Comparing Dean to Ivan
August 19, 2007
UPDATE at 4:30 PM: Based on the satellite imagery, it looks like Dean’s CPA to Kingston is going to come out very nearly identical to that of Ivan’s, about 40-45 nautical miles, which keeps Kingston out of the strongest winds.
The one stretch of Jamaica still in danger of getting the very worst wind-wise is the stretch that sticks out: the southern coast from Portland Point to Treasure Beach (Jamaica map). That will depend on which way the next couple of unpredictable wobbles go.
The other benefit of Dean’s faster motion that I neglected to point out is that it reduces the cumulative wind effects (i.e. the total amount of time the area is exposed to hurricane force winds). That serves to enhance building survivability.
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Looking at the forecast and the satellite loop, it looks like Dean’s closest point of approach to Kingston is going to be comparable to that of Ivan back in 2004. It may come slightly closer to the southwestern-most point of Jamaica, however.
The wind speed is comparable to what it Ivan’s was when it passed by Jamaica, however, the radius of hurricane force winds is a little bit larger (60 miles vice 45 miles).
A significant plus relative to Ivan is that Dean is moving much quicker (18 mph vice 8). Because rainfall accumulation is inversely related to forward speed (i.e. slower storm = more rain), Dean shouldn’t put nearly as much rain on Jamaica as did Ivan (25+ inches in many places).
Not to say that there won’t be widespread damage, but owing to the distance that it appears to be to keeping from land and its forward speed, Dean will give Jamaica a fairly significant break from the worst case scenario.
To briefly summarize tropical weather events in the western hemisphere since my last post:
Tropical Depression Three did become Tropical Storm Chantal but remained that for only twelve hours before becoming extra-tropical.
The disturbance east of the Lesser Antilles was at a relative peak when I wrote about the possibility of becoming the next tropical depression. After that, it had difficulty maintaining thunderstorm activity, which caused the low level circulation to open up. It then accelerated across the Caribbean and did become more impressive looking as time it did so. However, owing to its rapid movement it was unable to close off a circulation before going ashore. The description of it in Sunday morning’s Pacific Tropical Weather discussion was mildly amusing:
TROPICAL WAVE ALONG 88W N OF 5N AND IS MOVING W 10 TO 15 KT.
THIS WAS ABOUT AS STRONG AS A WAVE GETS BEFORE DEVELOPING INTO A
DEPRESSION. THE ONLY PROBLEM WAS IT MOVED OVER LAND..THUS
CONVECTION HAS BECOME SCATTERED. STRONGEST CONVECTION THIS
MORNING IS ON THE PACIFIC COASTS OF EL SALVADOR AND GUATEMALA
WITH ANOTHER BATCH OVER NRN GUATEMALA.
After that, the only feature of remote interest in the hemisphere was an area of low pressure that was moving westward across the Pacific. After plodding along for a few days with multiple competing circulations, one circulation became dominant on Wednesday, which led to the rapid development of Tropical Storm Flossie. This morning the storm intensified some more to become Hurricane Flossie, which is only the second hurricane to form in the Eastern Pacific basin this season.
Eastern Pacific Hurricane Flossie
There have been conflicting forecasts from the models on the future of Flossie relative to how it may effect the Hawaiian islands. The current NHC forecast for Flossie takes it south of the islands as a moderate tropical storm. Because of the potential threat, a hurricane hunter will fly to check it out on Monday.
While this was going on in the Pacific, tranquility ruled nearly all of the Atlantic basin, which is in it’s longest stretch without a hurricane since the time between the last hurricane of 2000 (October 20) and the first hurricane of 2001 (September 8). It’s been 302 days since Hurricane Isaac weakened to a tropical storm on October 2 of last year.
A big reason for the recent quiet in the Atlantic has been due to most of it being covered by dry stable air. From the visual satellite image below, one can see the western boundary of it, which runs from the southeast Bahamas southeast across the Lesser Antilles down to the Intertropcial Convergence zone. This area extends eastward along 10 N almost all the way back to Africa.
While most of the tropical Atlantic Ocean is dominated by the dry air, the Caribbean is moist. A few of the global forecast models have been forecasting something to form out of the western Caribbean and enter the southwest Gulf of Mexico by Tuesday of next week. It is a plausible scenario and the western Caribbean will undoubtedly be watched closely by the National Hurricane Center for any signs of the forecast becoming reality. One of the concerns about this scenario owes to a side-effect of the high pressure that has dominated the southeast and made it so #$%! hot: It’s resulted in a relatively cloudless Gulf of Mexico, which in turn raises Sea Surface Temperatures and increases the heat content that can serve as fuel for storms. There is a noticable difference in the Gulf of Mexico heat potential today compared to that of a week ago.
The other area that models have been forecasting development to occur in is at the opposite end of the Atlantic:
At the extreme right hand side of an image from this morning, one can see an area of thunderstorms that is coming off the coast of Africa. Over the past few days, an increasing number of models have forecast development out of this as soon as Saturday evening. The NHC will watch this closely to see if the thunderstorm activity is maintained as the system goes entirely ‘feet wet’; if it does it could be tagged as a system of interest by tomorrow morning.
After two and a half days of scrutiny that involved a sudden appearance followed by peaks and valleys in appearance, and seemingly missing its window for development, the area of low pressure northwest of Bermuda was designated as Tropical Depression Three by the National Hurricane Center this evening
Tropical Depression Three Advisory Number One
Tropical Depression Three Forecast Discussion
This comes as a bit of a surprise, because while the system became organized, it did not have much of the appearance of a pure tropical system. The analysts at the Satellite Services Division (who, along with the Tropical Analysis and Forecasting Branch analyze satellite imagery to provide estimates of storm position and intensity), had classified it as being sub-tropical this afternoon and extra-tropical this evening.
