Invest 97 likely to develop and be a rainmaker for Florida; ??? after that

July 20, 2010

Yesterday morning, the National Hurricane Center designated an area of disturbed weather east of Puerto Rico as Invest 97 and gave it a 20% chance of becoming a tropical cyclone within 48 hours.  As signs of organization have become apparent, that area immediately north-northeast of the Dominican Republic is currently being given a 60% chance of becoming a tropical cyclone in the next two days.

The 2PM best guess of a notional center was 19.5°N 68.2° W. I use the words ‘notional’ and ‘guess’ because there isn’t a single well defined center of circulation.  In the formative stages, forecasters will look at all of the data on han (visual satellite, satellite scatterometer, surface observations) and try to come up with a position estimate that either fits well with the data or at least  is  not outright conflict with one or more pieces.   Then, they run with that estimate (moving it along in a given direction and speed) until that becomes the well defined center or another feature pops up that is more suitable to be considered as the center.  Such uncertainty on where the center can develop can wreak havoc on the downstream forecast.

Assuming that a tropical cyclone does form in relation to the current best guess (and not further to the south or east) the track forecast ought to be straight forward.  Flow around a high pressure ridge would bring it west-northwest across the Bahamas and into southern Florida by Friday afternoon and to the Gulf coast around Sunday. However, if the storm were to be slower to develop and does so south and/or east of its current presumed center, then it would likely get caught under the ridge expanding and be pushed further to the west; perhaps missing the Florida Peninsula and ending up as far west as Louisiana or upper Texas.  The spread in the dynamic model guidance (initial landfall from West Palm Beach to the Keys; final landfall in the Panhandle to eastern Louisiana) reflects some of that uncertainty.

As far as intensity goes, I don’t see a  surprise category two or major hurricane as a threat for south Florida.  SHIPS  intensity guidance last night brought the storm towards 95 mph at the end of the 5 day forecast period.  However, that assumed a forecast track that took it into high heat content water along the south of Cuba.  Heat content to the north isn’t as plentiful, especially away from the Cuba coastline.  Between that,  a moderating amount of wind shear, and the dry air over the Bahamas, there should be enough inhibiting factors to keep a storm under hurricane strength should it proceed to make landfall between say Miami and West Palm Beach. Current SHIPS guidance, which assumes a track a bit further south than that, brings a storm to minimal hurricane force strength (75 mph) at landfall.  Remember, its guidance assumes the presence of a surface circulation already, so if one presumes the current absence of one, one would have to deduct a bit from its forecast.

After an assumed peninsular landfall and crossover into the northeastern Gulf of Mexico, the balance of the intensity forecast would be dependent on exactly how much land interaction there was and its effect on the circulation. However, I don’t think a ‘short-cut’ path across the the northeast corner of the Gulf would afford much opportunity for further intensification even if land-caused weakening were to be minimal. Because of that I would a second landfall in the panhandle to be at tropical storm strength as well.

Question marks start to pile up, though, if the storm were to go through the Keys and lazily swing to the west-northwest in the Gulf of Mexico.  On the plus side, the shear forecasts from GFS don’t show an especially hospitable enviroment nor does the Gulf heat potential chart show a fat hot spot.  However, more time at sea would give more time for conditions to change  and there is a warm spot in the northwest Gulf. Also, we would be quickly in the beginning of the “tropical storm winds forecast within 120 hours; must pack up shop” window for the Deepwater Horizon response team.

As far as preparation goes, south Florida residents should be ready for rain and windy conditions later this week.  Secure the gear adrift in your yard and if you’re in a low-lying area by mindful of the possibility of flash flooding.  People along the Gulf coast have time to watch and think while waiting.  Would be a good idea to do some of the little things like making sure the gas tank is topped off, etc.

A Hurricane Hunter is scheduled to examine the stormy area tomorrow afternoon (~ 2 PM EDT). I suspect that is when this will get classified as a tropical cyclone (either a depression or storm).  Also, in the evening, a Gulfstream-IV flight is scheduled to sample the upper-air enviroment to collect data for the models that would run late night/overnight Wednesday.  That ought to help clear up uncertainty on how the ridge is positioned and aligned, resulting in a better track forecast Thursday morning.

I intend to have my next post up early tomorrow morning with any short notes before then being posted to Friendfeed.


One Response to “Invest 97 likely to develop and be a rainmaker for Florida; ??? after that”

  1. Beth Unkefer Says:

    I enjoyed your analysis and comments-thanks! I look forward to future reports.
    Beth Unkefer

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