles south-southwest of the Cabo Verde
Islands is gradually becoming better organized.  Environmental
conditions are forecast to be conducive for additional development,
and a tropical depression is expected to form during the next
couple of days while the system moves west-northwestward across the
tropical Atlantic Ocean.
* Formation chance through 48 hours...high...70 percent.
* Formation chance through 5 days...high...90 percent.

2. A tropical wave is forecast to move off the west coast of Africa
in a few days.  Some development of the system is possible over the
weekend while the wave moves westward over the far eastern
tropical Atlantic Ocean.
* Formation chance through 48 hours...low...near 0 percent.
* Formation chance through 5 days...low...30 percent.

Forecaster Beven

[yoda]

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Thu Sep 6 2018

1. The circulation associated with an area of low pressure centered
several hundred miles west-southwest of the Cabo Verde Islands
remains broad and elongated, and the associated shower and
thunderstorm activity has not yet increased in organization.
However, environmental conditions are conducive for development, and
a tropical depression is expected to form within the next day or two
while the system moves slowly westward or west-northwestward across
the tropical Atlantic Ocean.
* Formation chance through 48 hours...high...80 percent.
* Formation chance through 5 days...high...90 percent.

[hawkeye_wx]

This morning's GFS and Euro are sending this system across the Caribbean.  Past runs have been turning it north. 

[Windspeed]

ECMWF continues to maintain W. Atl ridging enough to drive the TC into the E. Caribbean. It does also deepen the system into a hurricane by the time it tracks through the Leewards.

[yoda]

The system behind 92L is being named as PTC 8 by NHC at 11am... so this will likely be the "I" named storm

[nycwinter]

40 minutes ago, yoda said:

The system behind 92L is being named as PTC 8 by NHC at 11am... so this will likely be the "I" named storm

no it would be named helene. since it will be a storm first before 92 L...

[Windspeed]

92L probably isn't too far from being named either. The broad surface low has clearly tightened, though still impeded by strong mid-to-upper level easterlies.

[Windspeed]

This invest has now been classified TD9.

[Windspeed]

888 
WTNT44 KNHC 072058 CCA
TCDAT4

Tropical Depression Nine Discussion Number   1...Corrected
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Fri Sep 07 2018

Corrected bin number from 2 to 4

Satellite images indicate that the low pressure system over the
eastern tropical Atlantic has developed a well-defined center and
sufficient deep convection to be classified a tropical depression,
the ninth one of the 2018 Atlantic hurricane season and second one
of the day.  The initial intensity is set to 30 kt based on a 2.0
Dvorak classification from TAFB and earlier ASCAT data.

The depression has not moved much today, and it has even wobbled a
bit to the east recently.  This slow motion is a result of very
weak steering with broad troughing to the north and Tropical
Depression Eight to its east.  Little motion is expected through
tonight, but a westward motion with a gradual increase in forward
speed is forecast thereafter as a subtropical ridge builds to the
north of the cyclone.  The NHC track forecast lies closest to the
HCCA guidance and takes the system toward the Lesser Antilles by the
end of the forecast period.

The depression is experiencing some influences of easterly shear,
which is why the center is currently on the eastern edge of the
convection. Little change in intensity is expected during the next
12 to 24 hours since the cyclone is still in the developing stage
and because moderate shear is expected to continue.  However, the
shear is expected to decrease to less than 10 kt in about 24 hours.
These more favorable winds aloft combined with relatively warm
waters and a fairly moist airmass should allow for steady
strengthening beginning later in the weekend.  There is a large
spread in the intensity guidance, with the HWRF showing the system
becoming a major hurricane and HMON showing almost no strengthening.
The NHC intensity forecast lies a little below consensus models
since it appears that it will take some time for the system to
strengthen.


FORECAST POSITIONS AND MAX WINDS

INIT  07/2100Z 13.6N  34.9W   30 KT  35 MPH
 12H  08/0600Z 13.9N  35.2W   30 KT  35 MPH
 24H  08/1800Z 14.0N  36.2W   30 KT  35 MPH
 36H  09/0600Z 14.0N  37.6W   40 KT  45 MPH
 48H  09/1800Z 14.0N  39.3W   50 KT  60 MPH
 72H  10/1800Z 14.0N  44.0W   60 KT  70 MPH
 96H  11/1800Z 14.1N  48.9W   70 KT  80 MPH
120H  12/1800Z 14.5N  54.5W   75 KT  85 MPH

$$
Forecaster Cangialosi

 

[BlizzardNole]

Good grief -- they need a traffic cop in the Atlantic with all these systems.