The depression, which is not a threat to the United States, is given a 24 hour window to have a chance at becoming Tropical Storm Chantal before it completely sheds its tropical characteristics. I’m doubtful of this occurring… however, I was doubtful of this depression forming in the first place, so we shall have to wait and see.
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In the middle of last week , a couple of the global forecast models started to sporadically forecast tropical cyclone formation in the Tropical Atlantic. While it is rare that one of those forecasts actually verifies (i.e. it actually forecasts a storm that forms exactly as predicted), such forecasts are a useful indicator that conditions are changing to become more favorable for development and while maybe none of the particular tropical waves in existence will actualy develop, its likely that one of the next few will.
The first such candidate is currently about 800 miles west of the Windward Islands and is described thusly in this evening’s Tropical Weather Outlook:
A WESTWARD-MOVING TROPICAL WAVE LOCATED ABOUT 800 MILES EAST OF THE
SOUTHERN WINDWARD ISLANDS CONTINUES TO GENERATE SHOWERS AND A FEW
THUNDERSTORMS. ENVIRONMENTAL CONDITIONS APPEAR FAVORABLE FOR SOME
DEVELOPMENT OF THIS SYSTEM OVER THE NEXT COUPLE OF DAYS.
This afternoon, and early this evening, visual and infrared satellite imagery indicated a circulation center along 10° North, that was fairly devoid of convection (with Quikscat indicating that there was somewhat of a surface circulation). As the evening progressed, convection started to spring up 80-100 miles to the northwest and it appears that the circulation is reforming to the north-east of what existed earlier.
The system is in a somewhat favorable enviroment; wind shear is under 20 knots and the dreaded Saharan Air Layer, which had a strong grip on the basin for much of the month is not a factor at this time.
The big factor to its advantage, which is what causes the SHIPS intensity model to forecast it eventually become a hurricane, is the big jump in oceanic heat content as the system moves westward. Its current motion would bring it into the area of increased heat content on Wednesday. The area where the heat content starts to increase is coincidentally near the eastern edge of the area of responsibility for the Hurricane Hunters. If this system shows signs of development, a plane will check it out on Wednesday afternoon.
Quiet season?
July 11, 2007
(Note: After going back to look up the tidbit from Glen Reynolds that inspired this, I discovered that the ever alert Brendan Loy posted a response generally similar to what I had in mind. I press on regardless. Please bear in mind that this is not directed at Mr. Reynolds himself; it is the people that write stuff that give people like Glen the impression that things are slow/quiet that I’m aiming at. See Brendan’s post for one example.)
“GOOD THING IT SEEMS TO BE A SLOW SEASON” – Instapundit
Calling a hurricane season slow when it’s July is a little bit like calling a baseball game low-scoring when it’s only in the second inning. It is a particular folly when the season has not been a shutout, but rather are already two named storms in the books. This becomes evident when one examines where we stood on this day in the previous seasons of the current active cycle:
- 1995: Two named storms. The distinctive moment of the season (four storms at once) came in the last week of August.
- 1996: Two named storms (one of them a category three hurricane, which would put that season ahead,qualitatively.)
- 1997: Two named storms. Three more formed in July, but, more notably, none formed in August. El Niño conditions made for a quiet season.
- 1998: Zero named storms. All of the action, which included a category five hurricane, came after 27 July.
- 1999: One named storm. Nothing formed in July, but five major hurricanes formed later.
- 2000: Zero named storms. As in 1999, nothing formed in July, but the rest of the season was active.
- 2001: One named storm. The hurricanes didn’t come until September. (Because the one storm Tropical Storm the only tropical storm to have its name retired, one could rank 2001 ahead of this season qualitatively).
- 2002: One named storm. As in 2001, June, July, and August went by without a hurricane.
- 2003: Two named storms in the books, with a third in progress.
- 2004: Zero named storms. The”fun” didn’t start until July 31.
- 2005: The Season Like No Other is the only season that truly makes everything else look quiet. It was at this very time two years ago when the fifth storm, Emily got its name. Looking back on things, the first feeling I had that it was not a normal season was on July 9th, when Dennis became a category four in the Gulf of Mexico . That general sentiment was reflected in a choice quote 5 PM Emily discussion on July 12th : THE 2005 HURRICANE SEASON SEEMS TO HAVE LITTLE INTEREST IN CLIMATOLOGY. So, even in the most active season on record, it took getting into July to start to have a clue that something unique was going on.
- 2006: One named storm. El Niño conditions made for the quietest season since 2007.
So, out of the 12 seasons, 2007 is currently ahead of 7 quantitatively, and on par with an additional 3. Qualitatively, there’s only 3 seasons that clearly stand in front of it. If this season has been ’slow’ up to this point, it’s because hurricane seasons are usually slow before August.
A different approach to underline this point: When do the notorious storms from? Since the practice began in 1954, there have been 67 storm names retired. Here is how they break-out by the month in which they formed:
June: 3
July: 4
August: 23
September: 26
October: 10
November: 1
90% of the storms that had their name retired formed after July. From this it is clear that, lacking a strong El Niño already in progress, one has to wait until after Septembr before being able to reasonably state that it’s a quiet, uneventful season.
