Stu Ostro's Meteorology Blog

Trosum (tropical summary) -- Friday, August 31, 2012

By: stuostro, 9:38 PM GMT on August 31, 2012

Isaac poking northward into the strong, hot, dry ridge of high pressure which forced it west along the Gulf Coast and slowed its movement.

Image credit: UCAR


- Isaac's winds have continued to decrease and its rain pattern has become much more sporadic as its core moves away from the Gulf Coast. Nevertheless, it has been producing narrow bands of tropical downpours "training" over the same locations, such as in Pine Bluff, Arkansas, where significant flash flooding has occurred.

- While there will still be heavy showers scattered here and there to the south this weekend, as Isaac moves north and turns east rainfall will become more concentrated tonight and Saturday in the Mid-Mississippi Valley (Missouri, Illinois, southeast Iowa). Then during the remainder of the holiday weekend, Isaac's remnants will move across the Ohio Valley to the Appalachians.

- Isaac's ability to improve the overall drought situation will be quite limited; the exceptionally dry soil will be a limiting factor for flooding, however if too much falls too quickly that'd result in localized flash flooding.

- Although Isaac's wind energy is much lower than it was previously, it will continue to be sufficient for there to be a risk of very localized damaging wind gusts and short-lived tornadoes with the strongest showers and thunderstorms.


- Unlike Isaac, Tropical Storm Leslie will miss the Caribbean islands to the north. Steering currents will then become weak, though, and the storm won't move much next week.

- Thereafter, models are portraying the most likely scenario as one in which the core of Leslie stays east of New England; that's a long-range forecast and we'll need to keep an eye on future model trends. The ultimate track will depend on the strength/orientation of a dip in the jet stream predicted to develop over the United States and of a ridge of high pressure over the Atlantic north of the storm which will be slowing Leslie's movement.

- In any event, the track won't be too far from Bermuda, and the Canadian Maritimes stick out farther to the east than the U.S. East coast, so Leslie wouldn't have to be shoved as far west to put that region in play.

- With Leslie having a fairly large circulation which will churn up the Atlantic for days, ocean waves will reach the U.S. East Coast next week and cause an elevated risk of rip currents.


- By contrast, Kirk is tiny and headed even farther away from North America and Bermuda, but what's left of the hurricane will race to northwest Europe on Monday and could contribute some moisture and energy to the non-tropical system that will be absorbing it.


- Ileana, like many eastern Pacific hurricanes and tropical storms, will be moving over colder water, encountering more stable air, and weakening into a remnant low pressure system.

- The "monsoon trough" of low pressure offshore of Central America is active with a lot of thunderstorms, some of which are trying to consolidate and could become the next eastern Pacific tropical cyclone during the next few days.


- For the first time in a while, there are no tropical cyclones in the western Pacific.

Updated: 9:42 PM GMT on August 31, 2012


Trosum (tropical summary) -- Wednesday, August 29, 2012

By: stuostro, 4:20 PM GMT on August 29, 2012

Isaac relentlessly pounding the Gulf Coast on the anniversary of Hurricane Katrina

Image credit: Gibson Ridge


- Isaac continues to have its key characteristic of being very big in size, which is causing impacts to be felt across a much larger area, and be more significant, than would be the case with a small hurricane of the same category. The expanse of Isaac, and its slow movement, are resulting in places experiencing strong winds and bands of heavy rain hour after hour after hour, and those winds are pushing water onshore by way of the storm surge. Fortunately those water levels are overall much lower than they were exactly 7 years ago today, when Katrina made landfall, but there are communities in southeast Louisiana in which levees have been overtopped and water rescues are taking place.

- The relentless winds will cause power outages to continue to accumulate and will create adverse conditions for crews trying to quickly restore power. Expect widespread outages for days; more people will be directly affected by wind than any other impact. Rising water (from surge or rainfall) is dangerous, but so are falling trees and power lines -- be careful!

- Rainfall totals are adding up and will continue to do so, and the risk of flooding will increase. A threat of tornadoes will also exist.

- Very hot air for so late in the season in the northern and central Plains is associated with a strong ridge of high pressure which is blocking the hurricane from quickly exiting. Isaac will slooowly swirl northwestward through tonight and Thursday; then tomorrow night, Friday, and this weekend, what will then be Tropical Depression Isaac and finally just a remnant low will progress to the north and east across the mid-Mississippi Valley to the Ohio Valley, in a much weaker state but still with locally heavy rain and gusty winds.


- Yet another named storm, Kirk, the 11th of the Atlantic season, has formed but is on a track to stay out at sea.

- There's another one in the pipeline after that in the eastern Atlantic. Latest model trends have it also staying out at sea.


