Tropical weather analysis - August 18, 2012
Hurricane Gordon has rapidly intensified tonight, and is now just shy of being a major hurricane. As of the latest NHC advisory, the following was available:
Wind: 110 mph, with higher gusts
Location: 34.5°N 33.9°W
Movement: E at 22 mph
Pressure: 965 mb
Category: 2 (Saffir-Simpson Hurricane Scale)
An SSMIS microwave pass back near 2300 UTC showed that Gordon possessed a well-defined eye and an eyewall that was open to the south. Since that time, Gordon has maintained a well-organized satellite presentation, with the south side of the eye no longer exposed. The eye is also quite visible using conventional satellite pictures.
Figure 1. Latest infrared satellite image of Hurricane Gordon. Image credit: NOAA
I'm not easily impressed, but Gordon's abrupt intensification episode today was of great surprise. Let's look at a couple things here: first, it is sitting under SSTs of only around 26C. It is possible that these waters are a little warmer than the SHIPS sea surface temperature algorithm would suggest, as water temperatures of barely 80F are not typically thought to be major hurricane breeding grounds. On the other hand, waters warmer than 26C this far east is equally unlikely, and I lack direct evidence either way. One could also hypothesize that cold upper tropospheric temperatures are helping to produce a stronger vertical instability gradient and counteracting the otherwise negative effects of the cool waters, which seems fairly supported by earlier AMSU data. A similar synoptic situation occurred when Chris became a hurricane over the cold north Atlantic near 40°N back in June. Although such an event makes sense meteorologically, it is rather difficult to determine if these cold troposphere/lukewarm SSTs intensification sprees will actually occur. In many cases they don't.
Gordon has likely peaked its intensity, as it is quickly heading toward cooler water and high wind shear. However, given the cold upper-level temperatures alluded to on the AMSU pass, I have to assume the hurricane will maintain considerable strength as it approaches the Azores. In light of the strengthening trend that was observed today, hurricane warnings have been issued for the central and eastern Azores. Although Gordon is forecast to remain well south of Sao Miguel, tropical storm force winds will probably still be experienced there, especially if Gordon more northward than anticipated. In addition, it is forecast to pass close enough so that it could deliver a brief period of hurricane force winds could occur on Saint Maria. Rainfall will potential remains limited given the hurricane's fast forward motion.
The dynamical and statistical models -- in other words the whole shebang -- are in agreement that Gordon will dissipate by Wednesday or Thursday. It is still possible that Gordon will bring some remnant shower activity to sections of Portugal and Spain after it dissipates, although how much residual energy remains with post-tropical Gordon at that time remains to be seen.
It should be noted that any slight deviation to the north of the current forecast track could place the core of the hurricane over Santa Maria island.
Synoptic data indicates that Gordon continues to accelerate eastward, embedded within a well-defined westerly steering current between a strong low near the British Isles and the northern periphery of the Atlantic subtropical ridge. The combination of these features is expected to result in a turn to the east-northeast tomorrow. The global models remain in excellent agreement on this. As such, there has been no significant change to the synoptic reasoning or forecast track. Gordon is forecast to pass through the Azores during the early hours of Sunday morning as a minimal hurricane. Gordon is a rare hurricane threat for those islands, and preparations to protect life and property, particularly in the hurricane warning area, should be promptly completed. Winds could be especially dangerous in areas of elevated terrain. Near the end of the period, the hurricane is forecast to slow and turn toward the southeast as it comes under the influence of a building low- to mid-level ridge over the north Atlantic. My forecast agrees rather strongly with the National Hurricane Center's, based on multi-layer steering analyses as well as real-time synoptic trends.
5-day intensity forecast
INITIAL 08/19 0300Z 95 KT 110 MPH
12 hour 08/19 1200Z 90 KT 105 MPH
24 hour 08/20 0000Z 80 KT 90 MPH
36 hour 08/20 1200Z 65 KT 75 MPH
48 hour 08/21 0000Z 50 KT 60 MPH
72 hour 08/22 0000Z 35 KT 40 MPH...EXTRATROPICAL
96 hour 08/23 0000Z 25 KT 30 MPH...EXTRATROPICAL
120 hour 08/24 0000Z...DISSIPATED
5-day track forecast
Figure 2. My 5-day forecast track for Gordon.
Helene has dissipated inland over central Mexico. Helene is quickly approaching the rugged Sierra Madre mountain range, and some erratic motion is possible. The global models are less enthusiastic with regards to regeneration today, and atmospheric conditions aren't expected to be particularly conducive either, with a consistent flurry of northwesterly shear forecast over the northwestern Gulf of Mexico by the GFS. While it is still possible that Helene moves back over water in about three days, even the GFS which previously regenerated the system and meandered it off the Texas coast, now pulls it back into Mexico, providing it little opportunity to strengthen. The remainder of the global models are equally unenthusiastic, and some don't even show Helene moving back over water. Some residual mid-level energy and associated cloudiness could spread over portions of the northern Gulf Coast over the next five days, which could perhaps enhance the rainfall potential in those areas, but overall, Helene appears less likely to redevelop as the center surface center slides back into Mexico behind the advancing mid-level center.
Probability of development in 48 hours: Near 0%
A vigorous tropical wave over the eastern Atlantic centered about 600 miles west of the Cape Verde Islands is showing signs of organization. The low-level center appears to be consolidating along the south side of the convection, which has been showing some banding.
Figure 3. Latest infrared satellite image of Invest 94L. Image credit: NOAA
In the near-term, atmospheric conditions appear favorable for development. Underlying sea surface temperatures are warm, vertical shear is seemingly weak, and the overall environment is moist. However, I see an unfavorable environment -- at least as far as thermodynamics are concerned -- ahead of 94L. Water vapor imagery shows a large expanse of dry air covering the central Atlantic from 40W to 60W. The SHIPS responds to this by forecast ambient relative humidity values to decrease by 48 hours. Nevertheless, the upper-level wind pattern over the system is forecast to be anticyclonic, so 94L will probably not dissipate. I am calling for continued gradual development, and 94L could become a tropical depression over the next day or two. An alternate scenario is that it doesn't develop until at or west of 50W. This is probably the more realistic scenario given the pattern this year, but we shall see.
Regardless, this system poses a long-range threat to the Lesser Antilles, and interests there should be anticipating the possibility of a tropical storm affecting them in about 5 - 6 days. This system could either pass north of the Leeward Islands, or enter the eastern Caribbean. Given recent model depictions, I tend to side with the latter camp for now. It is uncertain if 94L would be able to make full transit of the Caribbean before recurving. Perhaps if it stays weak, or if Hispaniola significantly disrupts it. Its ultimate track also hinges heavily on the evolution of the synoptic pattern. The models have been trending back and forth between a strong ridge and strong troughing. I still think the pattern favors an eventual recurvature, but when and where remains to be seen.
Probability of development in 48 hours: 60%