Tropical weather analysis - August 9, 2012
Ernesto made landfall along the eastern coast of Mexico just west of Coatzacoalcos near 1800 UTC this afternoon as a 50 kt tropical storm. As of the latest NHC advisory, the following information was available on Ernesto:
Wind: 45 mph, with higher gusts
Location: 18.2°N 95.8°W
Movement: W at 11 mph
Pressure: 996 mb
Considering how long the storm has been overland, the cloud pattern is well-organized. However, the decay process has clearly begun, as satellite images show two large and well-defined convective bands emanating quite far from the center: one to the west, and one to the south, while convection is diminishing closer to the center. This suggests that Ernesto has lost its inner core structure.
Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA
Given the convection, Ernesto still poses a significant flood threat over the southern half of Mexico. These rains will be especially enhanced over areas of mountainous terrain. An additional 10 to 15 inches of rain will be possible across this area through the next few days. Even after Ernesto emerges into the Pacific, a large area of low-level southwesterly flow associated with the remnants of the large circulation will continue to funnel moisture into the country. Any remaining tropical storm force winds within the circulation are very likely only occurring over water. Given the recent downturn in convection, any sustained winds of that caliber are quickly diminishing. Ernesto is expected to dissipate over the next day or so as it encounters increasingly rugged terrain. However, the global models are pretty much unanimous in regenerating the storm over the Eastern Pacific. While rare, it can happen. The last such occurrence of an Atlantic-Pacific crossover storm was Hurricane Cesar-Douglas of 1996.
I will wait one more day before calling for regeneration in the Pacific, but I have a feeling I will need to be explicit with this tomorrow. The global models suggest a long-range threat to Baja California, close enough that watches and warnings may eventually be required for the southern tip of the peninsula.
Ernesto is well-embedded within the Gulf ridge, and is expected to continue moving westward into the eastern Pacific on Saturday morning.
5-day intensity forecast
INITIAL 08/10 0000Z 40 KT 45 MPH...INLAND
12 hour 08/10 1200Z 30 KT 35 MPH...INLAND
24 hour 08/11 0000Z 30 KT 35 MPH...DISSIPATING INLAND
36 hour 08/11 1200Z...DISSIPATED
5-day track forecast
Figure 2. My 5-day forecast track for Ernesto.
Although coastal warnings are still in effect as of the 0z intermediate advisory from the National Hurricane Center, these are there for precautionary measure, and will likely be discontinued with the next full advisory package due out in about an hour and a half.
Tropical Depression Seven
The tropical wave we have been tracking for the last several days has become a tropical depression. As of the latest NHC advisory, the following information was posted on the storm:
Wind: 35 mph, with higher gusts
Location: 13.7°N 43.8°W
Movement: W at 20 mph
Pressure: 1010 mb
Typical with developing systems this time of day, convection associated with the depression has decreased.
Figure 3. Latest infrared satellite image of Tropical Depression Seven. Image credit: NOAA
The long-term survival of the tropical cyclone is in doubt. Although some intensification is possible in the near-term, water vapor imagery shows a large area of dry air surrounds the storm, which could act as a deterrent to significant strengthening. In addition, satellite images show a large convective mass about 100 miles southwest of the depression, which appears to be limiting the storm relative inflow. Although the depression is expected to become a tropical storm over the next day or two, the GFS/SHIPS shows westerly shear increasing as early as Friday afternoon, and persisting as the system enters the Caribbean. Finally, the GFS and ECMWF, which I note did quite well with Ernesto, see the cyclone degenerating into a tropical wave, which is certainly possible given the very strong ridge to the north. It should be noted that upper-level winds are forecast to become more favorable as the system nears the western Caribbean in 5 or 6 days, at which time some significant strengthening is possible. That is, assuming there is anything left of the depression at that time.
The cyclone remains situated to the south of a rather strong low- to mid-level ridge. The global models are in agreement on building this ridge westward ahead of the tropical cyclone as large-scale troughing over the western Atlantic lifts out, and the flow across that region gradually becomes more zonal. In general, this should result in a pretty straightforward westward motion for the next few days. Longer-term, the models show a weakness in the western Atlantic ridge which could induce a more poleward motion at that time, but it will depend on the respective latitude of the system at that time. A weaker system ala Ernesto would tend to move in a more uniform westerly direction to the south of the ridge, while a stronger system would likely be strong enough to move through the purported weakness. Given that both are viable possibilities at this time, I will split the difference. It appears we could have another tropical cyclone approaching the western Caribbean in four to five days. This is just a bit south of the TVCN/TVCC consensus, in better agreement with the track from the National Hurricane Center.
