Hurricane Hunters find tropical storm winds in TD 9

The Air Force Hurricane Hunters are in Tropical Depression Nine, and have discovered a region of 40 - 45 mph winds at the surface, using their SFMR instrument. Flight level winds at their 1000 foot altitude spiked as high as 49 mph. The surface pressure was 1005 mb, a typical one for a weak tropical storm. Based on these measurements, it is highly likely that NHC will name this Tropical Storm Isaac at 5 pm EDT. A large area of dry air lies just to the north of the storm, as seen on water vapor satellite loops, and this dry air is interfering with development, allowing only a few clumps of heavy thunderstorms to fire up near TD 9's center, as seen on visible satellite loops. These loops also show some arc-shaped low clouds expanding away from the heavy thunderstorm area on the south side of the center, showing that TD 9 is ingesting dry air that is creating strong thunderstorm downdrafts, robbing the storm of moisture and energy. An analysis of upper level winds from the University of Wisconsin CIMSS shows that an upper level outflow channel has opened to the southwest, and another channel is attempting to open up to the north. A large clump of heavy thunderstorms several hundred miles to the southeast of TD 9 continues to compete for moisture, and is interfering with the low level inflow and upper level outflow of the storm. The center of TD 9 will pass about 50 miles to the north of buoy 41101 near midnight tonight. The next hurricane hunter mission into TD 9 is scheduled for 2 am Wednesday, and there will be a new mission launched every six hours. The NOAA jet is first scheduled to fly into the storm on Thursday afternoon, to do a large-scale dropsonde mission to aid model forecasts.


Figure 1. Satellite image of TD 9 and 96L taken at 10:45 am EDT August 21, 2012. The two storms are connected by a thin line of low clouds. Image credit: NOAA Environmental Visualization Lab.

Intensity forecast for TD 9
The latest 2 pm EDT run of the SHIPS model predicts that wind shear will be low to moderate, 5 - 15 knots, for the next five days. Ocean temperatures will warm from 28°C today to 29°C by Wednesday afternoon, and the total heat content of the ocean will increase sharply during that period. The low wind shear and warm waters will favor strengthening. The main impediment to development through Wednesday will be dry air to the north, though the storm will also have trouble separating from the clump of thunderstorms to its southeast. I expect that TD 9 will continue to struggle with dry air through Wednesday. The official NHC forecast is of a 60 mph tropical storm moving through the Lesser Antilles on Wednesday evening; I put the most likely range of strengths for TD 9 Wednesday evening at 50 - 60 mph. Once TD 9 enters the Eastern Caribbean, wind shear will be low, oceanic heat content high, and the storm should have had enough time to moisten the atmosphere to allow steady strengthening to a hurricane. Intensity forecasts 4 - 5 days out are low in skill, though, and it would not be a surprise at all to see TD 9 struggle like so many other storms have over the past two years, and remain a tropical storm over the next five days. Conversely, rapid intensification into a Category 2 hurricane five days from now, as the official NHC is suggesting may happen, would also not be a surprise.


Figure 2. Daily Oceanic Heat Content or Tropical Cyclone Heat Potential (TCHP) for TD 9. The forecast points are from the 11 am EDT NHC advisory, and the 24 hour forecast point shown here is for 8 pm EDT Wednesday. For tropical cyclones in favorable environmental conditions for intensification (i.e., vertical wind shear less than 15 kt, mid-level relative humidity >50 %, and warm SSTs [i.e., >28.5C]) and with intensities less than 80kt, values of ocean heat content greater than 50 kJ/cm^2 (yellow and warmer colors) have been shown to promote greater rates of intensity change. TD 9 will be crossing into such a region early Wednesday morning, and will enter a region of very high TCHP south of Hispaniola on Friday morning (the 72 hour forecast point.) Image credit: NOAA/RAMMB.

Latest model runs for TD 9
The latest set of 12Z (8 am EDT) model runs for TD 9 are fairly unified for the coming three days, showing a west to west-northwest track to a point just south of Hispaniola. Most of the models then predict a more west-northwest track across Southwest Haiti and into eastern Cuba, as TD 9 responds to a small trough of low pressure over the Southeast U.S. A notable exception is our best-performing model, the ECMWF, which keeps TD 9 south of Hispaniola, and takes the storm more to the west over Jamaica by Saturday, and then into the Yucatan Channel between Mexico and Cuba by next Tuesday. We'll have to wait another day to see where the center of TD 9 consolidates before judging which model solution is likely to be correct; reformations of the center closer to bursts of heavy thunderstorm often cause the center point to shift around in the early stages of development, leading to large changes in the forecast track many days later. TD 9 is a threat to affect Tampa during the Republican National Convention, August 27 - 30. I blogged about the climatological chances of a hurricane causing an evacuation of Tampa during the convention in a post last week, putting the odds at 0.2%. The odds in the current situation are higher, probably near 2%. It would take a "perfect storm" sort of conditions to all fall in place to bring TD 9 to the doorstep of Tampa as a hurricane during the convention, but that is one of the possibilities the models have been suggesting could happen.

Disturbance 96L off the coast of Africa
A large tropical wave in the eastern Atlantic about 550 miles southwest of the Cape Verde Islands (Invest 96L) is headed west at 15 mph. This disturbance has an impressive amount of spin, as seen on visible satellite loops, and a respectable amount of heavy thunderstorm activity that is increasing. The storm is under a light 5 - 10 knots of wind shear. Given that TD 9 has moistened up the atmosphere ahead of 96L, this disturbance should have less of a problem with dry air. In their 2 pm EDT Tropical Weather Outlook, NHC gave 96L a 60% chance of developing into a tropical cyclone by Thursday afternoon. This disturbance will track nearly due west the next three days, then is expected to turn more to the west-northwest late this week, bringing it close to the northern Lesser Antilles Islands by Sunday.

Disturbance 95L in the Gulf of Mexico near the Texas/Mexico border
A region of disturbed weather (Invest 95L) is in the Gulf of Mexico, just northeast of Tampico, Mexico. In their 2 pm EDT Tropical Weather Outlook, NHC downgraded the chances of 95L developing to 20%. The Air Force hurricane hunter aircraft scheduled to investigate 95L this afternoon was cancelled. Radar out of Altamira, Mexico does not show any organization to the precipitation echoes and visible satellite loops show that 95L is small and disorganized. The computer models show that 95L should drift westwards, and may move over Mexico on Wednesday.

I'll have a new post in the morning.

Jeff Masters