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Tropical weather analysis - August 4, 2013

By: KoritheMan , 6:58 AM GMT on August 04, 2013


Gil continues to weaken. As per the 0300Z NHC advisory, the following information was available on the storm:

Wind: 50 mph, with higher gusts
Location: 15.3°N 134.3°W
Movement: W at 9 mph
Pressure: 1001 mb

If the cyclone didn't run out of Gil yesterday, it certainly appears to have done so today. All of the deep convection has eroded, primarily due to northwesterly shear; in addition, recent total precipitable water imagery from the University of Wisconsin CIMSS webpage suggests that Gil may have entrained a little bit of dry air along its western periphery, likely reaffirmed by the shear. Satellite estimates are plummeting as fast as the Dvorak technique allows, and Gil may very well already be a tropical depression, rather than a tropical storm.

Figure 1. Latest infrared satellite image of Tropical Storm Gil. Image credit: NOAA's Satellite Services Division (SSD).

Although Gil is forecast to move southwestward into an environment of somewhat lighter shear, water vapor imagery continues to suggest northerly to northwesterly upper-level winds extending down to at least 13N; since Gil is not expected to get below that latitude, the potential for restrengthening later in the period appears diminutive. The GFS continues to suggest the environmental northerlies will decrease and Gil will find itself in a more favorable wind shear environment, but this model has stubbornly relaxed the shear in the area over the last several days to no avail. Given current trends, Gil will likely dissipate later today if convection does not return to the low-level center soon. In contrast to previous forecasts, I will forecast dissipation of Gil in 72 hours, although it could occur much sooner.

Gil remains south of a mid-level ridge, which is forecast to strengthen. Lacking any synoptic scale features that would cause a weakness of any sort, I will continue to forecast a rather aclimatological west-southwestward to southwestward motion over the next couple of days, with a turn back to the west thereafter. The model guidance remains in good agreement on this scenario, and even the GFS, which was previously a northern outlier near the end of the period, has come into better agreement with the other models in keeping Gil on a more southerly track.

Intensity forecast and positions

INITIAL 08/03 0300Z 15.3°N 134.3°W 45 KT 50 MPH
12 hour 08/03 1200Z 14.8°N 134.3°W 40 KT 45 MPH
24 hour 08/04 0000Z 14.4°N 135.5°W 35 KT 40 MPH
36 hour 08/04 1200Z 13.7°N 137.4°W 35 KT 40 MPH
48 hour 08/05 0000Z 13.2°N 139.5°W 30 KT 35 MPH
72 hour 08/06 0000Z 13.2°N 143.3°W 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
96 hour 08/07 0000Z...DISSIPATED

Track forecast

Figure 2. My forecast track for Gil.

Tropical Depression Eight-E

Tropical Depression Eight-E developed on Saturday from the persistent area of low pressure that has hung around big brother Gil for the last several days. As of the 0300Z NHC advisory, the following information was available on the cyclone:

Wind: 35 mph, with higher gusts
Location: 12.7°N 124.9°W
Movement: W at 10 mph
Pressure: 1008 mb

The satellite presentation is rather impressive, with a large area of very cold cloud tops exceeding -80C. The low-level center is very difficult to locate lacking any recent reliable microwave or scatterometer passes, but my best estimate, which is primarily based on extrapolation, is that it lies roughly in the center of the convective overcast, just to the north of the deepest convection.

Figure 3. Latest infrared satellite image of Tropical Depression Eight-E. Image credit: NOAA's Satellite Services Division (SSD).

The depression appears to be near the threshold of tropical storm strength, and it is probably one as I write this. Environmental conditions appear quite conducive to intensification, with the possibility of rapid intensification during the next 24-36 hours if the cyclone can develop an inner core. Water vapor imagery shows northwesterly upper flow to the north of the system, but the orientation of these winds relative to the cyclone center is such that the depression is experiencing less of a shearing influence and more of an expanding outflow mechanism, particularly in the northern quadrant of the circulation. This weakly diffluent environment is favorable for intensification, and the GFS builds an anticyclone over the depression over the next couple of days.

I am a little hesitant to forecast rapid intensification for more reasons than one, but most important of all, it's because, as the National Hurricane Center said in the 0300Z forecast discussion for the storm, Gil struggled immensely in roughly the same location as the depression less than a week ago, and memories of that are still fresh in my mind. While rapid intensification is not being explicitly indicated lacking any definitive proof that it's going to happen, I do suggest a quick rate of intensification over the next 72 hours, with the cyclone reaching a peak of 75 kt in 72 hours; this forecast is a little bit above the latest intensity consensus, and similar to the SHIPS model. Beyond 72 hours, sea surface temperatures cool along the forecast track, and the environmental airmass becomes drier. As a result, steady weakening is forecast at later time periods, and it is certainly possible this weakening could be more rapid than I have indicated below.

Like with Gil, the synoptic steering environment around the depression is well-defined and the mechanisms fairly straightforward. The tropical cyclone is south of a mid-level ridge, and appears to be pretty much on track with the 0300Z NHC forecast. A continued mostly westward motion is anticipated for the next 36-48 hours, after which the global models show the depression nearing a break in the subtropical ridge caused by a large upper low off the coast of California. The models show this feature amplifying and moving southward, dragging a mid- to upper-level trough with it. Even though the depression is expected to weaken late in the period, it may not do so fast enough to avoid threatening Hawaii next weekend, and interests there should begin to monitor the progress of this system over the next several days. The global model synoptic fields indicate that, should the depression weaken more abruptly on days four and five, it would have a propensity toward the lower-tropospheric flow, which would consequently induce a more westward motion well to the south of those islands. Even if the depression does somehow come close to Hawaii, it will likely meet the same fate as Flossie, and only bring rain showers to the archipelago.

Intensity forecast and positions

INITIAL 08/04 0300Z 12.7°N 124.9°W 30 KT 35 MPH
12 hour 08/04 1200Z 12.6°N 126.0°W 35 KT 40 MPH
24 hour 08/05 0000Z 12.6°N 127.1°W 45 KT 50 MPH
36 hour 08/05 1200Z 12.7°N 128.5°W 50 KT 60 MPH
48 hour 08/06 0000Z 13.3°N 130.3°W 65 KT 75 MPH
72 hour 08/07 0000Z 15.1°N 135.1°W 75 KT 85 MPH
96 hour 08/08 0000Z 16.3°N 138.3°W 65 KT 75 MPH
120 hour 08/09 0000Z 17.5°N 142.5°W 50 KT 60 MPH

Track forecast

Figure 4. My forecast track for Tropical Depression Eight-E.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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2. WunderAlertBot (Admin)
6:05 AM GMT on August 05, 2013
KoritheMan has created a new entry.
1. wxchaser97
7:23 AM GMT on August 04, 2013
Thanks Kori!
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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