01 December 2014

2014 Atlantic hurricane season summary

To those who have been subscribed and reading my posts since the humble beginnings in 1996, I want to thank you for your continued support and interest.  And to the many more recent readers, thanks for joining!

I have sent out approximately 950 tropical updates over the years.  Those updates have spanned 274 tropical storms, 122 hurricanes, 64 major hurricanes, and 36 retired storm names.  WHEW... my fingers are tired just thinking about that!

My season summary to wrap up the 2014 hurricane season is now available on the Washington Post's Capital Weather Gang blog, and again, thanks for reading and sharing!

Unprecedented lull in major hurricane landfalls continues as Atlantic season comes to a close

20 October 2014

Tropical disturbance brewing in the Gulf of Mexico

Just a day after Gonzalo transitioned to an extratropical cyclone south of Greenland, our attention shifts back to the tropics.  A very disorganized low pressure area is festering in the Bay of Campeche... my update on the system can be found on the Capital Weather Gang blog:

Tropical disturbance brewing in the Gulf of Mexico

18 October 2014

Gonzalo passes directly over Bermuda Friday night

The hit could not have been any more direct.  There was a moment when the large eye of the hurricane was centered so perfectly over the island that there were light northerly winds on the west side of the island and light southerly winds on the east side.  That was at 9pm local time, and the corresponding radar and satellite images are shown below:

(Bermuda Weather Service)
(Naval Research Laboratory Monterey)
The complete long-range and short-range radar loops from Bermuda (as well as earlier ones from Puerto Rico and Guadeloupe) can be found at http://andrew.rsmas.miami.edu/bmcnoldy/tropics/radar/

As you can tell from the lopsided satellite presentation, it was becoming sheared and beginning to show signs of an extratropical transition.  The radar and satellite images are from the same time (0100Z on the 18th).

During the day on Friday, I captured a series of webcam images from Bermuda's cruise ship terminal.  The three images span 4 hours, but do not include the worst of the storm (you couldn't see ANYTHING then)... the rapid deterioration of conditions is evident.

St. Davids experienced a wind gust close to 150mph, but for the most part, winds were characteristic of a Category 2 storm... still plenty strong to inflict severe damage.  It will take some time to sort out what all happened there, and as I write this, sunlight is just returning to the island.  There will likely be extensive damage to trees as well as piers/docks/marinas.  Reports indicate that about 85% of the island is without power.

Weather balloons are released twice a day at about 800 locations around the world every day to collect atmospheric data.  Bermuda is one of those sites, and at the regularly scheduled time, they were inside Gonzalo's large eye and released the radiosonde on time... what an incredible coincidence.  Here is that sounding for posterity:

The current satellite image and forecast track are shown here, and that track will take Gonzalo over very cold water beginning tonight, and it is expected to become an extratropical cyclone by Sunday afternoon as it zips off to the north central Atlantic hurricane graveyard.

This was a very bizarre week in an otherwise quiet season.  To have Tropical Storm (almost hurricane) Fay hit Bermuda last Sunday and then Hurricane Gonzalo hit on the following Friday night is the worst possible luck!  The debris from Fay was still not picked up completely when Gonzalo hit.

For now, the only area of possible interest in the coming days is in the Bay of Campeche, so I'll keep a close eye on it!

17 October 2014

Trouble in paradise: Hurricanes threaten Bermuda and Hawaii

Today's update covers Hurricane Gonzalo in the Atlantic, and Ana in the Central Pacific.  Gonzalo will impact Bermuda on Friday afternoon, and Ana will impact Hawaii on Friday night.  The update is available on the Capital Weather Gang blog... as always, thanks for reading and sharing!

Trouble in paradise: Hurricanes threaten Bermuda and Hawaii

16 October 2014

Hurricane Gonzalo one day away from Bermuda

As of 8am EDT today, Gonzalo is a Category 4 hurricane with 140mph sustained winds.  As I pointed out before, this is the strongest Atlantic hurricane in three years (it's tied with Ophelia 2011).  The last storm that was stronger (not tied) was Igor 2010.

