Tuesday, April 7, 2009

Weather in the ITCZ

Latest Position: 04/08/09 1500Z 4 08.049S 23 06.159W

Well with Internet problems, lots of squalls and poor sat phone reception yesterday, the blog never quite made it. It must be floating out in space somewhere!

I have had a pretty good time of it out here considering that I'm in the ITCZ now. The squalls have been frequent but not too strong so I am able to change course slightly and run with them and not reef which has been some good, fast sailing.

I thought you might be interested to see what David's forecasts are like. He has been keeping a close eye on the convection around Intrepid's course. This excerpt from yesterday's forecast has completely changed today but it will give you an idea of the level of expertise and experience that I am working with:

Current Conditions and Synoptic Outlook:
The overall synoptic situation further south has remained well-establishedand little changed from the previous forecast. The focus now is on the route across the ITCZ and the transition into the NE Trades. The ambient surface flow at Intrepid’s current position is generally ESE in the 11 to 15 knot range at this time, with plenty of localized ups and downs in wind strength, and shifts in direction in and around the squalls. Zac has now left the light to moderate, and relatively consistent SE regimine his wake, and has entered the southern band of trade wind convergence. Latest model solutions this evening show vertical velocities at 700mb that correlate well with the general distribution of large scale convection on tonight’s satellite imagery, which is very encouraging. They also show the widest band of predicted convection closer to the Brazilian coast, with a narrower band further to the east, along Intrepid’s intended track over the next few days.
So, it was a good decision to move Intrepid’s equatorialcrossing to 30°W, which was much further east than the original gate picked out for Intrepid. Latest satellite imagery shows that it will be very difficult to avoid someof the squall activity. Development today has been relatively rapid and fast moving, with 850mb flow keeping development tracking from east to west quite quickly.
This is in contrast to the rather slow moving convective masses that were much easier to tip-toe your way through when crossing the equator from N to S in the Pacific. Ambient wind directions should become more easterly at 3°S, then gradually backing to ENE at 2°S, and finally NE at the Equator. By 1°N there may be some NNE tendency and strengthening, but this should eventually settle into a consistent, moderate to strong NE flow further north. Of course this is a general description of the ambient surface flow, so, in practice, expect squalls to wreak havoc with your localized winds. Generally stronger winds mixing to the surface from aloft will tend to be ‘backed’ south of 3°S, and ‘veered’ north of 3°S.
Again, this is a gross generalization, with local outflows tending to fan out at the surface with complex interactions. Bear in mind though, that because of the upper flow, and the depth of convection, these squalls do have the potential to pack a sudden punch.
In the short term, (over the next 12 to 24 hours) it would make sense to head on a more NW or even NNW course, toward 3°S 23°W. This would minimize Intrepid’s time interacting with the largest convective complex. However, the disadvantage would be that it increases the overall distance sailed, and may make it slightly tougher to reach the equatorial waypoint at 30°W later, (because it would mean a slightly more downwind course in the light easterly ambient conditions). The chances are that you’ll get more fluky shifts than the general ambient flow, so in practice it shouldn’t make a lot of difference at all. Plus the sailing angle should improve as the equator is neared, enabling Intrepid to get back on track and gradually point her nose at Grenada without any penalty in boat speed.



Will post again later with more questions and a Clearpoint Weather graphic if I can figure it out!!!



Blogger John Gezelius said...

Well, how 'bout those Dodgers . . .

I don't claim to be a weather expert - the wx guy always briefed before me (the intel guy) at Air Force briefings. From what I can remember this forecast translates into nothing really bad and improving over the next couple of days . . .

Have you considered a university course of study in meteorology? You'd be well qualified from a practical perspective!

April 8, 2009 at 12:49 PM  
Anonymous Art Guy said...

Hey Zac,
Thank God (and David) for SAT weather forecasts. In the not too distant past you would have been largely left to your own devices and on scene/real time weather. It's great to follow your progress on Google Earth. Keep your spirits high, there are alot of people following your every move and wishing you the best. P.S. Try to catch a fish, would ya!

April 8, 2009 at 1:00 PM  
Blogger boyd_poe said...

wow zack: it reads like this fellow is a meteoroligist! really knows his stuff! it must make you feel a lot safer! by the way: is 850mb 850 millibars? that the way i read it. which is fairly high right?

April 8, 2009 at 1:18 PM  
Anonymous Anonymous said...

Hi Zac; I've been followinfg your trip with great interest. Wishing you all the luck in the world. I would like to know what ITCZ means.
Thanks, Charlene

April 8, 2009 at 1:41 PM  
Blogger Anita said...

Great progress Zac!!! Well done.
Keeping you in prayer.
Anita ~~_/)
Waterloo, NY
Captain SV "Wombat"

April 8, 2009 at 3:42 PM  
Anonymous Anonymous said...

The Intertropical Convergence Zone (ITCZ), also known as the Intertropical Front, Monsoon trough, or the Equatorial Convergence Zone, is a belt of low pressure girdling Earth at the equator. It is formed by the vertical ascent of warm, moist air from the latitudes north and south of the equator.

Within the ITCZ the average winds are slight, unlike the zones north and south of the equator where the trade winds feed. Early sailors named this belt of calm the doldrums because of the inactivity and stagnation they found themselves in after days of no wind. To find oneself becalmed in this region in a hot and muggy climate could mean death in an era when wind was the only effective way to propel ships across the ocean. 321


April 8, 2009 at 4:15 PM  
Blogger Croaker of FrogPond said...


Thanks for David's forecast. I've never seen one before and I think I understood about half of it. Yah, me! Now if I just could comprehend all the ramifications. I've always been fascinated by the weather and weather reports. I suspect that you and Dave may have just instigated a trip to the Walden Books! I guess I'm gonna get smart, then die!

It's always a joy to read your blog posts wnen the news is good. It's good to read them when the news is bad, but I prefer good news!

'Art Guy Said ..." said you should try to catch a fish. Time is running out for you to do so. You don't want the option of naming your book, Around the World without a Fish! Ernest Hemmingway would have apoplexy, or a six-toed cat with a crocheted tail!

... and on to Grenada!

The Croaker

April 8, 2009 at 5:39 PM  
Anonymous Anonymous said...

Having crossed the ICTZ several times in a 40' on a circumnavigation in the '80's, I think all that wx fcst verbiage borders on the ridiculous & likely won't make much significant/safe passage/convenient passage difference in your route???. I'm old school, now 63 - reading of your mom's cell phone ringing while she was at a soccer game some months ago because you sought a piece of info (wx?) was a 'new' way with which i could not identify. Saito benefited from knowing what was ahead at Cape Horn but ICTZ is entirely different.
Bob Lux

April 8, 2009 at 5:57 PM  

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