Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 1:23 AM GMT on July 31, 2013
Tropical Storm Gil quickly formed today. As of the latest NHC advisory (new one is due out in just under two hours), the following information was posted on the cyclone:
Wind: 40 mph, with higher gusts
Location: 12.7°N 116.1°W
Movement: WNW at 15 mph
Pressure: 1005 mb
The satellite presentation of Gil is fairly impressive, with a large curved band to the west. Microwave data throughout the day has suggested a formative mid-level eye feature with the possibility that a primitive eyewall could be attempting to form aloft. However, this feature has not been readily apparent in conventional satellite images. Convection has increased near the center, possibly signifying the beginnings of a central dense overcast.
Figure 1. Latest infrared satellite image of Tropical Storm Gil. Image credit: NOAA's Satellite Services Division (SSD).
For the most part, environmental conditions appear conducive for continued intensification, with very warm waters, weak vertical shear, and the small size of the circulation. One potential negative factor -- probably why the guidance is not forecasting rapid intensification -- is an area of dry air to the west of Gil on water vapor imagery; the airmass becomes particularly dry near 125W. One additional factor supporting the possibility of rapid intensification with Gil (in addition to the microwave passes) is an upper low to the north of the circulation moving west in tandem with the tropical cyclone. This ridge is forecast by the GFS to move westward in tandem with Gil for the next 48 hours while gradually weakening; the distance of the upper low is such that it will act less to shear the cyclone and more to assist in aiding its outflow. The SHIPS rapid intensification index as of 18z shows a 40% chance of an increase of 30 kt in the next 24 hours. None of the guidance forecasts to Gil to become anything more than a minimal hurricane, and even those appear to be confined to the outlying statistical guidance. Closer to the SHIPS guidance, which seems reliable in this case, my forecast assumes Gil will reach a peak of 70 kt in 72 hours; however, the intensity forecast at longer ranges could be conservative if the eye feature observed on microwave images translates down to the surface, leading to the formation of an inner core. Beyond day three, the models show westerly shear increasing, and sea surface temperatures will begin to cool. Gradual weakening is forecast at that time.
Gil is south of a mid-level ridge, and is moving west-northwestward. A blend of satellite and microwave fixes show that the cyclone more or less remains on track with the 2100Z NHC track prediction. The synoptic pattern ahead of Gil appears straightforward, with an upper low off the coast of British Columbia diving southeastward, bringing its trailing mid-level trough with it. The global models respond to this pattern by forecasting a general west-northwest motion for the next 72 hours, after which time the trough is forecast to lift and Gil is expected to turn toward the west. There could be a weak mid-level perturbation northeast of the Hawaiian Islands near the end of the period that could be capable of turning Gil northward closer to the state, but it is too early to be confident of its strength and potential impacts on the cyclone's poleward motion at those times.
Intensity forecast and positions
INITIAL 07/30 2100Z 12.7°N 116.1°W 35 KT 40 MPH
12 hour 07/31 0600Z 13.3°N 118.2°W 45 KT 50 MPH
24 hour 07/31 1800Z 13.9°N 120.6°W 50 KT 60 MPH
36 hour 08/01 0600Z 14.5°N 122.8°W 55 KT 65 MPH
48 hour 08/01 1800Z 15.3°N 125.3°W 65 KT 75 MPH
72 hour 08/02 1800Z 16.4°N 129.5°W 70 KT 80 MPH
96 hour 08/03 1800Z 16.6°N 132.4°W 60 KT 70 MPH
120 hour 08/04 1800Z 16.7°N 136.8°W 50 KT 60 MPH
Figure 2. My 5-day forecast track for Gil.
An area of low pressure centered a few hundred miles east of Tropical Storm Gil is also quickly organizing. Satellite images show evidence of convective banding to the north, and previous microwave data hinted at this as well.
Figure 3. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD).
The development potential/intensity forecast for this disturbance are rather mixed. None of the guidance suggests significant intensification, and several of the global models -- including the GFS -- fail to initialize or develop it. The ECMWF, which seems to rarely develop cyclones, does show development eventually. I don't see much evidence of vertical shear over the system at the moment, but the GFS assumes that there could be some northwesterly shear in the next couple of days, likely emanating from Gil's outflow. Complicating matters further is the possibility of a Fujiwhara interaction between this disturbance and Gil. Although the circulation of Gil is small, so this one, and the former's circulation envelope appears to be healthier overall, so I presume Gil will ultimately prevail.
Regardless, conditions do appear favorable for the present time, and I expect this disturbance to become a tropical cyclone in the next 24 hours. Preliminary track guidance suggests a motion to the west-northwest, which seems consistent with current trends and the large-scale pattern.
Probability of development in 48 hours: 60%
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.