Area in Bahamas will probably become Arthur tomorrow

Looks like the national hurricane center is getting ready to initiate advisories on this system, and we will probably have Arthur off the coast of Florida sometime tomorrow.  Here’s a visual band image and model tracks as the sun sets over the Atlantic . .  .
al91_30june_vis

Both of the main hurricane specific models (GFDL and HWRF) show Arthur becoming a hurricane, as does our in-house versions of WRF.  Here is NCEP’s HWRF track – if the storm does this it would cause around $100 Million in damage and around 2 Million people would experience tropical storm force winds or greater.  But remember: it’s only a model.
hwrf

Watching low off Florida

Conditions are starting to become more favorable for the system in the Bahamas,  AL912014 (see this post for what the ID codes mean), to become a tropical cyclone.  If it spins up as forecast it will be called Arthur.  NHC is saying 80% chance of this happening later this week, and by Thursday/Friday some of the models are now forecasting a minimal hurricane being just offshore the Georgia/South Carolina coast (or, in a couple cases, making landfall).  Here’s the spaghetti map.  The black line is the past track, the colored lines are the main computer track models.  Dark blue is GFS, purple is the older LBAR model, the cyan track is HWRF (which makes the storm a minimal hurricane), orange is a statistical model (based only on past storm tracks) called “CLIPER”.  The colored line skirting the GA coast is the BAMD (Deep Barotropic Advection Model), an older model, but for a weaker storm sometimes just as good as the more sophisticated models.

al91_30june

The models are a bit more aggressive at strengthening the storm this morning than yesterday, but it is unlikely the storm will reach the point of causing significant damage.  More likely it will be a nuisance for the holiday weekend.  Still, if you live in GA/SC/NC/VA, worth keeping an eye on.

Why do some storms spin up into a hurricane?

I’ve been playing around with the excellent VAPOR graphics package from the National Center for Atmospheric Research.  Modern meteorological models create massive amounts of data.  Managing it, much less making sense of it, is a challenge.  Here is a view of a simulation of the investigation area off the Southeastern US, AL912014.  This is the forecast for 8pm EDT June 29th, based on data as of 8pm June 28 (in other words, the 48 hour forecast – the label says “00:00 July 1st” because all weather data is in GMT, which is 4 hours ahead of Eastern Daylight Time).

This view shows the path air is taking around the storm in the lowest 500 meters (1500ft) of the atmosphere, color coded to indicate how much water vapor is in the air.  The vertical scale is exaggerated by a factor of 100.  Click to embiggen . . .
al91_48hrfcst_29june

You can see air being pulled in at the surface, but the the picture is pretty convoluted. Dry air to the north, and winds criss-crossing the storm are keeping it from spinning up.  By comparison, here is what a mature hurricane looks like (in this case, 1999’s Hurricane Floyd, which passed through the same area).

floyd_vapor2

 

Notice some key differences – especially the vertical development in the eyewall, and the huge area of organized winds feeding the center with warm moist air.  That is the mechanism by which a hurricane converts the potential energy of warm, moist air in to wind.  By using graphics like this, we can get a good picture of what is going on inside the storm, and hopefully improve our understanding and ability to forecast why some storms become hurricanes and some don’t.

 

Update on possible storm off SEUS (AL912014)

Here’s this morning’s track forecast map for the INVEST area off the coast of the southeast US coast, AL 912014 (see this post for a note on what the code names mean).  Most of the models show the storm drifting south before being caught up by another system and sent off into the Atlantic, but the Hurricane WRF model (HWRF) shows it drifting across Florida.

AL91_29junAM

None of the dynamical models are impressed with this storm.  HWRF and GFDL both keep the thing below tropical storm force.  The statistical models take it up to a 60mph tropical storm, but they are pretty unreliable in this situation.  That said, storm intensity is by far the most difficult aspect to forecast, and none of the models are really good at forecasting storm formation.  NHC says a 40% chance of a tropical storm in the next 2 days, 70% in five days.  I’d put it less than that, but given how close it is to shore, probably worth keeping an eye on (but not worrying too much about; it’s unlikely to cause a lot of damage either way).

INVEST area off the coast of South Carolina

There is an “INVEST”, or potential tropical cyclone formation area, just off the coast of South Carolina. This one has the ID code “AL912014″. The way tropical cyclone identification works is that each storm is assigned a code of the form XXNNYYYY, where XX is the ocean basin, NN is the storm number, and YYYY is the year. For the Atlantic, XX is AL; for the east pacific (off Mexico) it is EP, CP is the Central Pacific (near Hawai’i), while WP is the West Pacific (past the date line).   INVEST areas are too weak to have names, and often they don’t spin up.  The numbers from 90 to 99 are reserved for these potential storms, and the numbers are recycled during the year so you will see AL91 again.

NHC is saying a “medium” chance of a storm forming in the next few days.  Here is what the initial track models area looking like, along with an Infrared Satellite image (from about 1pm Saturday 28 June).  The tracks are all over the place for now, although the system is expected to drift south over the next day or so.  The questions is, will it turn back towards the coast (the BAMD line in blue, below), or turn out to sea?  No way to tell at this stage.  Will post more as things develop (or not!).

al91_tracks al91_28june