Paging El Nino -- what are you doing?

 

[Windspeed]

This has a shot at being a major hurricane by the time it reaches the E. Caribbean. The environment may not be so favorable after that however. Hopefully it slips south of last year's impacts, especially Dominica and north through Puerto Rico.

[Windspeed]

720 
WTNT44 KNHC 080255
TCDAT4

Tropical Depression Nine Discussion Number   2
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Fri Sep 07 2018

The depression continues to be impeded by moderate easterly shear
with the center of circulation decoupled to the east of a deep
convective banding feature.  The initial intensity is held at 30 kt
based on subjective intensity estimates from TAFB and SAB.

The dynamical-statistical intensity guidance, as well as the
large-scale models, indicate that the shear should diminish to less
than 10 kt within the next 24 hours.  A more favorable upper-wind
pattern along with warm oceanic temperatures is expected to promote
steady strengthening commencing after that time.  The intensity
forecast is based on a blend of the IVCN and the HCCA consensus
models and remains below the stronger HWRF at days 4 and 5.

The depression has been meandering during the past several
hours, as a result of weak low- to mid-level steering currents
created by a deep-layer trough digging southward over the eastern
Atlantic.  In 12 to 24 hours, a westward motion with a gradual
increase in forward speed is forecast thereafter as the trough lifts
northeast, and the subtropical ridge re-establishes to the north of
the cyclone.  The NHC track forecast lies closest to the HCCA and
TVCN guidance and takes the system toward the Lesser Antilles by the
end of the forecast period.


FORECAST POSITIONS AND MAX WINDS

INIT  08/0300Z 13.9N  34.9W   30 KT  35 MPH
 12H  08/1200Z 14.1N  35.6W   35 KT  40 MPH
 24H  09/0000Z 14.2N  36.7W   35 KT  40 MPH
 36H  09/1200Z 14.3N  38.4W   45 KT  50 MPH
 48H  10/0000Z 14.4N  40.4W   55 KT  65 MPH
 72H  11/0000Z 14.4N  45.2W   75 KT  85 MPH
 96H  12/0000Z 14.4N  50.2W   80 KT  90 MPH
120H  13/0000Z 14.9N  56.2W   85 KT 100 MPH

$$
Forecaster Roberts

 

[Windspeed]

12 minutes ago, WxWatcher007 said:

Looking like this could be Isaac at 5pm.

 

Little doubt. One look at the visible and even though the TC is still encounter mid-level easterly shear, that i abating. Convection canopy and MLC looks slightly more aligned. If you focus on low-level cumulus cloud flow on the south side of circulation, it is screaming east. This has got to be of TS intensity.

Edit: Oops. Board is lagging and messed up initial post.

 

[Jackstraw]

Consistent signal for some deep ridging over the conus in 10-12 day fantasy land.  If that verify's and it can survive the eastern Carib and slip underneath could give it an unhindered path into the south central gulf.   If Florence stays N there should be ridging in its wake to help keep Issac on a W/WNW track as it passes the through the Carib.  Fun times again this year.

[Windspeed]

510 
WTNT44 KNHC 082048
TCDAT4

Tropical Storm Isaac Discussion Number   5
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Sat Sep 08 2018

The structure of the cyclone, while still a shear pattern, is
improving, with the low-level center now near the convective
overcast and improved outflow in the eastern semicircle.  Satellite
intensity estimates have increased to the 30-45 kt range, with the
CIMSS satellite consensus technique now suggesting winds near 35
kt.  Based on these estimates, the initial intensity is increased to
35 kt and the depression is upgraded to Tropical Storm Isaac.

Isaac is now moving slowly westward, with the initial motion 275/6.
The subtropical ridge to the north of Isaac is forecast to steer the
system westward with an increasing forward speed for at least the
next 2-3 days.  After that time, there is some uncertainty in the
track forecast due to the formation of a mid- to upper-level trough
over the central Atlantic, with some of the track guidance showing a
decrease in forward speed and a more northerly motion near the end
of the forecast period.  The GFS and ECMWF models show a continued
westerly motion through that time, and the official forecast follows
that scenario, although with a slower forward speed than in the
previous forecast.  The new forecast is again similar to the
earlier forecast and lies near the various consensus models.

