16 October 2008

Omar becomes a major hurricane, TD16 dissipates.

At 03Z today (11pm EDT Wednesday), Omar was upgraded to a major hurricane, the 4th of the season.  Aircraft reconnaissance measured 700 mb flight-level winds of 117 knots and SFMR winds of 108 knots while penetrating the southeast eyewall just prior to the 3Z advisory.  Omar continued to intensify early this morning, briefly reaching Category 4 status according to the 9Z discussion before weakening a bit later this morning.  As of the 12Z update, Omar is listed as an 100-knot (Category 3) tropical cyclone with an estimated minimum sea level pressure of 967 mb.  You can follow Omar's trek through the northern Leeward Islands from Puerto Rico's long-range radar: http://einstein.atmos.colostate.edu/~mcnoldy/tropics/omar08/Omar_15-16Oct08_long.gif.  It is too early to know the extent of damage in the Caribbean, but the system likely caused substantial damage to several island groups including the Netherlands Antilles and French West Indies.
Omar is currently moving rapidly northeastward, being steered by a pronounced digging mid-latitude trough.  The latest motion estimate has it heading NE at 22 kts, up substantially from the estimated 6 kts of motion at the 9Z advisory yesterday.  Omar is predicted to gradually weaken as it moves northeastward over cooler waters, and it likely will begin to undergo extra-tropical transition by the end of the weekend.  Omar is the first major hurricane to form in the month of October since Hurricane Wilma in 2005.
TD16, although in a favorable synoptic environment, was never able to organize and develop.  This was largely due to its proximity to land.  The system has tracked west-southwestward since yesterday and made landfall in northern Honduras yesterday afternoon.  Flooding is the primary concern with the remnants of TD16.  Some of the global models hint at re-development of TD16 as a tropical storm in the eastern Pacific as it continues to drift westward.

Please visit my tropical Atlantic headquarters.

15 October 2008

Omar now a hurricane, TD16 scraping the coast...

At 03Z today (11pm EDT Tuesday), Omar was upgraded to a hurricane, the 7th of the season.  It's presently a 75kt storm with a minimum pressure of 982mb, and still intensifying gradually.  You can spot the eyewall now from San Juan's long-range radar: http://einstein.atmos.colostate.edu/~mcnoldy/tropics/omar08/Omar_15-16Oct08_long.gif

The motion has finally picked up, as predicted, and it's heading NE at 8kts, and it's expected to maintain that motion for the next 3 days.  Within the next day however, it will have its only encounter with land: Puerto Rico and the Virgin Islands.  The latest official forecast track takes it directly over the Virgin Islands early Thursday morning as a strong CAT1 hurricane, possibly CAT2.  But intensity forecasts are tough, especially when considering rapid intensification.  Yesterday evening, the central pressure fell 22mb in 12 hours, but that rate has slowed down since then.  It appears that moderate vertical shear is putting a brake on Omar's previous rapid intensification.  A hefty trough is digging down into the subtropics and tropics, and imposing shear over the system, as well as the northeast motion which will eventually take Omar out into the Atlantic's tropical cyclone graveyard.

TD16, which was located right at the "corner" of Nicaragua and Honduras yesterday, has drifted west and is now just miles off the eastern Honduras coast.  Its proximity to land is preventing it from organizing or intensifying.  As of 15Z, the intensity was 25kts and 1005mb, and heading W at 4kts.  The biggest threat with this system will be flooding and mudslides in Honduras, Belize, and Guatemala.

Please visit my tropical Atlantic headquarters.

14 October 2008

Nana dissipates, TD15 upgraded to Omar, and TD16 forms...

In the face of oppressive 35kt vertical shear, Nana has been written off... only a low-level swirl remains near 18N 44W.

On the other hand, what became TD15 yesterday has been upgraded to TS Omar, and the latest intensity estimate is 35kts and 1001mb.  It's basically stationary (still sitting between Dominican Republic and Aruba), and getting better organized by the hour.  Outflow is healthy, and a large area of cloud tops over the center are persistently -80C and colder.  This could become the season's 7th hurricane within a day or so -- it has the visual appearance of a storm that's on the verge of rapid intensification.  The stagnant motion won't be a big problem as far as the ocean is concerned... the heat content is extremely high in the Caribbean, so there's not really any cool water to be upwelled!  The forecast calls for the motion to pick up in a day, and begin heading NE toward a weakness in the subtropical ridge.  In 1-2 days, this will threaten Puerto Rico and the Virgin Islands.

