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Tropical weather analysis - November 2, 2013

By: KoritheMan , 6:12 AM GMT on November 02, 2013

Tropical Depression Eighteen-E

Tropical Depression Eighteen-E has not changed much as of the 0300Z NHC advisory:

Wind: 35 mph, with higher gusts
Location: 16.9°N 108.4°W
Movement: NNW at 3 mph
Pressure: 1006 mb

The depression is not well-organized, with satellite and microwave data depicting multiple embedded swirls encompassing the broader cyclonic gyre comprising the system. Satellite estimates do not support tropical storm strength, and earlier scatterometer data during the evening suggested that the strongest winds were located to the west of the center, undoubtedly due to 15-20 kt of easterly shear as diagnosed by UW-CIMSS. There are two clusters of convection currently ongoing west and north of the center, but I am not apt to call either of these features curved bands. Given the persistence of the northern cluster, I would look for a possible center reformation in that area, where the shear appears to be less.

Figure 1. Latest infrared satellite image of Tropical Depression Eighteen-E. Image credit: NOAA's Satellite Services Division (SSD).

The depression presents me with a tricky intensity forecast this evening. The easterly shear over the system will likely inhibit development in the near-term, but the 0z GFS shows a more favorable 200 mb wind pattern evolving during the next 24 hours, potentially allowing strengthening while over warm sea surface temperatures. However, it is notable that the upper air pattern in the 18z GFS looks much more unfavorable than the one at 0z, with the shear not forecast to decrease until about a day or less before landfall. I am not ready to jump on the 0z GFS as completely credible just yet, given the current hostile environment and lack of organization of the depression, so I will base my forecast more on the 18z GFS for now. If the shear decreases faster than forecast, the cyclone could reasonably get to 45 or 50 kt before making landfall. One other potential complicating factor for intensification is an area of dry air to the west of the depression. While this could delay development somewhat, the upper-level flow across the cyclone is easterly from a moister airmass to the east, a situation that is not conducive for significant dry air entrainment. After landfall, the depression is forecast to quickly dissipate over the rugged terrain of southwestern Mexico, although a remnant mid-level circulation capable of dropping heavy rains and causing flooding will likely linger for another couple of days. Another complicating factor is the large size of the depression, which may inhibit swifter intensification even if the shear abates as the 0z GFS forecasts.

The center has not exactly been easy to locate; fortunately, a couple of well-timed microwave passes gave an indication of where the center might be based on where the NHC officially has it. But again, I note that there are numerous embedded swirls within the cyclonic circulation comprising the depression, making any estimate of the center (my own is given below) highly uncertain; even the microwave passes were inconclusive. Assuming the depression does not reform to the north, it should continue a gradual recurve around the western periphery of the subtropical ridge which is forecast to shift eastward over the next 24 hours as a very large upper-level cyclone a few hundred miles off the coast of California moves northeastward and fragments into a trough. Landfall should occur over mainland southwestern Mexico in the Puerto Vallarta area in about two and a half days, possibly sooner. If the depression reforms to the north, the immediate track may bend a little bit to the left before the westerlies grab it; this would increase the threat to southern Baja, which currently lies west of the official NHC forecast track. My own track is a little to the east of the current NHC prediction, as I feel the upper cyclone will exert more of a poleward influence than currently expected.

Tropical storm watches and warnings will probably be issued by the government of Mexico for southern Baja California Sur and mainland southwestern Mexico later today, since the average lead time for a tropical storm warning in the eastern Pacific is 36 hours, while the average lead time for a tropical storm watch is 48 hours.

Intensity and forecast positions

INITIAL 11/02 17.0°N 109.0°W 0600Z 30 KT 35 MPH
12 hour 11/02 17.7°N 109.7°W 1800Z 30 KT 35 MPH
24 hour 11/03 18.4°N 110.3°W 0600Z 35 KT 40 MPH
36 hour 11/03 19.5°N 110.3°W 1800Z 35 KT 40 MPH
48 hour 11/04 22.9°N 108.3°W 0600Z 40 KT 45 MPH
72 hour 11/05 28.5°N 105.4°W 0600Z 25 KT 30 MPH...INLAND POST-TROPICAL/REMNANT LOW
96 hour 11/06 0600Z...DISSIPATED

Track forecast

Figure 2. My forecast track for Tropical Depression Eighteen-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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