August Ends With a Whimper for the Atlantic Hurricane Season

By: Dr. Jeff Masters , 3:19 PM GMT on August 30, 2013

The end of August is upon us, and with just one day remaining in the month, the remarkably quiet Atlantic hurricane season of 2013 is highly likely to be just the 6th season since the Hurricane Hunters began flying in 1944 without a hurricane forming by the end of August (the other years: 2002, 2001, 1988, 1984, and 1967.) Although there have been two tropical storms in August (Erin and Fernand), these storms were weak and short-lived, and August 2013 had one of the lowest Accumulated Cyclone Energy (ACE) totals on record for an August in the Atlantic. ACE is calculated as the square of the wind speed every 6 hours for every named storm with at least 40 mph sustained winds (scaled by a factor of 10,000 for usability.) Since the damage potential of a hurricane is proportional to the square or cube of the maximum wind speed, ACE is not only a measure of tropical cyclone activity, but also a measure of the damage potential. During the 20-year period 1981 - 2010, the Atlantic averaged 104 ACE units, and the 20-year average ACE by the end of August was 30. So far in 2013, we've managed just 9 ACE units, with only 1.9 of those occurring in August. Since the current active hurricane period we are in began in 1995, only 2002 had a lower ACE by this point in the year (4.9), and only 1997 had a lower August ACE (zero.) Both were El Niño years, when we expect hurricane activity to be low due to high wind shear. If we go back to the beginning of reliable Atlantic ACE statistics in 1966, when good satellite data first became available, only five other years had August ACE values comparable to 2013's. Three of those years were El Niño years, and the other two had ocean temperatures in the hurricane Main Development Region (MDR, from 10 - 20°N, 20 - 70°W) that were more than 0.5°C (0.9°F) cooler than in 2013. This year's combination of no El Niño, warm MDR SSTs, and an exceptionally low August ACE is an event unparalleled in the historical record, going back to 1966.


Figure 1. August Accumulated Cyclone Energy (ACE) totals in August 2013 were among the lowest on record for an August in the Atlantic. Other Augusts with low ACE in the Atlantic all occurred during El Niño years, or when sea surface temperatures (SSTs) were cool.

Why the quiet season?
The main reason for the quiet August has been the large amount of dry, stable air over the Atlantic. This dry air has two sources: the Sahara desert of Africa, and sinking air from aloft, which warms and dries as it sinks. Even so, I find it highly perplexing that activity has been so low when all of the other factors--lack of an El Niño, low wind shear, an active African Monsoon spitting out plenty of tropical waves, and above average ocean temperatures--have favored development. Instability has increased over the tropical Atlantic over the past few days, thanks to the influence of the MJO and a Convectively Coupled Kelvin Wave (CCKW) bringing rising air to the Atlantic. However, there do not appear to be any tropical waves positioned to take advantage of this increased instability and intensify to hurricane strength over the coming week (though 96L could become a tropical storm.) The next tropical wave with a shot at becoming a hurricane will not exit the coast of Africa until September 7. If we make it all the way to September 11 without a hurricane in the Atlantic, it will beat the record set by Hurricane Gustav of 2002 for latest date of formation of the season's first hurricane, going back to when the Hurricane Hunters first began flying in 1944. Even if we do get a first-half of September hurricane, the steering pattern features a strong trough of low pressure over the U.S. East Coast, which would have high odds of recurving any hurricane that manages to form out to sea, without affecting any land areas.


Figure 2. Vertical instability over the tropical Atlantic in 2013 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been much lower than average during most of August, due to outbreaks of dry air from Saharan Air Layer (SAL) , and an unusual amount of dry, sinking air in the tropical Atlantic. Low instability reduces the potential for tropical storm formation. During the last few days of August, instability has increased, which should raise the odds of tropical storm formation. Image credit: NOAA/NESDIS/CIRA.