5-day intensity forecast
INITIAL 08/10 0000Z 30 KT 35 MPH
12 hour 08/11 1200Z 30 KT 35 MPH
24 hour 08/12 0000Z 35 KT 40 MPH
36 hour 08/13 1200Z 40 KT 45 MPH
48 hour 08/14 0000Z 45 KT 50 MPH
72 hour 08/15 0000Z 45 KT 50 MPH
96 hour 08/16 0000Z 45 KT 50 MPH
120 hour 08/17 0000Z 50 KT 60 MPH
5-day track forecast
Figure 4. My 5-day forecast track for Tropical Depression Seven.
A vigorous tropical wave accompanied by a well-defined low pressure system has just emerged off the west coast of Africa. Since the system was just designated an invest about an hour ago and is still east of 20W, there is limited satellite coverage. Using available fixes, the system appears well-organized, and seems to already be on its way to becoming a tropical depression. With an estimated surface pressure of 1004 mb, there seems to be little in the way of inhibition for this system once it gets fully over water. The biggest impediment at this time appears to be a dry and stable airmass immediately ahead of the wave, accompanied of course by relatively cool sea surface temperatures. However, objective analyses of areal SSTs from both SHIPS and AOML suggest that water temperatures should remain more in the 27C range for the next day or so. This should allow the system to continue organizing. Afterward, cooler SSTs will likely halt the strengthening process. By Sunday, water temperatures are expected to slowly rebound, favoring a resumption of the earlier intensification.
Given the high latitude (15N) in which the system has emerged from the coast, as well as global model forecasts of a mid- to upper-level trough amplifying over the western Atlantic through the early part of next week, it appears that 93L is no threat to land at this time, with the possible exception of Bermuda. It is not unusual for systems originating so far east in the Atlantic to fail to make full transit. The last hurricane I recall doing so was Hurricane Dora in 1964, which went on to hit near Jacksonville, Florida as a major hurricane in early September.
Steady development is expected as the system moves west or west-northwest at 10 to 15 mph.
Probability of development in 48 hours: 40%
After peaking as 70 knot hurricane this morning, Gilma has weakened to a tropical storm as it moves across cooler waters. As of the latest NHC advisory, the following was posted on the storm:
Wind: 70 mph, with higher gusts
Location: 17.2°N 119.1°W
Movement: NW at 6 mph
Pressure: 992 mb
Gilma still has a well-organized appearance on satellite imagery. Also, an SSMI overpass just before 0z still indicated a low- to mid-level eye feature surrounded by a ring of convection. Since the cyclone structure does not appear to have deteriorated too significantly since that time, Gilma probably hasn't weakened much.
Figure 5. Latest infrared satellite image of Tropical Storm Gilma. Image credit: NOAA
Environmental conditions are quickly becoming unfavorable for sustenance of a tropical cyclone, with dry air lying just to the west of a storm. In addition, easterly shear is forecast to abruptly increase Saturday evening, and Gilma is expected to weaken more quickly from this point on. Dissipation as a tropical cyclone is expected in about three days.
Water vapor imagery shows a weak mid- to upper-level trough continues to linger off the west coast of the United States. This trough appears to be enforcing a weakness in the Pacific subtropical high near and west of 125W. Since Gilma is still a vertically deep system at this time, it is responding by moving in a slow northwesterly fashion. As a series of shortwave impulses move through the westerlies in this area, the large-scale cyclonic flow across the region is forecast to persist. Thus, the current northwest motion is expected to continue without appreciable acceleration as synoptic steering remains somewhat weak. After 48 hours, the track becomes a little more uncertain. The global models have come into better agreement today on redeveloping Tropical Storm Ernesto in the eastern Pacific on Sunday. While the projected development may be too quick in these models, Ernesto still possess the well-defined cyclonic structure it has had ever since its landfall along the Yucatan Peninsula. So it is my thinking that just enough westerly flow will persist south of the center of the area of disturbed weather -- or tropical cyclone -- to nudge Gilma just a little bit to the east. Since Gilma is expected to steadily weaken, I am not going to commit to a full on eastward motion as some of the models seem to be doing. However, Gilma is expected to slow down significantly near the end of the forecast period as it comes under the influence of Ernesto. This track is east of what it was yesterday.
5-day intensity forecast
INITIAL 08/10 0000Z 60 KT 70 MPH
12 hour 08/10 1200Z 55 KT 65 MPH
24 hour 08/11 0000Z 45 KT 50 MPH
36 hour 08/11 1200Z 40 KT 45 MPH
48 hour 08/12 0000Z 35 KT 40 MPH
72 hour 08/13 0000Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
96 hour 08/14 0000Z...DISSIPATED
5-day track forecast
Figure 6. My 5-day forecast track for Gilma.