30 August 2016

Storms headed for Florida and North Carolina

Tropical Depressions 8 and 9 are still taking aim at the U.S. -- TD8 will affect North Carolina, and TD9 will affect Florida.  Both could become tropical storms and get named later today.

TD8 is centered just 70 miles from Cape Hatteras and drifting toward the north-northwest.

A tropical storm warning is in effect for the Outer Banks from Cape Lookout up to the Oregon Inlet.  The bulk of the wind and rain from this system should remain offshore, so the biggest risk is for mariners.  It is forecast to strengthen slightly as it heads northeast away from the coast.  The worst weather for North Carolina will be today... conditions will improve by tomorrow.

TD9 is still moving toward the west... it has not made a turn toward the north yet.  It is centered 340 miles west of Key West and is nearly a tropical storm; peak winds are 35 mph.  The vertical wind shear has relaxed noticeably, but there is quite a bit of mid-level dry air to its west and north.

A tropical storm watch is not in effect yet, but will likely be issued later today for the Big Bend area of Florida.  Tropical storm conditions include heavy rain with rainfall totals over one foot in some places, sustained winds up to around 60-65 mph, isolated tornadoes, and moderate storm surge flooding to the right of the storm's center.

Elsewhere across the basin, Gaston is still spinning out there, 700 miles east of Bermuda, as a Category 2 hurricane.  It is expected to weaken as it heads eastward, possibly affecting the Azores by Saturday.

And finally, the easterly wave I mentioned yesterday is now officially over water and is still favored by models to develop.  In the near future, it will be surrounded by dry Saharan air, so I wouldn't expect any significant development in the next few days.  Identified as Invest 92L for now, it has not even reached Cabo Verde yet.  The earliest it would reach the Lesser Antilles is around September 5.

Again, the next few names on the list are Hermine, Ian, and Julia.  We could actually get Hermine AND Ian today (I'm not sure who wins the H name if they're both upgraded at the same time!).

29 August 2016

August wrapping up with three simultaneous tropical cyclones

Today's update is in three parts... two of which are on the Capital Weather Gang blog, and the third part will be included at the end of this post.

The last time there were three active tropical cyclones in the Atlantic was September 13, 2013 (at 12 UTC, there was TD Gabrielle, TS Humberto, and TS Ingrid).  If we nudge the criteria up to three simultaneous systems of at least tropical storm intensity, the last time was September 2, 2011 (Katia, Storm 20, and Lee).

Tropical storm in the forecast for N.C. during prime vacation week in Outer Banks

A tropical storm is brewing in the Gulf of Mexico; Florida landfall likely this week

(a few things to clarify in that second post... this is the same system that was known as 99L, and we've been watching it for about two weeks.  If it develops, it would be the 8th tropical storm.  And, it refers to the next three names on the list possibly being used in the coming week, though the third potential storm didn't make it in that post, but is included below for completeness.)

Now, the system that isn't in either of those posts is an African easterly wave of interest:

Although still over land, several leading models are forecasting this easterly wave to develop fairly quickly once it crosses the coastline tomorrow.  There is plenty of time to monitor it, since the earliest it would be close to the Lesser Antilles is around September 5.  However, this time of the season is when these African-born systems have become some of the most infamous major hurricanes.

28 August 2016

Three areas of interest, two of which could affect U.S. coast

There is a lot going on today... so I'll start in the east and work my way west.  Gaston, which formed back on August 22, is now a Category 2 hurricane, making it the strongest storm of the season so far.  An area of enhanced thunderstorms that was located near Bermuda has gotten much better organized in the past day and is now Tropical Depression 8.  And finally, "Invest 99L" is centered over the Florida Keys and about to enter the Gulf of Mexico.

Hurricane Gaston is centered about 600 miles east of Bermuda in the middle of the Atlantic and is forecast to turn to the north and gradually weaken this week.  But today, conditions favored an impressive intensification, and maximum winds are up to 105 mph.  Gaston is the season's third hurricane, and the first to reach Category 2 status.

Hurricane Gaston
The next system of interest is Tropical Depression 8, which is centered about 300 miles west of Bermuda, or about 430 miles east-southeast of Cape Lookout, NC.  It is experiencing rather strong wind shear, so the surface circulation is nearly exposed with minimal thunderstorm activity displaced to the west.

