Alastair Thompson Profile picture
Oct 11 18 tweets 11 min read
The collision of a Hurricane of the scale of #HurricaneJulia and Tehuantepecer [en.wikipedia.org/wiki/Tehuantep…] wind gap is not something you see everyday, setting the scene phase 2 of the #ExtremeWeather event over Central America. en.wikipedia.org/wiki/Tehuantep….
As you can see in the animation above it as if the storm has been blown up. But in fact it has been split in two - and there are now two components to watch. Potential Hurricane's on both side of the Central American Isthmus.

Left: Eastern Pacific Zone
Right: Atlantic Zone ImageImage
Here's the first 200 hours of previous run of the GFS model. And at the begin you can see the wind gap event as #HurricaneJulia's Western side came over the Yucutan peninsula pushing a high velocity mass of wet air through the gap.
Tis diagram from the #Wiikpedia page explains the phenomena. Effectively what just happened is that a large chunk of very unstable atmospheric water laden air was pushed through the gap at high speed and has immediately become a hurricane candidate of the Sth coast of Mexico. Image
This EPS Ensemble Sphaghetti Map shows the level of uncertainty in the weather models about what will happen next. Unfortunately neither of the new systems has yet been designated as an #Invest which means we do not yet have satellite floater imagery available to examine them. Image
The other factor is big blob of arctic air which is heading south east over the United states. This view shows the atmospheric height - which corelates to high barometric pressure.

The red bit is like a mountain range, the brown bit foot foothills & the blue like a crater. Image
And here's a 72 hour animation of the blob pushing south. This is a very macro sized view of the situation BTW. The dark blue bit at the top is the Arctic.
The previous animation shows a view of the height of the atmosphere at 5500m.

Here we see a atmospheric water view - as the water is coming from the arctic it is dry - and it is now colliding with tropical atmospheric water over the Gulf of Mexico.
The atmospheric physics here are colossal. Dry air has less mass so it cannot easily push super-saturated air in front of it and you can see here that the Tehuantepecer wind gap event continues for 5 days - eventually pushing the remnants of #HurricaneJulia through the gap.
In this latest GFS model the storm is putting up a titanic fight against the shear it is experiencing & most of the water is actually passing over southern Mexico. Eventually the vortex passes through the gap - and the Gulf Storm dissipates over the Pacific.
Here's a 5 day accumulating rainfall forecast. But what actually happens depends hugely on the development or not of the two storms.

Cyclone formation is very unpredictable.

Both will likely experience a lot of shear so their existence of development cannot be assumed.
In a best case scenario neither storm eventuates. And given the conditions this seems fairly likely - but the models are better at this than us.

Follow @NHC_Atlantic for updates and the #WX Hurricane community here on twitter. The next two storm names are Karl & Julia.
@NHC_Atlantic As I have been writing this thread #Invest93L has now been designated by @NHC_Atlantic - which is great as we can now take a closer look at it. I expect the Eastern Pacific Storm will probably be designated shortly based on its presentation.
[Correction: Looks like #Invest93L was designated shortly after #HurricaneJulia got hit by the Tehuantepecer.]

Left: CMISS NHC-IR imagery of #Invest93L which also covers the southern storm formation area.

Right: 2 newly designated storms in WestPac. ImageImage
In this CMISS Dvorak plot of #Invest93L data we can see the Southern Storm has much better development opportunities in relation to shear. The Northern storm is on the edge of 50kt shear which is way too much.
And here is a dry air over lay on top of Water Vapour satellite imagery [which as we know is being pushed south by the Arctic Blast].

The high shear and dry air issues ought to inhibit both storms from becoming too hazardous. Image
As the sun is just coming up over the area- I will post satellite imagery of both storms before closing off this thread.
In the meantime here is the modelling guidance - track and intensity - for #Invest93L [which can credibly also be considered to be #HurricaneJulia's remnants] ImageImageImage

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More from @althecat

Oct 13
[#NotYetAHurricaneKarl update Thread, waiting for sunrise ed. #ExtremeWeather #ClimateChangeNow ]

The ridge to the north of #Karl has moved south a fair bit and appears to be pushing #Karl to the south at the eastern edge of the offical NHC Track cone.
Here's another view where you can see the leading edge of the ridge that is pushing down on the hurricane capturing it near the entrance to the Tucanpecer's northern funnel.
Lookingclose at the last four hours @NHC_Atlantic may have a point about #NotYetAHurricane #Karl not quite yet being a hurricane - as predicted by the GFS it has collapsed and broadened out close to midnight as the models said.
Read 29 tweets
Oct 13
And now, finally, @NHC_Atlantic has put up a new advisory on #TSKarl #HurricaneKarl

...AIR FORCE RESERVE HURRICANE HUNTERS INVESTIGATING KARL... Image
Current satellite imagery shows why NHC thinks Karl will also be pushed through the Tehuantepecer wind gap for a 2nd time - its all a matter of timing. Karl is already clearly outside their cone, the railway lines show where a ridge is building as dry air pushes south. ImageImage
You can see the scenario in the latest GFS model here. But bear in mind that this is based on the assumption that Karl is still a tropical storm. And as you can see it is expected to decay very rapidly over night.
Read 8 tweets
Oct 12
Its now pretty clear what is going on. #HurricaneKarl #TSKarl is caught in an atmospheric river generated by #TD99E (in a similar way to how #HurricaneIan was) and headed towards florida - not Mexico.
The second overlay is a special "steering" product that allows you to see where a cyclone is headed - it focusses on the 500-850hpa wind range (1.5 to 5.5kms high winds ) which are particularly important for pushing cyclones. Image
And as you can see #HurricaneKarl is being steered towards northern Florida at present.
Read 6 tweets
Oct 12
This IR satelllite image [src: @TropicalTidbits] shows #TSKarl is unquestionably now #HurricaneKarl. It also shows it is not moving towards the Mexico coast line as per the current @NHC_Atlantic forecast - but moving NE. And the transformation into this was 3 hours ago.
Here are its first three hours as a hurricane.
And this is the most recent @NHC_Atlantic discussion bulletin for #TSKarl, issued 2 hours ago. ImageImage
Read 13 tweets
Oct 12
Image
These storms which are forming over Cuba and Haiti are 1200 to 2000kms away from #KARL but it is clear that the cause is #TSKarl, which still has not been designated as a hurricane even though its convection field now covers 454,000 kms.
Read 8 tweets
Oct 12
Alas it appears the shear has moved south following #Karl and it is already pulling the center apart, or at least tipping it over.
There is something very unusual (to me) in this #HurricaneKarl set up, sitting over southern Mexico - a small high pressure vortex, I didn't know something like this was possible.

Could this be artifact of the Tehuantepecer event. No wonder the #99E system is not developing.
Very very weird #Karl. Image
Read 6 tweets

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