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Tropical Depression Ernesto - 35 mph - 1004 mb - W 14 mph


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When I looked at the medium range guidance on Sunday, the ECMWF was taking the system into the Guyanas and the GFS/UKMET were most bullish taking it through the east-central Caribbean. The guidance is having issues with the strength of the low-level ridging to its northeast (which polar orbiter information could help with), as well as the degree of vertical wind shear it seems (which may not connected with the upper trough to its north in this case unless the system gains latitude). Even without the upper level trough to its north, there would still be shear due to the low level jet in its vicinity...a pattern more reminiscent of July than August. This is why it's usually not a good idea to pin hopes on the development of a system entering the Caribbean until it passes/survives past the 70th meridian. Climatologically, there is an upper level trough centered mid-Caribbean throughout the hurricane season (our friend the TUTT)...which is stronger during El Nino years...which this year is quickly becoming.

The LOL is more how the Euro and GFS just refuse to get on the same page with each other... yesterday the situation was almost entirely reversed.

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Getting close...

193600 1323N 05456W 9777 00276 0092 +209 +196 133042 043 032 007 00

Edit

closer

194000 1335N 05450W 9766 00291 0102 +196 +167 142047 049 044 016 00

The 49 knot flight level winds are probably enough to upgrade... the 44 knot SFMR wind is probably rain contaminated, however given the rain rate.

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When I looked at the medium range guidance on Sunday, the ECMWF was taking the system into the Guyanas and the GFS/UKMET were most bullish taking it through the east-central Caribbean. The guidance is having issues with the strength of the low-level ridging to its northeast (which polar orbiter information could help with), as well as the degree of vertical wind shear it seems (which may not connected with the upper trough to its north in this case unless the system gains latitude). Even without the upper level trough to its north, there would still be shear due to the low level jet in its vicinity. This is why it's usually not a good idea to pin hopes on the development of a system entering the Caribbean until it passes/survives past the 70th meridian. Climatologically, there is an upper level trough centered mid-Caribbean throughout the hurricane season (our friend the TUTT)...which is stronger during El Nino years...which this year is quickly becoming.

The low-level jet in the Caribbean is worrysome and has done in tropical cyclones in the past (Isidore 2002, Lili 2002, TD#2 2004, Earl 2004). However, the upper level flow in the Caribbean is still easterly with no TUTT feature in the Caribbean that the system might run into. This is an important distinction from the other systems I have listed above, which were origionally under favorable conditions, but the strong low-level jet caused them to move into more unfavorable upper level flow. It is also worth nothing that these systems had a fragile surface circulation that was not very large and was more suceptable to vertical wind shear. Ernesto already has a decent surface circulation (although somewhat elongated), and if it can develop good upper level outflow in the next 24 hours, it can strengthen the upper level anticyclone to the point the shear is not fatal.

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The low-level jet in the Caribbean is worrysome and has done in tropical cyclones in the past (Isidore 2002, Lili 2002, TD#2 2004, Earl 2004). However, the upper level flow in the Caribbean is still easterly with no TUTT feature in the Caribbean that the system might run into. This is an important distinction from the other systems I have listed above, which were origionally under favorable conditions, but the strong low-level jet caused them to move into more unfavorable upper level flow. It is also worth nothing that these systems had a fragile surface circulation that was not very large and was more suceptable to vertical wind shear. Ernesto already has a decent surface circulation (although somewhat elongated), and if it can develop good upper level outflow in the next 24 hours, it can strengthen the upper level anticyclone to the point the shear is not fatal.

The Yucatan did in Isidore. Claudette and Dolly would seem possible analogs, early season storms which fail to thrive in the Caribbean, but wound up making a decent, if not spectacular comeback in the Western Caribbean and Gulf.

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The Yucatan did in Isidore. Claudette and Dolly would seem possible analogs, early season storms which fail to thrive in the Caribbean, but wound up making a decent, if not spectacular comeback in the Western Caribbean and Gulf.

Many people don't remember but Isidore was a TD just east of the Lesser Antillies before it accelerated in the low-level flow right on the South American coastline. Claudette is another really good analog where the low-level flow put it from very favorable to marginal at best conditions. My argument is that this system can run way out into the Caribbean, but as long as the upper level outflow follows it, there is no TUTT feature that can impeed the upper level flow's progress.

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Many people don't remember but Isidore was a TD just east of the Lesser Antillies before it accelerated in the low-level flow right on the South American coastline. Claudette is another relaly good analog where the low-level flow put it from very favorable to marginal at best conditions. My argument is that this system can run way out into the Caribbean, but as long as the upper level outflow follows it, there is no TUTT feature that can impeed the upper level flow's progress.

It actually went over parts of Venezuela for a time...that didn't do it any favors.

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The SFC to 700 hPa shear roughly equals the SFC to 600 hPa shear, but I prefer measuring from the low levels to the bottom of a jet stream (not the top) when calculating things like that. But is that what you were asking?

Both of those soundings show deep easterlies through 600 hPa. Not sure what you mean by SFC to 700 hPa shear.

They probably won't cancel a recon mission if they are trying to confirm that the TD has a closed circulation ;)

Good morning turtle. I'm confused (scientifically) by your post. Perhaps you or another might clear things up for me.

You mention "SFC to 600 hPa" then refer to "the bottom of the jet stream." As I understand things, the jet stream is around 300 hPa in winter and 200 hPa in summer. Of course we have Low Level Jets too but I'll ignore them for this case. Would one typically (or ever) find the bottom of the jet stream anywhere near 600 hPa? From my amateur understanding the bottom of the jet should be around 400 hPa.

Additionally, when looking at the soundings posted earlier, I don't a jet stream, unless 10-15 kts qualifies as a "jet stream." Is it in meteorology that the jet stream is defined by hPa regardless of wind speeds or is it defined by an area of unusually high winds? I've always thought of the jet stream as an area of actual wind but perhaps in science it's technically a height regardless of wind speed?

Thank you.

Sorry if I'm late to the party, super busy all day.

Turtle, the sfc to 600 shear in the San Juan sounding is actually fairly low. There's a sfc-600 mb jet, but the shear within that particular layer is fairly small as there are strong easterlies throughout. The 850-400 mb shear, on the other hand, is fairly large, as the winds relax above 600. So the vertical change in winds, or shear, is large if you take the difference between the LL jet and the relative calm winds above.

It's also not technically correct to refer to it as a 'jet steam'. One typically refers to a jet stream when talking about the subtropical jet or mid-latitude jet, or some other feature at or above 300 mb. This, much lower feature is more of a low-level jet in response to warmer SAL air to the north, where easterlies develop in thermal wind balance in response to a low-level south-to-north temperature gradient in the vicinity of the SAL.

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