ATL: HERMINE - Post-Tropical - Discussion

[Kingarabian]

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.

Thank you this was a very informative post.

[Hurrilurker]

Appears to be losing some latitude still:

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

It definitely does appear so. If this thing stays south a bit longer and doesn't break up, things could get very interesting. It might miss the low to recurve entirely or partially and end up quite a bit more west/south than the models are showing right now. Even a little lost latitude can make a huge difference this far out.

The last track models I saw, almost all of them were tightly clustered, moving about due west at ~12°N before turning sharply northwest and diverging at about 50°W. But in the latest sat loops, it looks like it's crossing south of 10°. Maybe true motion being masked by convection right now though, we'll have to see tomorrow what it looks like.

[Gustywind]

000
AXNT20 KNHC 191034
TWDAT

Tropical Weather Discussion
NWS National Hurricane Center Miami FL
634 AM EDT FRI AUG 19 2016


Tropical wave extends from a 1009 mb low near 10N28W to 18N28W,
moving W at 15-20 kt within the last 24 hours. Intrusion of Saharan
dry air and dust to the wave environment limits the convection to
a cluster of heavy showers in the vicinity of the monsoon trough
W of the low pressure center or from 07N-11N between 32W and 37W. Environmental
conditions are expected to be conducive for gradual development,
and a tropical depression could form early next week while the
system moves westward across the tropical Atlantic Ocean.

[TheStormExpert]

8am TWO.

A tropical wave located about 500 miles southwest of the Cabo Verde Islands is producing a disorganized area of cloudiness and shower activity. Environmental conditions are expected to be conducive for gradual development, and a tropical depression could form early next week while the system 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...50 percent

[alienstorm]

It's look like this wave is falling apart as a result of either dry air or shear. Not very impressive this morning and I think the Euro is on to something that it will fall apart.

[Kazmit]

It's look like this wave is falling apart as a result of either dry air or shear. Not very impressive this morning and I think the Euro is on to something that it will fall apart.

I doubt it. While it might not develop now, I don't think it will die off for good.

[Aric Dunn]

Looking much better in the low levels this morning. though the euro still just does not like it.

[OuterBanker]

It's look like this wave is falling apart as a result of either dry air or shear. Not very impressive this morning and I think the Euro is on to something that it will fall apart.

Yes, it doesn't look very heathy now. But, what I see is that the convection that was associated with it to the sw has dissapated. The barren true llc is quite evident now with some convective banding starting to wrap on the nw side. Look at the visable,not the avn.

[SouthFLTropics]

Agreed...Visible satellite tells the story this morning. The rotation is clearly there and it is starting to brew. With that said I think it is safe to say that it is going to take some time. Still very far south in latitude and it needs to separate itself from the ITCZ. Until that happens it will continue to be robbed of energy.

SFT

[StormHunter72]

Looks vigorous on satellite nice structure. Could be the real deal. Maybe lol

Hopefully it goes fishing.

[sunnyday]

I'd like to know what WXman is thinking about this one....

[Blown Away]

Maybe I'm wrong, but I noticed when something significant could happen, the pro's get very quiet... I guess analyzing all the details to make accurate statements??

[centuryv58]

Maybe I'm wrong, but I noticed when something significant could happen, the pro's get very quiet...

I've noticed that too. Maybe just coincidence?

[Aric Dunn]

THe lack of convection is a temporary thing as is quite normal for the way the system originated. expect convection to build on the periphery and work closer to the center of the broad circ at which time it will begin to tighten a LLC. still another 12 to 18 hours until we start to see enough organization. NHC likely wont do anything till late tomorrow unless it organizes faster. which is possible i suppose.

[TheStormExpert]

FAR from anything significant.

[stormlover2013]

Doesn't look to impressive right now

[Aric Dunn]

FAR from anything significant.

not far at all. its coming along quite normally.

[tarheelprogrammer]

Starting to believe that the ECMWF is right about this wave. Seems to lack any convection near the center and that is with it being attached to the ITCZ still. It has a nice circulation but I can now see why the ECMWF is slow to develop this and by the time it develops it runs into Hispaniola. If it develops like the GFS shows it could be trouble but right now I would not bet against the ECMWF winning this model battle (lol yes a battle of models).

[SouthDadeFish]

I've noticed that over the past 12-24 hours there has been strong inflow into the convective mass from the north. It is likely that this inflow contained some relatively dry air, which helped to weaken the convection. A pronounced outflow boundary can been seen on visible satellite imagery propagating to the SE, which is a sign of dry air. It will likely take some time for the air to moisten again before new convection can blossom. It seems this was expected by many of the dynamical models, which forecast little strengthening over the next 2-3 days.

[Aric Dunn]

I've noticed that over the past 12-24 hours there has been strong inflow into the convective mass from the north. It is likely that this inflow contained some relatively dry air, which helped to weaken the convection. A pronounced outflow boundary can been seen on visible satellite imagery propagating to the SE, which is a sign of dry air. It will likely take some time for the air to moisten again before new convection can blossom. It seems this was expected by many of the dynamical models, which forecast little strengthening over the next 2-3 days.

yeah, its also quite a large broad circulation which in and itself takes time to consolidate.