Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 2:44 AM GMT on May 08, 2014
A broad area of low pressure accompanied by widespread cloudiness and thunderstorms continues moving toward the coast of southern Mexico. In yesterday's forecast, I was pretty solidified that we would see Tropical Storm Amanda heading toward the coast, but instead the circulation was not as well-defined as I was led to believe, as evident by a 1623Z ASCAT pass, which I will happily display below.
Figure 1. A 1623Z ASCAT pass captured the disturbance. Notice the two competing areas of vorticity (the actual mean center is the swirl farther to the south), and the lack of a well-defined center within the disturbance.
Satellite images show an extensive area of convection, some of it fairly deep, but recent microwave data and last-light visible imagery suggest that the low-level center remains hoisted to the south of the most intense shower activity.
Figure 2. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD).
Upper-level outflow is becoming restricted to the south, signifying an abrupt but well-forecast increase in southerly shear; this is due an amplifying downstream upper-level trough. This upper-level trough is also forcing a severe weather episode across portions of the Rockies/central plains region, with a myriad of large hail reports. Anyway, the increase in shear is further evident with a homogeneous comparison of microwave data (shown below), with the most recent microwave data showing the mid- and upper-level circulations displaced quite a bit.
Figure 3. Homogeneous comparison of the 37 GHz and 91 GHz channels on the latest SSMIS microwave pass, taken at 0022Z (current time rounded is roughly 0130Z as of this writing). The 37 GHz channel is more sensitive to the lower levels, while the 91 GHz channel is more sensitive to the mid- and upper troposphere. The low-level center can be fairly easily identified in the first image roughly around 15.6N 106.1W. In the second image, the circulation is located closer to 16N 104.8W.
While the recent acceleration of the system theoretically leaves open the possibility of the lower- and mid-level centers becoming somewhat better aligned vertically again, shear over the system is already about 20 kt over the system, and the GFS/SHIPS assume it will increase to over 30 kt in the next 24 hours. There is no physical way the low-level center will bolster the accelerator to 30 kt during this period, so the end result is that we're left with the mid-level center rapidly racing northeastward across southern Mexico, enhancing precipitation and flooding in that area while the low-level center gets strung out, left behind, and gradually weakens at or near the coast tomorrow. Indeed, the global models have unanimously become less aggressive in terms of tropical cyclone development from this area, and it looks like 1990's Hurricane Alma will maintain the status quo of the recordholder for earliest formation of the first eastern Pacific named storm (May 14, tied with Tropical Storm Aletta's record set in 2012).
To add insult to injury, satellite data show arc clouds emanating westward away from the system, which is a classical sign of dry air entrainment, likely from the subsident portion of the upper-level trough, and probably exacerbated by the local marine layer.
Even if this system does somehow consolidate into a short-fuse tropical storm, environmental conditions are quickly becoming unfavorable for development as the system approaches the coast. Regardless of development, heavy rainfall and flooding remain a threat across southern Mexico, particularly over areas of enhanced terrain. Any threat of locally gusty winds will be extremely short-lived and diminishing at the time of arrival as the overall circulation decays under the increasing shear and land interaction.
Moisture from this system may interact with the approaching frontal boundary and enhance heavy rainfall over portions of the southern plains and possibly the upper midwest/Great Lakes region through the weekend.
Probability of development in 48 hours: 40%
Probability of development in 120 hours: 40%
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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