Ominous satellite image of Hurricane Gonzalo as of 9:15am EDT.  Bermuda is marked in red in the upper right corner.
Gonzalo completed the eyewall replacement cycle that I described in yesterday's update, and as is typical, it weakened slightly during the structural reorganization, then once the transition was complete, it re-intensified.  Additionally, every time an ERC occurs, the wind field expands in size (meaning that stronger winds cover more area).

Tropical storm (yellow) and hurricane (red) force wind swaths for Gonzalo since it formed through Thursday morning.  Notice how the wind field expands with time.  (NOAA)
It has also stopped moving westward, meaning that the recurvature toward Bermuda has begun.  As of 9am EDT, it's about 500 miles south-southwest of Bermuda and moving toward it at 9mph... a faster motion will begin today.  Tropical storm force winds extend up to 150 miles from the center (Bermuda is just 350 miles from those winds).

Models remain in tight agreement on the track over the next four days... it will make it closest approach to Bermuda early Friday afternoon and then make its closest approach to Newfoundland late Saturday night.  Both encounters will be quite significant.

Forecast from NHC's 8am EDT intermediate advisory. (NOAA)
As far as intensity goes, Gonzalo is still expected to be a major hurricane when it passes by Bermuda, which would be just the 4th major hurricane on record to hit Bermuda during October. The last 5 major hurricanes (and their intensities at closest approach) to pass within 100 miles of Bermuda are:
     - Fabian 2003:    120mph
     - Edna 1953:       120mph
     - Unnamed 1948: 130mph
     - Unnamed 1947: 120mph
     - Unnamed 1939: 130mph
Gonzalo could be slightly weaker than Fabian when it reaches the island, but still a major hurricane.

You will be able to monitor the storm via radar and surface observations as it approaches.  There's also a pier station with 6-minute reporting and water levels.

Stay tuned for updates, and you can always find the latest forecast and advisories from the National Hurricane Center.

12 October 2014

Fay hits Bermuda, and Gonzalo in the making?

To catch up from Friday morning's post, STD 7 was upgraded Subtropical Storm Fay at the next advisory on Friday afternoon.  Then on Saturday morning, Fay officially transitioned from subtropical to tropical, and headed north toward Bermuda as expected.  It also strengthened to just below hurricane intensity (60kts) and passed directly over Bermuda on Sunday morning.

Early morning visible satellite image of Tropical Storm Fay (7am EDT).  Bermuda is the red speck just south of the circulation center.  Thin wispy outflow cirrus are evident on the side of the storm. (NASA)
Radar image of Fay shortly after "landfall". (Bermuda Weather Service)
In the coming days, Fay will weaken and become extratropical in the face of decreasing sea surface temperatures and increasing wind shear.

What's next?

The disturbance that I mentioned on Friday that was east of the Lesser Antilles has indeed gotten better organized.  It is now centered roughly 350 miles east of the Leeward Islands and moving west at 12mph.  It is nearing tropical depression status, and could be upgraded to TD8 today or tomorrow.  If and when it becomes a tropical storm, the name will be Gonzalo.

Visible satellite image of the disturbance east of the Leeward Islands.  (NOAA)
Models are still in excellent agreement on it developing, and on a northwestward track toward Puerto Rico.  Beyond that, it appears likely that a trough will steer it toward the north. 

Forecast tracks from the Oct12 06Z model suite.  There are 4 global models and 5 regional models shown here. (U. Albany)
With very low shear and very warm SSTs ahead of it, this could potentially develop quickly and be close to hurricane intensity by the time it passes east of Puerto Rico, so it's worth paying attention to!

10 October 2014

TD7/Fay could form today

[11am UPDATE: This was upgraded to Subtropical Depression 7]

Over the last several days, an upper-level low pressure system positioned north of the Leeward Islands has gradually been acquiring more and more tropical characteristics, including a surface circulation and thunderstorm activity wrapping more evenly around the center. Model guidance is in fairly good agreement on this feature developing into a subtropical or tropical storm, and the next name on the list is Fay.

The thermal and wind structure is on the border between subtropical and tropical, and a case could be made for either designation right now.  But since both subtropical storms and tropical storms get named, it would still become Fay (or a subtropical/tropical depression first).

In an average season (using 1981-2010), we would have 10 named storms and an ACE of 88.3 by this date... but this year we're at 5 named storms and the ACE is 35.8.  IF this gets named today, it will be the latest date for the 6th named storm since 1994 (Florence formed on November 2).

