Tag Archives: Atlantic basin

Forecasting, Tropical

Emily Organizes; Gulf Threat Decreases

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Tropical Storm Emily struggled through most of last night and today with disorganization, an aftereffect of its multivortex structure as an unnamed disturbance.  However, it has become better stacked today, with convection blowing up over its center.  It still has a long way to go, despite its more pleasing appearance in satellite photos.

Steering in the short term is straightforward.  Emily has been generally on the left side of the forecast track for most of the day, and it is now expected to make landfall in the Dominican Republic as a tropical storm.  Weaker systems generally weather the mountains better than stronger ones, provided that they do not linger in the area; therefore dissipation of the system seems comparatively unlikely.

 

Models conclude that trough will miss Emily

The models have largely converged on a scenario in which the trough that is to weaken the Bermuda High will be gone before it can force the full recurvature of Emily.  The ridge is expected to build back in, and the GFS shows the hurricane being trapped off the coast of Florida, unable to move ashore because of another ridge, stalling until a shortwave trough lifts it away.  The GFDL and HWRF models, which take their input from the GFS, both show a very close approach to the east coast of Florida, with the HWRF showing near-hurricane-force winds onshore.  The NOGAPS shows this same scenario without the stall.  In this scenario, a landfall on the Outer Banks of North Carolina occurs, followed by a pull up the Atlantic seaboard (offshore) and out to sea.

The Canadian model shows a very weak system, probably no more than a mild tropical storm, making landfall on the east coast of Florida and then being merged into the shortwave.  I should observe that the Canadian model now shows the first low strengthening to 988 mb at sea and reducing the Bermuda High to its winter stage (the Azores High), which does not seem remotely reasonable to me for an August system.  I am not putting a lot of faith in this aspect of the Canadian solution.

The European model also shows a “screwy” solution, amplifying the shortwave trough to 992 mb at sea, while completely dissipating Emily over Hispaniola.  Although unlikely in my opinion, dissipation of the tropical storm is certainly possible, as yesterday’s blog entry said, but I am having great difficulty believing that either of the baroclinic low pressure systems involved in this will reach 990 mb levels.  The first trough is currently located in New England producing a severe weather risk, and it is analyzed at 1002 mb.

Bottom line, I am giving a highly skeptical eye to anything that destroys the Bermuda High at the beginning of August and amplifies low pressure cores to winter levels, especially when they have not been doing this consistently.  Any land-free recurvature of Emily depends entirely on such “bombs,” and the approach to the East Coast will be so close that a weaker trough or shortwave will make all the difference in the world in what wind speeds are felt onshore and whether landfall occurs.

 

Gulf Coast threat decreases… for now

As should be apparent, the threat to the Gulf Coast states has decreased over the course of the day (with a caveat).  The current thinking is that the trough will lift Emily northward enough to miss an entrance into the Gulf of Mexico.  However, this could change if the storm stays south and west enough, or the trough is weaker than expected at sea.

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Ringing In the 2011 Hurricane Season

I think it’s safe to say that I am joined by a substantial part of the Southeast and Midwest in bidding a very loud “GOOD RIDDANCE” to the 2011 spring severe weather season, even as a student meteorologist. This season took a toll on me in a way that I honestly did not expect. It is painful to watch such atmospheric carnage unfold when the career you have embarked upon, whether as a forecaster or a research scientist, is intended to minimize such tragedies. Even when it is fairly universally recognized that the disaster was not in any way the fault of meteorologists, that only leaves a sense of helplessness. So yes, I am quite ready to say “good riddance” to the tornadoes, at least in this particular corner of the world, for a few months. The Southeast, along with much of the rest of the East, is embroiled in a heat wave at the moment, a fairly sure sign that a summer pattern has taken hold. On that cue, enter hurricane season. Hurricanes were my first atmospheric “love” and even the season of 2005 did not change this for me. It is with a sense of excitement that I start opening up my tropical web browser bookmarks regularly again.

I don’t see a lot of point in making a specific numerical forecast for this year’s hurricane season. Suffice it to say that my best guesses that I formed in winter are unchanged, and that I expect an active season with a higher-than-average chance of American strikes, unlike last season. I am not expecting a transition to El Niño, which would tamp down activity in the Atlantic, but I am not entirely sold on the expectation that the ENSO state will remain neutral throughout the season.  I think there are close to even odds that it will begin to return to La Niña conditions again by autumn, albeit milder than those of last winter.  However, either pattern will promote tropical activity.

