Tropical weather analysis - August 6, 2012
Ernesto continues cruising through the Caribbean Sea as a tropical storm. As of the latest NHC advisory, the following was posted in regards to the storm's status:
Wind: 50 mph, with higher gusts
Location: 15.0°N 79.9°W
Movement: W at 15 mph
Pressure: 1003 mb
Category: Tropical storm
Ernesto is still not exceptionally well organized, but it is definitely improving. Although an earlier SSMIS overpass just after 0000 UTC suggested that the lower- and middle-tropospheric vortexes were displaced in a west-to-east fashion, as alluded to above, Ernesto appears to be organizing a little bit, and it is presumed that these centers are starting to realign.
Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA
Whatever shear was there is diminishing, as upper-level outflow is now evident in the western semicircle, a first in quite awhile for Ernesto. The SHIPS and GFS forecast a slow reduction in the shear, which should presumably lead to intensification, but Ernesto could surprise. It is expected that Ernesto will attain hurricane status prior to moving across the Yucatan Peninsula late Tuesday or early Wednesday, but assuming the cyclone follows the current forecast track, it will likely miss the warmest waters of the western Caribbean. Thus, rapid intensification is not being explicitly shown at this time (although it remains a possibility). Not that I have any skill in predicting such an episode, anyway. Although temporary weakening is expected to begin as Ernesto moves across the Yucatan Peninsula, reintensification is possible once the cyclone emerges over Bay of Campeche -- if it does so. At that point, Ernesto could become become a hurricane again before moving into Mexico. After its second landfall, Ernesto should quickly dissipate over the rugged terrain of central Mexico, although the threat for heavy rainfall and flooding will persist even after dissipation.
Ernesto remains south of a rather strong subtropical ridge extending to about the longitude of the tropical cyclone. Dynamical models and synoptic data suggest this ridge is beginning to weaken somewhat with the approach of a shortwave trough over the eastern United States. While this trough is not particularly strong, it should be sufficient to induce some poleward component of motion to the cyclone. In a couple of days, another shortwave currently dropping out of southern Canada is forecast to be vastly more potent. This trough and its attendant frontal zone are poised to slide eastward through the Ohio Valley and the mid-Atlantic region. This was the trough that was originally projected to turn Ernesto northward in the Gulf of Mexico. At this point, Ernesto will be too far south to feel an appreciable northward tug from this trough. I would not be surprised if subsequent forecasts have to be shifted southward. A good analogue for Ernesto at this point is Hurricane Debby of 1988, which moved across the Yucatan Peninsula and went on to hit eastern Mexico as a minimal hurricane.
An alternate scenario is that Ernesto runs into Central America, though this seems highly unlikely since the storm appears to be organizing as well as slowly gaining latitude. Should Ernesto move slower or farther south than forecast, a significant flood threat could manifest over portions of Central America through the latter part of the week.
5-day intensity forecast
INITIAL 08/06 0600Z 45 KT 50 MPH
12 hour 08/06 1800Z 50 KT 60 MPH
24 hour 08/07 0600Z 55 KT 65 MPH
36 hour 08/07 1800Z 65 KT 75 MPH
48 hour 08/08 0600Z 70 KT 80 MPH...INLAND
72 hour 08/09 0600Z 65 KT 75 MPH...OVER WATER
96 hour 08/10 0600Z 75 KT 85 MPH
120 hour 08/11 0600Z 35 KT 40 MPH...INLAND
5-day track forecast
Figure 2. My 5-day forecast track for Ernesto.
Watches and warnings
A TROPICAL STORM WARNING IS IN EFFECT FOR...
A TROPICAL STORM WATCH IS IN EFFECT FOR...
* GRAND CAYMAN
* THE COAST OF HONDURAS FROM THE HONDURAS/NICARAGUA BORDER WESTWARD
TO PUNTA CASTILLA
A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN
A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...IN THIS CASE WITHIN THE NEXT 24 TO
INTERESTS IN BELIZE AND THE YUCATAN PENINSULA OF MEXICO SHOULD
MONITOR THE PROGRESS OF ERNESTO. A TROPICAL STORM OR HURRICANE WATCH
MAY BE NEEDED FOR THESE AREAS MONDAY MORNING.
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
Tropical Storm Florence has continued to weaken over the eastern Atlantic as of the latest NHC advisory:
Wind: 40 mph, with higher gusts
Location: 16.2°N 37.9°W
Movement: W at 13 mph
Pressure: 1008 mb
Category: Tropical storm
Florence looks deathly ill, with no deep convection to be found anywhere within the cyclone vortex. However, satellite imagery suggests that the circulation and low-level wind field remain well-defined.
