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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Early of course, but consistent so far...
144hr NOGAPS is interesting in W. Carribean.
Or a Georges hit
All of the models show them merging to some extent then going on to develop. In any case, many of the models initiate 31L, it's starting to look like our system may end up further south than we expected...
132hrs
Uhh...polo?
We do know that November 27th will be a very dark and cloudy day in Tally.
I think they are supposed to move. PGI30L is moving slow, and 31L is movin' along.
Isn't that 220 mph about I would guess that's wind gusts but still that means cat 5 I dont want another Isabel
Most importantly look at the wind barbs out ahead of it...Lower level wind flow
I lived in Tallahassee for 2 winters. It gets a lot colder up there than here in Tampa.
Drak going to Tally ? Woo-hoo!!
You be wearing shorts and flip flops till ... say ... mid-December.
Now that is funny!!! LOL
Hey CT; likewise!
That product looks to be a little too sensitive.
Old version:
Huh...That's interesting. First Levi came out of the closet, now Drak is making a synopsis of tally regions...
Slightly farther north and east than the 12z run, which brought the storm to a landfall near Cape Canaveral. This means the storm could travel up the East Coast.
150hrs
West...
Also shows a weak TD just off Africa.
Sensitive? It's at a smaller scale. You can see the dry air wrapping around the south side on MIMIC-TPW.
Yes with the Western edge of the ridge all the way in the GOM
GFS 12z, August 24 00z
The new run is farther north and east, the storm is stronger, and the Bermuda high is more elongated.
12z run storm near landfall:
CONCLUSION: The storm could travel in a track that would allow it to threaten New York City.
The 06Z and 12z GFS affirms my belief that a landfall somewhere on the eastern seaboard is most likely with the storm.
Ode to the Bustcaster (aka "Ode to DestinJeff")
We started with high expectations,
And watched as the season progressed,
Then shared in each others' frustrations
As shear helped to kill each invest.
We studied each blob in the ocean;
We analyzed models like geeks;
We worried at each little motion,
Complaining "No cyclone!" for weeks.
But then we remembered: it's early!
No bustcasting yet; it's too soon.
We mustn't get sullen and surly,
Engaging in feeble lampoon.*
So listen: the atmosphere's humming;
The season is starting. Just wait.
I promise you, cyclones are coming.
(Best hope they don't come to your state.)
* - That line's for you, Jeff. ;-)
You know we complain about forecast errors of the NHC in 2004 / 2005, when the storm was originally forecast to make landfall at one point and eventually made landfall at another point within the 300 mile wide cone of error. Here's what the warnings were like for the latter of these OBX storms:
Preparations
On September 14, forecasters issued storm warnings from Jacksonville, Florida to Beaufort, North Carolina. Hours later, the warnings were extended to Virginia. As the storm moved closer to land, forecasters predicted that the hurricane would hit North Carolina in 12 hours. Because of this, hurricane and gale warnings were issued from North Carolina to Boston, Massachusetts.[3]
That's practically the entire US east coast!
174hrs
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