Tropical weather analysis - August 9, 2012
Ernesto has weakened to a tropical storm, but has held up rather nicely through its adventure across the Yucatan Peninsula. As of the most recent NHC advisory, the following was posted on this storm:
Wind: 70 mph, with higher gusts
Location: 18.8°N 93.0°W
Movement: W at 16 mph
Pressure: 993 mb
Category: Tropical storm
Satellite images show that Ernesto is organizing once again. Convective banding has increased around what appears to be a developing eye. Doppler radar data from Sabancuy, Mexico confirms this, and also shows this feature is slowly becoming better defined. However, neither the satellite images nor the radar fixes suggest a formative eyewall.
Figure 1. Latest infrared satellite image of Tropical Storm Ernesto. Image credit: NOAA
Atmospheric and oceanic conditions appear conducive for intensification until the center reaches the coast this afternoon. Based on earlier SSMIS data, Ernesto does not appear to have been too disrupted by its Yucatan landfall. Dissipation is forecast to occur rather quickly after landfall. Although some of the global models still suggest the possibility of regeneration over the Eastern Pacific, the GFS shows some rather strong easterly shear impinging on whatever is left of Ernesto. Thus, this possibility appears unlikely for now.
Ernesto is south of a mid-level ridge over the Gulf of Mexico. Seeing as though Ernesto is so far south and has accelerated, it now appears unlikely that any appreciable slowing of the forward speed will occur prior to landfall. A gradual bend to the west-southwest is expected over the next 12 hours, and Ernesto should cross the coast within the hurricane warning area in about 12-18 hours. After landfall, the forward speed may slow slightly due to interaction with rugged terrain.
5-day intensity forecast
INITIAL 08/09 0600Z 55 KT 65 MPH
12 hour 08/09 1800Z 75 KT 85 MPH...NEAR COAST OF MEXICO
24 hour 08/10 0600Z 50 KT 60 MPH...INLAND
36 hour 08/11 1800Z 30 KT 35 MPH...INLAND
48 hour 08/12 0600Z 25 KT 30 MPH...DISSIPATING INLAND
72 hour 08/13 0600Z...DISSIPATED
5-day track forecast
Figure 2. My 5-day forecast track for Ernesto.
A brief period of sustained Category 1 hurricane force winds is expected near where the center comes ashore. Given the motion, it is likely that the strongest winds will be felt in the northwest quadrant of the storm. The biggest threat with Ernesto is the potential for heavy rains capable of causing flash flooding, especially in mountainous areas. Given the forecast motion of the tropical cyclone, I expect 10 to 15 inches of rain over the remainder of Ernesto's existence, with locally higher amounts.
Preparations should be completed in the warning area quickly, as conditions will begin to deteriorate later this morning.
Watches and warnings
A HURRICANE WARNING IS IN EFFECT FOR...
* VERACRUZ TO CHILITEPEC MEXICO
A HURRICANE WATCH IS IN EFFECT FOR...
* NORTH OF VERACRUZ TO BARRA DE NAUTLA
A TROPICAL STORM WARNING IS IN EFFECT FOR...
* NORTH OF VERACRUZ TO BARRA DE NAUTLA
* EAST OF CHILITEPEC TO CAMPECHE ALONG THE GULF COAST OF THE YUCATAN
PENINSULA OF MEXICO
A HURRICANE WARNING MEANS THAT HURRICANE CONDITIONS ARE EXPECTED
SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN 12 HOURS.
PREPARATIONS TO PROTECT LIFE AND PROPERTY SHOULD BE RUSHED TO
FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.
A well-defined tropical wave is centered over the eastern Atlantic between the Cape Verde Islands and the Lesser Antilles. Although the system is generating some rather vigorous convection, the areal coverage of this convection is rather small. In addition, the low-level center appears to be located north of the strongest thunderstorm activity, which means that the system is not well-organized yet. There are hints on microwave imagery that the center could actually be a little farther than the 06z ATCF coordinates, which place it 14.6N 39.3W. This is suggested by recent satellite fixes as well, though it is always difficult to detect such motions when convection is involved.
