Tropical weather analysis - August 23, 2012

By: KoritheMan , 3:48 AM GMT on August 24, 2012

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Isaac

Tropical Storm Isaac continues moving through the eastern Caribbean. As of the latest NHC intermediate advisory, the following was posted on the storm:

Wind: 45 mph, with higher gusts
Location: 16.7°N 68.7°W
Movement: WNW at 18 mph
Pressure: 1001 mb

Isaac still remains weak and relatively disorganized according to satellite and radar data from San Juan. However, the central pressure has come down a bit, and the winds have responded by coming up a little. There still appears to be some sort of mid-level center orbiting around to the south of the cyclone, but overall, based on convective trends, the northern center appears to be the dominant one. This would also agree well with aircraft reports and extrapolation of earlier microwave data.



Figure 1. Latest infrared satellite image of Tropical Storm Isaac. Image credit: NOAA

Having said that, the overall circulation of the tropical cyclone remains broad, and we saw it intensify about the same time last night, only for it to weaken again. This was also a common motif for Ernesto when it was in this area three weeks ago. Convection has resurged near the center this evening, and I dare say, again, that Isaac may finally be intensifying. However, dry air still appears to be interrupting the development process, and the storm will soon have Hispaniola to contend with.

Other than that, broad anticyclonic flow still appears to be established over Isaac. The SHIPS/GFS still call for weak shear throughout the forecast period, and since Isaac appears to be battling little vertical shear, this feels like a reasonable prognosis. In fact, the GFS maintains a large anticyclone above Isaac up until landfall on the United States Gulf Coast, which is a pattern considered highly conducive to intensification. So far, the biggest impediment to significant strengthening of Isaac has been a combination of dry air and the enormous size of the circulation envelope. Although not explicitly shown here, depending on how Isaac's inner core fares after tangling with Hispaniola and Cuba, rapid intensification is a possibility in the Gulf of Mexico, and Isaac could become a major hurricane at its United States landfall. Interests along the Gulf Coast from New Orleans to Panama City should be prepared for a hurricane landfall. Peninsular Florida may not see a landfall at this point, but because of the location of the storm relative to the coast, high winds, heavy rains, and tornadoes will pose a serious threat even as Isaac barrels west.

Synoptic data indicates that the trough over the eastern United States is relatively weak below 600 mb, but remains strong above that. In addition, water vapor imagery shows that an upper low has formed over northern Florida, which will finally serve as a forcing mechanism to move the trough east. This is already happening, and the western extent of the Atlantic ridge appears to be breaking down a little. As a result, Isaac is expected to turn northwestward soon, and should maintain this motion as it traverses the Gulf of Mexico. Model guidance is in reasonable agreement on this, especially for during the early portion of the forecast period.

The model consensus has continued to shift westward today, especially at longer ranges. This appears to be due to the purported weakness in the subtropical ridge being farther west than originally anticipated. Some of the models have a weakness over the central plains, which is a classical setup for a strike on Louisiana or Mississippi. Others have see the weakness being farther east, which of course favors a strike farther east. Regardless, it appears that Isaac will be a hurricane threat to some portion of the United States Gulf Coast next week, and residents living there should review their hurricane preparedness plans now.

However, it would be most premature not to focus on the immediate threat -- vulnerable Haiti, which is facing a very serious flood situation, especially given the sheer size of Isaac. As much as 10 to 20 inches of rain could fall over the impoverished mountain nation over the next couple of days, with locally higher amounts. Needless to say, this is likely to produce some serious flash flooding and mudslides, and unfortunately, death. In 2004, Hurricane Jeanne killed 3,000 people from floods. I don't anticipate that the death toll will be quite that high with Isaac, but the vast majority of residents are living in tents, especially following the earthquake in 2010, residents there are particularly susceptible to floods. Not to mention nearly all of the country is deforested, which allows water and mud to more easily flow down mountainsides.

Based on model trends, my forecast track has shifted considerably westward since yesterday, and now shows a long-range threat to the central Gulf Coast. However, there is still room for error here. The upcoming 0z models should have data from tonight's synoptic surveillance mission that was conducted by the NOAA G-IV jet. This should give us a much better idea on the track, and any shifts after that will likely only be within a very specific 100 mile threat area. I may update the track subsequent to the arrival of these models.

5-day intensity forecast

INITIAL 08/24 0300Z 40 KT 45 MPH
12 hour 08/24 1200Z 40 KT 50 MPH
24 hour 08/25 0000Z 40 KT 45 MPH
36 hour 08/26 0000Z 60 KT 70 MPH...INLAND OVER EASTERN CUBA
48 hour 08/27 0000Z 55 KT 65 MPH...OFF THE NORTH COAST OF CUBA
72 hour 08/28 0000Z 60 KT 70 MPH...OVER WATER
96 hour 08/29 0000Z 70 KT 80 MPH
120 hour 08/30 0000Z 85 KT 100 MPH

5-day track forecast



Figure 2. My 5-day forecast track for Isaac.

