Tropical weather analysis - August 10, 2012
Tropical Depression Seven
Tropical Depression Seven continues to race across the central Atlantic. As of the most recent NHC advisory, the following was available on the system:
Wind: 35 mph, with higher gusts
Location: 13.7°N 51.9°W
Movement: W at 24 mph
Pressure: 1009 mb
Satellite images suggest that the low-level center remains elongated in a southwest to northeast fashion. Oh, how I wish we still had QuikSCAT.
Figure 1. Latest infrared satellite image of Tropical Depression Seven. Image credit: NOAA
At this stage, it appears that speed shear -- not vertical -- is the primary inhibitor for organization since there are no obvious signs of strong vertical shear in the nearby environment. Broadening the horizon a bit, I see a large upper low several hundred miles north of the depression, near 26.8N 50.5W dropping rapidly southward toward the latitude of the tropical cyclone. The global models forecast this feature to gradually elongate and move westward, leaving behind a fractured piece of energy to reinforce the mid-oceanic trough. The net result should be a steady increase in southwesterly to westerly shear as the cyclone continues racing westward. This sort of unfavorable upper-level pattern is not atypical of El Nino years. Recall in 2009 that tropical storms Ana and Henri struggled against similar conditions in roughly the same area. The GFS and ECMWF continue to show the depression opening up into a wave over the eastern Caribbean, which is a viable possibility given current trends. In fact, it may even happen sooner, especially if the cyclone does not slow down.
An additional negative is the small size of the depression, which will make it quite susceptible to subtle environmental changes. Given current trends, and with no significant slowdown foreseen, I have revised my forecast significantly, and now show the system dissipating in about three days. This is in good agreement with the forecast from the National Hurricane Center, and is also harmonious with the unbeatable GFS/Euro consensus. Nevertheless, the system could still reach tropical storm status, but this prospect is becoming increasingly less likely with time.
It should be noted that the long-range pattern favors the remnants of the depression possibly getting pulled into the western Gulf of Mexico once it reaches the western Caribbean, where the GFS suggests upper-level winds could become more favorable (albeit not anticyclonic) for development. However, I assume there will be too little left of the system to take advantage of the more favorable atmospheric parameters at that time.
Synoptic data suggests that the strong subtropical ridge that is steering the depression so quickly to the west is in no hurry to budge. Indeed, all of the global models build it westward with time. Thus, a continuation of the rather fast and unclimatological westward motion is forecast, and the depression is expected to arrive in the Windward Islands as early as Saturday evening, possibly sooner. After about 72 hours, the cyclone -- or its remnants -- could encounter a break in the western extent of the ridge. The depression is expected to respond by moving more toward the west-northwest. Although a fairly significant break in this ridge is forecast, it is anticipated that the system will be too weak to fully take advantage of it.
5-day intensity forecast
INITIAL 08/10 2100Z 30 KT 35 MPH
12 hour 08/11 0600Z 30 KT 35 MPH
24 hour 08/12 2100Z 30 KT 35 MPH
36 hour 08/13 0600Z 30 KT 35 MPH
48 hour 08/13 2100Z 30 KT 35 MPH
72 hour 08/14 2100Z...DEGENERATED INTO TROPICAL WAVE OVER EASTERN CARIBBEAN SEA
5-day track forecast
Figure 2. My 5-day forecast track for Tropical Depression Seven.
No doubt as a course of least regret, the NHC has opted to issue tropical storm watches for much of the Lesser Antilles. Given the rapidity of the forward motion, it is likely that any heavy squalls will contain periods of tropical storm force winds, especially in gusts.
Watches and warnings
A TROPICAL STORM WATCH IS IN EFFECT FOR...
* GUADELOUPE AND MARTINIQUE
* ST. LUCIA
* ST. VINCENT AND THE GRENADINES
A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE IN 24 TO 48 HOURS.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
A tropical wave over the eastern Atlantic ("93L") has lost organization since yesterday. While I'll admit I didn't quite expect this, it's also not that unusual either; few systems develop along or east of 20W due to cool sea surface temperatures that generally prevail in this region. This wave is embedded within a region of cool sea surface temperatures, 26.3 according to the SHIPS at 18z. The surrounding environment is also rather dry, which suggests that any short-term development potential will be slow. By Monday, 93L will be approaching warmer waters, and could start to organize a bit as it begins to approach 50W. Under a light and seemingly diffluent upper wind environment, conditions would normally be a go for strengthening. However, none of the global models are that enthusiastic with intensification. There is a large surge of dry air on water vapor imagery over much of the central and eastern Atlantic which looks like it will be hard to eradicate, so perhaps that's it. On the other hand, the purely statistical models intensify the system almost immediately after the 72 hour point, bringing to a strong tropical storm. I tend to side with the less aggressive global model camp given the abundance of dry air seen on water vapor images.