Figure 3. Latest infrared satellite image of Tropical Storm Florence. Image credit: NOAA
Coincident with the loss of convection, Dvorak numbers have quickly plummeted, ranging anywhere from 1.0 to 1.5, the very bottom of the scale. Given the degraded satellite presentation and lack of inner core convection, I prefer the lower end of these estimates. I highly doubt there are any remaining tropical storm force winds anywhere within the circulation given the cool waters and absence of convection, but using the National Hurricane Center operational estimates for my blog, I am forced to follow restrictions.
If convection does not return soon, Florence could degenerate into a remnant area of low pressure later today. Water vapor and upper shear data from UW-CIMSS show a swath of southwesterly vertical shear along and west of 40W extending to around 70W. The SHIPS model correctly recognizes this shear, as does the GFS. The tropical cyclone is forecast to encounter these winds within about 36 hours. With 25 - 30 kt of zonal shear forecast to infringe on the cyclone, Florence will not survive, even though the forecast track takes it into gradually warmer water. In addition to the factors mentioned above, dry air is seen to the west of the storm on water vapor images.
Most of the global models quickly lose the circulation as of 0z, and I strongly agree with this. Given current trends, dissipation is now forecast to occur much sooner. Although vertical shear is shown by the GFS to be decreasing at day five and beyond as Florence approaches 70W, there will likely be too little left of Florence for any regeneration to occur.
Florence is south of a well-established subtropical ridge. A break in the ridge is evident west of 40W. Although Florence is now a shallow system devoid of deep convection, the global models suggest there will still be enough of a weakness for the storm to move more toward the west-northwest with time. However, I remain on the south side of the guidance envelope. Not that it matters much.
5-day intensity forecast
INITIAL 08/06 0600Z 35 KT 40 MPH
12 hour 08/06 1800Z 25 KT 30 MPH
24 hour 08/07 0600Z 25 KT 30 MPH
36 hour 08/08 1800Z 20 KT 25 MPH...POST-TROP/REMNANT LOW
48 hour 08/09 0600Z 20 KT 25 MPH...POST-TROP/REMNANT LOW
72 hour 08/10 0600Z...DISSIPATED
5-day track forecast
Figure 4. My 5-day forecast track for Florence.
An area of disturbed weather centered about 500 miles southwest of Acapulco is producing only minimal shower activity. However, satellite images and earlier microwave data suggest that the circulation is fairly well-defined. The system is currently experiencing easterly shear, which is not uncommon for weak Eastern Pacific tropical entities. A ragged curved band feature has developed in the thunderstorms on the western side.
Figure 5. Latest infrared satellite image of Invest 92E. Image credit: NOAA
Atmospheric conditions appear favorable for development, and this low has the potential to become a tropical depression over the next day or two. However, the SHIPS model suggests that cooler waters awaits the system in about two days. Given that the global models are forecasting a poleward bending cyclone, this forecast may not be quite so unreasonable. This system does not currently pose a threat to land.
Probability of development in 48 hours: 50%
Atlantic getting active
Now would be a good time to remind everyone to ensure they have a hurricane preparedness kit. Yes, the last few years have been fairly kind to us. But eventually, we will see another year where a major hurricane -- or worse, a series of major hurricanes -- batters the United States mainland. Central America and Mexico have taken the brunt for us the last few years, but this cannot be expected to last. A lot of our luck can be attributed to the persistent drought conditions that have generally been prevalent across the southern United States during the last couple years.
Anyway, the GFS, complimented by the NOGAPS -- and to a loose extent the CMC -- forecasts the development of Tropical Storm Gordon between the west coast of Africa and the Cape Verde Islands. While this would be unusually far east for a tropical cyclone even by Cape Verde standards, the GFS tends to do well in sniffing out storms in the long-range. Additionally, it has been consistent enough in this solution over the last few days for me to believe it. The formation is forecast to occur in about five days.
Broadening my horizons a bit, the GFS has been consistent in developing at least one or two more storms behind Gordon during the upcoming two week forecast period. While things will probably not pan out exactly as the GFS is saying, the MJO should be at least neutral in our area of the world throughout much of this month, with the possibility of a significant upward surge in September. This means that despite the oncoming El Nino, the 2012 tropical season is not quite over, and I anticipate that these next two months will feature anywhere between 4 and 6 storms.