Tropical weather analysis - August 3, 2013

By: KoritheMan , 7:14 AM GMT on August 03, 2013

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Invest 91L

An area of low pressure located about 70 miles east of Cape Canaveral, Florida is associated with the remnants of Tropical Storm Dorian. Satellite and radar data suggest that the associated thunderstorm activity has been steadily increasing over the last several hours. Based on the limited surface observations available, along with scatterometer-derived surface wind data, 91L appears close to being a tropical depression, if it is not already one now.

Figure 1. Latest infrared satellite image of Invest 91L. Image credit: NOAA's Satellite Services Division (SSD).

There is some northwesterly shear over the system, and my best gander at the location of the low-level center is along the northern edge of the convective canopy. Despite the northwesterly shear, atmospheric conditions appear at least marginally conducive for some additional development, and this system may well become a tropical cyclone later today. By Sunday, the global models suggest that 91L will become entangled with an approaching cold front. There is the possibility of baroclinic intensification as an extratropical cyclone thereafter, but those processes are more suited for near-term forecasts, and not short to medium range speculation.

Water vapor imagery shows that the initial shortwave trough over the eastern United States has pretty much departed, with a sharp gradient of northwesterly mid-level winds over much of that area. The imagery also reveals another impulse -- this one all the way up in Quebec -- that is likely to reamplify the weakness in this area and cause the system to move northward later today, followed by a turn toward the northeast thereafter. Since the trough appears to have departed a little sooner than forecast, I would presume that there will be a temporary rebuilding of the subtropical ridge to the north of the low pressure center during the next 12-24 hours, which may allow the center to come farther west, on a track closer to the Outer Banks of North Carolina; this is supported by UW-CIMSS steering imagery as well, which does indeed show that the ridge over the central Atlantic has shifted westward in the last six hours, closer to the center of Invest 91L. While I am not currently anticipating a landfall, I do expect some westward shifts in the dynamical guidance over the next cycle or two, and the margin for error is enough that interests along the Outer Banks should monitor the progress of this system.

The circulation associated with this feature is small, and the shear will keep the heaviest precipitation and strongest winds to the southeast and east of the low-level center. Because of this, even if the system becomes a tropical cyclone again, watches and warnings will not be required for any portion of the Florida east coast. Rainfall will also not be a significant issue, and the left side of Northern Hemispheric tropical cyclone is typically considered to be subsident as well.

Probability of development in 48 hours: 60%


Gil dropped below hurricane strength yesterday. As of the 0300Z NHC advisory, the following information was available on Gil:

Wind: 65 mph, with higher gusts
Location: 14.9°N 130.6°W
Movement: W at 12 mph
Pressure: 995 mb

Gil was completely devoid of deep convection earlier, but a new burst has developed over the low-level center in the last several hours. Satellite estimates suggest that Gil is still a strong tropical storm, and it is likely that the temporary cessation of deep convection from the cyclone center earlier was not sufficient to weaken the storm just yet.

Figure 2. Latest infrared satellite image of Tropical Storm Gil. Image credit: NOAA's Satellite Services Division (SSD).

Water vapor imagery suggests that Gil continues to experience westerly shear, and the corresponding outflow signature in that direction is rather flat as a result; in addition, there was some dry air entrainment into the circulation earlier, and that may continue to be the case as the westerlies continue.

The intensity forecast is quite a challenge this morning. The GFS suggests that the environment could become more favorable over the next day or so, but this model is clearly not handling the shear in the near-storm environment particularly well, so I am not sure how logical it is to rely on it in this instance. Since there are no signs of the shear easing up over the next couple of days, I have chosen to forecast additional weakening during the first 48 hours of the forecast. After that time, the GFS -- hopefully performing a little better this time around -- shows the environment becoming more favorable as Gil heads farther south, which would agree well with high cloud motions derived from water vapor imagery. Thus, some restrengthening is shown at that time. In about four days, the SHIPS model shows water temperatures cooling a little (but still remaining at or above 26C), which may not allow for robust intensification. An alternate scenario is that Gil succumbs to the presently hostile environment and runs out of Gil in the next 48 hours.

Recent satellite fixes suggest that Gil's center is a little north of the 0300Z NHC advisory position, which I reflect in my 0600Z of the storm below. The cyclone appears to be moving pretty much due west, and this motion is expected to continue in the short-term. Within the next 12-24 hours, the global models strengthen the ridge to the north of Gil, which is expected to force the cyclone to turn toward west-southwest. The GFS and ECMWF have somewhat differing positions in where the Gil vortex ends up in five days, but for all practical purposes, there is a solid enough consensus that confidence in the forecast track is relatively high.

One fly in the ointment is the possibility of a Fujiwhara interaction of sorts with a tropical disturbance to the east. Since Gil still appears to be the stronger of the system, and thus far shows no signs of interacting with the disturbance, my forecast is basically an update of the NHC forecast track.

Although Gil will enter the central Pacific on Tuesday, it will remain far to the south of Hawaii, probably far enough to not even provide any locally increased surf.

Intensity forecast and positions

INITIAL 08/03 0600Z 15.2°N 131.2°W 55 KT 65 MPH
12 hour 08/03 1800Z 15.2°N 132.3°W 55 KT 65 MPH
24 hour 08/04 0600Z 14.8°N 133.9°W 45 KT 50 MPH
36 hour 08/04 1800Z 14.4°N 135.8°W 40 KT 45 MPH
48 hour 08/05 0600Z 13.8°N 137.8°W 40 KT 45 MPH
72 hour 08/06 0600Z 13.4°N 140.0°W 45 KT 50 MPH
96 hour 08/07 0600Z 13.3°N 142.4°W 45 KT 50 MPH
120 hour 08/08 0600Z 13.3°N 144.9°W 45 KT 50 MPH

Track forecast

Figure 3. My forecast track for Gil.

Invest 90E

A persistent area of low pressure continues to linger several hundred miles east of Gil. Convection associated with this area is not well-organized, although earlier scatterometer data and recent microwave pictures suggest a fairly robust surface circulation. Thus, if convection can persist and organize, it will not take a lot of time for this system to become a tropical cyclone.

Figure 4. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD).

Since Gil is now in a better established steering regime, the distance between the two system is increasing, raising the potential for development with 90E. The disturbance is expected to move westward for the next 24-36 hours, after which time a turn toward the west-northwest is forecast as the system approaches a small break in the subtropical ridge, in agreement with the guidance.

Cooler waters await the system in about four days, so if it's going to develop, it has to do it within the next 72 hours. This system is not a threat to any landmasses.

Probability of development in 48 hours: 50%

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2. WunderAlertBot (Admin)
6:58 AM GMT on August 04, 2013
KoritheMan has created a new entry.
1. nigel20
3:57 PM GMT on August 03, 2013
Great blog discussion, Kori!
Member Since: November 6, 2010 Posts: 11 Comments: 7888

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.