High Pressure in the Atlantic

[SouthFloridawx]

If I recall correctly, one of the factors we had an average amount of storms last year was also partly due to the fact that High Pressure in the Atlantic was above normal. We saw very large domes of high pressure averaging in the high '20s and '30. Is there anyway to compare the average level of High Pressure and comparing it to previous years? This year from what I have noticed so far is pressure in the low '20s. If anyone could point me towards where I can get the data or archives of something of this nature. This could also be a good indicator of the number and strength of the storms, and if we have higher than normal pressure out in the East Atlantic we may see lowered SST'S.

[web]http://www.nhc.noaa.gov/tafb/ATSA_18Z.gif[/web]

[wxmann_91]

http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl

Research away!

[Matt-hurricanewatcher]

Of course this season will be more active then the 30 year norm. But I don't expect a close to 95 type season. Things are very unfavorable for that kind of season to set up. Hey I do expect 98,99,2000,01,03,04 like season. Really so because the la nina forming.

[turtlehurricane]

An aspect of the high pressure systems was that they stirred up African dust (SAL) , probably the main factor with them actually.

[wxmann_91]

I wonder where AJC3's post went. He does have a good point in that SLP's aren't always a good indicator. For example, suppose you have an anomalously strong cold front which stalls over the central Atlantic. High pressure dives in behind the cold front. Well, storms are still going to recurve. They don't ever get to that high pressure. Now, if you were looking purely at SLP charts, that would not be so evident. But on geopotential height charts, it would be clear, since the cold front over the central Atlantic would be associated with a longwave trough over the western Atlantic. In this case, the high over the western Atlantic is a continental high, and the true Azores/Bermuda High is shunted east.

BTW, the above scenario, is exactly was what happened with Florence.

[Ptarmigan]

Of course this season will be more active then the 30 year norm. But I don't expect a close to 95 type season. Things are very unfavorable for that kind of season to set up. Hey I do expect 98,99,2000,01,03,04 like season. Really so because the la nina forming.

1998, 1999, 2000, 2001, 2003, and 2004 were very active season. I tend to think this hurricane season will more like be 1998, which had 3 major hurricanes or like 2004 with 6 major hurricanes. The North Atlantic Oscillation should be another indicator, if it is neutral or negative, it is more favorable for hurricanes.

[AJC3]

I wonder where AJC3's post went.

After re-reading his post, I deleted my response.

His question wasn't so much about how the strength of surface high pressure ridge affects the track of storms - rather it was more about how it affects the number of systems that form, as well as how strong they get. I sorta misread what exactly he was saying - hence the long-winded diatribe about deep layer steering when I probably should have focused on the lower levels.

However, this point remains - since t-waves are most strongly reflected from about 850MB through about 700 or 600MB, he's probably still better off looking at lower level (1000-850MB or 1000-700MB, etc) mean height fields.

[SouthFloridawx]

Yes I am aware of the fact that waves and depressions are steered by the Low and Low-Mid Layers. Thanks for the info you guys, I think I'm going to put some time into this and see what comes out of it. I was thinking that this may not mean much for the Western Atlantic, but after some further thought... If a stronger ridge at the lower levels, it may act to push the waves farther to the west, allowing them to make it to the Caribbean. If you have a weak ridge it would be more influenced by shortwaves and associated cold fronts.

Perhaps I should also look at the average positions and amounts of shortwaves as an indicator also. I'm not quite sure what I'm going to get out of all this, but it may prove interesting.

[MiamiensisWx]

This is a nice discussion on steering layers. Identifying steering layers at the various millibar levels for easterly waves, developing tropical cyclones, and mature storms (tropical, extratropical, and non-tropical) is a crucial forecasting aspect. Thanks for posting this topic, SouthFloridawx!

For fully developed storms (tropical storms and hurricanes), you should take a closer look at the middle and upper levels for steering (500 millibars or greater in terms of heights) when the systems are in the Atlantic. For easterly waves, surface charts (which you posted) identify the steering and surface ridging (and sea-level pressures) for these low-level systems.

[MiamiensisWx]

If I recall correctly, one of the factors we had an average amount of storms last year was also partly due to the fact that High Pressure in the Atlantic was above normal. We saw very large domes of high pressure averaging in the high '20s and '30. Is there anyway to compare the average level of High Pressure and comparing it to previous years? This year from what I have noticed so far is pressure in the low '20s. If anyone could point me towards where I can get the data or archives of something of this nature. This could also be a good indicator of the number and strength of the storms, and if we have higher than normal pressure out in the East Atlantic we may see lowered SST'S.

[web]http://www.nhc.noaa.gov/tafb/ATSA_18Z.gif[/web]

The negative factor for eastern Atlantic tropical development in 2006 was the southerly dip of the mid-level jet in the central Atlantic. This led to high westerly shear (in addition to unfavorable shear from nearby lows and upper-level systems) and a mean trough in the central tropical Atlantic. This also led to the recurvatures of developing systems and Florence, Helene, and Gordon. Another negative factor in 2006 was the strong easterly wind bursts on the south side of the strong subtropical ridge at the upper levels (500 millibars) near the Azores and Canary Islands in the eastern North Atlantic. Although easterly wind flow at the surface due to ridging (either at the surface or middle and upper levels) can cool SST anomalies in the tropical Atlantic (and Caribbean, Gulf, and throughout the basin) heat content beneath the surface remains warm (74 degrees or greater) and is very sufficient for development. It was sufficient in 2006 and I doubt cooler SSTs would reduce development. A strong upper-level ridge in the eastern Atlantic can aid in drier air at the middle levels across the MDR (this occurred in 2006) and reduce Cape Verde and tropical Atlantic development.

In addition, in other circumstances a stronger ridge at the upper levels (and surface) can help development by providing less westerly wind exposure and instead produce more favorable easterlies and lower pressure which is conducive for tropical and subtropical development. Subtropical systems can also develop from TUTTs and mid-level troughs extending southward into the tropical and subtropical latitudes. Many variables play more significant roles. For example a favorable QBO phase might aid in tropical development by producing lower sea-level pressures in the MDR and Gulf and Caribbean.

I hope this helps.