Tropical Depression 8
TD8 is forecast to intensify further as it heads toward the North Carolina coast, though probably as nothing more than a borderline tropical storm.  The map below shows the official NHC forecast of tropical storm force wind probabilities over the next five days.  There is a slight chance that the NC Outer Banks could experience tropical storm conditions Monday night into Tuesday.  Depending on how soon it reaches tropical storm status, it could be named Hermine or Ian (keep reading for explanation).

Last but not least, Invest 99L.
You've been hearing and reading about 99L for a long time now... in fact, it came off the African coast 12 days ago.  The cryptic name is nothing more than a temporary identifier given to disturbances of interest.  Once it becomes a tropical cyclone, it is given a unique number (08L for example, for the 8th TC of the season).  If it reaches tropical storm intensity, it is also given a unique name (Hermine, for example).  The "invests" are numbered and reused cyclically from 90-99, and the L denotes the Atlantic.  So there will be another 99L this season, but not at the same time as the current 99L.

99L has been a troublesome system to predict.  Model guidance has often forecasted intensification, but has been consistently wrong so far.  It has at times looked quite healthy on satellite images, only to get shredded apart hours later.  It's now just a disorganized mess between Florida and Cuba producing some gusty winds and areas of thunderstorms.

99L is also forecast to organize once it enters the Gulf on Monday.  So here comes the race for the name Hermine.  Either TD8 or 99L could be the next named storm... which is Hermine.  The name after Hermine is Ian, so both could get used in the coming days!  Hermine and Ian are the 8th and 9th names on the list this season... though the average is only 5 named storms by September 1st.

There is pretty good agreement among global and regional models that this system could intensify significantly in the Gulf this week.  Anywhere from LA over to Tampa Bay should be watching this very closely for possible impacts from Wednesday through Friday, particularly the Big Bend area of Florida which is extremely vulnerable to storm surge.

Five-day forecast tracks from several select models (global and regional).

26 August 2016

Threat of significant South Florida impact greatly reduced

The same disturbance that we've been watching since it left the African coast back on the 16th, "99L", is still not even a depression, and is further from becoming one today than it ever has been.  It has entered an area of strong vertical wind shear, and is very disorganized.  The approximate center is north of eastern Cuba, though you wouldn't know it by looking at a satellite image.

Models continue to track the "center" of this system (open wave) over the southern Florida peninsula/Keys, though none do so at any intensity worth worrying about.  Quite a few are unable to track a coherent vortex for the entire forecast period.

However, shear is expected to relax in a couple days, and it could actually regain some organization while near Florida, but not to the point of reaching hurricane intensity.  One model (HWRF, orange line in the map below) still favors this system to cross south Florida/Keys and then intensify to a substantial hurricane once it enters the Gulf... that cannot be ruled out yet, so we'll keep a close on it.

As the wave axis approaches, winds will pick up and chances for heavy rain greatly increase... generally Sunday into Monday for south Florida.  At this point, winds are not expected to reach tropical storm strength.

Five-day forecast rainfall accumulation over south and central Florida. (NOAA/WPC)

24 August 2016

Tropical wave over Virgin Islands presents threat to Florida, Gulf Coast

I'm on vacation this week, but given the activity in the Atlantic, I wanted to share an update with you written by Jason Samenow from the Capital Weather Gang:

Tropical wave over Virgin Islands presents threat to Florida, Gulf Coast

Throughout the week, if I don't get a chance to write an update, I will share one with you from a colleague!

23 August 2016

Fiona is nearly gone, Gaston has formed, and TD8/Hermine is on deck

The disturbance that was identified as "90L" (far eastern Atlantic) was upgraded to Tropical Depression 7 on Monday afternoon, and then again to Tropical Storm Gaston on Monday evening.  Although Gaston is expected to become the season's third hurricane sometime tomorrow, models are still in very good agreement on it turning/recurving to the north even further east than Fiona did -- so safely in the middle of the ocean.

Tuesday morning sunrise over Tropical Storm Gaston.
The western tropical wave, still identified as "99L", is the one I've been discussing that left the African coast LAST Tuesday (16th).  It is still disorganized, but is gradually becoming better organized, and is now approaching the Lesser Antilles.  The next name on the list is Hermine.