Global and regional models also agree on a future track: continue to drift slowly to the northwest, then begin turning to the northeast by Sunday.  It is not near land now, but could make a close approach to Bermuda on Sunday (that's the black speck near 32N 65W on the map below).

Elsewhere, there's a tropical wave centered near 15N 50W (about 730 miles east of the Lesser Antilles) today that is heavily favored by global models to develop into something significant in the coming days as it heads west-northwest.  If and when it shapes up a little more, I will give a full update on it.

Although the season has been quite inactive, it is important to note that October can still produce some very intense hurricanes.  It's not over 'til it's over, so stay tuned.

08 October 2014

Sea Level Rise in Miami and Miami Beach

Since many people who read my blog reside on or near the coast, I think this non-hurricane-related post would be of general interest.  Although the focus is on data and observations from Miami, the theme and lessons apply just about everywhere.

Water, Water, Everywhere: Sea Level Rise in Miami

02 October 2014

Will quietest hurricane season in two decades continue in October?

Today's update on the status of the hurricane season so far and a look ahead can be found on the Capital Weather Gang blog:

Will quietest hurricane season in two decades continue in October?

As always, thanks for reading and sharing!

17 September 2014

Edouard becomes first major hurricane since 2012

On Tuesday morning at 11am EDT, Edouard was upgraded to a Category 3 hurricane (115mph).  A major hurricane is defined to be any Category 3-4-5 hurricane, and the last one was Sandy when it made landfall on eastern Cuba on October 25, 2012.

However, Edouard only held that intensity for 6 hours.  The last two major hurricanes (Sandy 2012 and Michael 2012) also each held that intensity for just 6 hours.  In one day, Hurricane Rina in October of 2011 racked up as much time as a major hurricane as all other storms combined did in the subsequent 1,064 days!

Visible satellite image over Edouard from 8:45am EDT. (NASA)
As of today at 5am EDT, Edouard's intensity is back down to 90mph.  It is centered about 600 miles northeast of Bermuda and heading northeast at 20mph.
It will continue to weaken over decreasing water temperatures and increasing wind shear as it accelerates to the northeast toward the Azores.

Edouard's activity brings the seasonal ACE (Accumulated Cyclone Energy) up to about 52% of average for this date.  So although 4 out of the 5 storms became hurricanes so far, the overall numbers and intensities are falling well short of average.

Only 1 of the 4 hurricanes made landfall anywhere (Arthur), but NONE of them have existed in the tropics!!  All four hurricanes formed north of 24N, a sign that the weak El Nino is suppressing activity in the tropical Atlantic, as expected.  The figure below shows the typical influence of El Nino on hurricane activity in the East Pacific and in the Atlantic... 2014 couldn't fit this any more perfectly!

Typical influence of El NiƱo on Pacific and Atlantic seasonal hurricane activity. (NOAA)
Elsewhere across the basin, there's an easterly wave about to exit the African coast today, and models generally develop it over the next few days, but not very aggressively.

15 September 2014

Edouard becomes strongest Atlantic hurricane in nearly 700 days

Since my last post on Friday morning, Edouard was upgraded to a Category 1 hurricane on Sunday afternoon, then again to a Category 2 hurricane early Monday morning.  Eduoard is the fifth named storm and the forth hurricane... the last time 4 out of the first 5 storms became hurricanes was 1996 (which coincidentally used the same name list as 2014)!

The intensity is currently estimated at 105mph, making it slightly stronger than Arthur back in July of this year, stronger than any storm in all of 2013, and tied with Sandy on the morning of October 29, 2012: 686 days ago.

As of 5am EDT today, Edouard is located about 700 miles east-southeast of Bermuda in the middle of the Atlantic.  The forecast track is shown below - recurving by 60W and staying very far away from any land.

Additional strengthening is likely, and within a day or two, Edouard could become a Category 3 (major) hurricane.  It would be the first major hurricane in the Atlantic since Sandy BRIEFLY reached that status when making landfall on eastern Cuba on October 25, 2012.

