26 September 2006

Stubborn wave nearly a depression...

The easterly wave that left the African coast on Sept 18 is still intact, and has been on the threshold of being upgraded to a Depression for the past couple days.  It's had persistent deep convection, but no luck closing off a surface circulation.  There's a broad 1009mb Low associated with it, located at about 23N 50W.  It's in about 15kts of westerly wind shear, and over almost 28C water.  The next number/name on deck is 9/Isaac.

There's an impressive wave just exiting the African coast today at about 11N 19W which is quite interesting.  Otherwise, the basin is quiet.  As we head into October, the favored cyclogenesis locations are the western Caribbean and off the US east coast: http://www.nhc.noaa.gov/gifs/october.gif -- a sign of the "Cape Verde season" shutting down due to higher shear over the central and eastern parts of the basin.

2006 is on par with climatology so far, almost down to the day.  Using a 1944-2005 average, we typically have 7 named storms, 4 hurricanes, and 2 major hurricanes by this date, and we've actually had 8, 4, and 2.

Please visit my tropical Atlantic headquarters.

20 September 2006

Gordon and Helene both weakening...

Gordon passed over the Azores at 08Z this morning as a 65kt hurricane.  The strongest sustained wind reported at Santa Maria Island was 50kts, the lowest pressure reported was 990mb, and as of 15Z, the intensity was lowered to 50kts and 995mb.  It is holding onto its tropical characteristics, but is expected to gradually transition to extratropical as it passes north of Portugal and toward England over the next couple days.

Helene has weakened a bit to 90kts, and the satellite presentation is less than ideal.  The eye is no longer open, and cloud tops are not as cold as they have been.  The SSTs are ~28C, vertical shear is 10-15kts,  but there is a respectable amount of dry air to the southwest of the storm that could be entraining in.  The latest intensity estimate is 90kts and 958mb, moving NW at 10kts.  A gradual weakening trend is expected as SSTs decrease and shear increases.

The tropical wave I mentioned yesterday just off the African coast continues to get better organized.  Though convection is sparse and scattered, the low-level circulation is improving.  It's presently located at about 9N 29W, or approximately 600km SSW of the Cape Verdes.  This wave was generated over far eastern Africa on Sept 12-13, and has had a history of potent activity as it raced across the continent.  The next number/name on deck is 9/Isaac.

Please visit my tropical Atlantic headquarters.

19 September 2006

Gordon and Helene stirring up the central Atlantic...

Since my last update on Thursday, nothing new has formed, but Gordon has continued on a trip toward the Azores as a strong hurricane, intensity fluctuating between CAT1 and 3 (presently 90kts and 970mb).  It is heading directly for the Azores at 24kts and will hit them on Wednesday morning.  It has now been a hurricane for nearly 7 days.

Helene has intensified substantially since Thursday... it was upgraded to the 4th hurricane of the season on Saturday, and then the 2nd major hurricane on Monday morning, and still hanging on to that at 100kts and 960mb.  It too is safely out in the central Atlantic, no threat to the US, but the Azores may get another rough ride in about a week.

Elsewhere, there's a tropical wave near 25W that's presently not very convective.

Please visit my tropical Atlantic headquarters.

14 September 2006

Gordon strengthens, Helene forms...

At 03Z last night, Gordon was upgraded to 105kts, making it the first major hurricane of the season (CAT3+).  Twelve hours later, it's maintaining that 105kt intensity, with an MSLP of 955mb.  Just 24 hours ago, it was 80kts and 977mb, then 48 hours ago it was 50kts and 1000mb.  This didn't touch any rapid intensification records, but was still impressive.

The satellite presentation is spectacular: a large 40-mile-wide clear eye, symmetric convection, and few spiral rainbands.  However, things are about to change... looping the recent satellite imagery reveals an insurgence of westerly vertical wind shear, which will eventually reach a critical level and disrupt Gordon's major hurricane lifestyle.  Typically, once the shear is >20kts, the vertical coherence of the vortex is upset, and this is forecast to occur within the next 12 hours.  In about two days, it could already be weakened to a tropical storm.

It's forecast to maintain its current intensity for the next 6-12 hours, then gradually start weakening as it heads NNE-NE into cooler water and higher shear.

TD8 was upgraded to TS Helene, the 8th named storm of the season, at 03Z last night.  Although it's a VERY broad and diffuse surface circulation, the estimated intensity is 40kts and 1003mb.

