ATL: ISAAC - Post-Tropical - Discussion

[Buck]

Very impressive envelope this thing has developed.

[Gustywind]

Increasing T numbers up to 3.5 for TS Isaac...

09/1145 UTC 14.5N 38.7W T3.5/3.5 ISAAC
09/0545 UTC 14.5N 37.7W T3.0/3.0 ISAAC

[KWT]

I'm wondering whether this is a repeat of Beryl. Its getting a classic look though it is quite small it seems. The inner core looks a little far on the Eastern side still but it's definitely forming looking at the microwave. Seems like it's on its way to Hurricane strength.

[cycloneye]

12z Best Track up to 55 kts.

AL, 09, 2018090912, , BEST, 0, 143N, 386W, 55, 999, TS

[NDG]

The reason why the models show Isaac weakening when it reaches the Caribbean is because of Florence's huge outflow that it will have by then, creating northerly shear over the system, hope they are right.

[cycloneye]

The reason why the models show Isaac weakening when it reaches the Caribbean is because of Florence's huge outflow that it will have by then, creating northerly shear over the system, hope they are right.

I wonder why one model sees the pattern different.Maybe not too strong outflow?

[Gustywind]

The reason why the models show Isaac weakening when it reaches the Caribbean is because of Florence's huge outflow that it will have by then, creating northerly shear over the system, hope they are right.

I wonder why one model sees the pattern different.Maybe not too strong outflow?

Very interresting question Luis.

[Gustywind]

The reason why the models show Isaac weakening when it reaches the Caribbean is because of Florence's huge outflow that it will have by then, creating northerly shear over the system, hope they are right.

I wonder why one model sees the pattern different.Maybe not too strong outflow?

From the NHC disco..."Beyond a few days, however, the SHIPS model shows a notable increase in shear, in part due to the outflow from Florence, which should end the strengthening trend and induce some weakening".
Lets hope that this scenario occurs if not... could have some troubles in tape,?, let's keep our fingers crossed.

[cycloneye]

BULLETIN
Tropical Storm Isaac Advisory Number 8
NWS National Hurricane Center Miami FL AL092018
1100 AM AST Sun Sep 09 2018

...ISAAC STRENGTHENING OVER THE EASTERN TROPICAL ATLANTIC...


SUMMARY OF 1100 AM AST...1500 UTC...INFORMATION
-----------------------------------------------
LOCATION...14.3N 39.1W
ABOUT 1470 MI...2370 KM E OF THE WINDWARD ISLANDS
MAXIMUM SUSTAINED WINDS...65 MPH...100 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 9 MPH...15 KM/H
MINIMUM CENTRAL PRESSURE...999 MB...29.50 INCHES



Tropical Storm Isaac Discussion Number 8
NWS National Hurricane Center Miami FL AL092018
1100 AM AST Sun Sep 09 2018

WindSat microwave data from several hours ago show that Isaac has
developed a well-defined low-level ring in the 37-GHz channel, and
the center is embedded in the middle of a small central dense
overcast. TAFB and SAB Dvorak estimates have increased to T3.5
given Isaac's improved structure, so the initial intensity is raised
to 55 kt.

Isaac's future intensity is the most challenging part of the
forecast. Since sea surface temperatures will be warm (between
27-28 degrees Celsius) and vertical shear will be almost
non-existent (less than 10 kt) for the next 36-48 hours, the tiny
storm will be prone to a possibly significant increase in intensity
during the next couple of days. For that period, the NHC intensity
forecast is above the guidance, and above the previous forecast, due
to relatively high numbers from the various rapid intensification
indices. However, in 2-3 days, the global models are showing
northwesterly shear steadily increasing and potentially reaching a
maximum of around 30 kt by Thursday. The big question is the
timing of this shear and how much it will affect Isaac's intensity
before the cyclone reaches the Lesser Antilles. The HWRF, which
brings Isaac to major hurricane intensity despite the forecast
shear, appears to be an outlier, and is therefore pulling up the
ICON intensity consensus and HCCA. As a result, the NHC intensity
forecast is below ICON on days 4 and 5. This new forecast shows a
more peaked maximum intensity, with Isaac possibly increasing and
then decreasing in strength at faster rates than shown here.

While the microwave data indicate that Isaac's center is slightly
south of previous estimates, the current motion estimate is 270/8
kt. A low- to mid-level ridge is expected to move westward in
tandem with Isaac for much of the forecast period, causing the
cyclone to accelerate toward the west, reaching a forward speed of
around 15 kt by 36 hours, and then holding that speed through day
5. With the exception of the UKMET and HWRF models, the guidance
envelope is tightly packed from north to south, and the speed
differences noted over the past few days have decreased. The
updated NHC track forecast is just a little south of the previous
one to account for the adjusted initial position, and lies slightly
south of the multi-model consensus in the direction of HCCA and the
Florida State Superensemble.