Visible satellite image of Hurricane Edouard from 8:15am EDT.  (NOAA)
The disturbance that was passing over southern Florida on Friday entered the Gulf and dissipated.  Aside from Edouard, there are no other areas of interest today.

12 September 2014

Edouard forms, and possible Gulf storm next week?

At 11pm EDT on Thursday (03Z on Friday), TD6 was upgraded to Tropical Storm Edouard.  This is latest date for the fifth named storm since Ernesto formed on September 22, 1994.  But, climatologically, it's actually right on schedule... the average date for the fifth named storm is September 11!  And if it becomes a hurricane soon (which it should), it would be well ahead of the average date for the forth hurricane, September 28.  (these dates utilize the full 1851-2013 period of record... they change if a different period is chosen)

As of 5am EDT on Friday, Eduoard's intensity was estimated at 40mph and it is moving west-northwest at 15mph.  It is battling moderately strong wind shear and dry air, but over the next 3-5 days the shear should relax and Edouard is forecast to intensify to a minimal hurricane out in the open central Atlantic.

Visible satellite image from 8:30am EDT over the tiny and sheared Tropical Storm Edouard. (NASA)
The track forecast with cone of uncertainty can be found here; the recuravature around 55W is supported by all models.

Next... the disturbance that was over the northwest Bahamas yesterday is now inland over the southern Florida peninsula.  However, dry air and shear (sound familiar?) have taken their toll on the small embryo circulation.  But what happens when it enters the Gulf of Mexico?

Visible satellite image from 8:45am EDT.  (NOAA)
First, a look at the past 24 hours of rainfall over Florida (8am-8am).  Parts of Palm Beach County got nearly 5", and 1-3" fell across portions of Broward and Miami-Dade Counties.  The remainder of southern Florida will likely be similar as the weak system drifts slowly west today.

Estimated rainfall totals from 8am Thu through 8am Fri. (NOAA)
The disturbance will enter the Gulf of Mexico by early Saturday morning, then  models (the colored lines on this map show track forecasts from 9 skillful models: the first 4 are global and last 5 are regional) diverge on how quickly it curves back to the north.  As of now, conditions in the Gulf this weekend and next week appear to be only slightly conducive for development.  SSTs are very warm, there is less dry air, but the wind shear should remain strong.  Through the next 5 days, models generally forecast this to become a tropical storm, but none bring it up to a hurricane.  It certainly can't be dismissed though... if the longer-range shear outlook is wrong, the intensity forecast could be very wrong.  The next name on the list is Fay.

11 September 2014

Update on two tropical disturbances

11am EDIT: The eastern disturbance upgraded to Tropical Depression 6.

The quick version: neither one is a depression yet, but the eastern one is very close and the western one should remain weak but bring wet weather to FL in the coming days.

First, a look at the one that is over the Bahamas and headed for Florida.  It has not gotten much better organized, and time is running short for it to do anything.  It's now centered over the far northwestern Bahamas, and thunderstorm activity remains sparse.  Although the water temperature under it is plenty warm, the vertical wind shear is picking up and should restrict any rapid organization.

Visible satellite image from 9:30am... the approx center of the surface circulation is marked with a red X. (NASA)
So, it will continue drifting west and just bring enhanced rainfall and some gusty winds to parts of central and south Florida over the next 2-3 days.  A three-day rainfall forecast valid between Thursday morning and Sunday morning is shown below.  After crossing the FL peninsula, whatever remains will enter the Gulf, but there's too much uncertainty to speculate that far out... we'll have to wait to see if there's anything intact after crossing Florida.

72hr rainfall forecast, valid Thu 8am - Sun 8am. (NOAA/WPC)
I have a long and updating radar loop from Melbourne FL at http://andrew.rsmas.miami.edu/bmcnoldy/tropics/al92/AL92_11-12Sep14_MLBlong.gif.

Now onto the eastern disturbance, which is close to becoming a tropical depression or tropical storm.  It's centered about 800 miles west of the Cape Verde islands this morning... and moving west-northwest at about 15mph.

Visible satellite image from 9:15am.  (NRLMRY)
It is forecast the gradually strengthen, possibly reaching hurricane intensity early next week, but also recurve by 50W, never coming close to any land.  If named, the next name on the list is Edouard.