- Tropical Storm Tembin is headed for South Korea tomorrow.


- Tropical Storm Ileana is headed out to sea.


Good thing it's less unstable than average

By: stuostro, 2:57 AM GMT on August 29, 2012

Isaac has had two characteristics that have been a constant throughout its life: its struggles to strengthen, and its big size. The latter unfortunately more than compensates for the former. Isaac is a large and dangerous tropical cyclone and represents about as serious a threat as one will ever see from a low-end Category 1 hurricane in this location.

But what about the former? Here are a current map and time series graphs of vertical instability in the atmosphere relative to average, showing it being much less than average over the Gulf and Caribbean. I'm not sure to what extent this is dry air vs. something else going on in the atmosphere, but it is consistent with the difficulty Isaac has had in having and maintaining deep, symmetric convection.

Images credit: Regional and Mesoscale Meteorology Branch (RAMMB)

If instability had been above average, Isaac may very well have become a higher, perhaps a much higher category of hurricane. Which is scary, given its size and how dangerous it is as it is. The wind will be relentless, and result in widespread power outages continuing to accumulate and create adverse conditions for crews trying to restore power quickly. A long-duration strong onshore flow onto this very surge-prone coastline will cause the storm surge to be much higher compared to that from a small Category 1. The size and slowing movement of Isaac will produce excessive rainfall amounts, and flooding of that nature in addition to storm surge flooding.

And, having said that about Isaac's struggles, it is, alas, at the last minute the healthiest it has been. Look at the difference in symmetry between 24 hours ago last night (top image below) and tonight (top image), and the central pressure is down to 968 millibars... It's going to be a long night and a long couple of days ...

Images credit: NOAA/NESDIS

Updated: 3:00 AM GMT on August 29, 2012


Trosum -- Monday night August 27, 2012

By: stuostro, 3:03 AM GMT on August 28, 2012

Didn't have time today to do a more in-depth analysis of Isaac; will try tomorrow. For now, here is my overall take on it as well as a summary of other tropical goings-on.


- Isaac has had two important characteristics throughout its entire life: its big size, and its struggles to strengthen. Both are still present. That struggle can still be seen on satellite imagery.

- That will provide a limit to how much the storm can intensify compared to if it were in a more conducive atmosphere, but regardless, Isaac continues to pose a very serious threat even though it's still a tropical storm and not yet a hurricane.

- The wind speed is still likely to increase, and that size will cause impacts to affect a much larger area and last much longer than with a tiny storm.

- Isaac's expanse caused bands of thunderstorms with extremely heavy rain all the way over on the east coast of the Florida peninsula Monday even though the center of circulation was pulling even farther away over the Gulf. A calendar-day rainfall record for August was set at Vero Beach and a 2-day record for August was set at West Palm Beach.

- On the central part of the northern Gulf Coast, initial direct effects from outer fringes will arrive onshore early Tuesday and conditions will go downhill from there; Isaac will still be producing significant impacts through Wednesday; and on Thursday the effects will be progressing farther inland.

- This won't be a case like with some landfalling tropical cyclones in which the main event is just a couple hours around the time of landfall and then the show's over, like was the case with Beryl in the Jacksonville area in May. The relentless winds will result in power outages continuing to accumulate and create adverse conditions for crews trying to quickly restore power. A strong onshore flow onto this very surge-prone coastline will persist in some locations through more than one high tide cycle, something more typical of nor'easters. The size and slowing movement of Isaac will produce excessive rainfall amounts, and flooding of that nature in addition to storm surge flooding.


- The next system, Invest 97L, is showing signs of organization but is on a track to stay out at sea.

- There's another one in the pipeline after that which just came off Africa. Models differ on whether it'll take a track similar to Isaac into the Caribbean, or miss the Leewards to the north.


- Typhoon Bolaven fortunately has weakened quite a bit from when it was at peak intensity, but is still large in size as its center heads toward North Korea while its circulation also impacts South Korea.

- Typhoon Tembin has done a loop and is affecting Taiwan again, which it already hit.


- An area of concentrated convection well offshore of Mexico became organized and persistent enough to be classified as Tropical Depression Nine-E and has since been upgraded to Tropical Storm Ileana. It is headed out to sea.


Size matters

By: stuostro, 1:24 AM GMT on August 27, 2012

For my top "Weather images of 2008" blog, I chose the bottom half of the set below as #1 for the year. I'll never forget the feeling I had when looking at that GFS model forecast for the wind field of Ike.

Looking at the current GFS model forecast for the wind field of Isaac (for Tuesday morning, top image) over the Gulf brought a feeling of deja vu. [Yes, I know, the ECMWF forecast track is different, but it also predicts a large wind field for Isaac, and with the GFS I can do an apples-to-apples graphical comparison.]

