2012 Atlantic Hurricane Season Birdseye Discussion #9A (Special Update)

By: NCHurricane2009 , 6:00 PM GMT on April 29, 2012

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...APRIL 29 2012...
Caribbean Sea tropical disturbance emerges over the Florida Straits while upper-level winds quickly become more conducive for tropical development. Surface observations from south Florida show no signs of development at this time...but full birdseye discussions will resume immediately if this changes. This discussion supercedes full update #9 which had discounted potential for tropical development.

The special feature section of full update #9 mentioned the disturbance over the last several hours transitioning to a more subtropical state while supported by divergence E of the cut-off upper trough axis located over the E Gulf of Mexico/Yucatan. It was also mentioned that CIMSS products from University of Wisconsin showed a decrease in the southerly vertical shear associated with calm upper winds near the upper trough axis. However...tropical/subtropical development was discounted in full discussion #9 as the computer models showed an eventual de-amplification of the upper trough...which would have re-increased the vertical shear and reduced the supportive divergence east of the upper trough axis.

Figure 1 below instead shows what has happened to the upper trough between full discussion #9 (181Z Apr 28) and this special update #9A (1315Z Apr 29). The thunderstorms have gained great concentration to the NE of the upper trough axis...resulting in a new warm core upper ridge axis and associated cirrus outflow (which also suggests a trend from subtropical back to a more tropical state). In turn...this new upper ridge axis has caused the upper trough definition to become much more amplified by 1315Z today...to the degree that there is even a new cut-off upper low just N of W Cuba! The NW to SE tilting of the amplifying upper trough has changed the upper wind direction over the Florida Straits to a more easterly direction...which is reducing the vertical shear as the upper winds become more aligned with low-level easterly trade winds. The sharpening upper trough has also increased the divergence over the Florida Straits...and a surface trough has formed as a result as of 0600Z TAFB (kudos to the GFS model which had predicted this would happen as of full discussion #9).

The fate of this disturbance still will be associated with how this upper trough evolves. First off...radar imagery from S Florida (Figure 2) shows a spin developing SE of Key Largo. An isolated rain band moving SW suggests another possible spin due S of the Florida peninsula. This could mean the new 0600Z TAFB surface trough is developing a closed circulation supported by the divergence of the upper trough. Observations in S Florida at this time do not support this theory with stiff surface easterly winds across the board and surface pressures remaining steady at a high 1017 mb. When compared with Figure 1...the GFS computer model is initialized with an under-represented amplitude of the recently-sharpened upper trough. However...the GFS model has the upper trough getting even sharper thru 2100Z today. The GFS then gradually de-amplifies the upper trough while moving it east into Florida tomorrow April 30...and then dissipates the upper trough altogether by May 1. This means westerly vertical shear should re-increase and the favorable upper divergence reduce. This is why I am still iffy on tropical development as the window of favorable upper winds should last only today according to the models. However..one has to be a bit skeptical of the models as they did not predict today's dramatic amplification of the upper trough described in the third paragraph of this update. If surface observations from Florida or elsewhere indicate surface pressure drops and a developing closed circulation...then I will be resuming full birdseye discussions. Otherwise...this is my last discussion until hurricane season starts June 1 2012.

In lieu of these updates...I have to update the rainfall forecast from previous full discussion #9. The 200 mb GFS run has the upper trough getting even sharper thru 2100Z today...and then gradually dissipates the upper trough while moving it east thru May 1. The 850 mb GFS model run has the Florida Straits surface trough turn westward into the south-central Gulf of Mexico (while steered by the low-level ridge axis due N) and dissipates it May 2 (since the supporting upper trough is dissipated by May 1). So does the rainfall move east while supported by the divergence of the dissipating upper trough? Or does the rainfall move west while associated with surface convergence of the surface trough? I'll go with something in the middle...and say the rainfall will be largely stationary and gradually weakening with both the surface and upper troughs dissipating by May 1 and 2.

Steady heavy rainfall/flooding potential in the west Bahamas/S Florida will persist south of the red line in Figure 2 (line running from West Palm Beach to Belle Glade to Naples). Rainfall to the north of this line will be largely limited. Some of the rain shield may expand northward beyond the red line only if the thunderstorm mass expands in size under the supportive upper ridge axis outflow shown in Figure 1.

The Florida Keys may not see as much rain. Right now the Keys are in a dry slot to the SW of the radar rotations in Figure 2. They will be entering the rain mass NE of the rotations once the rotations travel westward. However...it is entirely possible the Keys never escape the dry slot...which is induced by upper-level convergence on the back side of the upper trough. With the models suggesting the upper trough moving eastward...the dry slot may follow suit and expand northeastward. If this dry air mass indeed expands northeastward...it will also begin to end the rainfall from south to north on the mainland of S Florida.


Figure 1: Visible image of Florida Straits disturbance...complete with surface analysis in red and upper-level analysis in light blue.


Figure 2: Radar image from south Florida taken around 1315Z today.