FORECAST POSITIONS AND MAX WINDS

INIT 09/1500Z 14.3N 39.1W 55 KT 65 MPH
12H 10/0000Z 14.4N 40.8W 65 KT 75 MPH
24H 10/1200Z 14.4N 43.3W 75 KT 85 MPH
36H 11/0000Z 14.4N 46.3W 85 KT 100 MPH
48H 11/1200Z 14.3N 49.1W 85 KT 100 MPH
72H 12/1200Z 14.2N 54.8W 80 KT 90 MPH
96H 13/1200Z 14.5N 60.5W 70 KT 80 MPH
120H 14/1200Z 15.0N 66.5W 60 KT 70 MPH

$$
Forecaster Berg

[SoupBone]

I was talking about the shear last night. And the Euro and GFS due west motion is caused by a high parked over Cuba, not allowing it to gain latitude, ultimately crashing it into Honduras and Nicaragua.

[NDG]

I was talking about the shear last night. And the Euro and GFS due west motion is caused by a high parked over Cuba, not allowing it to gain latitude, ultimately crashing it into Honduras and Nicaragua.

The almost due west motion across the Caribbean is also due to the models showing it to weakening and getting caught in the low level easterlies across the Caribbean, lets not forget the Euro has been west biased. The ridging north of it doesn't look that strong because of Florence so if it stays stronger across the Caribbean it is highly unlikely it will crash into C.A. Time will tell.

[ConvergenceZone]

Hopefully this won't get too strong prior to reaching islands. After the islands, it appears they are forecasting it to be a rain event only. So hopefully convection will wane in the Caribbean as well so that after it hits the islands, there won't be much left of it.......
Best of luck to the islands....

[AtlanticWind]

GFS weakens Isaac over next 36 hrs.
Hard to figure why it would do this

[SoupBone]

GFS weakens Isaac over next 36 hrs.
Hard to figure why it would do this

Not hard at all. High Caribbean shear currently in the area and forecast to remain as it tracks across.

[SoupBone]

I was talking about the shear last night. And the Euro and GFS due west motion is caused by a high parked over Cuba, not allowing it to gain latitude, ultimately crashing it into Honduras and Nicaragua.

The almost due west motion across the Caribbean is also due to the models showing it to weakening and getting caught in the low level easterlies across the Caribbean, lets not forget the Euro has been west biased. The ridging north of it doesn't look that strong because of Florence so if it stays stronger across the Caribbean it is highly unlikely it will crash into C.A. Time will tell.

Not that it can't change, but shear across that area of the Caribbean has been parked there all season and has been fairly stout. Our discussions this morning (with our mets) was focused on this feature. They were obviously cautious about anything beyond their 5 day mark where they feel comfortable, but if the shear stays put, they think it's not going to gain latitude. We'll see in a few days.

[northjaxpro]

I am worried that Isaac will become a very strong hurricane. Hoping somehow he doesn't get powerful. Praying for all in the Caribbean as the people down there definitely don't need this.

[ConvergenceZone]

I am worried that Isaac will become a very strong hurricane. Hoping somehow he doesn't get powerful. Praying for all in the Caribbean as the people down there definitely don't need this.

I agree. Even though the carib is forecast to really rip it apart, the concern is the islands before it gets to the carib...

[cycloneye]

I am worried that Isaac will become a very strong hurricane. Hoping somehow he doesn't get powerful. Praying for all in the Caribbean as the people down there definitely don't need this.

Agreed 100%. Imagine that still almost one year from Maria,thousands of homes are with the blue tarps.That says it all about how are things in PR so we dont need even a Tropical Storm to threat us.

[NDG]

I am worried that Isaac will become a very strong hurricane. Hoping somehow he doesn't get powerful. Praying for all in the Caribbean as the people down there definitely don't need this.

Yeah, very good chance of it becoming a major hurricane because of fairly good UL environment over the next 2-3 days or so, storms in this area usually become stronger than what the global models forecast it to be.

[AtlanticWind]

GFS weakens Isaac over next 36 hrs.
Hard to figure why it would do this

Not hard at all. High Caribbean shear currently in the area and forecast to remain as it tracks across.

Not talking about when it reaches carribean,
GFS weakens it starting now,when conditions appear favorable