Quiet in the Atlantic
A tropical wave located about 900 miles east of the Lesser Antilles Islands is moving westward at 15 mph, and has changed little over the past two days. The wave has a modest amount of spin, but only a limited amount of heavy thunderstorm activity. In their 8 am EDT Friday Tropical Weather Outlook, NHC left the 5-day odds of formation of this disturbance at 20%, and the 2-day odds at 10%. Wind shear is a moderate 10 - 20 knots over the system, but there is an area of dry air and dust from the Saharan Air Layer (SAL) to the north and west that is interfering with development. Our three reliable models for predicting genesis, the UKMET, GFS, and European models, do not develop the disturbance during the next five days. The wave will likely spread heavy rains and gusty winds to the Lesser Antilles Islands by Monday, when the wave will encounter an area of high wind shear positioned over the Northern Lesser Antilles Islands. This high wind shear will likely decrease the odds of development beginning on Monday.


Figure 3. MODIS satellite image of the tropical disturbance 650 miles east of the Lesser Antilles, taken at 2:30 pm EDT on August 30, 2013. A small closed circulation had developed well to the west of an area of increasing heavy thunderstorms. Image credit: NASA.

A well-organized tropical wave emerging from the coast of Africa (Invest 96L) is headed west-northwest towards the Cape Verde Islands. This wave is expected to take a west-northwesterly track into an area with drier air and higher wind shear, and would likely not be able to make the long trek across the Atlantic to threaten North America or the Caribbean Islands. In their 8 am EDT Friday Tropical Weather Outlook, NHC put the 5-day odds of development at 60%, and the 2-day odds at 40%. The GFS model predicts that 96L will develop, but the UKMET and European models show little or no development.

Wunderblogger Lee Grenci has an excellent new post, "Atlantic Hurricane Season: The Saharan Air Layer and Vertical Wind Shear".

I plan on having a relaxing Labor Day weekend, but will do at least one update. Have a great weekend, everyone!

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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1200. BahaHurican
11:12 PM GMT on August 31, 2013
..
Member Since: December 31, 1969 Posts: Comments:
1199. KEEPEROFTHEGATE (Mod)
5:23 PM GMT on August 31, 2013
Member Since: December 31, 1969 Posts: Comments:
1198. vis0
4:39 PM GMT on August 31, 2013
CREDIT: NASA (WV ch.3 has its 2 least WV areas contrast enhanced 2% to better show anemic wispiness. ch.5 has
magenta Lat. Long. lines added) CLICK ON play button = play, CLICK outside the play button plays in pop up re-size the popup to your liking.






If one looks at SATs from the similar 3-5 analog yrs* notice the wispiness of the pTwF (tropical cyclonic formations). Most
eyes go to the bright/strong colours.

In those other years the connected moisture at the wispy level was strong or densely connected. That is important as that's
the part of the pTwF (tropical cyclonic formations)
that carries it through the thick and thin of its journey. Like the embryo in the egg needs the egg white. Also for the tropical
cyclonic formations to "inject" itself into/under a HIGH or sinking air it uses that wispy level as to be strong enough to enter under the HIGH
but not too strong clash with the HIGH. In time this allows for a TS to form and/or moisturize the air so its more buoyant for the following tries,
Another analogy, in reproduction when the egg is inseminated (HEY its science)* it does
so with the most aggressive yet delicate manner. For a pTwF to go under a protective HIGH it needs a strong continuous (as in
surrounding the entire cyclonic area) wispy level and i see anemic wispiness via water vapor channels.

*Some might know this but just in case there is a great young mind reading: learn from nature its how most great deductions are/were
done by observing life around you that has not been influenced by mankind. As i stated months ago i post SAT images i tweak not just copy n paste
& secondly post info that should be known already but just in case its not i share it.
2 minds are better/greater than 1 BUT ONLY when they are connected.


Member Since: December 31, 1969 Posts: Comments:
1197. GatorWX
3:50 PM GMT on August 31, 2013
Member Since: December 31, 1969 Posts: Comments:
1196. Relix
3:50 PM GMT on August 31, 2013
Quoting 1177. Gearsts:
uhh ok...


