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Unseasonably stout wave in Caribbean bringing enhanced showers and tstorms for several days, but lacks model support for development 5/26
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CoconutCandy
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'Explosively' Developing Invest 97C Nearing the Hawaiian Islands
      #89628 - Wed Sep 22 2010 02:43 AM

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.



------------------------------------------------------



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.



========================================
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."
========================================

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 11:46 PM)


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CoconutCandy
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Re: Developing Invest 97C Near the Hawaiian Islands [Re: CoconutCandy]
      #89639 - Wed Sep 22 2010 10:55 PM

Interesting developments with Hawaii's Invest 97C since yesterday ...

Although convection did flare somewhat during the overnight hours (local basin time!), it wasn't nearly to the degree and coverage that I had anticipated with my last post. It appears that there was *just enough* SW'erly upper level wind shear to impede the development of the type of 'Bursting Convection' I had mentioned.

In fact, it appeared that, due to the shear, the LLCC had elongated NE to SW, with the deepest convection restricted to the 'eastern side' of the disturbance. Essentially, the disturbance was not very well 'aligned in the vertical', and I actually thought that the system was becoming *completely decoupled*, with the elongating LLCC 'outpacing' the convection, and diving towards the SW, leaving behind the dwindling convection in it's wake.

A series of passive microwave satellite images from polar orbiting satellites portrayed an increasingly lop-sided system, with one image in particular suggesting that the (elongated) LLCC had already crossed west of 155 degrees west longitude.



However! After carefully studying this mornings animated visible satellite imagery, it's becoming increasingly evident that a *NEW* LLCC is forming, co-located under the deepest convection. As is sometimes the case with developing systems before they 'consolidate', 2 LLCC's will form at some distance from one another, with one or the other eventually becoming the dominate one. Just such appears to be the case this morning. ( Remember, Hawaii is 6 HOURS behind EDT !! )



Careful scrutiny of the 1 km zoomed animated loops this morning reveals distinct, curvilinear banding features developing to the west and south of this new LLCC, which remains distinctly still to the EAST of 155 west.

I surmise that the next 'fix position estimate' will RELOCATE the center just east of 155 W, rather than continued 'fixing' on the fading, elongated LLCC, still drifting off to the SW.

After increasing the 'probability of development' to 'HIGH' (60%) at 8pm last evening, Hawaii time, they've backed off slightly to a 'Medium' (50%) probability as of 8am this morning.

Recent model guidance suggests a continued WNW track for another day or so, followed by a gradual turn towards the WSW. And Intensity guidance still does not bring this disturbance up to storm strength. But I suspect the models are still 'fixing' on the now-waning elongated LLCC, still drifting off to the SW. We'll see what guidance does later today, once it begins to focus on the newly developing LLCC.

To my eyes, it seems that the system is now moving slowly towards the NW or even NNW, bringing it ever so slightly closer to the Big Island of Hawaii. And, since Hawaii (especially the Big Island and Maui) is still in a serious drought (complements of last winters' moderate El Nino episode, which typically brings drought conditions to the Islands), perhaps the BEST thing that could happen is for this little disturbance to bring some much needed beneficial rains to these southern Islands!

----------- UPDATE ----------------

The 'elongated' LLCC, instead of fading, has persisted and has become much more symmetrical (circular) and increasingly showery over the past few hours. However, the meager deep convection (thunderstorms) are (at this writing) only present in the NE quadrant.



Meanwhile, the "New" LLCC that I had alluded to above ALSO persists, and by far, MOST of the deep convection remains revolving around THIS center, as is plainly discernible in animated visible imagery.



Hmm .... A case of the 'Dueling' LLCC's !! Wonder which one will win out and become the dominate circulation center ??

OR! Perhaps what I've been noticing and calling a 'new' LLCC is *actually* a (remnant?) MID-LEVEL circulation center; an artifact of last nights convective flareup !! The LOW level center remains the 'fix position' for the center of the disturbance.

OR! Is it possible that the circs will bifurcate (split) with one heading generally west and the one to the NE will drift slowly WNW or NW and become 2 *SEPARATE* Invests ??? THAT would be a FIRST, in my personal recollection.

Any thoughts or comments about this unusual invest? Think it still has a shot at developing into a depression? Sooner, or later?

More coming soon ... stay tuned!

...

--------------- New Theory ---------------

Actually, it's just occurred to me. I now think that what we're seeing is *NOT* 2 LLCC's, but rather, the LLCC 'outpacing' the *mid-level circulation center* as it progresses westward.

The deeper convection still seems to be more associated with this 'supposed mid level center', while the LLCC is mainly exposed, except in the NE quad, as mentioned above, where there is some meager convection that might be considered to be associated with the LLCC proper.

Still a few hours from sunset here in Hawaii, so it will be interesting to see what becomes of all this in the overnight hours, local basin time.

Sure would be NICE if the Big Island could get some much needed rain out of this!


Edited by CoconutCandy (Thu Sep 23 2010 03:15 AM)


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