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24. NCHurricane2009
2:03 PM GMT on April 30, 2012
Quoting klew136:


Well, woke up to more solid rain, wind and barometer dropped to 1013.0, and noaa says rain for two more days. It kinda looks to me that south of key west is a little circulation, just check barometer again 1012 or 29.88 now it seems as if it dropping, when it is light outside I am going to see what kind of clouds are up in the sky.


Thanks for the update on this...still watching this disturbance carefully. The upper trough is beginning to shift east like the models said it would. I also see the little circulation south of Key West...but that circulation is about to get exposed to strong westerly shear on the back side of the upper trough as the upper trough shifts east.

Can't watch it in detail till I get back from work late this afternoon/evening. I am actually typing this from work :)
Member Since: September 15, 2009 Posts: 466 Comments: 3654
23. klew136
9:02 AM GMT on April 30, 2012
Quoting klew136:


pressure went up 1014 clouds are clearing out, still some rain off and on, by the way the clouds seem to be kinda too high for a depression to start, usually when a depression is around the clouds are very low. This is an interesting system or should I say .systems, hopefully by Wednesday we should be clear of rain. The winds are brisk and breezy though changing up from ne to east Three days of rain is TOO much. Great chatting with you, gonna go to bed work tomorrow. Will keep in touch if I get a weather alert.:)


Well, woke up to more solid rain, wind and barometer dropped to 1013.0, and noaa says rain for two more days. It kinda looks to me that south of key west is a little circulation, just check barometer again 1012 or 29.88 now it seems as if it dropping, when it is light outside I am going to see what kind of clouds are up in the sky.
Member Since: September 5, 2008 Posts: 0 Comments: 130
22. klew136
12:38 AM GMT on April 30, 2012
Quoting NCHurricane2009:


Wow...pressure keeps dropping and winds rising. I'll be heading out shortly to go grocery shopping (oh boy...yippy). It'll be interesting to see what the numbers are when I get back in a couple of hours...

Thanks for posting the numbers continually...its getting me riveted....


pressure went up 1014 clouds are clearing out, still some rain off and on, by the way the clouds seem to be kinda too high for a depression to start, usually when a depression is around the clouds are very low. This is an interesting system or should I say .systems, hopefully by Wednesday we should be clear of rain. The winds are brisk and breezy though changing up from ne to east Three days of rain is TOO much. Great chatting with you, gonna go to bed work tomorrow. Will keep in touch if I get a weather alert.:)
Member Since: September 5, 2008 Posts: 0 Comments: 130
21. NCHurricane2009
9:48 PM GMT on April 29, 2012
Quoting klew136:
1013 gusts now31


Wow...pressure keeps dropping and winds rising. I'll be heading out shortly to go grocery shopping (oh boy...yippy). It'll be interesting to see what the numbers are when I get back in a couple of hours...

Thanks for posting the numbers continually...its getting me riveted....
Member Since: September 15, 2009 Posts: 466 Comments: 3654
20. klew136
9:35 PM GMT on April 29, 2012
1013 gusts now31
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19. klew136
9:22 PM GMT on April 29, 2012
http://www.ndbc.noaa.gov/station_page.php?station=M LRF1&unit=E. This the one is ocean side right off islamorada
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18. klew136
9:14 PM GMT on April 29, 2012
roswellweather.net/buoy/Florida/Florida.php
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17. klew136
9:12 PM GMT on April 29, 2012
Florida Weather Buoy Data Map
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16. klew136
9:04 PM GMT on April 29, 2012
Quoting NCHurricane2009:


Thanks for the surface pressure drop updates. It looks like its been steady at roughly 1 mb/hour across the region.

Surface winds have also begun to show more curvature as if a surface rotation is trying to get going. Instead of blowing out of the E across the board...they are now starting to blow out of the NE at Key West. If Miami gets winds out of the SE...I am going to get really excited....
Winds are starting to get gusty we are now getting rain in forms of bands
Member Since: September 5, 2008 Posts: 0 Comments: 130
15. NCHurricane2009
8:55 PM GMT on April 29, 2012
Quoting klew136:
1014 now


Thanks for the surface pressure drop updates. It looks like its been steady at roughly 1 mb/hour across the region.

Surface winds have also begun to show more curvature as if a surface rotation is trying to get going. Instead of blowing out of the E across the board...they are now starting to blow out of the NE at Key West. If Miami gets winds out of the SE...I am going to get really excited....
Member Since: September 15, 2009 Posts: 466 Comments: 3654
14. klew136
8:49 PM GMT on April 29, 2012
1014 now
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13. NCHurricane2009
7:26 PM GMT on April 29, 2012
Quoting MAweatherboy1:
Thank you NCH... I don't think this has much chance to develop but still fun to track!