In addition, the area of disturbed weather off the Nicaraguan coast that I mentioned in yesterday's update was upgraded to TD16, and appears to be on its way to becoming TS Paloma within a day.  It too is over very water water with exceptional heat content, but a limiting factor in its intensification COULD be the proximity to land.  The official forecast takes it from its current position near the "corner" of Nicaragua/Honduras and tracks it west along the coast of Honduras into Belize in three days.  If it does manage to keep a bit further offshore and intensify notably, this could be really bad news for Honduras, which suffered greatly from a stalled major hurricane 10 years ago... Mitch.

Please visit my tropical Atlantic headquarters.

13 October 2008

Nana and TD15 form...

By the weekend, a large area of disturbed weather (hefty easterly wave) in the central Atlantic had already separated into two distinct areas.  The separation occurred in the middle of last week, and this from the same wave I first mentioned on October 1 when it was over Guinea.  On Sunday afternoon, the western portion has been upgraded to TS Nana, the 14th named storm of the season.  It never looked all that organized, but various satellite tools supported upgrading it.  It has since been downgraded to a Depression, and the latest intensity is 30kts and 1007mb.  It's located near 17N 40W and crawling WNW at 6kts.

As of 15Z today, a new Tropical Depression formed in the eastern Caribbean Sea, between the Dominican Republic and Aruba. Intensity is 30kts and 1005mb, and is expected to strengthen as it heads generally NE over the next 5+ days.  In the short term however, steering is virtually non-existent, so it should remain stationary or drift erratically until it gets picked up by an approaching trough in a couple days.  It should pass over Puerto Rico on Wednesday evening, perhaps as a hurricane.  If this seems like a strange track, it is!  Typically, we think of a storm like this heading toward the Yucatan Peninsula or Cuba, but this late in the season, mid-latitude troughs can dig pretty far south and influence tropical cyclone tracks even at 15N.  The next name on the list is Omar.
You can follow the storm on radar when it's close enough...
From Curacao: http://www.weather.an/sat_img/radar.asp#
or from Puerto Rico: http://radar.weather.gov/radar.php?rid=JUA&product=N0Z&overlay=11101111&loop=yes

Elsewhere, there's an interesting area of disturbed weather just east of Nicaragua.  A large area of cloud tops to -70C (and some to -80C) are just 2-3 degrees offshore and the whole area is moving north.  There's an embedded 1006mb Low.  This will also be watched very closely... and should it also get named eventually, the name after Omar is Paloma.

Please visit my tropical Atlantic headquarters.

07 October 2008

Marco comes and goes...

Yesterday morning, TD13 formed in the far south portion of the Bay of Campeche.  It was a small disturbance, but fairly potent for its size.  By 21Z yesterday, it was upgraded to TS Marco, the 13th named storm of the season, based on aircraft recon into the storm.  It intensified to 55kts and made landfall earlier today just north of Veracruz, Mexico.  Marco is now a tiny cluster of weakening thunderstorms over Mexico, barely recognizable as a tropical storm.

Please visit my tropical Atlantic headquarters.

01 October 2008

Laura dissipates...

Convection has dissipated in TS Laura, and the circulation is now over 13C SSTs.  The estimated intensity at the final advisory is 40kts and 995mb.  While 13C is cold, what matters more to a storm is the difference between the "input" air temperature near the surface and the "exhaust" air temperature aloft.  In higher latitudes, the SSTs drop off, but so do the tropopause temperatures (the tropopause is the top of the layer of the atmosphere in which all of our weather occurs).  If the tropopause temperature gets colder faster than the SSTs, the storm "thinks" it's in a good environment!  That's the primary reason we just had a fully tropical system over such cold water.

The circulation is now located just east of Newfoundland and heading north.  It will start recurving to the east with the next mid-latitude trough in a day or two and become a potent extratropical storm affecting the UK this weekend.

Elsewhere, there's a strong easterly wave trekking across Guinea in western Africa.  It is traceable back about 5 days over the Ethiopian Highlands.  A majority of forecast models develop this system once it exits the coast in about 3-4 days.  And finally, there's an easterly wave centered near 14N 40W which is currently poorly organized but could slowly develop.

Please visit my tropical Atlantic headquarters.