An active Atlantic hurricane period coming
The remnants of Tropical Depression Five are no longer a threat, done in by high wind shear and close proximity to land. However, an active period for Atlantic hurricanes is likely for the remainder of August, as the global atmosphere undergoes a major change to the circulation pattern that has dominated Northern Hemisphere weather during July and August. A large trough of low pressure is gathering strength over Europe, and is expected to push eastward. By Thursday, this trough should be able to push away the blocking ridge of high pressure that has given Russia its worst heat wave in history. The shift in circulation has already weakened the large region of sinking air that has brought dry, stable conditions to the tropical Atlantic over the past month. Vertical instability, which was unusually low since late July, has now returned to near normal levels over the tropical Atlantic (Figure 1), though it remains quite low over the rest of the North Atlantic. Instability is measured as the difference in temperature between the surface and the top of the troposphere (the highest altitude that thunderstorm tops can penetrate to.) If the surface is very warm and the top of the troposphere is cold, an unstable atmosphere results, which helps to enhance thunderstorm updrafts and promotes hurricane development. Since SSTs in the Atlantic were at record highs and upper tropospheric temperatures were several degrees cooler than average in July, enhancing instability, something else must have been going on to reduce instability. Dry air can act to reduce instability, and it appears that an unusually dry atmosphere, due to large-scale sinking over the Atlantic, was responsible for the lack of instability. Now that vertical instability has returned to near normal levels, Atlantic hurricane activity should increase to at least average levels over the next two weeks. This is particularly true since SSTs are at record highs and vertical wind shear is at average to below average levels over the tropical Atlantic.
Figure 1. Vertical instability (in °C) over the Caribbean (left) and tropical Atlantic between the Lesser Antilles Islands and coast of Africa (right) in 2010. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere became much more stable than normal in both regions at the end of July. This lack of instability also extended to the Gulf of Mexico and North Atlantic Ocean between Europe and North America. However, in the past few days, vertical instability has returned to normal, thanks to a major pattern shift in the global atmosphere. Image credit: NOAA/CIRA.
Figure 2. The climatology of Atlantic hurricane activity shows a sharp rise in activity around August 18.
Analysis
August 18 historically marks the point where Atlantic hurricane activity makes a major spike upwards (Figure 2.) On average, we can expect to see two named storms and one hurricane during the last half of August. The last half of August usually sees a moistening of the atmosphere off the coast of Africa, as the the African monsoon kicks into high gear. This year is no exception (Figure 3.) The dry Saharan Air Layer (SAL) has retreated to the north, leaving a moist atmosphere conducive for tropical cyclone development off the coast of Africa.
It would not be a surprise to see atmospheric instability increase to above-average levels by early next week as the major atmospheric pattern shift progresses. Will this usher in a hyperactive period of Atlantic hurricane activity next week, with a parade of three or four simultaneous storms strung out across the Atlantic? Probably not, since the Madden-Julian Oscillation (MJO) only marginally favors upward motion over the tropical Atlantic, and is not forecast to change much over the next ten days. The Madden-Julian oscillation is a pattern of enhanced rainfall that travels along the Equator from west to east. The pattern has a wet phase with large-scale rising air and enhanced thunderstorm activity, followed by a dry phase with large-scale sinking air and suppressed thunderstorm activity. Each cycle lasts approximately 30 - 60 days. When the Madden-Julian oscillation is in its wet phase over a hurricane-prone region, the chances for tropical storm activity are greatly increased. The bottom line: I expect we will see 2 - 3 named storms in the Atlantic by the end of August, including one hurricane. Where these storms might develop and move is difficult to say. It currently appears that the global shift in circulation will bring near-average steering currents to the Atlantic over the next ten days, with a series of troughs of low pressure capable of recurving hurricanes, moving across the Atlantic. The GFS model is indicating, though, that during the few days of August, these troughs may weaken, making recurvature of hurricanes less likely, and increasing the probability of landfalling storms.
The GFS, NOGAPS, and ECMWF currently predict that one or two tropical storms will form between Africa and the Lesser Antilles Islands during the period 4 - 10 days from now. The NOGAPS model is predicting the development of a strong tropical disturbance near the coast of Honduras late this week.
Figure 3. Saharan Air Layer (SAL) analysis shows that the dry air and dust of the SAL (orange colors) lies well to the north of the hurricane breeding grounds off the coast of Africa, near the Cape Verdes Islands. Image credit: University of Wisconsin/NOAA Hurricane Research Division.
Smoke bedeviling Moscow again
Light easterly winds over the past few hours have brought smoke from wildfires back into Moscow today. Temperatures at Moscow's Domodedovo airport hit 29°C (84°F) today, which is 11°C (20°F) above average. The latest forecast for Moscow predicts that just one more day remains for Russia's greatest heat wave in recorded history. On Thursday, a strong trough of low pressure will move through European Russia, finally bringing below average temperatures.
Jeff Masters
Reader Comments
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at the end of the run it has the storm moving westward around 25N 63W, but it appears that based on the ridge to its north a recurve would happen after that time
I know, I like my peaches on the tree.
Here's the NOGAPS at 60 hours, it clearly shows the well-organized wave emerging.
Now if we advance to 180 hours we see that the NOGAPS develops that tropical wave into a potent major hurricane.
I would say that this is probably the most impressive type of development indicated by the NOGAPS this entire year. And yes, even though this isn't the best model, it's always good to look for a second opinion.
If you would like to view the run, click here. If it doesn't work, click here.
it is much further south close to 10N
It also develops the Carribean wave, and takes it into the Yucatan, something the folks that got hit by Alex and TD 2 might need to keep their eyes on, just in case. Also note the wave just coming off of Africa, as well.
And even there, there were people who predicted Dean would hit N.O.
Yep. That's the 2nd storm tht's been popping in and out. ;)
MARK
15.07N/67.45W
Agreed. What octant(s) is the MJO forecast to go in?
Are you quoting Dr. Masters?
12Z ECMWF (Much weaker trof and no split)... this time more ridging with a possible bridge trying to develop between the A/B High and the GOM high.
Would like to see more runs... but as of now it does appear the Negative NAO might be strong enough to prevent re curvature until it reaches the Bahamas regions which is when a trough should start digging in through the Central CONUS... but plenty of time to watch this and seeing how things evolve and change which is guaranteed.
Man, You repeat stuff well.
I guess it's agree to disagree.
Typical NHC forecaster, then entire southern and eastern US coast! ;)
Trying to head towards 2. We will see if it makes it?? I find MJO forecasts to be rather low confidence. I take it a day at a time with the MJO. MJO is starting to look a bit better this week for the tropical Atlantic, so this could have an affect on a very active end of August.
It appears to me that what models are developing is the tropical wave over eastern Africa. Any thoughts?
Again, I am not saying the MJO has been in downward motion, BUT it hasnt been as favorable as some have been saying. All you have to do is look at the Satellite maps everyday and see that stable air out there.
Would like to see more runs... but as of now it does appear the Negative NAO might be strong enough to prevent re curvature until it reaches the Bahamas regions which is when a trough should start digging in through the Central CONUS... but plenty of time to watch this and seeing how things evolve and change which is guaranteed.
I know a lot of love goes out to StormW and a couple of others (as it should) but I wanted you to know that I very much respect your posts and what you bring to the blog.
Yes! I've been waiting for someone to say that all day! Nothing like forecasting a storm's track before it moves off the coast of Africa.
Ah, okay. I wasn't aware of that. Thanks for the enlightenment! ;)
More memory.
One and Two is favorable, although, like you said, we just don't know with the MJO forecasts. ;)
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