Oh. Its going to Cayman Islands then!
Member Since: December 31, 1969 Posts: Comments:
1195. JRRP
3:46 PM GMT on August 31, 2013
Quoting kmanislander:
Looks like an anticyclone now building overhead as well. Could be game on


that will help the wave
Member Since: December 31, 1969 Posts: Comments:
1194. Vlad959810
3:45 PM GMT on August 31, 2013
Quoting 1132. stoormfury:
strange no talk of recon. hope the islanders are watching this closely
No need, it is not even close to a TD status.
Member Since: December 31, 1969 Posts: Comments:
1193. kmanislander
3:41 PM GMT on August 31, 2013
Quoting 1184. GatorWX:


I agree this is looking better and better this morning, but isn't that anticyclone hundreds of miles away in the wrong direction? Sorry, not trying to be critical, lol. Just observing.


No, it is actually right on top of it.
Member Since: December 31, 1969 Posts: Comments:
1192. hydrus
3:41 PM GMT on August 31, 2013
Quoting 1187. wunderkidcayman:

And also the fact that it tracks it further W and it continues to move W it's something that if this does play out the NW Carib need to watch and the E half of the GOM
Exactly. Always a serious scenario when conditions are favorable.
Member Since: December 31, 1969 Posts: Comments:
1191. wunderkidcayman
3:41 PM GMT on August 31, 2013
Quoting 1182. 62901IL:

Ominous? Can you post the run?

Link

Quoting 1183. kmanislander:


Go back on the vort by 3 hours and compare both the colour and symmetry. More orange,less yellow and more symmetrical. Not dramatic but improving.

Yep

Quoting 1184. GatorWX:


I agree this is looking better and better this morning, but isn't that anticyclone hundreds of miles away in the wrong direction? Sorry, not trying to be critical, lol. Just observing.

Which one is the question the one to its E or the one on top of it

Member Since: December 31, 1969 Posts: Comments:
1190. WunderAlertBot (Admin)
3:40 PM GMT on August 31, 2013
JeffMasters has created a new entry.
1189. CybrTeddy
3:40 PM GMT on August 31, 2013
UKMET seems to initialize off the low correctly as being already closed.
Member Since: December 31, 1969 Posts: Comments:
1188. Gearsts
3:38 PM GMT on August 31, 2013
Member Since: December 31, 1969 Posts: Comments:
1187. wunderkidcayman
3:38 PM GMT on August 31, 2013
Quoting 1179. hydrus:
The HWRF has been looking rather ominous lately. I do hope it does not come to fruition.

And also the fact that it tracks it further W and it continues to move W it's something that if this does play out the NW Carib need to watch and the E half of the GOM
Member Since: December 31, 1969 Posts: Comments:
1186. Gearsts
3:37 PM GMT on August 31, 2013
Quoting 1184. GatorWX:


I agree this is looking better and better this morning, but isn't that anticyclone hundreds of miles away in the wrong direction? Sorry, not trying to be critical, lol. Just observing.
No
Member Since: December 31, 1969 Posts: Comments:
1185. SLU
3:36 PM GMT on August 31, 2013
Quoting 1162. 62901IL:

Any buoys in that "large and closed circulation"?


They are to the north of the center

Quoting 1163. Gearsts:
But the wave is still far from the islands.


If you look at the visible loops, you can see the low level clouds near the south eastern Caribbean getting drawn into the large circulation which is as a result of a very large inflow.

Quoting 1165. hydrus:

Interesting to watch..Things will pop soon I believe. .Slow going sure..


It deserves an invest designation IMO.
Member Since: December 31, 1969 Posts: Comments:
1184. GatorWX
3:36 PM GMT on August 31, 2013
Quoting 1173. kmanislander:
Looks like an anticyclone now building overhead as well. Could be game on



I agree this is looking better and better this morning, but isn't that anticyclone hundreds of miles away in the wrong direction? Sorry, not trying to be critical, lol. Just observing.
Member Since: December 31, 1969 Posts: Comments:
1183. kmanislander
3:36 PM GMT on August 31, 2013
Quoting 1174. GatorWX:


Going strictly off those maps, how so?


Go back on the vort by 3 hours and compare both the colour and symmetry. More orange,less yellow and more symmetrical. Not dramatic but improving.
Member Since: December 31, 1969 Posts: Comments:
1182. 62901IL
3:35 PM GMT on August 31, 2013
Quoting 1179. hydrus:
The HWRF has been looking rather ominous lately. I do hope it does not come to fruition.

