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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....along with the usual disagreements and bickering.
Mommy bought me a laptop so I can sneak on to the blog during nap time and recess......LOL
LMAO! Don't be so sure it's started everywhere yet!
3 more weeks to go here in VA, though I will say I've been rather proud of our 'local' jr mets this year - no big drama, LOL.
Hi Levi32, that blog entry I promised last night is finally here, check out my blog.
This is so weird. Was,am just reading abut that.
The environment was still characterized by easterly shear, and the depression took 30 h to reach storm strength; this occurred at 1200 UTC 14 August, when the system was centered about 1250 n mi east of Barbados. Although the cyclone’s satellite presentation remained ragged, Dean strengthened early the next day as it turned to the west-northwest, still moving briskly. Dean would continue on this heading, to the south of a deep-layer ridge of high pressure, for the next seven days.
During this period of rapid deepening Dean’s forward motion slowed to about 15 kt; its heading, however, remained remarkably constant throughout the Caribbean as high pressure was maintained to the north of the hurricane, while a mid- to upper-level low over south Florida on 18 August retreated westward in tandem with the tropical cyclone. The center of Dean passed about 180 n mi south of Puerto Rico early on 18 August, and continued west-northwestward in the direction of Jamaica.
Kinda makes you pull your head out of the MDR and take a bigger look. For me, anyway!
you are correct too sir....
Good to see yall on....
Taco :o)
thanks
Nice post, thanks.
Okay..that was too funny! You definetly lighten up the blog..
Morning Taco, just thought I'd drop in for a few minutes.
Been noticing that with the GFS for the past three or four runs. In my opinion, the GFS may be onto something since as the strong trough of low pressure that will break the Russian heat wave moves east, it will allow for the Azores-Bermuda High ridge to build across the Atlantic. Definitely something to watch in the coming days.
Yep, that's funny right there LOL!
Keep 'em coming ....maybe these bustcasters need a dose of "The Chart"
Now, that is funny!
I'm out in the field today and working on work Laptop checking when the next rain comes in....
Taco :o)
He he he - Dr Masters already gave 'em one.
Bravo and good idea :)
I spewd gatorade all over my puter screen and key board....Thanks for that..FAH
And IFF a frog had wings he wouldn't bump his butt every time he hopped.
I am so glad you are here!!!!
+1
And if global models don't show systems as strong as they "should" be, what would that translate to?
DOOM!!! :) JK
There is really no way to translate the model intensity. They just do a poor job on intensities. Some times they nail it, but most of the time they over due it or under do it.
Just in time for the peak of the season!
So, if they decide to name one Obama, we better head for the hills.......sorry, couldn't resist.
Dont forget that if Florida was not there and the ULL was not there NOLA would have had a major hurricane from Bonnie (not that it would have been Bonnie from what you are saying) and also if the ULL was not there we could have had 2 landfalls from TD5 as a hurricane in New Orleans both times... so much for not active huh???
Well, according to GFS, Virginia is the one that is doomed.
Amen to that.
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