Did I mention checking tropics-related web bookmarks once more?  Well, it turns out that the Atlantic basin is following the “official” calendar right on schedule, so I have reason to look at the tropics regularly already.  There is a disturbance in the Caribbean Sea that is the first really interesting possibility for tropical development.

Here is an image of how this disturbance looked at 11:00 PM Thursday night:

For reference, here is a true visible satellite image of the same disturbance a few hours earlier, which shows the mid- to low-level circulation of this system better:

There are two “blobs” in the Caribbean, but the first image makes it clear that the one to watch is the one closer to Central America. That is the one that, according to loops of visible satellite images, has visible rotation occurring, and it is analyzed as a low pressure center in official maps:

The area of convection to its east is associated with a tropical wave that is expected to merge with the low, adding energy and moisture to the brew.

This system is interesting for so early in the season because it has some atmospheric variables in its favor despite the calendar. The low has been developing low-level convergence (winds drawing together) and upper-level divergence, both of which are conducive for tropical cyclogenesis, though these areas of convergence and divergence need to become better aligned with each other:

As the images indicate, the area of convergence is the chief culprit in the misalignment. The divergence is occurring above the area of convection, indicating that the low is developing a system for ventilating itself.

An analysis of vorticity shows that the system has positive vorticity at the 850 and 700 mb levels and that the two levels are basically aligned, which in a tropical cyclone (or proto-cyclone) is positive for development:


850 mb vorticity

700 mb vorticity

Shear is all right above the system but not especially favorable in the surrounding environment:

This is no surprise for this early in the season, but the National Hurricane Center expects the environment to become more favorable for this system in the next couple of days. An examination of the GFS model indicates that they are probably correct in this expectation; though shear is expected to be prohibitive of any tropical activity in the Gulf of Mexico, it is supposed to lighten up around the low pressure center.

Incidentally, the GFS doesn’t seem to do much with this cyclone other than letting it churn in place. Don’t expect a hurricane out of this! At best, I’d say it might rate a tropical depression. It is primarily an interesting feature to watch for so early in the season, a harmless storm that we tornado-weary weather folk can observe without anxiety. Tropical cyclogenesis is a fascinating, somewhat mysterious, and awe-inspiring phenomenon, and instances like this that are not the classic “Cape Verde wave in September starts to spin in the middle of the Atlantic” pattern are particularly interesting because the process of genesis for them is not cut-and-dry like the well-known central Atlantic tropical wave process is. This system may very well be a harbinger, but that remains to be seen. For now, it’s a neat feature to watch.

Tropical

Getting Ready: An Early Preview of the 2010 Hurricane Season

From the point of view of a snow lover, it was an excellent winter. But that is now long over for those of us in the Eastern United States. Many areas have already hit 90°F! Here in MS we have not, but I anticipate that some spot in the Gulf Coast states will reach this wretched milestone early in May.

Before I get to the topic about a future event, I feel compelled to talk about one closer to the present. Severe weather season is upon us, though it is off to a slow start. The Southeastern states are arguably past the springtime peak and seem to have gotten off quite light, but we must not forget that it is the months of May and June when so many major tornado disasters have occurred. Jarrell, TX… Moore, OK… the May 2003 outbreak sequence… Greensburg, KS… Parkersburg, IA… those are all F5 or EF5 events except for 2003 (and even it had a tornado that was considered by some to have been underrated as F4). The list goes on, and it does not require an EF5 tornado to do massive, tragic damage. But it is very difficult to forecast severe weather more than a few days in advance, let alone a month or more.

Hurricane season is a different matter, and it is quite possible to make long-range forecasts about the overall activity level of a particular season, especially now that we are merely a month and a half away from the beginning of the Atlantic season. 2010’s hurricane season is not, I believe, going to be anything at all like 2009’s.

The strong El Nino event that gave the Eastern U.S. such a cold and snowy winter (and killed off much Atlantic hurricane activity) is fading fast. The majority of ENSO models predict a return to ENSO-neutral conditions by the June-July-August period (link: PDF).

However, El Nino has left its mark. As is typical following a significant El Nino event, sea surface temperatures in the eastern Atlantic are well above average, and in fact, the anomalies for this year are greater than the anomalies in April of 2005, a year that had record heat across the ocean. In the far eastern part of the ocean, there are areas that are already at 30°C.