Enhanced infrared satellite image of 99L approaching the Lesser Antilles.
The morning model track guidance is shown below, and the majority of models now keep it north of the Caribbean and on a trajectory that would bring it near the Florida peninsula late next weekend into early next week (6-8 days).

The intensity is still a huge unknown since it isn't even a depression yet, but IF it starts to develop, it would have time to reach hurricane intensity and is worth keeping a very close eye on.  Some models have consistently shown it crossing the Florida peninsula and then strengthening in the far northeast Gulf of Mexico as it heads north.  It's too soon to trust any exact scenario now, but it is also prudent to be aware of the possibilities now that we're within the one-week time frame (global models have skill that far out!).

And finally, I suspect the final advisory will be written on Fiona today... it is now a small swirl essentially devoid of any thunderstorm activity.  It is not near any land.

19 August 2016

Worth monitoring: Atlantic wave that could become Tropical Storm Gaston

Today's update includes a brief mention of Fiona, but also a closer look at what could be the next storm in the lineup: Gaston.  The post is available on the Capital Weather Gang blog:

Worth monitoring: Atlantic wave that could become Tropical Storm Gaston

17 August 2016

Tropical Storm Fiona will likely form in eastern Atlantic today

The easterly wave I mentioned in yesterday's update is now Tropical Depression 6... and should be upgraded to Tropical Storm Fiona later today. My update is on the Capital Weather Gang blog:

Tropical Storm Fiona will likely form in eastern Atlantic today

16 August 2016

Possible action brewing in the eastern Atlantic

We're at the time of year when all eyes are on Africa for generating tropical cyclone embryos... African easterly waves.  One exited the coast about three days ago and is now close to becoming a tropical depression.  Another strong one is still inland and should cross the coastline in about four days (models favor its development).  For additional background on AEWs, please refer to my post from a few weeks ago: "Africa is Awakening".

Visible satellite image of the eastern tropical Atlantic and northern Africa.  The two features of interest are southwest of the Cape Verde islands and also the blob of clouds passing into Burkina Faso. Yellow coloring indicates the presence of dust.  (CIRA/RAMMB)
Focusing on the one over water, it is centered near 10°N 30°W and is in an environment that would support development, at least for another couple of days.

Models generally agree that it will become at least a tropical depression in the near future, but a track toward the northwest will take it over slightly cooler water and into drier air for a few days.  Then after that potential speedbump, it could re-intensify over the central Atlantic, posing no threat to land.  The next name on the list is Fiona.  [Fiona was a new name introduced in the 2010 season to replace Frances which was retired after the 2004 season.]

Forecast tracks from a variety of numerical models of varying complexity (not all lines should be weighted equally or even considered... but that's a whole different topic!).

01 August 2016

Strong tropical wave cruising through Caribbean Sea

The average formation date of the fifth named storm is August 30, but this year, it will likely arrive early.  The African easterly wave we've been tracking for a bit over a week now is much better organized and is very close to being upgraded to Tropical Storm Earl.  An aircraft reconnaissance plane will fly into it later today to try to find a closed surface circulation -- its rapid motion makes it harder to meet this requirement.  It is centered south of Hispaniola and is moving quickly to the west.  I marked the approximate center with a blue X in the satellite image below.  It is a compact system, but rainbands are affecting Haiti and Dominican Republic today.

Environmental conditions are favorable for further development... vertical wind shear is forecast to remain in in the 5-15 kt range throughout the week, and sea surface temperatures are adequate now and will increase as the storm moves west.

Models continue to agree on a westward track across the rest of the Caribbean, bringing it to the Yucatan peninsula or Belize on Wednesday.  It would weaken as it crosses land, so whatever emerges in the Bay of Campeche will have some reorganizing to do before it poses a major threat somewhere else.  At this point, a south Texas impact looks very unlikely, but it's far too early to completely rule it out.

I plotted the tracks of the past fifteen "E" storms just for reference (2001-2015)... seven of them became hurricanes, and the one that most closely resembles this year's storm is 2012's Ernesto.  The strongest was 2005's Emily, which became a 140-kt Category 5 storm near Jamaica.  Both Ernesto and Emily traveled the full length of the Caribbean and both hit the Yucatan peninsula as hurricanes.