Based on global model forecasts, 93L should begin to approach a weakness in the subtropical ridge lying near 50W in a few days, which should presage recurvature. The system possibly remaining a weaker entity will not increase the odds of an eventual strike on the lower 48, it would just delay the inevitable.
Regardless of development, locally heavy rainfall and strong gusty winds are likely over much of the Cape Verde archipelago over the next 24 hours.
Probability of development in 48 hours: 20%
Tropical Storm Gilma continues to slowly weaken over cool waters. As of the latest NHC advisory, the following information was posted:
Wind: 60 mph, with higher gusts
Location: 19.2°N 119.6°W
Movement: NNW at 5 mph
Pressure: 998 mb
The cyclone continues to do rather well for itself considering all available data suggests that it is over sub-26C SSTs. A recent AMSUB microwave overpass taken just after 2100 UTC suggests that the earlier eye feature which had been evident throughout the day has decayed, and only a small area of convection west of the center -- perhaps the fragments of an eyewall -- remain. However, recent infrared pictures suggest a more banded pattern to the convection, which could herald the onset of a more pronounced weakening trend.
Figure 3. Latest infrared satellite image of Tropical Storm Gilma. Image credit: NOAA
Environmental conditions are not favorable for Gilma to maintain its strength for much longer. Strong northeasterly shear is evident on water vapor images just to the north of the tropical storm. The SHIPS/GFS successfully recognize this and forecast Gilma to run into it within the next 12-24 hours. This shear is forecast to persist through the greater part of Sunday. That, coupled with a thermodynamic environment that is only marginal at best -- and finally -- progressively cooling sea surface temperatures -- suggests that Gilma is not long for this world. Dissipation is forecast in about 36 hours.
Gilma is moving in a poleward fashion, embedded in a weakness in the Pacific subtropical high caused by a weak shortwave trough over Nevada. A second trough is amplifying off the west coast of the United States, and this feature is expected to reinforce the broad cyclonic vorticity in this area. As a result, Gilma is expected to continue moving toward the north-northwest without significant acceleration. By Sunday, the system is forecast to begin to slow. I realize this is in contrast to the current forecast from the National Hurricane Center, but given the vigorous nature of the remnants of Ernesto, and the relative consistency in the global models to pull the circulation eastward, I am having a hard time believing some sort of interaction will not happen.
5-day intensity forecast
INITIAL 08/10 2100Z 50 KT 60 MPH
12 hour 08/11 0600Z 40 KT 45 MPH
24 hour 08/11 2100Z 30 KT 35 MPH
36 hour 08/12 0600Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
48 hour 08/12 2100Z 25 KT 30 MPH...POST-TROP/REMNANT LOW
72 hour 08/13 2100Z 25 KT 30 MPH...POST-TROP/REMNANT LOW
96 HOUR 08/14 2100Z...DISSIPATED
5-day track forecast
Figure 4. My 5-day forecast track for Gilma.
A large area of disturbed weather, primarily associated with the remnants of Ernesto, is centered off the southwest coast of Mexico. This large system is already showing signs of organization, and conditions appear favorable for a tropical depression to form in the area. Convection is beginning to develop closer to the center, while the large outer band that was seen encompassing the western semicircle earlier is dissipating. This is a pattern that is typical of an organizing tropical cyclone.
Figure 5. Latest infrared satellite image of Invest 94E. Image credit: NOAA
The SHIPS analyzes strong easterly shear over the system throughout the next five days, but analysis of satellite imagery suggests this is probably storm relative. The GFS builds a small anticyclone above the system, which is certainly a viable solution since sea surface temperatures are expected to remain quite warm throughout the entirety of the forecast period. I believe this system will eventually become a hurricane, although the large size of the circulation dictates that it will take time to spin up.
In the long-range, the models suggest a threat to southern Baja, with the GFS actually bringing the system inland Wednesday evening. Either way, this system will probably come close enough to the coast that watches or warnings will eventually be required, and interests across the southern and central Baja Peninsula, as well as southwestern Mexico, should carefully monitor the progress of this system over the next several days.
Regardless of development, heavy rainfall and possible flooding will continue over portions of southern Mexico over the next day or two.
Probability of development in 48 hours: 80%