Conditions appear favorable for gradual strengthening during the
next 72 h as Isaac moves westward across the warm waters of the
tropical Atlantic.  In best agreement with the SHIPS model for the
next three days, the new intensity forecast is raised from the
previous forecast during this time.  From 72-120 h, the system is
expected to encounter westerly wind shear, which is expected to
cause some weakening.


FORECAST POSITIONS AND MAX WINDS

INIT  08/2100Z 14.5N  36.6W   35 KT  40 MPH
 12H  09/0600Z 14.6N  37.7W   40 KT  45 MPH
 24H  09/1800Z 14.7N  39.5W   50 KT  60 MPH
 36H  10/0600Z 14.7N  42.0W   60 KT  70 MPH
 48H  10/1800Z 14.7N  44.6W   65 KT  75 MPH
 72H  11/1800Z 14.5N  50.0W   75 KT  85 MPH
 96H  12/1800Z 14.5N  55.5W   75 KT  85 MPH
120H  13/1800Z 15.0N  61.0W   70 KT  80 MPH

$$
Forecaster Beven

 

[hlcater]

In other news, the HWRF is a huge fan and bombs Isaac to C5 on approach to the Leewards. Not really sure how it does this in the face of 20kts southwesterly shear. Maybe due to super robust convective bursts that help lower the pressure despite shear induced asymmetry? After all, there's 5000 CAPE available in this area averaged sounding--plenty of juice for robust thunderstorms.

 

 

[Windspeed]

Yeah I am a little puzzled at the HWRF's intensity, however, it is possible the shear axis from the Florence's outflow channel will stay north of Isaac due to a cutoff low in the central Atl., east of Bermuda. Also, a TUTT may develop over the Greater Antilles, which, if Isaac is positioned just right, there could be a sufficient upper ridge east of the Leewards to support major hurricane intensity. Granted, Isaac may encounter an unfavorable environment as it traverses deeper into the Caribbean. Unfortunately, it may have already impacted hard hit areas from last year before it reaches a more unfavorable environment.

[Windspeed]

251 
WTNT44 KNHC 090242
TCDAT4

Tropical Storm Isaac Discussion Number   6
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Sat Sep 08 2018

Isaac is strengthening this evening.  Satellite images indicate that
the deep convection has been increasing in intensity and coverage
with better defined banding features.  The center is not located in
the center of the convection, however, due to some easterly shear.
An ASCAT pass around 00Z showed maximum winds in the 40-45 kt
range.  These data are also in line with the latest Dvorak
classifications of 3.0/45 kt from TAFB and SAB.  Based on these
estimates, the initial intensity is raised to 45 kt.

Isaac will likely continue to strengthen during the next few days as
the storm remains over warm waters and moves into an environment of
decreasing wind shear.  Beyond a few days, the SHIPS model shows a
notable increase in shear, in part due to the outflow from Florence,
which should end the strengthening trend and cause some weakening.
With the exception of the HMON and COAMPS-TC models, the remainder
of the intensity guidance is higher this cycle.  The NHC intensity
forecast is raised from the previous one, but it is a little lower
than the HCCA and IVCN guidance.

The storm is moving due westward at 7 kt.  The track forecast seems
fairly straightforward.  A strengthening subtropical ridge to the
north of the system should cause Isaac to move westward at an
increasing forward speed during the next several days.  This
scenario is supported by the usually more reliable GFS and ECMWF
models, and the NHC track forecast is near a blend of those aids.
Based on the current forecast, Isaac will be near the Lesser
Antilles in 4 to 5 days and interests there should monitor the
progress of this system.

FORECAST POSITIONS AND MAX WINDS

INIT  09/0300Z 14.4N  37.5W   45 KT  50 MPH
 12H  09/1200Z 14.4N  38.7W   50 KT  60 MPH
 24H  10/0000Z 14.5N  40.9W   60 KT  70 MPH
 36H  10/1200Z 14.5N  43.5W   70 KT  80 MPH
 48H  11/0000Z 14.5N  46.1W   75 KT  85 MPH
 72H  12/0000Z 14.5N  51.5W   85 KT 100 MPH
 96H  13/0000Z 14.6N  57.0W   80 KT  90 MPH
120H  14/0000Z 15.0N  62.9W   75 KT  85 MPH

$$
Forecaster Cangialosi

 

[USCAPEWEATHERAF]

Updated the thread

[Windspeed]

273 
WTNT44 KNHC 101436
TCDAT4

Hurricane Isaac Discussion Number  12
NWS National Hurricane Center Miami FL       AL092018
1100 AM AST Mon Sep 10 2018