Trivia for today: the last time the fifth named storm occurred so late in the season was 20 years ago!  Ernesto was named on Sept 22, 1994.

10 September 2014

Two weak disturbances to watch

Tropical tidbit 1: The last time there was not a named storm in the Atlantic on September 10 was in 2000! And even then, a subtropical depression formed (not named).  So, the last time there was not any tropical or subtropical cyclone at all on this date was 1992.

Tropical tidbit 2: You may recall a previous bit of trivia I shared: the last time the A, B, and C storms all became hurricanes was also in 1992.  AND, the A storms in 1992 and 2014 both made landfall on the U.S. as hurricanes.

As I mentioned in my update yesterday, there are two areas of interest right now: one over the Bahamas and one just west of the Cape Verde islands.  Neither are very impressive, but are worth watching over the coming days.

First, the system over the Bahamas... it's actually being generous to even consider it for discussion, but because of its proximity to Florida, I'll give some highlights.  It's not from an easterly wave, but rather an upper-level Low that interacted with a weak surface trough over the past few days.  It occasionally spawns some widespread thunderstorms, but this morning isn't one of those times, as you can see in the benign-looking satellite image here:

However, global models do show the disturbance persisting (not developing) and heading west into Florida.  It should result in nothing more than some welcome rain for the area in the coming 1-3 days.

Total rainfall forecast in south Florida valid from today through Monday. (NOAA/WPC) 
The second system of interest is an easterly wave that exited the African coast this past Saturday.  It is now centered about 450 miles west of the Cape Verde islands, or about 1900 miles east of the Windward islands, and is extremely disorganized.

As has been the story for the entire season so far in the deep tropics, a copious amount of low-mid level dry air is choking it off and limiting any development... the SST and wind shear are both favorable for development. Global and regional models alike forecast it to continue moving WNW and eventually get better organized once it escapes the Saharan Air Layer. According to a consensus of leading models, getting a hurricane out of this one is actually not out of the question by Sunday-Monday.

Infrared satellite image (grayscale background) with a depiction of the Saharan Air Layer overlaid (yellow-red).  The center of the disturbance is marked with a red I. (CIMSS)

09 September 2014

Peak of hurricane season comes quietly this year

Although the Atlantic is fairly calm today, if you average activity over the whole 163 years of records, September 9 is actually the climatological peak!

There are numerous ways to define "activity" of course, and there are numerous time periods one could use to create the average.  But for this claim, I'm using a standard metric called Accumulated Cyclone Energy (ACE), and the full 1851-2013 period.

 It is not a sharp peak, but rather a broad hump that spans about two weeks (roughly the first half of September).  On average, Sep 2-16 generates 1/4 of the entire season's ACE!

[ACE is calculated for each named storm every six hours.  The higher the peak wind and the longer it lasts, the more ACE a given storm will accrue.  It's the sum of the squares of the six-hourly peak winds,... for example, if a storm's intensity is estimated to be 65 kt in an advisory, the ACE is 4,225 kt^2, but it's commonly shown in units of 10,000 kt^2, so that value would become 0.4225.]

Now let's look at just the average counts of tropical cyclones through the season (tropical storms, hurricanes, and major hurricanes).  The pattern is similar, but additional interesting details come through.  Major hurricanes (Category 3+) are almost exclusively found in Aug-Sep-Oct.

Rather than breaking it down into daily counts, the next figure shows the average cumulative counts.  This is handy if you wish to see that the third hurricane typically occurs by September 12, for example.

These figures and dates do change if the period used for the climatology changes.  One could look at more recent times, like 1950-2010, 1981-2010, or whatever is desired!  However, the fewer years that go into the averages, the more noisy the plots will become and the less robust the results will be.

Now to wrap up with current activity.  There are no named storms, but there are a couple areas of interest that could form in the coming 2-5 days.  The 5-day formation outlook from NHC highlights where those areas are now (X) and where formation might occur within the next five days (shaded blobs).  Note that these are NOT track forecasts!  The yellow area is given a 20% chance of development, and the red area a 70% chance.  I will probably send out a more complete update on these disturbances tomorrow if they continue to develop.