It is predicted to remain over warm water with low to moderate shear, which should allow it to achieve an intensity similar to Gordon's within 4-5 days.  Again, the majority of models turn the storm more NW in 3-5 days, as the weakness in the subtropical ridge allows it to slip up that way (the same one responsible for steering Gordon into the north central Atlantic).

For those who keep tabs on the season's Net Tropical Cyclone Activity (NTC), 2006 is at 50.2% as of 15Z this morning, with the largest contribution coming from Gordon at 16.5%.  For comparison, we were at nearly 150% last year at this date.

Please visit my tropical Atlantic headquarters.

13 September 2006

Gordon rapidly intensifying...

The final advisory was written on Florence yesterday afternoon, as the extratropical transition raced to completion.  It was a named storm for almost 8 days.

Gordon was a 50kt TS (1000mb) yesterday morning, and just 24 hours later, is a 80kt hurricane with 977mb MSLP -- and this appears to be in the midst of a rapid intensification phase.  The storm is small, symmetric, has a beautiful open eye, and still in reasonable shear.  The SST is 29C, BUT the vertical shear is expected to pick up quite a bit in a day or so, so it has little time to strengthen.  It is forecast to move north through a weakness in the subtropical ridge.

TD8 is a large circulation, and has been slow to get better organized.  The forecast is for gradual intensification and a gradual curve to the NW over the next 5 days.  This solution is agreed upon by most models (not all).  Latest intensity estimate is 30kts and 1007mb, tracking W at 15kts.

Please visit my tropical Atlantic headquarters.

12 September 2006

Florence still a hurricane, Gordon and TD8 form...

Florence is now a minimal hurricane, and beginning an extratropical transition... still a warm-core cyclone, but the wind field is expanding, convection has departed the inner core, and it's becoming asymmetric.  The latest advisory has the intensity at 65kts and 982mb, and moving NE at 20kts.

At 21Z yesterday, TD7 was upgraded to TS Gordon, and it's now a 50kt storm, on the way to becoming the season's third hurricane.  Currently located near 23N 58W, it's forecast to more or less follow in Florence's footsteps and recurve to the north by 60W, safely to the east of Bermuda this time.  It should reach hurricane strength, but probably not have time for much more than that before increasing wind shear takes its toll.  Gordon is a compact storm, so in the near future, rapid intensity changes are not out of the question.

At 15Z today, the well-organized wave I mentioned immediately off the African coast was upgraded to TD8, just southeast of the Cape Islands at 12.5N 23W.  The satellite presentation is truly remarkable for a storm so far east, and the likelihood of it being upgraded to TS Helene later today is high.  The future should be just a general WNW track over the next 5+ days, with the potential for significant development.  The initial intensity is 25kts and 1007mb.  It's still close enough to land and a major field program that it will have some aircraft recon and dropsondes at this early phase of development... how exciting!  The vertical shear is forecast to remain VERY low over the next 5 days, with SSTs around 27.5-28.5C.
Coincidentally, this storm has a LOT in common already with Hugo '89 in terms of early genesis, African coastal exit location and date, etc.

Please visit my tropical Atlantic headquarters.

11 September 2006

Florence passes over Bermuda, TD7 forms...

Florence was upgraded to the second hurricane of the season early Sunday morning, about 500km away from Bermuda.  It passed very near the island with an intensity of 80kts and 972mb this morning.  Although still transmitting surface observations sporadically, the radar has been down since ~10Z.  As of this writing, the strongest gust has been 78kts, and sustained at 57kts...

TXKF 111555Z 21057G78KT 3SM -RA BR BKN008 BKN022 OVC120 26/25 A2914 RMK SLP867

This is more or less the end of Florence, and although it may maintain or even gain a little intensity over the next day or so, it will be due to baroclinic enhancement, and it is destined for an extratropical transition in about two days.  This is also the time it will be giving gale-force winds to Newfoundland.

The wave I was mentioning on Saturday to the southeast of Florence was upgraded to TD7 on Sunday night, and is now estimated at 30kts and 1009mb.  It's moving WNW at 8kts and is expected to be upgraded to TS Gordon later today.  The long-range track forecast indicates that it will slide north through the same weakness in the subtropical ridge that Florence did.

