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 morning 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 20-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.