It remains to be seen how strong Isaac's maximum sustained winds get, and the diameter of tropical storm force winds (yellow line) is not forecast by the model to be quite as extreme as Ike's, but it's big, and Isaac has always been large even when disorganized, and its wind field is going to be quite large over the Gulf and when it makes landfall. (The white line is the diameter of sustained winds of ~25 mph.)

Isaac is in the same Big Tropical Cyclone species as was Ike, in contrast to, say, 2007's Humberto.

The size matters for wave heights, surge height, geographical expanse of coastal flooding and wind impact, duration of the effects in any given place, and depending on the situation it can matter for rainfall too.

Image credit:

Updated: 1:26 AM GMT on August 27, 2012


Trosum -- Sunday August 26, 2012

By: stuostro, 4:41 PM GMT on August 26, 2012

Hi folks,

As time permits, I'll post separate, geekier entries like the one I posted last night, while also posting these overall trosums (tropical summaries) that I do for The Weather Channel...

Image credit: NASA Earth Science Office


- Isaac is hitting the Keys and South Florida with bands of heavy rain and very gusty winds. As the day progresses, those squall bands will swing around to include locations farther north in the peninsula, and include the potential for tornadoes to form. Tonight into tomorrow as the center moves by into the Gulf and the wind direction swings around, the peak storm surge will occur in southwest Florida.

- Although still fighting things outside and inside itself such as dry air and wind shear, it appears as if Isaac is beginning a process of getting better organized than it has been throughout its life so far, and is poised to strengthen over the Gulf of Mexico and then head toward the northern Gulf Coast. We expect Isaac to become a hurricane in the process, exactly which category t.b.d.

- Size matters with tropical cyclones, and this one has had BIG in its "DNA" ever since it was a tropical wave in the Atlantic, and it's still large in size, and that's going to matter as it moves across the Gulf and onto the coast and inland. Wind, rain and storm surge impacts will affect a much larger area than with a tiny cyclone, and surge/waves will be higher. Dangerous rip currents will be generated all the way to South Texas.

- Thus, this is a case in which even if it's "only" a Category 1 or 2 hurricane, it would still pose a serious threat.

- Initial direct effects from outer fringes could arrive onshore on parts of the northern Gulf Coast as early as late tomorrow and tomorrow night, and conditions will deteriorate on Tuesday.

- There has been a general trend for most models to focus more on the central than eastern Gulf Coast for a landfall location. That raises the probability of a Louisiana landfall, yet still with uncertainty in details and with the large expanse of effects, residents and visitors east across Mississippi, Alabama, and the Florida Panhandle and along Apalachee Bay, and west to the upper Texas coast, also need to be very aware of the latest information on Isaac.


- The next system, Invest 97L, is showing signs of organization but is on a track to stay out at sea.

- Models show the one after that, a tropical wave now coming off the African coast, as having the potential to develop and track across the Atlantic to a position not far from the Leeward Islands by Labor Day weekend.


- The center of Typhoon Bolaven has tracked right over Okinawa Island, and is now on a path to North Korea, with its large circulation also affecting South Korea.

- Typhoon Tembin has stalled and is doing a loop, to be headed back to Taiwan, which it already hit.


- An area of concentrated convection well offshore of Mexico is being monitored for signs of further organization.


Isaac a fascinating -- and dangerous -- storm

By: stuostro, 12:11 AM GMT on August 26, 2012

I don't think I'm exaggerating when I say that this is one of the more interesting tropical cyclones I've observed since starting to forecast them professionally more than 30 years ago.

It's been a big complicated mess meteorologically right from the start. During the morning weather briefing I lead at TWC, a few days ago I remarked that it seemed more like a monsoon depression than a tropical storm, i.e. one of those big gyres that occur in the western Pacific and Indian Ocean in which the sphere of influence is ginormous and the center is real broad rather than tight like in a tropical cyclone. (And at the time, aircraft recon was having trouble finding tropical storm force winds, thus my reference to a depression.)

At times throughout its life, Isaac has been fighting dry air and unfavorable upper-level winds, but at other times, especially when the outflow improved, it was hard to digest exactly why it was struggling to intensify, other than that huge systems tend to not spin up as quickly as tiny ones.

Finally Friday night, it started getting a tight core of convection, but it was then disrupted by land. Since then it has maintained a tighter center of circulation than a few days ago, but a new complication arose: a system to its west already producing locally heavy rain over South Florida and the Keys, and connected to Isaac (as well as to the feature producing heavy rain from North Carolina up across the Mid-Atlantic).