Apr. 12, 2010:

Apr. 12, 2005:

Note: All graphics in this post are created by NOAA and are therefore public domain. I have downloaded the graphics current for 04/12/2010 to my server to avoid taking U.S. Government bandwidth. Links to the pages where these graphics were found will not point to the same images at dates in the future.

If this continues and shear decreases as expected, this year may be quite good for long-tracked Cape Verde systems. Indeed, these temperatures are apparently a record in terms of warmth.

The Gulf of Mexico is below average, but this is because of the cold winter. With surface temperatures reaching into the upper 70s and low 80s in the Gulf states for the foreseeable future, and little cloudiness to moderate the effect, it’s likely that this body of water will warm up. Indeed, observing the sea surface temperature anomaly maps for the past few weeks indicates that this warmup is occurring already.

A limiting factor at present may be wind shear, which is above the climatological average:


(Link takes you to the current shear map on NOAA.)

This will continue to be a limiting factor for cyclone development if it persists into the early season. However, as the El Nino fades, shear should decrease. Indeed, the current above-average level of wind shear may only be a temporary event, as overall it has been below average for much of the past several months:


(Link takes you to the current shear graph on NOAA.)

The Bermuda High, an area of high pressure that extends to the western Atlantic, is not yet established. The location of this feature will be important to watch, as it determines whether long-tracked Cape Verde hurricanes tend to strike land—and what landmasses that they strike—or recurve to sea. The farther west it goes, the more likely that such hurricanes will hit a coastline, but too far west and storms tend to be steered south of the United States, as was the case in 2007.

In short: Sea surface temperatures are likely not going to be a problem in 2010. I think the features to watch, here in the pre-season, will be the evolution of ENSO, the location of the Bermuda High, the warmup of the Gulf and far western Atlantic near the Bahamas, and the levels of wind shear as compared to climatology. If the ENSO level decreases to neutral by the peak of the season and shear decreases to the climatological average (and these two factors are very intimately connected, I should note), but sea surface temperatures continue to remain high, I fully expect to see some beasts brewed up and for “Category 5” to make a reappearance in this basin for the first time since 2007.

Unless the ENSO prediction models are mostly wrong, the Atlantic coasts are not going to get off light this year. It’s impossible yet to determine what areas are likely to be targeted, since we do not know how far west that the Bermuda High will set up, but at present I would go out on a limb and say that somebody is in for a bad year. It’s time to start getting ready.

Tropical

The blob race is on

Our three invests have been intensifying all day. All of them have developed banding features and notable rotation. The race is on to see which one is deemed TD 12 first.

96L is a massive system. The cyclonic rotation is very evident on it and I am surprised that it hasn’t been called a TD yet. The size of this storm’s rotation could be an impediment to rapid development, as well as its latitude.

97L has made quite the comeback today from the exposed swirl it was earlier. Models take this to a hurricane, and I wouldn’t be surprised to see it happen.

94L is sheared on the west side, but it too has put on a burst of convection today. It seems headed for Mexico and doesn’t appear likely to have the chance to develop into a powerful system.

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Tropicornucopia

Well, the Atlantic has awakened from dormancy once again. We have four features to track, three of which are a potential threat.

First, Subtropical Storm Jerry got named today. Its satellite photos show a classic subtropical storm. It is forecast to develop tropical features before merging with an extratropical system. I’m not sold on the tropical transition happening, but whether it does or not, it’ll provide extra energy to the system it is merging with and may prove to be a threat to shipping interests if it develops into a nasty high seas cold-core storm. Either way, it’s heading out.

Closer to home is 94L in the Gulf of Mexico, and the computer models are taking this one anywhere and everywhere, although it should be noted that none of them take it farther northeast than Texas. The SHIPS forecasts it to become a hurricane. I would disregard this if not for the inconvenient little fact of Humberto. Shear in the Western Gulf is forecast to be high, though, so SHIPS may indeed be out to lunch. At the moment the system doesn’t look like much; it would have to get organized soon to reach that intensity. On the whole, I’d deem this a rainmaker for Texas–which is unfortunate, as it does not need it.