Isaac's cloud pattern is currently a bit ragged-looking, with an
irregular CDO and limited convective banding.  Dvorak intensity
estimates from both TAFB and SAB are 65 kt, and that value will be
used for the advisory.  There is significant uncertainty in the
intensity forecast for this system.  Vertical wind shear over the
hurricane is forecast to be weak to moderate during the next day or
so, which should allow Isaac to strengthen somewhat.  By 48 hours,
however, the global models depict significantly stronger
northwesterly shear, associated with an upper-level trough over the
central Atlantic, affecting the tropical cyclone.  Most of the
guidance does not show Isaac strengthening over the Caribbean.  The
HWRF model is an outlier, strengthening Isaac into a major hurricane
later in the period, and it is the main contributor to high
consensus forecasts at days 3-5.  The NHC intensity forecast is
above the model consensus through 72 hours and below it by the end
of the period.

Isaac continues to move westward, or 275/12 kt.  A subtropical
ridge should be maintained to the north of the tropical cyclone for
the next several days, so a continued westward track seems likely.
The U. K. Met. Office model is an outlier and shows Isaac turning
northwestward and northward to the northeast of the Leeward Islands.
The official track forecast, like the previous one, follows the
other reliable global models and is also close to the HFIP corrected
consensus guidance.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the forecast is
greater than normal.  Although Isaac is forecast to begin weakening
when it approaches the Lesser Antilles, it is still expected to be
at or near hurricane intensity when it reaches the islands.

2. Interests in the Lesser Antilles should continue to monitor Isaac
during the next few days.

FORECAST POSITIONS AND MAX WINDS

INIT  10/1500Z 14.7N  43.9W   65 KT  75 MPH
 12H  11/0000Z 14.9N  45.9W   70 KT  80 MPH
 24H  11/1200Z 15.0N  48.7W   75 KT  85 MPH
 36H  12/0000Z 15.0N  51.4W   75 KT  85 MPH
 48H  12/1200Z 15.1N  54.0W   75 KT  85 MPH
 72H  13/1200Z 15.4N  59.4W   70 KT  80 MPH
 96H  14/1200Z 15.5N  64.5W   60 KT  70 MPH
120H  15/1200Z 15.7N  70.0W   50 KT  60 MPH

$$
Forecaster Pasch

 

[yoda]

Hurricane Isaac Discussion Number  13
NWS National Hurricane Center Miami FL       AL092018
500 PM AST Mon Sep 10 2018

Isaac has not become better organized today.  The system continues
to display a ragged CDO with limited banding features.  Dvorak
T-numbers from both TAFB and SAB continue to support an intensity of
65 kt.  It is not clear why Isaac has failed to strengthen.  Cirrus
motions and microwave imagery suggest some shear over the tropical
cyclone, which may have had some influence.  Isaac should not
experience a significant increase in shear until about 48 hours, so
some strengthening is called for up to that time.  Thereafter, the
dynamical guidance shows a significant increase in shear, which
should reverse the intensity trend.  The HWRF model continues to be
an outlier in predicting significant intensification over the
Caribbean, whereas the other models do not.  The official intensity
forecast is above the model consensus over the first half of the
period, and below the consensus during the latter part of the
period, considering the HWRF could be making the consensus too high
at days 3-5.

Isaac continues moving westward or 270/12 kt.  There continue to be
a few differences among the track guidance, in particular the U.K.
Met, which shows the system turning northwestward to northward well
to the northeast of the Leeward Islands.   The GFS takes the
system into the Caribbean but shows a more west-northwestward
motion.  The official forecast stays close to the ECMWF and the
corrected consensus predictions, which is very similar to the
previous NHC track.  Given the spread in the guidance, the
confidence in the details of the track forecast beyond the first
couple of days is larger than usual.

Key Messages:

1. Isaac is a small hurricane and uncertainty in the intensity
forecast is greater than normal.  Although Isaac is expected to
begin weakening when it approaches the Lesser Antilles, it is still
likely to be at or near hurricane intensity when it reaches the
islands.

2. Interests in the Lesser  Antilles should continue to monitor
Isaac during the next few days.  Watches will likely be required for
portions of the Lesser Antilles tomorrow.

