Tonight, we’re tracking a Low Pressure Area along the western Caroline Islands that appears will threaten the Cagayan Valley late Monday/early Tuesday.
First, let’s address some nomenclature. Gather a group of meteorologists in a room and chances are, there will be slight variances in the forecast. This science is not quite as objective as many would like… there is still an art to it. …and with that art, is each forecaster’s creative license. In the case of this system, the lead weather agency, Japan Meteorological Agency (JMA), is carrying this system as a Low Pressure Area as of 0600 UTC on Thursday. Meanwhile, the United States’ Joint Typhoon Warning Center (JTWC) declared the disturbance as Tropical Depression Fifteen (15W) earlier on Thursday. Following suit, the Philippine Atmospheric, Geophysical & Astronomical Services Administration (PAGASA) also identified the area as a tropical depression. However, since the storm still is east of the Philippine Area of Responsibility (PAR)–which begins at 135°E–it remains without a Filipino name and the full suite of tropical cyclone advisories are not being produced.
So with that said, models project this system to continue to strength. JMA is forecasting this to become a tropical depression on Friday. Meanwhile, the low should transit into the PAR late Thursday or early Friday where it will be christened Luis. If this storm develops further into a tropical storm (which the models indicate), JMA will name the storm Kalmaegi, a name donated by North Korea representing a sea gull.
Now, let’s get into some meteorology: Let’s address the current state of this low / tropical depression / area of disturbed weather. At 1200 UTC Thursday, Dvorak analyses for the low indicated that the convection is wrapping a little better. JTWC analyzed the wrap at 2.5/10, which results in a Data-T of 1.5… the basis of the Final-T of T1.5/1.5/S0.0/18H or approximately 45 km/h. (Side bar: Usually JMA will lead by declaring a disturbance as a depression before JTWC does. The fact JTWC is calling this a tropical depression before JMA and as a T1.5 is a bit… aggressive.) There is significant divergance in the placement of the center. For the sake of simplicity, we’ll use the same JTWC Dvorak analysis, which places the center 500 km NNW of Yap, Micronesia… or 745 km NNE of Melekeok, Palau.
Another challenge is the forward motion with this system. JTWC is observing it moving NW at 37 km/h, which is rather fast for a tropical depression. Usually at these speed, tropical waves have a difficult time completely wrapping and establishing a west wind. Conversely, JMA mentions in their 0600 UTC Summary that the low was moving W slowly.
This establishes very low confidence in the initialization of the forecast. If we have such difficulty determining where the storm presently is, this creates challenges in the latter points of the forecast. Thankfully, earlier today at 0046 UTC, we had a very clean pass from ASCAT which helps us determine what the low level winds are like. At that time, it did show a closed or nearly closed circulation (transitioning from the “cusp” stage of a tropical wave to a tropical depression). Additionally, 45 km/h winds were indicated in the NE quadrant of the low. Based on this data, it appears JTWC is on-point with calling this a tropical depression.
Now, as we look forward, the low will continue to move WNW along the periphery of the subtropical ridge that extends from the central Pacific. Meanwhile, a developing area of low pressure south of Japan coupled with broad low pressure extending from interior China through Luzon will influence the system toward the Philippines. Caveat: Forecast models and general statistics show that this is uncertainty in the forecast, especially at the Day 5 point. That said, between this developing low south of Japan–which eventually develops fronts–and the broad low pressure that’s already established, the confidence that this storm will affect Luzon is growing. A large majority of the models have pin-pointed a typhoon to affect the Cagayan Valley for Monday/Tuesday. At this time, it appears that the storm should not intensify into a major typhoon considering it’s still in it’s infancy in the western Carolines, but it still bears watching.
More discussion is included in the video link below. Of course, keep it tuned here as we’ll continue to monitor the progress of this storm over the next few days.