Yeah...if the upper wind forecast in the models verify...this disturbance will only look fun today and start dying tomorrow. Paragraph 4 said this could be my last post until June 1...but I've said that many times before & changed my mind. These early-bird disturbances sure are interesting this year....
Member Since: September 15, 2009 Posts: 466 Comments: 3654
12. klew136
7:25 PM GMT on April 29, 2012
Quoting klew136:


will do so thanks for the explanation.
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11. klew136
7:25 PM GMT on April 29, 2012
Quoting NCHurricane2009:


Well....I have seen that the surface Atlantic subtropical ridge has been strong...with its western lobe extending thru north Florida. The surface ridge has kept surface pressures high...even in your area.

As to why the surface ridge exists? A surface ridge is created by some form of upper-level convergence. The cut-off upper trough that is supporting this disturbance has been producing upper SW winds across Florida. These SW upper winds have been converging with mainstream W winds over the SE US...and hence why we have a surface ridge over north Florida.

But the upper divergence over the Florida Straits has dramatically increased today. Do you see the split upper flow with the blue arrows in Figure 1? This new divergence may allow surface pressures to start dropping. In fact if they do...this will be a crucial sign we have some tropical development. That would be awesome if you let us know....
Member Since: September 5, 2008 Posts: 0 Comments: 130
10. MAweatherboy1
7:18 PM GMT on April 29, 2012
Thank you NCH... I don't think this has much chance to develop but still fun to track!
Member Since: February 11, 2012 Posts: 83 Comments: 7735
9. NCHurricane2009
7:16 PM GMT on April 29, 2012
Quoting klew136:
The barometer has been holding at 1016 and this seems kind of high for the weather we are having, any explanation would be appreciated.


Well....I have seen that the surface Atlantic subtropical ridge has been strong...with its western lobe extending thru north Florida. The surface ridge has kept surface pressures high...even in your area.

As to why the surface ridge exists? A surface ridge is created by some form of upper-level convergence. The cut-off upper trough that is supporting this disturbance has been producing upper SW winds across Florida. These SW upper winds have been converging with mainstream W winds over the SE US...and hence why we have a surface ridge over north Florida.

But the upper divergence over the Florida Straits has dramatically increased today. Do you see the split upper flow with the blue arrows in Figure 1? This new divergence may allow surface pressures to start dropping. In fact if they do...this will be a crucial sign we have some tropical development. That would be awesome if you let us know....
Member Since: September 15, 2009 Posts: 466 Comments: 3654
8. klew136
7:05 PM GMT on April 29, 2012
Quoting NCHurricane2009:


Thanks...

I hope you weren't planning to do stuff outdoors today. Hopefully that dry slot lurking to your SW can move in if everyone down there hates the rain....


Only one day off and it is Sunday, have been trying to go fishing for the last three weeks, nothing but wind and rain for the weekends. It is pouring out right now. Finally got to walk the dogs about an hour ago, was only drizzling. The noaa report says rain through tuesday, hopefully they are wrong, everything is so damp and wet, it feels more like a tropical depression. I have lived here for 34 years, kinda of an early start for the rainy season, but i least we won't have an early drought.
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7. klew136
7:02 PM GMT on April 29, 2012
The barometer has been holding at 1016 and this seems kind of high for the weather we are having, any explanation would be appreciated.
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6. NCHurricane2009
7:01 PM GMT on April 29, 2012
Quoting klew136:


Islamorada


Thanks...

I hope you weren't planning to do stuff outdoors today. Hopefully that dry slot lurking to your SW can move in if everyone down there hates the rain....
Member Since: September 15, 2009 Posts: 466 Comments: 3654
5. klew136
6:58 PM GMT on April 29, 2012
Quoting NCHurricane2009:


You're right....a closer look at the above radar image shows the rain shield over the upper Keys. Yeah...its really the western keys and Key West that may not get as much rain...

Out of curiosity...Which Key are you at?


Islamorada
Member Since: September 5, 2008 Posts: 0 Comments: 130
4. NCHurricane2009
6:56 PM GMT on April 29, 2012
Quoting klew136:
The Florida Keys may not see as much rain.

We have had rain through the night and still raining 4.62 inches of rain. You may have been talking about Key West --- but the rest of the Keys, especially upper keys have had alot of rain.


You're right....a closer look at the above radar image shows the rain shield over the upper Keys. Yeah...its really the western keys and Key West that may not get as much rain...

Out of curiosity...Which Key are you at?
Member Since: September 15, 2009 Posts: 466 Comments: 3654
3. klew136
6:50 PM GMT on April 29, 2012
The Florida Keys may not see as much rain.

We have had rain through the night and still raining 4.62 inches of rain. You may have been talking about Key West --- but the rest of the Keys, especially upper keys have had alot of rain.
Member Since: September 5, 2008 Posts: 0 Comments: 130
2. NCHurricane2009
6:21 PM GMT on April 29, 2012
Quoting nigel20:
Thanks much NCH2009


No problem...this is a complicated situation and I hope the above discussion made sense...
Member Since: September 15, 2009 Posts: 466 Comments: 3654
1. nigel20
6:16 PM GMT on April 29, 2012
Thanks much NCH2009
Member Since: November 6, 2010 Posts: 11 Comments: 7974

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