ATL: KATIA - Post Tropical - Discussion

[LarryWx]

Also notice the almost complete lack of any Sal. The last few systems that came off africa had a good deal of moisture that was well north to the southern desert.. not if there was measurable rainfall lol but its sure stopped the flow of dust. Dry air wont be a problem with the next few waves ... at least not from the SAL

I'm assuming this is one reason the models develop it so much so early. Then again, this projected quick strengthening just increases the chance for a recurve east of the U.S. east coast based on climo considerations. For this to be perceived to have a better shot at making it all of the way across, it needs to stay weak/sheared and/or move a little south of due west while still in the east Atlantic as per climo.

The most important climo based stat is that 5 out of 6 storms that first become a TD east of 50W
don't ever hit the continental U.S. Some dissipate, some go too far south (into MX or Central America), and a large majority recurve.

[Gustywind]

Latest from NRL:

20110829.1315.12LTWELVE.30kts-1008mb-97N-270W.100pc.jpg
20110829.1200.12LTWELVE.25kts-1009mb-93N-257W.100pc.jpg

[hipshot]

Note sure if the OT or not but here goes. This may be a stupid question but it just struck me that if the main wind pattern in the northern hemisphere is west to east, why do these waves move east to west? They come out above or in the ITCZ so they are not in the southern hemisphere. Stupid question?

[wxman57]

I'm assuming this is one reason the models develop it so much so early. Then again, this projected quick strengthening just increases the chance for a recurve east of the U.S. east coast based on climo considerations. For this to be perceived to have a better shot at making it all of the way across, it needs to stay weak/sheared and/or move a little south of due west while still in the east Atlantic as per climo.

The most important climo based stat is that 5 out of 6 storms that first become a TD east of 50W
don't ever hit the continental U.S. Some dissipate, some go too far south (into MX or Central America), and a large majority recurve.

That's right. It's moving at only about half the speed the pre-Irene disturbance was in the same area. Easterly trade winds are much lower now due to weaker high to the north. That means less low-level shear, although the satellite image of the depression right now does indicate it is encountering a little easterly shear.

[poof121]

Note sure if the OT or not but here goes. This may be a stupid question but it just struck me that if the main wind pattern in the northern hemisphere is west to east, why do these waves move east to west? They come out above or in the ITCZ so they are not in the southern hemisphere. Stupid question?

I'm no expert, but as far as I know, it's west to east in the mid-latitudes, and east to west in the low latitudes/tropics.

[Gustywind]

Latest from SSD...

29/1145 UTC 9.6N 26.7W T1.5/1.5 12L

[RL3AO]

Note sure if the OT or not but here goes. This may be a stupid question but it just struck me that if the main wind pattern in the northern hemisphere is west to east, why do these waves move east to west? They come out above or in the ITCZ so they are not in the southern hemisphere. Stupid question?

The northern hemisphere has three main belts. The tropics (about to 30N) moves east to west. The mid latitudes, which most of our weather comes from goes west to east, and the polar region which is kinda irregular in terms of winds.

[otowntiger]

Aric: But wxman57 said that the lack of SAL means that the ridge is very weak or non-existant. I realize that SAL precludes development but it is also apparently an indicator of a blocking high that is now not evident.

[Blown Away]

Wxman57, due to the persistent EC trough do you think it will be difficult for a W bound system to effect the EC?

[wxman57]

I'm hearing through the grapevine that TD 12 may be passing beneath a region that might not be so favorable for development by tomorrow. Supposedly, a region of positive 200hPa (mb) vorticity anomalies will be passing over 12 for the next few days. Theoretically, this may suppress convection for a while. You can see this region on the graphic below. It's the reddish-brown area just reaching 30W now.

[Aric Dunn]

Aric: But wxman57 said that the lack of SAL means that the ridge is very weak or non-existant. I realize that SAL precludes development but it is also apparently an indicator of a blocking high that is now not evident.

Weaker than when Irene was in that area but by no means not existent. Plenty of ridging to keep on a westerly track as the models point out its the development of that mid to upper level low that retrogrades west that weakness the western side of the ridge allowing for a turn to the WNW.

[LarryWx]

Not to mention its at total odds with the GFS/ECM, which at this range tend to be the two models that need to be most closely watched.

