ATL: HERMINE - Post-Tropical - Discussion

[AJC3]

It has 25kts-30kts of shear waiting for it to the southwest...

Is all easterly UL winds, if the TW picks up significant forward speed towards the west, which the GFS shows it approaching the windward Island by Wednesday morning, traveling west at an average speed of 15 mph, it cancels out a lot of that UL easterly shear.
18Z SHIPS shows shear to be well under 15 knots over the next 5 days as it travels westward.

Code: Select all

SHEAR (KT)        14     8     8    11    12     7     9     6     9    12     5     9     8

Gang, either there is 25-30kt of shear or there isn't.

Shear is a vector difference, which already takes into account a lower level wind vector. The 850 wind vector in the deep tropics, is usually (but not always) a decent proxy for speed of forward motion. The only way that a shear calculation will be significantly off is if the storm motion vector is markedly different from the low level wind vector used in the shear calculation. If the OP was talking about an upper tropospheric wind vector, then, and only then, will the direction of that wind vector, relative to the direction of the storm motion vector play a role in the total magnitude of the shear.

I.e. if a model is advertising 25-30kt of easterly shear in the MDR, that means the upper level easterlies are that much stronger than the lower level easterlies, which are generally 15-20kt. This means the upper tropospheric easterlies used in the shear calculation would be over 40kt. Shear is shear.

OK, now that my mini-rant is over, here's a little two-question true or false quiz to see if you were listening...

True or false: In the MDR, 25-30kt of upper level westerly shear over a system moving westward at 15kt is "worse" than 25kt of upper level easterly shear.
True or false: In the MDR, 25-30kt of upper level westerly wind over a system moving westward at 15kt is "worse" than 25kt of upper level easterly wind.

Carry on.

[Aric Dunn]

gfs not looking good for PR

[cycloneye]

I can see an increase in the % at 8 PM.(20%-40%) Is only my opinon.

[NDG]

It has 25kts-30kts of shear waiting for it to the southwest...

Is all easterly UL winds, if the TW picks up significant forward speed towards the west, which the GFS shows it approaching the windward Island by Wednesday morning, traveling west at an average speed of 15 mph, it cancels out a lot of that UL easterly shear.
18Z SHIPS shows shear to be well under 15 knots over the next 5 days as it travels westward.

Code: Select all

SHEAR (KT)        14     8     8    11    12     7     9     6     9    12     5     9     8

Gang, either there is 25-30kt of shear or there isn't.

Shear is a vector difference, which already takes into account a lower level wind vector. The 850 wind vector in the deep tropics, is usually (but not always) a decent proxy for speed of forward motion. The only way that a shear calculation will be significantly off is if the storm motion vector is markedly different from the low level wind vector used in the shear calculation. If the OP was talking about an upper tropospheric wind vector, then, and only then, will the direction of that wind vector, relative to the direction of the storm motion vector play a role in the total magnitude of the shear.

I.e. if a model is advertising 25-30kt of easterly shear in the MDR, that means the upper level easterlies are that much stronger than the lower level easterlies, which are generally 15-20kt. This means the upper tropospheric easterlies used in the shear calculation would be over 40kt. Shear is shear.

OK, now that my mini-rant is over, here's a little two-question true or false quiz to see if you were listening...

True or false: In the MDR, 25-30kt of upper level westerly shear over a system moving westward at 15kt is "worse" than 25kt of upper level easterly shear.
True or false: In the MDR, 25-30kt of upper level westerly wind over a system moving westward at 15kt is "worse" than 25kt of upper level easterly wind.

Carry on.

I understand what you are saying but where they have the 25-30+ knots of windshear is just to the south & southeast of Fiona where surface winds are from a westerly component, south of the 15th latitude, so of course winshear is going to be analyzed that high with easterly UL winds in the 20-30 knots.

[AJC3]

I understand what you are saying but where they have the 25-30+ knots of windshear is just to the south & southeast of Fiona where surface winds are from a westerly component, south of the 15th latitude, so of course windshear is going to be analyzed that high with easterly UL winds in the 20-30 knots.

Which is why, when it comes to using shear charts, it's best to look at overall wind shear on the periphery of the system, rather than over, or very close to it, since the environmental winds used in the shear calculation get "contaminated" by the system's own circulation. Hence my caveat that the low level (850 MB) wind vector being used is "usually" a good proxy for the storm's motion. When and if the westerly 850 MB winds on the southern periphery of a system's circulation that's moving westward at a pretty decent clip (as you mentioned) are being used to calculate the shear vector, you can bet that the system doesn't actually have 25-30kt shear headed for it.

[NDG]

I understand what you are saying but where they have the 25-30+ knots of windshear is just to the south & southeast of Fiona where surface winds are from a westerly component, south of the 15th latitude, so of course windshear is going to be analyzed that high with easterly UL winds in the 20-30 knots.

Which is why, when it comes to using shear charts, it's best to look at wind shear on the periphery of the system, rather than over it, since the environmental winds get "contaminated" by the system's own circulation. Hence my caveat about the low level (850 MB) wind vector "usually" being a good proxy for the storm's motion.

