July 22, 2011

A Tropical System For the Gulf To Watch

Filed under: Forecasting,Tropical — Erin @ 10:27 pm

A tropical wave, designated 90L by the National Hurricane Center, is worthy of being watched by the Gulf Coast states. This system is arguably the first tropical system of real interest to the Gulf states in the U.S., as Tropical Storm Arlene was regarded as a Mexican storm (correctly so) almost from its inception, and Tropical Storms Bret and Cindy were never a threat to any land areas.  However, 90L is in a situation that will strongly favor its reaching the Gulf of Mexico, where conditions are favorable for development.

The system has been steadily increasing its convection over the course of the day, and with this increase has come an improvement in its cyclonic structure.  Cyclonic curvature is evident on satellite (Fig. 1), and upper-level divergence (Fig. 2) indicates good ventilation for the system.  Lower-level convergence (not shown) is not so impressive, indicating that the system needs to form a strong low-level circulation to be considered a tropical cyclone.  This is usually the last step that developing tropical cyclones take.

90L is in a simple steering regime, being located south of the Bermuda High.  In about 3 days, a trough associated with a cyclone is expected to be located off the East Coast of the U.S., eroding the high somewhat.  It was previously assumed that this temporary weakening of the ridge would result in 90L being drawn north for a recurvature.  However, recently, it has become likely that the trough will be weaker than previously believed.  90L is also expected to take longer to develop owing to shear and likely land interaction.  The net result will be a stronger ridge and a weaker tropical system, and the consensus is that 90L will be forced into the Gulf of Mexico (Fig. 3).

90L will have to pass through an area of 20-knot wind shear (Fig. 3, Fig. 4), which is moderate, but will inhibit strengthening for as long as the system is located under that wind regime.  The GFS model does not indicate a sharp spike in wind shear over the course of 90L’s trek toward the Gulf of Mexico.

Unless the expected path drastically changes, 90L should enter the Gulf in about four or five days.  Models are unreliable for storms like this in the long range, and it should be noted that some of the models, like the GFS, are not particularly impressed with this system in the first place.  However, the cyclone-specific model HWRF does develop 90L into a 60 mph tropical storm, keeping it south of Cuba by the end of its run (126 hours out).  For my part, I am disinclined to accept a forecast of zero land interaction at this point.  However, the salient point is that any interaction with Cuba or Hispaniola will have a profoundly negative effect on 90L’s short-term intensity even if it becomes a tropical storm before reaching those areas, and avoiding those landmasses will result in a stronger cyclone that has not been delayed by a reorganization after being disrupted.

My gut forecast for a week or more out (in other words, break out the salt!) is that this system will become a tropical cyclone of moderate intensity (I’ll say Category 1, max, because of mild levels of shear in the Gulf even though the temperatures are well over 90 degrees in many areas) and that it will make landfall somewhere west of Pensacola.  I will have updates about this system if it continues to be a concern.


Figure 1: Shortwave infrared satellite of 90L, late Friday night


Figure 2: Upper-level divergence for 90L, late Friday night


Figure 3: Google Earth overlay of model tracks and shear for 90L, late Friday night


Figure 4: Wind shear tendency, late Friday night

June 2, 2011

Ringing In the 2011 Hurricane Season

Filed under: Tropical — Erin @ 11:36 pm

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.

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