Agreed. The CMC is generally an inferior model. I'm still going with a 90% chance of a recurve east of the continental U.S. The western Atlantic is forecast to be pretty void of the high pressure needed to bring a TC far enough west.

90% recurve 10 days out, i think you're over shooting the runway.

Considering that the history of tracks over the last 50 years shows that a large majority of storms that form east of 50W recurve east of the continental U.S. AND that the GFS has consistently been showing a recurve, I don't see why going with 90%, even this early, would be considered overshooting the runway. If I had said something like 98 or 99%, I could see going that high this early as overshooting.

[tolakram]

latest loop, saved

[SFLcane]

I'm hearing through the grapevine that TD 12 may be passing beneath a region that might not be so favorable for development by tomorrow. Supposedly, a region of positive 200hPa (mb) vorticity anomalies will be passing over 12 for the next few days. Theoretically, this may suppress convection for a while. You can see this region on the graphic below. It's the reddish-brown area just reaching 30W now.

[img]http://www.atmos.albany.edu/student/janiga/rtmaps/hov_plots/200_vp_30d_anom/vp_hov.200.p30d.lt0.dom3.gif[ /img]

Then things (may) become a tad interesting if doesn't spin up quickly.

[WeatherGuesser]

Lets keep it on topic. This isn't he "Complain about JB thread".

That's fine as long as people stop quoting everything he says as if he's the sole source of knowledge. I'm really only interested in what the NHC says.



Speaking of models, has anyone compared the early models to where Irene actually went? Would that help with reliability in any of these early models for where 12 might end up?

[gatorcane]

Looking at the satellite loop tolakram posted above, looks like there are some hints of a more north of due west motion commencing.

Wxman, thanks for the graphic, and it will be interesting to see how that plays out over the next couple of days. The easterly upper-level shear is also quite strong as you mentioned, so if the system stays weaker, could end up more west of what the current model guidance has progged.

[WeatherGuesser]

latest loop, saved

Already has some fair rotation, eh?

[SFLcane]

TPC 11am...

THE INITIAL MOTION IS WESTWARD OR 275/13. THERE IS A MID-LEVEL
RIDGE TO THE NORTH OF THE DEPRESSION AT THIS TIME...BUT A MID- TO
UPPER-LEVEL LOW IS FORECAST TO CREATE A WEAKNESS IN THE RIDGE NEAR
40-50W OVER THE NEXT COUPLE OF DAYS. THIS FEATURE SHOULD CAUSE THE
TROPICAL CYCLONE TO TURN TOWARD THE WEST-NORTHWEST WITHIN THE NEXT
DAY OR SO. LATER IN THE FORECAST PERIOD...GLOBAL MODELS SHOW THE
WEAKNESS IN THE RIDGE SHIFTING WESTWARD AND THIS SHOULD PREVENT THE
CYCLONE FROM MOVING SIGNIFICANTLY POLEWARD DURING THE FORECAST
PERIOD. THE OFFICIAL TRACK FORECAST IS SIMILAR TO THE PREVIOUS ONE
AND ON THE SOUTHERN SIDE OF THE GUIDANCE ENVELOPE.

[LarryWx]

Lets keep it on topic. This isn't he "Complain about JB thread".

That's fine as long as people stop quoting everything he says as if he's the sole source of knowledge. I'm really only interested in what the NHC says.



Speaking of models, has anyone compared the early models to where Irene actually went? Would that help with reliability in any of these early models for where 12 might end up?

The GFS started showing a Caribbean hit as well as a continental U.S. hit starting with the 6Z 8/15 run and didn't ever stop showing those hits until it actually hit...that is 49 straight runs with those areas getting hit! (I have all of this documented.) Overall, the GFS verified to have been a bit too far west despite this overall quite impressive long range performance.

[wxman57]

Speaking of models, has anyone compared the early models to where Irene actually went? Would that help with reliability in any of these early models for where 12 might end up?

You can check the model plot archive on Colorado State's website here:

http://euler.atmos.colostate.edu/~vigh/ ... c/archive/

Storm ID is aal09_xxxx. Here's the first model plot, taking Irene west of FL:
http://euler.atmos.colostate.edu/~vigh/ ... ck_gfs.png

Models were too far south and west with Irene with almost every run.