FORECAST POSITIONS AND MAX WINDS

INIT  10/2100Z 14.4N  45.0W   65 KT  75 MPH
 12H  11/0600Z 14.5N  47.1W   70 KT  80 MPH
 24H  11/1800Z 14.6N  49.8W   75 KT  85 MPH
 36H  12/0600Z 14.6N  52.3W   75 KT  85 MPH
 48H  12/1800Z 14.8N  54.7W   75 KT  85 MPH
 72H  13/1800Z 15.3N  60.2W   70 KT  80 MPH
 96H  14/1800Z 15.5N  65.5W   60 KT  70 MPH
120H  15/1800Z 15.5N  70.5W   50 KT  60 MPH

$$
Forecaster Pasch

[yoda]

Tropical Storm Isaac Discussion Number  14
NWS National Hurricane Center Miami FL       AL092018
1100 PM AST Mon Sep 10 2018

Several microwave images that arrived after the issuance of the last
advisory revealed that the low-level center of Isaac has raced ahead
of the main convective mass, and the system is poorly vertically
aligned. The system also recently passed very near NOAA buoy 41041,
which confirmed that the center is on the western side of the
central dense overcast. The maximum winds observed by the buoy were
only about 30 kt, with a minimum pressure of near 1001 mb. Due to
the western adjustment of the center location, the latest Dvorak
classification from TAFB is lower, and a blend of the TAFB final-T
and CI-numbers supports a lower initial intensity of 60 kt. It is
worth noting that this estimate is more uncertain than normal due to
limitations of the Dvorak technique, and most of the other intensity
estimates are higher. Given the high pressure and light winds at the
buoy, I am inclined to favor the lower estimate at this time.

Confidence in the track forecast is slightly higher now, and the
UKMET is the only outlier that does not bring Isaac westward into
the Caribbean within the next 72 h or so. There is still some
disagreement on the speed of Isaac, especially once it crosses the
Leeward Islands, and this seems largely related to the cyclone's
intensity. The ECMWF and GFS depict a weakening storm that simply
continues westward, while the HWRF and CTCI show strengthening as
Isaac slows down and turns more west-northwestward. The NHC track
forecast for now continues to favor the typically more reliable
global models and shows a steady westward motion through day 5. The
updated NHC forecast is a little faster than the previous advisory,
due primarily to the westward adjustment of the initial position,
and remains close to HCCA.

A slight adjustment has been made to the intensity forecast, but
confidence in the intensity forecast remains quite low. Due to the
current structure of Isaac, it seems less likely that the tropical
storm will intensify more than what is shown by the intensity
guidance over the next 24-48 h. The NHC forecast has therefore been
lowered slightly, but it is still near the top end of the guidance,
and calls for Isaac to be a hurricane when it approaches the
Leeward Islands later this week. It is also worth noting that the
small size of Isaac could make it more susceptible to sudden
short-term changes in intensity that are nearly impossible to
forecast. The intensity spread is tremendous by the end of the
forecast period, with multiple models showing Isaac becoming a major
hurricane around day 4 or 5, and others showing outright
dissipation. The NHC forecasts have been favoring a weaker solution
thus far due to an expected increase in shear beginning in a couple
of days, and I see no reason to make a dramatic change at this
point.

Key Messages:

1. Isaac is expected to be at or near hurricane intensity when it
approaches the Lesser Antilles. However, confidence in the forecast
is lower than normal.

2. Interests in the Lesser Antilles should continue to monitor
Isaac during the next few days. Watches will likely be required for
portions of the Lesser Antilles tomorrow.

FORECAST POSITIONS AND MAX WINDS

INIT  11/0300Z 14.5N  46.9W   60 KT  70 MPH
 12H  11/1200Z 14.5N  49.0W   60 KT  70 MPH
 24H  12/0000Z 14.6N  51.7W   65 KT  75 MPH
 36H  12/1200Z 14.8N  54.3W   65 KT  75 MPH
 48H  13/0000Z 15.0N  56.9W   65 KT  75 MPH
 72H  14/0000Z 15.4N  62.4W   60 KT  70 MPH
 96H  15/0000Z 15.5N  68.0W   55 KT  65 MPH
120H  16/0000Z 15.5N  72.5W   50 KT  60 MPH

$$
Forecaster Zelinsky

[MJO812]

https://twitter.com/webberweather/status/1039603018214395905/photo/1

In regards to this storm

[the ghost of leroy]

1 minute ago, Snow88 said:

https://twitter.com/webberweather/status/1039603018214395905/photo/1

In regards to this storm

we just have to hope that john hope's graveyard kills it.

[Akeem the African Dream]

isaac is going to be my peak season florida storm

[MJO812]

22 hours ago, Akeem the African Dream said:

isaac is going to be my peak season florida storm

Or the gulf like the Euro op and ensembles show