Western Pacific Tropical Analysis: July 26, 2017

Since my last post a week ago, an amazing five tropical storms have developed across the Tropical Western Pacific. Typhoon Noru and Tropical Storm Kulap developed over the basin’s subtropics, very small Tropical Storms Roke and Sonca developed on either side of Luzon, and the recently named Tropical Storm Nesat has emerged from the monsoon trough in the western Philippine Sea. Noru, Sonca, and Nesat are still active currently, although Tropical Depression Sonca hardly counts since it is now well inland over Southeast Asia. It is possible that additional storms may develop in the coming days, but thanks in part to weak steering currents, the establishment of a reverse-oriented monsoon trough, and possible storm interactions, the predictability in the current forecast period is remarkably low.

***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.***

Having existed for over five days now, Typhoon Noru is the longest-standing storm in the basin and, if some of the guidance solutions are to be believed, could possibly still be hanging around ten days from now. As of 1500Z July 26, a satellite bulletin from JTWC placed Noru at 30.18ºN, 154.59ºE. JMA’s and JTWC’s latest intensity estimates for Noru are 70 kt and 80 kt, respectively, making it the first typhoon of the 2017 Pacific Typhoon Season. Noru has spent its life thus far looping around the subtropical waters of the Western Pacific.

Noru’s steering has been dominated by two factors the past several days, a blocking mid-latitude ridge and Tropical Storm Kulap. The former has prevented Noru from escaping north into the mid-latitudes, while the latter resulted in Fujiwhara interaction between the two systems. The interaction helped to keep Noru further east in some more favorable conditions while Kulap was flung westward to Noru’s north, where it met its demise in more hostile conditions. With Kulap now gone, Noru is again able to move west and then west-southwest as the primary steering mechanism transitions from a mid-latitude blocking ridge to a subtropical ridge anchored over the northern East China Sea between China, South Korea, and Japan. Members of guidance are actually in good agreement in the near term with this scenario. Conditions along the way will likely be similar to what the system is experiencing now: enough dry air to keep a lid on intensification, but favorable enough otherwise. I would not be surprised to see Noru remain within 10 kt of its current intensity as it carries out its west-southwestward dip over the next four days.

From then on, the forecast gets more dicey. After completing the west-southwestward dip, Noru will be over warmer waters than those found near 30ºN where the system currently is, but also in close proximity to Nesat and a strengthen monsoon trough extending eastwards. In other words, steering becomes very complex, and forecasting beyond four days out becomes a nightmare. The current trend in guidance has Noru heading back east beginning around day five around 25ºN while possibly interacting with new monsoon trough development, but previous solutions have shown everything from escaping north to interacting with Nesat until one or the other’s demise. Some guidance (particularly American guidance) continues to blow Noru up into an exceptionally powerful storm, but I remain skeptical of such solutions. This skepticism has paid off thus far, as Noru has yet to become a major typhoon. Forecasting intensity with so much uncertainty with everything else remains a fool’s errand anyhow, but if forced to put out an intensity forecast for the taus beyond hour 96, it would probably remain very near the current analyzed intensity.

The following tweet from Philippe Papin explains not only the uncertainty with Noru, but in the entire basin far better than I can.


I want to briefly touch on Tropical Depression Sonca. Sonca made landfall along the Vietnam coast over 24 hours ago from the time of this entry, but the system has held together amazingly well as it treks inland across Southeast Asia. As mentioned, JMA is still classifying Sonca as a tropical depression despite having crossed about half way to the Bay of Bengal. As expected from a system well inland, Sonca’s biggest threat will come in the form of heavy rain.

Tropical Storm Nesat is the newest of the currently classified systems in the Tropical Western Pacific. As of 1800Z July 26, a JTWC satellite bulliten placed Nesat at 17.40ºN, 127.64ºE. Nesat is the system I first mentioned in my July 16th entry that would consolidate along the monsoon trough in late July. Nesat’s development has been just about the only thing that has been predictable in the Tropical Western Pacific recently, and keeping with the recent theme, that predictability only goes down from here.

As seen in the loop above, Tropical Storm Nesat is characterized by strong, cold central core convection. However, it appears that the system may be taking on the Central Cold Cover pattern, or CCC pattern. Such patterns are usually a result of some mid-level shear undercutting the anvil, and bands 9 and 10 from Himawari-8 do indicate that light northerly shear of this variety is occurring. A recent GPM pass also indicates that most of Nesat’s convective structure is confined to the south of the center. Considering Nesat’s present state, development in the immediate short term if any should be slow to occur.

Nesat is currently moving with a slow north-northwestern motion. This general motion to the northwest or north-northwest should continue for the next couple of days. At the time, it appears that will remain far enough away from Noru to avoid any direct interaction with the typhoon, but such a scenario cannot be completely ruled out at this time, especially if Nesat tracks a little slower than currently expected. As it stands right now, Nesat is expected to be near Taiwan by tau 72. Northerly and then easterly shear may be something that Nesat has to contend with its entire life, but it is possible that the system could intensify into a typhoon before landfall.

92W is one of the two invests currenly declared across the Tropical Western Pacific. Located in the South China Sea, 92W is developing along the same monsoon trough that Tropical Storm Nesat emerged from. It is possible that development could occur with this invest, but easterly shear, partially from the larger and stronger Nesat, would likely keep any development weak before moving inland into southern China.

Invest 93W is the other active invest. The system is located well east in the basin’s subtropics, near where Kulap originated from. At the moment, I do not expect development due to the magnitude of vertical wind shear, but this is an area worth keeping an eye on, especially once Noru moves WSW and helps draw out monsoon southwesterlies across the basin.

As mentioned early on in this post, more development is possible over the next several days as the monsoon trough begins to extend well east across the entirety of the basin. The monsoon trough is expected to take on a reverse-orientation (more north the further east along the monsoon trough, opposite of a typical setup), extending the eastern end well above 20*N. Such a scenario offer up not only an opportunity for multiple new storms, but also interactions between these yet to develop system. Guidance almost always struggles with such a scenario, but most members agree on the general idea. The specifics remain to be seen though, and I’m not even going to pretend to know how it will all play out. Partially for entertainment purposes, the 12Z July 26 GFS run is provided below.

Thank you all for being patient with me getting this new post up as I have been dealing with moving into a new place. The next update should be up this weekend. Until the next entry is posted, analyses and updates in forecast philosophy will arrive in the comments section.


2 thoughts on “Western Pacific Tropical Analysis: July 26, 2017”

  1. Noru is still struggling with massive amounts of dry air, keeping the convection weak, shallow, and unorganized. The 00Z RJAO (Chichijima) sounding featured a rather dry profile above 850 mb, and this is the airmass that is circling Noru.


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