Ominous? Can you post the run?
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1181. CybrTeddy
3:35 PM GMT on August 31, 2013
Also, watch out for the CATL wave. NAM, HWRF, UKMET, CMC are all showing development still. Given the GFS and Euro has done statistically speaking incredibly poor the last few days for 120+ hour forecasts, I'm going to place my faith in the UKMET right now with slow development in the Caribbean being not out of the question.
Member Since: December 31, 1969 Posts: Comments:
1180. Kyon5
3:35 PM GMT on August 31, 2013

Quoting 1173. kmanislander:
Looks like an anticyclone now building overhead as well. Could be game on

If the anticylone manages to fight off the shear, then this could become Gabrielle.
Member Since: December 31, 1969 Posts: Comments:
1179. hydrus
3:35 PM GMT on August 31, 2013
Quoting 1173. kmanislander:
Looks like an anticyclone now building overhead as well. Could be game on

The HWRF has been looking rather ominous lately. I do hope it does not come to fruition.
Member Since: December 31, 1969 Posts: Comments:
1178. hurricanes2018
3:35 PM GMT on August 31, 2013
Quoting 1173. kmanislander:
Looks like an anticyclone now building overhead as well. Could be game on

I think you are right
Member Since: December 31, 1969 Posts: Comments:
1177. Gearsts
3:33 PM GMT on August 31, 2013
Quoting 1170. wunderkidcayman:

Yep much closer look and yep it's S of 15N and its very clear
uhh ok...
Member Since: December 31, 1969 Posts: Comments:
1176. wunderkidcayman
3:33 PM GMT on August 31, 2013
Quoting 1173. kmanislander:
Looks like an anticyclone now building overhead as well. Could be game on


I totally agree but I still say no true development till further W
Member Since: December 31, 1969 Posts: Comments:
1175. hydrus
3:33 PM GMT on August 31, 2013
Quoting 1136. kmanislander:


I do, but with one caveat. In loop imagery we can see upper level winds out of the SW ahead of the system which is shear. How it deals with this and whether shear values continue to fall will make all the difference as to how well it does. It is clearly capable of generating convection as we can see but it is being displaced to the NE of the low center due to winds being out of the SW aloft.
Things can get " funky " now I like to say. I have seen large areas of shear drop to near nothing in just 6 hours, allowing systems to form unexpectedly.
Member Since: December 31, 1969 Posts: Comments:
1174. GatorWX
3:32 PM GMT on August 31, 2013
Quoting 1168. kmanislander:
Looking better 3 hours later.





Going strictly off those maps, how so?
Member Since: December 31, 1969 Posts: Comments:
1173. kmanislander
3:32 PM GMT on August 31, 2013
Looks like an anticyclone now building overhead as well. Could be game on

Member Since: December 31, 1969 Posts: Comments:
1172. RascalNag
3:31 PM GMT on August 31, 2013
At least so far, the CMC might actually be right about this thing developing.

Of course, if you throw enough crap on the wall, something's bound to stick at some point.
Member Since: December 31, 1969 Posts: Comments:
1171. kmanislander
3:30 PM GMT on August 31, 2013
Quoting 1166. Hurricanes305:
There is no question that we should see at least an invest this afternoon low level circulation is well defined.


97L by this evening or perhaps before then would not surprise me.
Member Since: December 31, 1969 Posts: Comments:
1170. wunderkidcayman
3:30 PM GMT on August 31, 2013
Quoting 1150. Gearsts:
15nLink

Yep much closer look and yep it's S of 15N and its very clear
Member Since: December 31, 1969 Posts: Comments:
1169. Tazmanian
3:29 PM GMT on August 31, 2013
Quoting 1166. Hurricanes305:
There is no question that we should see at least an invest this afternoon low level circulation is well defined.



if wind shear drops off this thing could take off in a hurry
Member Since: December 31, 1969 Posts: Comments:
1168. kmanislander
3:29 PM GMT on August 31, 2013
Looking better 3 hours later.



