Keeping the status quo so far this year, The Tropical Western Pacific will likely remain relatively quiet for at least the next week. Upper level subsidence will continue to rule, and any disturbance that does manage to gain at least minimal organization will likely struggle with a PV Streamer-enhanced Tropical Upper Tropospheric Trough. A disturbance originating from the tail end of a lifting trough south of Japan and a potential disturbance on a monsoon trough near the Philippines have some chances at development, but an absence of tropical cyclogenesis is the most likely outcome over the next seven 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 biggest reason I am currently so bearish with development prospects has to do with upper level subsidence. Guidance is persistently painting the Tropical Western Pacific south of 25*N or so with heights of 1-2 standard deviations above normal (seen below in the EPS 500 mb heights and normalized anomalies from 12Z, July 9). Now, this is in the tropics, so given the relatively small variability in heights aloft, a 1 or 2 SD increase in heights isn’t resulting in a death ridge. However, these heights are a symptom of something else occurring: upper level subsidence. This sinking aloft helps to squelch typical tropical convection. It isn’t an end all of potential tropical cyclones, but it takes a disturbance featuring more robust low level convergence to overcome this impediment. Additionally, an anticyclonic wave break in the Mid-Latitude North Pacific should reenforce the Tropical Upper Tropospheric Trough, stretching it east/west across much of the basin between 15-20*N (illustrated in 18Z GFS’s depiction of 355K cyclonic PV from July 9). The long and short of it all is that the Tropical Western Pacific is not particularly favorable for tropical development at the moment.
The first area of interest that has at least some small potential for development currently exists south of Japan (pictured and annotated below). An area of convection has developed with a bit of mid level vorticity left behind by a parent mid-latitude trough.
Steering currents should remain weak for the next couple of days in the subtropical waters south of Japan, so the orphaned disturbance will not be moving much. Modest northerly shear of around 15 kt and a punch of dry air from the mid-latitudes (both pictured from July 9 18Z GFS output below) should keep the disturbance from developing before rejoining the mid-latitude flow in 3-4 days, but since it will be sitting over water as warm as 30*C, it doesn’t hurt to keep an eye on it. Still, I put development chances well below 10%.
Part way through the week, a surge in the southwest monsoon from the Bay of Bengal should help sharpen the monsoon trough in the vicinity of The Philippines (pictured below via 12Z ECMWF MSLP and 850 mb winds initialized July 9). This should lead to increased convection over the South China Sea and western Philippine Sea. A surge in the southwest monsoon, even a more modest one like currently modeled, is often a winning formula for tropical cyclogenesis this time of year, but considering the upper level subsidence that exists across most of the basin, it’s not surprising that guidance is only lukewarm with development prospects this go around. Canadian guidance is the most bullish with development (as is typical), while American guidance is very bearish (no GEFS members develop a system). European guidance lies somewhere in the middle. Short term trends with European guidance have trended towards some sort of development just east of The Philippines, but it not at a confidence inspiring level as of yet. The operational ECMWF picked up development for the first time on the July 9 12Z cycle with at least modest ensemble support (12Z July 9 EPS MSLP normalized spread pictured below). Considering any development would still be 5-6 days from now, more consistency will be needed in successive guidance suites to accurately ascertain proper development prospects, but I’d probably put a 30% chance or so of development from the monsoon trough on either side of the Philippines by the end of the week.
My schedule does not look particularly heavy this week, so I may be able to fit in a mid week update this week. If not, the next entry will arrive around this time next week. Until the next entry is posted, analyses and updates in forecast philosophy will arrive in the comments section.