Next is 96L, in my opinion, the most interesting. This one is looking good, with banding already showing up in the satellites. 96L is a potential Cape Verde system located in the Atlantic, and any interaction with land is many days away… however, SHIPS forecasts it to be a major hurricane by the time it reaches the Caribbean. I see nothing in the short term to prevent this from happening and expect this system to develop. Models are in agreement on a WNW track, with the global models forecasting a more NWly bend at the end, but the GFDL keeping it going WNW. At this point it’s tough to tell whether it will recurve at sea. For some reason, the HWRF and GFDL don’t develop this system beyond a tropical storm, but I think this is unrealistic, and am thus disinclined to put a lot of trust in their current solutions, including track, which at this time of year is very dependent on the strength of the system. The bottom line, keep an eye out. This one could be an East Coaster. Not saying it will, of course, but I want to point out the possibilities.

97L is in the Central Atlantic and is forecast by the models to enter the Caribbean. This one was recently declared and currently doesn’t look so hot, although better than 94L. If it wants to become anything, it needs to get its act together before it hits the infamous “dead zone” in the Caribbean. I’m unable to pull up model runs for this system yet. However, if it does develop, it seems that it is likely to go south of the big shear masses that are forecast to develop over the Caribbean.

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Dean strengthening, TD5 in Gulf

Good morning!

TS Dean has maintained its intensity of 45 knots overnight. Satellites from around 0900Z indicate that Dean is trying to wrap convection around an “eye”-like structure, which may in time develop into a true eye. The system has taken on the classic recognizable “comma” shape of a tropical cyclone, and in the face of low shear, should develop into a symmetrical and photogenic hurricane. Dean is in very low shear now, having escaped the 20kt that limited its intensification previously. This is good for development. However, since it’s happening during the day, Dean’s ability to utilize this may be limited. We’ll have to see.

NHC’s forecast has taken a westward shift, in line with what the computers are indicating. As I said in my previous blogs, I am on board with a Caribbean –> Gulf storm and do not intend to change this prediction unless the ridge is eroded too much for it to happen.

The GFDL and HWRF, our two tropical cyclone-specific models, are both in unfortunate agreement in their 00Z runs of taking Dean to a borderline Cat 3/Cat 4 in the central Caribbean and sending it south of Hispaniola. If this pans out, there is a chance that he will enter the Gulf as a Cat 3 or Cat 4 without ever having hit a large landmass.

I’m not happy about this. In such a scenario, we’d have to hope for shear towards landfall, because the very hottest Gulf waters are right offshore.

Closer to home, TD5 has formed in the Gulf, although according to the NHC it seems to have some problems getting together. Well, it’s a new system. That’s what usually happens. Quite honestly, in my opinion, TD5 could landfall in Texas as anything from a depression to (if conditions are perfect and it takes advantage) a low-end Category 1 hurricane. The NHC is forecasting a 40kt landfall. I think that, if TD5 gets organized in time, it has the potential for rapid development, and this forecast could be too low. With the depression, everything comes down to the timing of its organizing.

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Dean holds steady

Tropical Storm Dean has held its intensity in the face of a small pocket of 20kt shear. Although its convection dropped off significantly during the diurnal minimum, this is cyclical, especially for a weak system. Dean used the day to do something more important than blowing up blobs of clouds, start developing the classic tropical cyclone structure. It has feeder bands forming, and as soon as it gets out of the pocket of shear, I’m expecting it to take off. The timing on this is important: If it breaks into the minimal shear area to its west at night (night for it, a few hours ahead of people in the Americas), the effect will be more pronounced in a short time. This is because of diurnal max. Dean is skipping along a tad south of due west at 20 mph.

I think it’s now fair to start discussing where Dean will end up and how strong he will be. I highly doubt that he will recurve, and I am not sold on a U.S. East Coast landfall either (including Florida’s east side). In the short term, I think it’s fair to say that Dean will be a weather-maker for the islands off Venezuela. I also have a bad feeling that he will get into the Caribbean. The water there is even hotter than in the Gulf. A major hurricane seems highly likely if this happens. Many people in the U.S. see a powerful hurricane hit Hispaniola, and see the mountains tear it up and weaken it (a good example is Georges of 1998), so that it has a hard time restrengthening before its final landfall, and they cheer. But the island nations can’t afford a hit any more than New Orleans can. The 12:00 run of the HWRF showed a Hurricane Allen-like system barreling headlong into the Dominican Republic at peak intensity. Needless to say, this would be horrific. The one good thing about Dean is that, since it’s a long-track Cape Verde storm, there’s plenty of time to watch it, and prepare for landfall.