Figure 3. Latest infrared satellite image of Invest 92L. Image credit: NOAA
The GFS forecasts an increase in westerly shear beginning in about two days, but there is little evidence of westerly flow on water vapor images at this time. I do see some moderate southerly flow from 5N 50W all the way to 15N and 55W, probably associated with an equatorial anticyclone. It is possible that if the anticyclone shifts eastward, the upper flow could transition from southerly to westerly, but I don't think this is what the GFS is seeing that will cause the shear. Instead, an upper low centered about 500 miles east of Bermuda is rapidly dropping southward. Giving it more than a cursory glance, it appears this upper low is forecast to amplify and cause westerly shear over the system. While I will not completely rule this out, 92L may be too distant to it when it makes its closest to approach, so that it doesn't get completely destroyed. In addition, if 92L is moving in a more southerly fashion like the satellite images are indicating, that is further reason to believe it will escape the shear. Dry air does still linger though, and this may retard rapid development.
The future track of the system is uncertain, but right now, the synoptic pattern favors a strong ridge building westward ahead of the system as large-scale troughing over the western Atlantic lifts out. It is possible that the aforementioned upper low could impart some sort of weakness to the ridge in a few days, but 92L will be well west of that weakness when it begins to manifest. At this point, it seems physically unlikely, barring rapid development (which appears equally unlikely) that 92L will go anywhere other than the Caribbean, but one should always be aware. Thereafter, the track becomes quite muddled. It is possible the system could eventually enter the Gulf of Mexico. Alternatively, it could stay weak like Ernesto and move generally westward. Moderate westerly shear is forecast to affect the system if it enters the Caribbean, but it will all depend on the timing, as some of the models are quicker with its arrival toward the Leeward Islands than others.
Interests in the Leeward Islands should monitor the progress of this system through the early part of next week.
Development is anticipated, and this system could become a tropical depression over the couple of days.
Probability of development in 48 hours: 70%
Gilma has become a hurricane. As of the latest NHC advisory, the following was posted on the storm:
Wind: 75 mph, with higher gusts
Location: 16.1°N 118.4°W
Movement: W at 9 mph
Pressure: 987 mb
Category: 1 (Saffir-Simpson Hurricane Scale)
Gilma looks like a typical low-end Category 1 hurricane on satellite images, with a well-defined central dense overcast that has expanded since the previous advisory. An eye is not identifiable at this time.
Figure 4. Latest infrared satellite image of Hurricane Gilma. Image credit: NOAA
Gilma will cross the 26C isotherm in about 12 hours. Thus, time is quickly running out for strengthening. In addition, easterly shear is forecast to abruptly increase in about 72 hours, possibly in response to a broad area of disturbed weather associated with the remnants of Ernesto. It could also be associated with a building ridge over the Bay of Campeche and southern Mexico in the wake of Ernesto. Either way, Gilma will probably not last through five full days. Dissipation is shown at day four.
Although Gilma is currently moving west, a gradual turn to the northwest with a reduction in forward speed is likely. A mid- to upper-level trough is seen off the west coast of the United States. This feature is expected to weaken the subtropical ridge north of Gilma, which the global models agree will result in the northwestward motion. A turn to the west is expected by Sunday as Gilma becomes a shallow system and becomes steered by the low-level flow.
5-day intensity forecast
INITIAL 08/09 0300Z 65 KT 75 MPH
12 hour 08/09 1800Z 70 KT 80 MPH
24 hour 08/10 0300Z 65 KT 75 MPH
36 hour 08/11 1800Z 50 KT 60 MPH
48 hour 08/12 0300Z 40 KT 45 MPH
72 hour 08/13 0300Z 35 KT 40 MPH
96 hour 08/14 0300Z 30 KT 35 MPH...POST-TROP/REMNANT LOW
120 hour 08/15 0300Z 25 KT 30 MPH...POST-TROP/REMNANT LOW
5-day track forecast
Figure 5. My 5-day track forecast for Gilma.
An area of low pressure centered about 500 miles south-southwest of Acapulco has become a little better organized. However, most of the associated convection is located west of the center due to moderate easterly shear. This shear is forecast to increase significantly over the next day or so, ultimately increasing to over 30 kt. Even by Eastern Pacific standards, where most storms move west, this is rather significant.
Figure 6. Latest infrared satellite image of Invest 93E.
Given the recent increase in organization, I think the NHC's probability of 30% seems a little low. Either way, 93E is not a threat to any land areas.
Probability of development in 48 hours: 40%