Elsewhere, a very well-organized tropical wave exited the African coast this morning after the trek across the continent since its origin 6 days ago over the Ethiopian Highlands.  The wave has an embedded 1008mb surface Low located near 12N 20W.  There's light shear over the system and warm SSTs, so this could quickly become TD8.

As you may recall, we had a strong hurricane off the northeast US coast on the memorable morning of September 11, 2001 (Erin), and that has happened several times since then, so I attached an collage of the four storms we've had on the mornings of other September 11ths, also not far from the coast, including today.

Please visit my tropical Atlantic headquarters.

09 September 2006

Florence nearly a hurricane, heading for Bermuda...

Since Friday afternoon, environmental conditions near Florence changed noticeably, and the storm was quick to notice.  Convection became deeper and more centralized, the dry air that was plaguing it seems to be absent, the vertical shear has dropped to around 10kts, and the SSTs are 29C+.  As of 15Z, the intensity was 60kts and 992mb.  A plane is /en route/ as I write this, and could very well find that it is now a hurricane.  Motion is NW at 11kts.

The forecast is for some substantial strengthening over the next two days, then quickly becoming extratropical and recurving into the north central Atlantic.  The official intensity forecast brings the storm up to 90kts over Bermuda, but they should be prepared for even worse, as rapid intensification is certainly possible today, tonight, and/or tomorrow given such ideal conditions.  Bermuda has a Hurricane Watch and Tropical Storm Warning, and a direct landfall is expected there early Monday morning as a CAT2 hurricane.  This is especially bad because it was only three years ago that the island was devastated by Hurricane Fabian.

There's also a wave behind Florence that bears watching, located at about 21N 50W... conditions are marginally favorable for development there.  The next name on deck is Gordon.

Please visit my tropical Atlantic headquarters.

07 September 2006

Florence slowly gaining strength...

It appears that Florence has been battling some mid-level dry air as well as moderate vertical shear, and as such, has been slow to reach hurricane intensity.  It's also a huge storm, and those are typically slower to intensify too.  The low-level center is completely exposed and there's even minimal convection elsewhere.  The intensity as of 15Z today is 45kts and 1000mb, position is 19.8N 53.4W, and motion is WNW at 7kts.

It is expected to recurve later this weekend, perhaps affecting Bermuda early next week.  It is also forecast to become a CAT2 hurricane by then, but given improved conditions, intensification beyond that is certainly possible.  Reaching the US is rather unlikely, thanks to a developing significant weakness in the subtropical ridge allowing it to slide north well offshore.  It only has 4 days to reach high intensity; after that, the SSTs and vertical shear become extremely detrimental.  Today, Friday, and Saturday however, all factors point to reaching at least CAT1 hurricane.

The strong tropical wave that was trailing Florence has more or less caught up to it and is being distorted/filamented by Florence's giant circulation.  A new wave exited the African coast today -- it was generated in eastern Africa back on Sept 2.

Please visit my tropical Atlantic headquarters.

05 September 2006

Florence forms in eastern Atlantic...

On Sunday afternoon, a strong tropical wave was upgraded to TD6, then further upgraded to the sixth named storm of the season, Florence, on Tuesday afternoon.  As of 21Z today, Florence was located at 17.4N 48.5W (roughly 1/2-way between Cape Verdes and Leewards) and tracking WNW at 10kts.  It's not very convectively active, but is forecast to gradually enter more favorable environmental conditions.   Satellite-estimated intensity is 35kts and 1005mb.

The longer-range forecast is very interesting -- a WNW track and possibly becoming the season's first major hurricane in a few days, yet still quite far from any land.

There is also a potent tropical wave on the heels of Florence, which exited Africa on Sept 1, just a couple days after pre-Florence did, and now located near 15N 35W.  This stands a pretty high chance of becoming the next depression/storm.  The next name/number on deck is 8/Gordon.  Florence and pre-Gordon (?) could also move close enough to each other to experience a Fujiwhara Effect... a well-known phenomenon involving interacting vortices in fluids.

Lastly, a brand new wave is introduced on the African coast today, after a very convective past few days over the continent... it was generated over the Ethiopian Highlands back on August 30 or so.  This also bears watching over the next few days -- we are in the prime of the Cape Verde season, so waves are climatologically favored to develop.

Please visit my tropical Atlantic headquarters.