Aloha Friends!
Been away most of the season from posting and commenting, but it's nice to be back with something interesting to share with you!
A weak, low level vortex embedded in the Central Pacific Monsoonal Gyre these past few days has begun to develop 'explosively' in the past 12 to 18 hours or so.
And a Tropical Cyclone Formation Alert (TCFA) has recently been issued for this compact, robust little Invest. So apparently the Central Pacific Hurricane Center (CPHC), the Joint Typhoon Warning Center (JTWC), the Navel Research Lab (NRL) and the Fleet Numerical Meteorology and Oceanography Center (FNMOC) feel there are justifications to warrant the posting of a formation alert.
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Additionally, the CPHC has just revised their 'Graphical Tropical Weather Outlook' product to reflect the increasing probability of tropical cyclogenesis, now at 30%, or the beginning of the 'Medium' range.
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CPHC: "An area of showers and thunderstorms is located about 535 miles south southeast of Hilo, Hawaii, and has been moving west northwest near 10 mph. Although thunderstorms have decreased in coverage during the day, slow development is possible. There is a medium chance, 30 percent, of this system becoming a tropical cyclone during the next 48 hours."
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NOTE the forecaster wrote "Although thunderstorms have decreased in coverage during the day ..." to implicitly acknowledge the typical diurnal convective minimum (the sun's heating of the upper levels of the troposphere) and a general warming of the cloud tops. But I suspect that, with the rapidly approaching sunset (local basin time) and the overnight diurnal convective maximum (when the lack of the overhead sun causes the upper troposphere to cool, thus slightly increasing the lapse rate, and promoting the development of bigger, stronger, deeper and more intense thunderstorms) we may very well see some "Bursting Convection", with large cells attaining very cold cloud top temperatures, perhaps in excess of -80 degrees C or more. In a nutshell: Look for large and strong and persistent, or at least cycling, thunderstorms to form very near or over the already very well developed low level circulation center (LLCC).
Pressures have already fallen a least 1 mb since this morning (Hawaii time!) and the anticipated deep, sustained thunderstorms overnight, overlying the LLCC, will surely drop it a few mb's further. It *appears* that tropical cyclogenesis is already well underway.
Early Model Guidance (which can be a bit sketchy while a developing system is still in the 'Invest' stage) seems in fairly good consensus, tracking the system generally NW or WNW through the next day or two, then almost unanimously turns the system towards the WSW or even the SW, as the system will apparently traverse into an area of increasing SW'erly shear, thus collapsing the deep convection and allowing a shallow, low level system to be 'steered' by the prevailing NE'erly trade wind flow.
The SHIPS intensity forecast, too, reflects this scenario by *slightly* strengthening the disturbance initally, but then weakening it, only to restrengthening it again as the shallow system (by that time well WSW of the Hawaiian Islands) finds itself in increasingly warmer Sea Surface Temperatures (SST's) and decreasing upper level shear once again.
The BIG question is, at this point, will the upper shear between the Tropical Disturbance and the Hawaiian Islands slacken, thus allowing a continued NW or WNW motion, or will the disturbance become sheared sufficiently to allow the trades to drive a shallow system towards the SW?
----------- More when I can find the time !! -------------
Edited by CoconutCandy (Wed Sep 22 2010 07:46 PM)
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