The global tropics remain active today, but little has changed since yesterday. Hurricane Hilary weakened to a tropical storm last night, and has changed little in strength today. Tropical Storm Irwin has also changed very little in strength. The eastern Atlantic disturbance was briefly designated Invest 97L last night, but was quickly dropped from the Tropical Weather Outlook after it failed to organize.

Tropical Storms Hilary and Irwin still expected to merge early next week

As of 2:00 p.m. PDT (5:00 p.m. EDT), Tropical Storm Hilary was centered near 19.7°N 119.2°W and was moving northwestward at about 9 mph. Maximum sustained winds were 55 knots (65 mph) with an estimated minimum pressure of 996 mb. Although the structure of Hilary has improved slightly this afternoon, the convection is not very deep. Hilary is currently located over sea surface temperatures below 26°C (78.8°F). Although the wind shear is not very strong and the environment is very moist, the cool sea surface temperatures should cause Hilary to continue to slowly weaken as it moves west-northwestward to northwestward over the coming days. The official National Hurricane Center (NHC) forecast calls for Hilary to become a post-tropical remnant low by Monday afternoon as it absorbs what is left of Tropical Storm Irwin.

As of 2:00 p.m. PDT (5:00 p.m. EDT), Tropical Storm Irwin was centered near 14.9°N 125.0°W and was moving northward very slowly at about 2 mph. Maximum sustained winds were 50 knots (60 mph) with an estimated minimum pressure of 998 mb. The structure of Irwin has changed very little today as it has remained nearly stationary. Irwin is likely to change little in intensity during the next day or so as it moves very slowly northward. Some slight re-intensification cannot be ruled out in the short term, as the cyclone is located over sea surface temperatures above 27°C (80.6°F).  Irwin is likely to increase its forward speed by late tomorrow as it turns to the north-northwest, and Irwin should be merged into Hilary’s circulation on Monday or Tuesday. It is likely that Hilary will be a post-tropical remnant low by the time the two cyclones merge.

Nesat becomes a typhoon, expected to strike northern Taiwan on Saturday

Severe Tropical Storm Nesat intensified into the second typhoon of the 2017 season on Friday morning, and is now expected to become the first landfalling typhoon of the season. As of 00:00 UTC Saturday, Typhoon Nesat was centered near 22.5°N 123.1°E and was moving northwestward. Maximum 1-minute and 10-minute sustained winds were 80 knots (90 mph), equivalent to a strong category 1 hurricane on the Saffir-Simpson scale. The estimated minimum pressure was 960 mb. Nesat is expected to strike northern Taiwan around 12:00 UTC Saturday (8:00 p.m. local time). An eye-like feature has been intermittently visible on satellite images the past few hours, and Nesat could strengthen a little more before landfall. All preparations should have been completed in Taiwan.

Invest 97L not expected to develop, East Pacific disturbance may develop

Late on Thursday evening, the National Hurricane Center (NHC) designated Invest 97L over the eastern tropical Atlantic after an ASCAT pass revealed a broad circulation. After convection decreased today, the NHC lowered the 5-day development chance from 30 percent to 20 percent at 8:00 a.m. this morning, before dropping it from the Tropical Weather Outlook (TWO) at 2:00 p.m. EDT. This was one of the shortest-lived invests I have ever seen. Ex-97L has not become any better organized today, and dry air is expected to limit any development. Aside from 97L, a new tropical wave is currently emerging off the African coast, and this system may be worth watching for possible development by the middle of next week. The NHC has not yet mentioned this system in their TWO, and any imminent tropical cyclone development in the Atlantic basin is unlikely.

In the Eastern Pacific, yet another area of low pressure is expected to form several hundred miles south of Mexico by the middle of next week. The NHC gives this system a near zero percent chance of development within 48 hours, and a 20 percent chance of development within five days. The 12z Friday ECMWF and GFS model runs predicted development of this system, while the 12z Friday CMC and UKMET runs did not. Based on the East Pacific being very favorable for tropical cyclone development recently, and the ECMWF and GFS models both predicting development, I give this system a 50 percent chance of developing within the next ten days. While the GFS is stronger and predicts a major hurricane turning west-northwestward away from land, the ECMWF is weaker and farther east with development, and not far from the Baja California peninsula. Therefore, residents of Baja California should monitor this system in case it develops farther east than expected.

I will be back tomorrow for another post on these systems.