Visible satellite images of the two disturbances are shown here for reference:

02 September 2014

Dolly forms in Bay of Campeche, landfall tonight

Over the past week, an easterly wave has been making its way across the Caribbean.  As it neared the Yucatan peninsula on Sunday, it began to get better organized.  Shortly after it crossed the peninsula and entered the Bay of Campeche, it was upgraded to Tropical Depression 5.  Then, at 5am EDT on Tuesday, it was upgraded to Tropical Storm Dolly.

Today, as of 8am EDT, Dolly is centered 145 miles east-southeast of La Pesca, Mexico and is moving west-northwest at 13mph.  At this rate, landfall will occur late Tuesday evening (local time).  Maximum sustained winds are 50mph and additional intensification is possible up until it makes landfall. The greatest threat from this storm will be the heavy rain and resulting mudslides and flash floods.

Dolly is the fourth named storm of the season, and ushers in the climatologically most active couple of weeks of the Atlantic hurricane season.  In the figure below, a timeline of the average tropical storm and hurricane counts are shown, and today's date is highlighted by a green line.  This season, the overall activity as measured by ACE (Accumulated Cyclone Energy) is just 54% of average as of today.

Dolly's origin and track is interesting because past storms in this "lineage" have been similar.  Prior to Dolly, Diana was the female name in this list (it was retired in 1990).  But, going back to 1990, Diana 1990, Dolly 1996, and Dolly 2008 all crossed the Yucatan peninsula and strengthened as they moved west... also note that all three of them were hurricanes at landfall.

Elsewhere across the basin, global models have been bullish on developing an easterly wave that's still over Africa, but is expected to enter the Atlantic on Thursday.  This wave had its origins over eastern Africa back on August 28th.  The next name on the list is Edouard.

Today is a very significant day in hurricane history.  On the evening of September 2, 1935, the infamous Labor Day Hurricane made landfall on Florida's Upper Keys (near Islamorada) as a Category 5 hurricane... and it remains the most intense U.S. landfall on record.  Maximum sustained winds were 185mph (stronger than Andrew and Camille); it generated a 18-foot storm surge that totally inundated the low-lying islands, and the incredible winds leveled everything in its path.  It was responsible for over 400 deaths.

25 August 2014

Cristobal heading north and out to sea

Since my previous post on Saturday afternoon, TD4 was upgraded to Tropical Storm Cristobal on Sunday morning.

Cristobal is the latest third named storm to form since Charlie in 1992 (named on Sep 22).  You may recall from previous posts that 1992 was also the last year that both the A and the B storms became hurricanes.  Well, if Cristobal strengthens into a hurricane (which it could), 1992 is ALSO the last time the A, B, and C storms all became hurricanes!  And yet another similarity between 1992 and 2014 is that the A storm made landfall on the U.S. as a hurricane (though Andrew was a Category 5 and Arthur was a Category 2).

As of Monday morning, Cristobal is drifting north at about 3mph, and is centered just north of Mayaguana in the eastern Bahamas.  The intensity estimate as of 8am EDT is 60mph with a 994mb central pressure.  It is forecast to become a hurricane within the next day or two.

Visible satellite image from 8:15am EDT.  I overlaid the past track in light blue and marked the current center location with a light blue X.  (NASA)
As you can see in the image above, the surface center is exposed, and all of the thunderstorm activity is displaced to the south due to strong northerly wind shear.  The sea surface temperature and ocean heat content are extremely favorable for further intensification, but due to the shear, the NHC intensity forecast currently brings the storm up to a minimal Category 1 hurricane.

The northward turn that was in question for so many days did finally happen on Sunday, and was the favored solution by the majority of models.  There was only a slight chance for a westward track into south Florida, but given the potential impact there, it was worth monitoring that possibility very closely.  The map below shows the surface pressure in line contours and the 500mb heights (steering features) in shaded contours as of this morning... Cristobal will head northeast into the trough (yellow).

Surface pressure and 500mb height contours valid at 8am EDT today.  (tropicaltidbits.com)
Now, models have come into strong agreement on Cristobal's future.  Once it pulls away from the eastern Bahamas, it is forecast to pass west of Bermuda on Wednesday as a hurricane, then zip out into the north central Atlantic where it will become a potent extratropical cyclone by the weekend.