Member Since: December 31, 1969 Posts: Comments:
1167. hydrus
3:28 PM GMT on August 31, 2013
<
Member Since: December 31, 1969 Posts: Comments:
1166. Hurricanes305
3:27 PM GMT on August 31, 2013
There is no question that we should see at least an invest this afternoon low level circulation is well defined.
Member Since: December 31, 1969 Posts: Comments:
1165. hydrus
3:27 PM GMT on August 31, 2013
Quoting 1161. SLU:
West winds reported from Barbados, Grenada, St. Lucia, Trinidad and Tobago. No question that a large and closed circulation exists.

Interesting to watch..Things will pop soon I believe. .Slow going sure..
Member Since: December 31, 1969 Posts: Comments:
1164. hurricanes2018
3:27 PM GMT on August 31, 2013
two tropical storm moving west!! watch out!
Member Since: December 31, 1969 Posts: Comments:
1163. Gearsts
3:24 PM GMT on August 31, 2013
Quoting 1161. SLU:
West winds reported from Barbados, Grenada, St. Lucia, Trinidad and Tobago. No question that a large and closed circulation exists.
But the wave is still far from the islands.
Member Since: December 31, 1969 Posts: Comments:
1162. 62901IL
3:23 PM GMT on August 31, 2013
Quoting 1161. SLU:
West winds reported from Barbados, Grenada, St. Lucia, Trinidad and Tobago. No question that a large and closed circulation exists.

Any buoys in that "large and closed circulation"?
Member Since: December 31, 1969 Posts: Comments:
1161. SLU
3:22 PM GMT on August 31, 2013
West winds reported from Barbados, Grenada, St. Lucia, Trinidad and Tobago. No question that a large and closed circulation exists.
Member Since: December 31, 1969 Posts: Comments:
1160. KEEPEROFTHEGATE (Mod)
3:21 PM GMT on August 31, 2013
Quoting 1158. 62901IL:

Will they transmit data to the public?

Are they flying in to the area that NHC has dubbed as 0%?



9. PATTERN: N-S RACETRACK WITHIN BOX BOUNDED BY 19.5N
55.0 W, 10.0N 55.0W, 10.0N 45.0W, 19.5N, 45.0W.

Member Since: December 31, 1969 Posts: Comments:
1159. KEEPEROFTHEGATE (Mod)
3:20 PM GMT on August 31, 2013
Quoting 1149. kmanislander:


It has a closed wind field already. If the convection builds sufficiently it would not take much to go from 0% to TD LOL.
I know but it being kept in check for now
Member Since: December 31, 1969 Posts: Comments:
1158. 62901IL
3:20 PM GMT on August 31, 2013
Quoting 1151. wunderkidcayman:

Not USAF RECON
NASA and NOAA RECON flying today

Will they transmit data to the public?
Quoting 1152. KEEPEROFTHEGATE:
flying a race track pattern n too s doing sampling

Are they flying in to the area that NHC has dubbed as 0%?
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1157. JRRP
3:19 PM GMT on August 31, 2013
Member Since: December 31, 1969 Posts: Comments:
1156. HuracandelCaribe
3:19 PM GMT on August 31, 2013
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1155. hurricanes2018
3:19 PM GMT on August 31, 2013
that funny I see three tropical storms on here!
Member Since: December 31, 1969 Posts: Comments:
1154. pcola57
3:19 PM GMT on August 31, 2013
Sorry Gator..
Gotta run..
I'll try to check them out later on..
BBL.. :)
Member Since: December 31, 1969 Posts: Comments:
1153. weatherman994
3:19 PM GMT on August 31, 2013
Bust Season all the way.
Member Since: December 31, 1969 Posts: Comments:
1152. KEEPEROFTHEGATE (Mod)
3:18 PM GMT on August 31, 2013
Quoting 1148. 62901IL:

is Recon flying in today? And if so, where?
flying a race track pattern n too s doing sampling
Member Since: December 31, 1969 Posts: Comments:
1151. wunderkidcayman
3:18 PM GMT on August 31, 2013
Quoting 1148. 62901IL:

is Recon flying in today? And if so, where?

Not USAF RECON
NASA and NOAA RECON flying today
Member Since: December 31, 1969 Posts: Comments:
1150. Gearsts
3:18 PM GMT on August 31, 2013
Quoting 1147. wunderkidcayman:

It's actually more near 14.5N
New sfc map has it closer to 13N
15nLink
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather