Tropical weather analysis - June 21, 2012
Chris briefly became a hurricane earlier today, the first of the 2012 season. I wasn't quite expecting this, as I had the inkling that the cool water the storm is sitting underneath would prevent full mixing of the winds from aloft to the surface. Perhaps the eye persisted long enough so that the winds in the eyewall were ultimately able to support hurricane force winds. It would have been most interesting to have a reconnaissance aircraft and a couple of dropsondes in the storm during the time of peak intensity this morning. As of the most recent NHC advisory on the storm, the following was posted:
Wind: 70 mph, with higher gusts
Location: 42.4°N 42.9°W
Movement: NNE at 14 mph
Pressure: 990 mb
Category: Tropical storm (Saffir-Simpson Hurricane Scale)
I should note that three named storms and a hurricane before July, while highly unusual, is not necessarily indicative of an active season. History has shown that early season storms forming in the MDR typically portends an active season, but all of our storms have formed outside this region, in a small hole of lighter wind shear outside the westerlies. Indeed, even the potential Gulf tropical storm will form outside this region, in a climatologically favored area. If El Nino develops as predicted, even a weak one should be enough to stagnate (relatively speaking) activity a little later in the season.
And for those of you doubting that there are any actual signs of an El Nino, I present you with this fanciful graph from NOAA:
Figure 1. Latest sea surface temperature (SST) anomalies for the Northern Hemisphere. Notice the large swath of warm waters extending along the equatorial Pacific, the classic signature of an El Nino. Image credit: NOAA
I'm not really expecting a strong El Nino, and if by some chance we see most of the warming in the central Pacific, there would theoretically be less wind shear over the Atlantic. This was the case in 2002 and 2004, where the former saw Lili hit the central Louisiana coast, and the latter saw four major hurricanes impact the Florida coast.
Anyway, satellite images show that Chris retains a well-defined and symmetrical wind field, but the cold waters are clearly taking their toll. There is virtually no deep convection anywhere within the circulation, and the cyclone appears to be becoming entangled in the developing extratropical low to its south.
Figure 2. Latest infrared satellite image of Tropical Storm Chris. Image credit: NOAA
There have been no recent AMSU microwave overpasses to assess the vertical temperature profile in the storm, but I imagine that Chris is quickly losing its warm core. Personally, I feel Chris will be declared extratropical in the next advisory. Global models continue to indicate that Chris will become absorbed into the baroclinic system in about 24-36 hours. In the meantime, the cyclone, or its extratropical remnants, are forecast to execute a broad cyclonic loop around this low.
5-day intensity forecast
INITIAL 0000Z 06/22 60 KT 70 MPH
12 hour 1200Z 06/22 55 KT 65 MPH...EXTRATROPICAL
24 hour 0000Z 06/23 50 KT 60 MPH...EXTRATROPICAL
36 hour 1200Z 06/23...ABSORBED BY LARGER EXTRATROPICAL SYSTEM
5-day track forecast
Figure 3. My 5-day forecast track for Chris.
A large area of disturbed weather over the southern Gulf of Mexico north of the Yucatan Peninsula has the potential to become a tropical depression or tropical storm over the next day or two as it moves toward the central Gulf of Mexico. I mentioned yesterday that the low might try and relocate along the Yucatan Peninsula. I just wasn't buying a convectiveless swirl being the dominant system. Interestingly, the models have generally come west today, although the GFS still shows a threat to the west coast of Florida.
Satellite images show a large but slowly organizing disturbance. Indeed, the system is beginning to develop an outflow channel to the north and west, which probably heralds the onset of more favorable upper-level winds. An upper low over Brownsville is moving westward, which should lessen the shear over the Gulf. This should allow the system to continue to organize, and I anticipate that this system will be a tropical depression by tomorrow evening. The global models generally support this.
Figure 4. Latest infrared satellite image of Invest 96L. Image credit: NOAA
One thing to note about this disturbance is that it is large. While such systems usually take awhile to spin up, they are also more resilient to vertical shear and subtle changes in the surrounding environment. A belt of westerly shear is forecast to be over the Gulf of Mexico north of about 25N, but it is unclear how much of this is related to 96L, or to the subtropical jet. A thorough examination of the GFS upper-level forecast fields indicates that a weak cyclonic shear axis could remain over the northern Gulf of Mexico for the next few days. This feature was evident in the model at 300 mb, but not further down the troposphere. This suggests that it is not storm relative. However, vertical shear conditions can change rather quickly, and with the system as large as it is, the moisture associated with it is causing a lot of latent heat energy to be released. As such, it remains possible that, when and if 96L builds an anticyclone (which the GFS does forecast to happen, at least for the first 48 hours), it could quickly change the surrounding environment and push the shear further north. Nevertheless, I doubt the system retains an anticyclone all the way to the coast, and there is likely to be some westerly shear over the northern and central Gulf Coasts, which would probably weaken the system in the event it makes landfall that far north. The shear is forecast to be weaker farther west, closer to the western Gulf Coast, which would potentially allow for a stronger system to impact that area.
If the system moves slower than anticipated and meanders before it actually gets moving, a hurricane cannot be ruled out. But I would rather remain conservative considering the time of year, the large size of the disturbance, and the uncertainty in the intricacies of the upper-level shear.
The track forecast for this system is much more difficult. One positive sign is that we finally have a well-defined center to track, which theoretically will make things easier. However, I do not like the initialization in most of the models. The closest to reality appears to be the ECMWF, but even that model is a bit too slow. The CMC also had a fairly good initialization, but flunked shortly afterward. I think the GFS threat to Florida is based on a faulty premise -- examination of the forecast fields indicate that the low was initialized about a degree or so too far east. As I said, it is not climatologically favored to get a sharp recuravture toward the western Florida coast this early in the year, and I think areas farther west, from northeast Mexico to the Florida panhandle, are at greater risk. The westward shift in the guidance gives me more confidence in this. However, it is simply impossible to delineate a specific threat area this far in advance. All we can be certain of in the meantime is a slow northward movement toward the central Gulf of Mexico. Water vapor imagery and real-time steering data show the ridge over Texas weakening with the approach of a shallow trough. Although this trough is forecast to lift out, a residual weakness is forecast to remain in western Atlantic, which favors a slow motion toward the north or northwest. Thereafter, the system will either slide under the ridge over the central plains and move into Texas, or it will become entangled with a secondary trough forecast to dive out of Canada and into the Ohio Valley.
There is no absolutely no way to determine which location along the Gulf Coast will see a landfall from this, and despite my claims, the Florida coast is still at risk.
Regardless of development, heavy rainfall should continue over portions of the Yucatan Peninsula, western Cuba, the Florida Keys, and perhaps extreme south Florida over the next day or so. Long-range radar out of Key West shows bands of heavy rain moving cyclonically around coastal areas. Observations in these areas indicate some gusty winds, 30 to 35 mph.
A reconnaissance aircraft is scheduled to investigate the system tomorrow, if necessary.
Probability of development in 48 hours: 70%