Image credit: UW-Madison SSEC

And, as the National Hurricane Center noted in their discussion, it is connected to another area of vorticity to the southwest of it.

Image credit: UW-Madison SSEC/CIMSS

Despite all of these complications, it has nevertheless managed to produce significant impacts, from flash flooding in Trinidad and Tobago and in Puerto Rico, and from flooding and wind in Haiti where fatalities have unfortunately been reported.

And now, by way of Cuba and the Bahamas, its large circulation takes aim on the United States, first in the Keys and the Florida peninsula and then the east-central part of the northern Gulf Coast. (Another interesting aspect has been the model track forecasts, and in particular the way the ECMWF's and GFS's have evolved, but that's a whole other topic!) Hopefully Isaac will remain a disorganized mess and relatively weak through its trek across the Gulf, but indications are that it'll finally be able to become stronger, and between that and its size, Isaac poses a serious threat.

Updated: 12:13 AM GMT on August 26, 2012


Tropical Summary for Friday, August, 24

By: stuostro, 10:45 PM GMT on August 24, 2012

Hi everyone! As a weather geek since infancy (literally -- I had a childhood phobia of thunder and lightning which created a fascination and obsession with weather!), I'm honored to have been invited to become a blogger for Weather Underground!

I will be posting a variety of types of entries, generally on the short side unless time permits for longer ones. I will also be posting tropical cyclone summaries like this one...

Image credit: NERC Satellite Receiving Station, Dundee University, Scotland


- On the one hand, Isaac is still struggling to have persistent, deep, symmetric convection (thunderstorms). On the other hand, today it has become better organized: a more well-defined core has tightened up, aircraft reconnaissance has found flight-level winds that were stronger than had been present previously, and the central pressure (with which wind speed has a relationship) has dropped.

- This is occurring as Isaac approaches Hispaniola, and also an increasing amount of heavy rain is wrapping around the center to the north and northeast, and heading toward the Dominican Republic and Haiti with life-threatening urgency to the expectation of flash flooding and potential for mudslides/landslides. The storm has caused flash flooding, mudslides, downed trees and power outages in Puerto Rico yesterday and today.

- In addition to less ambiguity of where the true center of the storm is, i.e. it's not as broad and discombobulated anymore, Isaac also is making a decisive move more toward the northwest rather than west as it had been going. That, along with a steering feature evident on satellite imagery -- a trough of low pressure dipping south over the eastern Gulf of Mexico -- suggests that Isaac will move readily across Cuba to the Florida Straits and, with its large size and effects extending well in advance of the center, bring them into South Florida and the Keys beginning as early as late tomorrow night and Sunday morning.

- After Isaac enters the Gulf, the model ("European") which is normally thought to be the most accurate but had an epic FAIL with Debby has been erratic with its Isaac track forecast, shifting between a central and eastern Gulf Coast destination, whereas our other main model has been steadier and on the east side, into the Florida Panhandle and quickly. Thus there are still model differences, but regardless, the bottom line is that a tropical storm or hurricane is expected to hit the eastern or central Gulf Coast during the early-middle part of next week, and residents and visitors should stay abreast of the latest forecasts and information.

- Effects from Isaac in the U.S. will include wind, storm surge, high surf, and rainfall, and will be experienced for a longer time by people in the path of Isaac than would be the case with a storm which is tiny in size and dissipates quickly upon making landfall, such as was the case with Beryl when it hit Florida in May. Details of impacts in any given location will depend on the exact track, intensity, and structure of the storm at that time. Effects won’t be confined to the coast, and Isaac will also be capable of spawning tornadoes.


- The system which became Joyce did so briefly but just enough to tie the record for the second earliest date on which the 10th tropical storm of the season formed. It quickly met its demise and is now just a remnant low.


- Weather Underground reports that Typhoon Tembin brought 24 inches of rain in 24 hours in Hengchun, in the southern tip of Taiwan, breaking a 100-year record. Tembin is moving slowly and still rotating bands of heavy rain into southern Taiwan.

- Typhoon Bolaven is very intense -- the equivalent of a Category 4 hurricane -- and has a very large expanse of tropical storm force winds as it heads toward Okinawa this weekend.


- No tropical cyclones present or imminent, though an area of thunderstorms in the "monsoon trough" of low pressure offshore of Guatemala is being monitored.

Updated: 11:04 PM GMT on August 24, 2012


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Ad Blocker Enabled

Stu Ostro's Meteorology Blog

About stuostro

Proud to be a weather-obsessed weather geek! \m/ Senior meteorologist at The Weather Channel. If not a meteorologist, would be a DJ ♫

Local Weather

68 °F

Recommended Links