Beyond the Caribbean, as I have stated, I have a really bad suspicion that this is ultimately a Gulf problem. I’m not going to change this guess until and unless the real, observed weather conditions change as Dean continues on its path and gets closer to the point where it must “decide” whether to curve out of the Caribbean or not. Fluctuations of models, which have had it from Central America to an Atlantic fish storm, don’t qualify. Real, large-scale changes in weather do.

The NHC, which has a tendency to underforecast the intensity of hurricanes, has Dean as a 95kt Category Two system in five days. Again, how quickly Dean gets its act together will determine when (NOT “if”) it becomes a hurricane and thus when it may hit Cat-2 or higher intensity. It has followed the NHC’s predicted intensity forecast thus far, and tropical storms usually take several advisories to get fully “with it” and reach a level of stability that permits them to develop eyes and become hurricanes. I see no reason to deviate significantly from the NHC forecast, with the one caveat that Dean might rapidly intensify after it becomes a hurricane.

Note: If anyone tries to tell you that Dean, or any other new TS, has an eye at any intensity under about 55 kt, they don’t know what they are looking at. Former hurricanes that made landfall, of course, are a different matter, and the eye structure could persist long after it lost hurricane-force winds. But newly-formed storms don’t start to develop eyes until they approach hurricane intensity.

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Flossie and proto-Dean

Against all predictions, Hurricane Flossie in the Central Pacific exploded to a Category 4 and is posing a threat to Hawai’i, although it is expected to weaken and avoid a direct hit on the islands. It seems to the casual observer that the EPac has had all the activity this year. It might prompt the question, does the advent of Flossie signal a change in the ENSO pattern?

NO!

While it is true that, usually, the most intense hurricanes and typhoons in the Pacific basin occur during El Ni

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TD4, 91L, and Flossie

As expected, former invest.90L strengthened in to Tropical Depression 4 this morning. The system is currently embedded in moderate shear. This has caused the depression to be elongated, as the satellite images show. It will keep it from becoming very strong for now. At a brisk pace of 20 mph, it should move out of this shear in a day or two, and we can then expect explosive intensification. I expect this system to become a hurricane by Thursday night. As for where it will go from there — I don’t know, neither do you, and neither do the computers!

A system in the north Caribbean/southern Gulf of Mexico is intensifying its convection. Shear is low in the area. It’s possible this may form into something, but I’m not expecting it.

Hurricane Flossie in the Pacific is starting to appear a little more ragged in its satellite, hopefully signaling the long-expected weakening. However, because it was a Category Four for so long, it has built up a large surge, which will threaten Hawaii in the form of waves as the hurricane approaches the islands.

Tropical

The race to be Chantal

Well, the month of July didn’t pass without a tropical cyclone forming! Former invest 98L was declared Tropical Depression 3 tonight by the NHC. I think they may have missed the boat on this and in reanalysis, the storm could well be analyzed as having formed earlier (as a subtropical depression that transitioned into a full TD today). It may well get the name Chantal before the Central Atlantic disturbance 99L, but if so, it won’t last long as a tropical system. The system is also not forecast to be strong at sea. The only threat is to Europe as an extratropical cyclone.

Closer to home, invest 99L has formed from a central Atlantic tropical wave, and is heading WNW into the Caribbean. It is fairly strong for a tropical wave, and is developing outflow, as one can see from the satellite picture — especially to the north. After having much of its convection sheared off today, it is rebuilding itself. More importantly, the new convection is forming directly over the surface low, as opposed to the earlier convection, which was lopsided and was hindering further organization. This is a MAJOR leg up for 99L. If this continues, I suspect it will be named TD Four by tomorrow afternoon, perhaps even late morning. The real question is whether this system or TD 3 will get the C name.

The DSHIPS intensity model forecasts it to become a hurricane within 3-4 days.

As some may know, I don’t treat any computer model as a deity, because I think that they all miss things. And so it is with this model for this storm. I don’t quite buy this forecast of intensity. I do think the storm will develop, and will probably eventually become our first hurricane of the season, and will unfortunately threaten the Gulf and Caribbean, but not that soon. There is a fair amount of dry air in its path, and shear, while favorable, isn’t ideal — 10-15 knots in some spots.

Anyway, this is definitely a system to keep an eye on if you live anywhere from Central America to Florida. (Wide range, I know, but it’s pretty far out!) Like clockwork, we move from July into August, and it looks like the Atlantic is transitioning as well.