Monday morning's suite of model and consensus guidance.  (UW-Milwaukee)
Elsewhere, there is a weak easterly wave located about 1400 miles east of the Windward Islands, but environmental conditions will inhibit development for the foreseeable future.  This wave can be tracked back to the African coast on August 21.

23 August 2014

Tropical Depression 4 forms north of Haiti

The disturbance we've been watching struggle across the Atlantic for the past 13 days is now north of Haiti... and has just been upgraded to Tropical Depression 4.  Aircraft reconnaissance missions have been carried out frequently, but up until now, it has either been lacking enough centralized thunderstorm activity or a closed circulation that extends all the way to the surface.  Both are requirements for a tropical cyclone (that generic term includes tropical depressions, tropical storms, and hurricanes). 

Because it has remained so poorly organized, models have had a hard time with it.  Stronger cyclones are steered by different layers of the atmosphere than weaker cyclones, so if a model is too bullish on the intensity in the analysis or forecast, it will likely get the track wrong.  For the past several days, this track forecast has revolved around whether or not a weakness in the subtropical ridge is strong enough to allow the disturbance to slide northward into it, or if it will not "feel" that weakness and continue a west-northwest heading.

The difference between those scenarios is rather big: it either hits south Florida or recurves over the Bahamas and stays well away from the US... with some 'wiggle room' in between.  I would still estimate a 10-15% chance of the south Florida option happening, which is noteworthy because it would only be 3 days away and the Bahamas are infamous for producing rapid intensifiers.  The wind shear is low, the SST is very warm, the only obvious obstacle now is its proximity to Hispaniola.

The 12Z suite of "late" models, including the 20 GFS ensemble members.

The eastern Bahamas are now under a tropical storm warning, and the official NHC forecast (with cone of uncertainty) is shown below.  Their forecast brings it up to tropical storm intensity tonight, and then hurricane intensity on Tuesday... right in the middle of the pack in the figure above.  If you're in south Florida, this should not be ignored.  There is a slight possibility that soon-to-be Cristobal could make landfall there as a minimal hurricane in just three days.  If you're in coastal SC or NC, you should also be paying very close attention to this in the coming days.

21 August 2014

Tropical disturbance slowly organizing, model tracks shift

Since yesterday's post, the area of interest east of the Lesser Antilles has gotten slightly better organized, and more interestingly, the models have all shifted their forecasts north.  Keep in mind that this is not yet a tropical depression, so there is tremendous uncertainty in any forecast you see (hard to forecast "it", when "it" isn't even a tropical cyclone).  And even with a bona fide tropical cyclone, model forecasts beyond five days contain a lot of uncertainty.

Visible satellite image from 8:15am EDT.  I added a red L to help locate the center of the disorganized disturbance.  (NRLMRY)
The northwestward motion that we've been seeing is due to a weakness in the subtropical ridge... or in other words, rather than being pushed west or west-northwest like storms typically would in this location, it's moving north much quicker than usual.  It is forecast to pass over the northern Leeward Islands early Friday morning, but shouldn't be more than a gusty and wet day there.

Though too complicated to explain in a caption, the steering feature I'm describing can be found in the area north of Puerto Rico and Hispaniola.  The line contours are the current 500mb heights, and the shaded contours are the height anomalies. (tropicaltidbits.com)
As far as the latest model forecasts go, a storm entering the Gulf of Mexico now looks like a long-shot, whereas yesterday it looked relatively likely.  Models  seem to have a better grip on the structure of that subtropical ridge and show a northward turn sometime around Sunday when the "storm" is near Hispaniola.  BUT, any change in the strength of that ridge will make a huge difference in track... if it builds, the storm would start moving more west, and if it weakens further, the storm will nudge further north.

Tracks from the 06Z model runs... the first four are global models and the last four are regional hurricane models. (U.Albany)

Of historical note, today is the 7-year anniversary of Hurricane Dean's landfall on Mexico's Yucatan peninsula.  Dean hit near Chetumal with sustained winds estimated at 170+ mph.  It remains the strongest landfalling hurricane in the Atlantic since Andrew hit south Florida in 1992.

Enhanced infrared satellite image of Hurricane Dean making landfall during the early morning hours of August 21, 2007. (NRLMRY)