I agree. Current h85 & h20 wind vectors. GFS shows that after tomorrow 99L will a get a kick forward towards the west, so that's what I am saying that the current windshear analyzes to its west will not come into effect once it picks up speed with easterly winds above it in the mid and upper levels.

[cycloneye]

Almost nailed it.

A tropical wave located about 300 miles southwest of the Cabo Verde
Islands is producing an elongated area of cloudiness and
disorganized shower activity. Environmental conditions are expected
to be conducive for gradual development of this disturbance through
early next week while it moves westward at about 15 mph across the
tropical Atlantic Ocean.
* Formation chance through 48 hours...low...10 percent
* Formation chance through 5 days...medium...40 percent

[Kazmit]

Almost nailed it.

A tropical wave located about 300 miles southwest of the Cabo Verde
Islands is producing an elongated area of cloudiness and
disorganized shower activity. Environmental conditions are expected
to be conducive for gradual development of this disturbance through
early next week while it moves westward at about 15 mph across the
tropical Atlantic Ocean.
* Formation chance through 48 hours...low...10 percent
* Formation chance through 5 days...medium...40 percent

Nice. Looks like it has a fair chance of becoming Gaston.

[Hurricaneman]

And maybe dangerous in the islands but can't say for sure

The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or STORM2K. For official information, please refer to products from the National Hurricane Center and National Weather Service

[BobHarlem]

There's a while to track this one.. Not sure where to put this, but that area next to Bermuda flared up a lot today.

[abajan]

If that NHC cone of probability pans out, I hope possible future Gaston runs out of gas before it reaches this region, because it's pointed right at Barbados!

[NDG]

There's a while to track this one.. Not sure where to put this, but that area next to Bermuda flared up a lot today.

viewtopic.php?f=31&t=118165

[p1nheadlarry]

It has 25kts-30kts of shear waiting for it to the southwest...

Is all easterly UL winds, if the TW picks up significant forward speed towards the west, which the GFS shows it approaching the windward Island by Wednesday morning, traveling west at an average speed of 15 mph, it cancels out a lot of that UL easterly shear.
18Z SHIPS shows shear to be well under 15 knots over the next 5 days as it travels westward.

Code: Select all

SHEAR (KT)        14     8     8    11    12     7     9     6     9    12     5     9     8

Gang, either there is 25-30kt of shear or there isn't.

Shear is a vector difference, which already takes into account a lower level wind vector. The 850 wind vector in the deep tropics, is usually (but not always) a decent proxy for speed of forward motion. The only way that a shear calculation will be significantly off is if the storm motion vector is markedly different from the low level wind vector used in the shear calculation. If the OP was talking about an upper tropospheric wind vector, then, and only then, will the direction of that wind vector, relative to the direction of the storm motion vector play a role in the total magnitude of the shear.

I.e. if a model is advertising 25-30kt of easterly shear in the MDR, that means the upper level easterlies are that much stronger than the lower level easterlies, which are generally 15-20kt. This means the upper tropospheric easterlies used in the shear calculation would be over 40kt. Shear is shear.

OK, now that my mini-rant is over, here's a little two-question true or false quiz to see if you were listening...

True or false: In the MDR, 25-30kt of upper level westerly shear over a system moving westward at 15kt is "worse" than 25kt of upper level easterly shear.
True or false: In the MDR, 25-30kt of upper level westerly wind over a system moving westward at 15kt is "worse" than 25kt of upper level easterly wind.

Carry on.

false true.

[Gustywind]

000
AXNT20 KNHC 182355
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
755 PM EDT THU AUG 18 2016


Tropical wave extends from 12N28W to 20N28W moving W at 15-20 kt.
A 1009 mb low is centered at the southern extent of the wave that
remains embedded within the Monsoon Trough. The wave coincides
with 700 mb troughing between 25W-32W with 850 mb relative
vorticity maximized along the wave axis near 13N and in the
vicinity of the low. Scattered moderate from 08N-12N between 24W-
34W.

[AJC3]

True or false: In the MDR, 25-30kt of upper level westerly shear over a system moving westward at 15kt is "worse" than 25kt of upper level easterly shear.
True or false: In the MDR, 25-30kt of upper level westerly wind over a system moving westward at 15kt is "worse" than 25kt of upper level easterly wind.

false. true.[/quote]

Well done, thank you.

[Aric Dunn]

True or false: In the MDR, 25-30kt of upper level westerly shear over a system moving westward at 15kt is "worse" than 25kt of upper level easterly shear.
True or false: In the MDR, 25-30kt of upper level westerly wind over a system moving westward at 15kt is "worse" than 25kt of upper level easterly wind.

false. true.

Well done, thank you.

hehe. shear is shear

[cycloneye]

00z Best Track.Low is to the east of convection.

AL, 99, 2016081900, , BEST, 0, 107N, 271W, 25, 1009, LO

[NDG]

Perhaps 99L is more organized than they they think, but ASCAT only caught the western half of the circulation which could still be elongated west to east.

[gatorcane]

Appears to be losing some latitude still:

http://rammb.cira.colostate.edu/ramsdis ... display=12

[SoupBone]

Appears to be losing some latitude still:

http://rammb.cira.colostate.edu/ramsdis ... display=12

Weren't there a few models or at least one model for a few runs suggesting it would bury itself into SA?