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.

April 12, 2010

Getting Ready: An Early Preview of the 2010 Hurricane Season

Filed under: Tropical — Erin @ 10:28 am

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.

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