Western Pacific Tropical Analysis: August 2, 2017

Typhoon Noru continues to spin across the Tropical Western Pacific, and the system has been joined by Tropical Storm Nalgae well to the east. Nalgae appears to be no threat to land, but the same cannot be said about Noru. Noru appears to be headed on a collision course for southern Japan and possibly South Korea. Noru has come back down to earth after a period of explosive intensification a few days ago, and some guidance solutions showing similar strengthening are likely overdone, but some strengthening remains possible before final landfall. Aside from the two active storms, no additional tropical activity appears likely over the next several days.

***NOTE: While I would consider myself well-learned in meteorology, I am still a student with more to learn before becoming a degreed meteorologist. This forecast is not from an official source and should not be treated as such. For official information, please refer to your local weather agency.***

The ever persistent Typhoon Noru is now approaching two weeks as a named system, an impressive feat. Noru has also generated the most ACE of any Pacific Typhoon since Goni and Atsani of 2015, and it will likely be passing them soon as well. As of 21Z August 2nd, the center of Noru’s now very large eye was located at 27.1ºN, 135.3ºE per JMA. JMA and JTWC’s latest intensities for the system are 85 kt and 95 kt, respectively. These are at or just below (respectively again) each agency’s T5.5 intensity. However, considering the particularly large eye and somewhat lackluster convective appearance, I have a suspicion that these intensities are perhaps a little generous. Regardless, Noru has come back down to earth after explosively intensifying into the first SSHWS category 5 tropical cyclone of 2017 a few days ago.

The defining feature of Typhoon Noru at the moment is the system’s massive eye. The eye both on infrared and microwave imagery measures about a degree and a half, or 90 nautical miles, in diameter. While not the largest tropical cyclone eye observed, it is in the upper percentile. This eye size is even more rare considering it lacks a remnant inner eyewall within it. With such a large core, the upper level outflow needs to be working overtime to properly vent the system. This has not been the case recently. Analyses have shown outflow to the system’s northwest restricted by a passing trough and associated PV streamer. This trough is now departing though, and there are already some early signs that the outflow is beginning to recover.

The aforementioned trough has been a major source of uncertainty in previous forecasts. Noru’s primary steering mechanism for the past several days has been a subtropical ridge to the north. The trough that is in the process of passing by weakened the ridge, and it was initially uncertain if enough of a weakness would be created to allow Noru to escape poleward. However, it now appears that Noru will not be escaping poleward. As the mid-latitude trough departs, the subtropical ridge should restrengthen some to Noru’s north, forcing the system slowly westwards towards the northern portion of Japan’s Ryukyu Islands. This west motion should continue for at least the next two days and at most three. After that, Noru is expected to be picked up by the next trough and recurve to the northeast. The exact recurve point is not yet known, but it does appear like it will occur either in the vicinity of the northern Ryukyus or in the East China Sea just to the west. This means landfall could still come in a variety of locations from anywhere on Kyushu to eastern South Korea, but considering that I expect Noru to continue maintaining an exceptionally large core, the worst of the typhoon could spread unusually far from the centerline. At the moment though, I expect a western Kyushu landfall, near the center of the guidance envelope.

As has been the case with Noru for all its life, the intensity forecast remains far from a walk in the park. Much of guidance has been blowing Noru back up into a very powerful system in the vicinity of the northern Ryukyus. American guidance in particular has been insistent on a system well below 900 mb. However, I continue to remain skeptical of such solutions, perhaps even more now than ever before. As mentioned previously, I expect Noru to maintain an exceptionally large core for the remainder of its life, which will need near perfect outflow for any strengthening to occur. Additionally, upwelling could become an issue for the large, slow moving storm. A strenghening signal does appear to be nearly unanimous in the vicinity of the Ryukyus, and this does seem to be in response to improving upper air conditions. The degree of strengthening is almost certainly overdone though. The HWRF more or less maintains the large cored Noru as a higher end SSHWS category 2 up until landfall, which appears to be a reasonable intensity forecast to me.

The other active system is Tropical Storm Nalgae, with formed from the reverse oriented monsoon trough well to the east of Typhoon Noru. In fact, the location of Nalgae’s birth is almost exactly the same as Noru’s. Nalgae is expected to strenghen some, perhaps nearly or just meeting the criteria for a minimal typhoon before being picked up by the same trough that just missed picking up Noru, sending the system north into the mid-latitude North Pacific.

Development over the rest of the Tropical Western Pacific looks unlikely over the next several days. There is an outside shot that Invest 98W, located to Tropical Storm Nalgae’s east, could organize enough to become classified, but considering the hostile conditions currently assaulting the invest, I do not expect this outcome to transpire.

My next post is slated for either Saturday or Sunday. Until the next entry is posted, analyses and updates in forecast philosophy will arrive in the comments section.

1 thought on “Western Pacific Tropical Analysis: August 2, 2017”

  1. I forgot to mention it in the parent post, but I’d probably analyze Noru at 80-85 kt (1 minute sustained), or between the T4.5 and T5.0 intensities right now, based largely on a surprisingly tight SATCON consensus.

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