Watches and warnings


A HURRICANE WATCH IS IN EFFECT FOR...
* HAITI

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* DOMINICAN REPUBLIC
* HAITI
* CUBAN PROVINCES OF CAMAGUEY...LAS TUNAS...GRANMA...HOLGUIN...
SANTIAGO DE CUBA...AND GUANTANAMO
* SOUTHEASTERN BAHAMAS INCLUDING THE ACKLINS...CROOKED ISLAND...LONG
CAY...THE INAGUAS...MAYAGUANA...AND THE RAGGED ISLANDS
* TURKS AND CAICOS ISLANDS

A TROPICAL STORM WATCH IS IN EFFECT FOR...
* CUBAN PROVINCES OF CIEGO DE AVILA...SANCTI SPIRITUS...AND VILLA
CLARA
* ANDROS ISLAND AND THE CENTRAL BAHAMAS INCLUDING CAT ISLAND...THE
EXUMAS...LONG ISLAND...RUM CAY...AND SAN SALVADOR

A HURRICANE WATCH MEANS THAT HURRICANE CONDITIONS ARE POSSIBLE
WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 24 TO 36 HOURS.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA WITHIN 36 HOURS.

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

INTERESTS IN JAMAICA...THE REMAINDER OF CUBA...THE REMAINDER OF THE
BAHAMAS...SOUTH FLORIDA...AND THE FLORIDA KEYS SHOULD MONITOR THE
PROGRESS OF ISAAC.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA IN THE UNITED STATES...
INCLUDING POSSIBLE INLAND WATCHES AND WARNINGS...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR LOCAL NATIONAL WEATHER SERVICE FORECAST
OFFICE. FOR STORM INFORMATION SPECIFIC TO YOUR AREA OUTSIDE THE
UNITED STATES...PLEASE MONITOR PRODUCTS ISSUED BY YOUR NATIONAL
METEOROLOGICAL SERVICE.



Joyce

Joyce formed today from what was Tropical Depression Ten. And now, Joyce is already back to a tropical depression as of the latest NHC advisory:

Wind: 35 mph, with higher gusts
Location: 16.3°N 43.7°W
Movement: WNW at 14 mph
Pressure: 1008 mb

Satellite and microwave fixes show a highly disorganized tropical cyclone. There is little convection near the center, which is displaced over 100 miles south of the mid-level center due to strong southerly shear associated with a persistent central Atlantic upper cold low.



Figure 3. Latest infrared satellite image of Tropical Depression Joyce. Image credit: NOAA

The upper low is forecast to loosen its grip in about two days, but there could be little left of Joyce at that time given the current environment and storm structure. Dry air is also forecast to afflict the system over the next few days, although I have a feeling the SHIPS is overdoing it a bit. Should it survive, Joyce has the potential to become a hurricane prior to or during recurvature, although I am not explicitly showing that at this time. My intensity forecast remains the same as that of the National Hurricane Center for now.

Joyce remains situated between a weak low- to mid-level ridge to the east and an upper-level trough to the west. The guidance suggests that this ridge will break down further over the next few days as the upper trough moves eastward. Both in a synoptic and teleconnection fashion, this would make sense, and it appears that Joyce will recurve. Guidance is very well clustered, which leads to greater confidence in the forecast. It should be noted that the four day forecast point brings a moderate tropical storm very near Bermuda, and interests there should monitor the progress of Joyce in case it survives the current hostilities.

5-day intensity forecast

INITIAL 08/24 0300Z 30 KT 35 MPH
12 hour 08/24 1200Z 30 KT 35 MPH
24 hour 08/25 0000Z 30 KT 35 MPH
36 hour 08/25 1200Z 30 KT 35 MPH
48 hour 08/26 0000Z 35 KT 40 MPH
72 hour 08/27 0000Z 40 KT 45 MPH
96 hour 08/28 0000Z 45 KT 50 MPH
120 hour 08/29 0000Z 50 KT 60 MPH

5-day track forecast



Figure 4. My 5-day forecast track for Joyce.



Eastern Atlantic tropical wave

A tropical wave, probably soon to be designated Invest 97L, is over the eastern Atlantic between Africa and the Cape Verde Islands. Shower activity has increased a bit over the last few hours, but remains displaced south of the wave axis. This system appears to be getting at least part of its convection from the Intertropical Convergence Zone (ITCZ), which suggests that it is not a well-organized entity yet. Upper-level winds are somewhat conducive to development of this system over the next couple of days, although there are still some indications that it could run into some southwesterly shear beyond that time, especially if it moves faster or loses the weak anticyclone the GFS suggests will build over it. I expect this wave to move generally westward as low-level ridging builds in behind Joyce.

Probability of development in 48 hours: 20%



Eastern Pacific disturbance

A tropical disturbance has developed in the Eastern Pacific several hundred miles south of the Gulf of Tehuantepec. Shower activity is poorly organized, and some northeasterly shear appears to be affecting the system. However, environmental conditions appear quite conducive for additional development, the current shear notwithstanding. This system should move west-northwestward, and southern Mexico may need to watch it over the next few days.

Probability of development in 48 hours: 20%

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2. LakeWorthFinn
4:58 AM GMT on August 24, 2012
Thanks Kori :)
Where are you located?
Member Since: October 6, 2005 Posts: 67 Comments: 7309
1. TexNowNM
4:16 AM GMT on August 24, 2012
Okay, now I have a better understanding why there is so much uncertainty concerning the ridge situation. Thanks.
Member Since: October 14, 2008 Posts: 0 Comments: 179

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.