Two tropical depressions in the Eastern Pacific have strengthened into named tropical storms: Tropical Depression Nine-E has strengthened into Tropical Storm Hilary, and Tropical Depression Ten-E has strengthened into Tropical Storm Irwin. Hilary is likely to become a major hurricane, while Irwin also has a chance to become a hurricane. In the West Pacific, Tropical Storm Noru has strengthened into a typhoon, a category 2 on the Saffir-Simpson Hurricane Scale, making it the first typhoon of the 2017 season and second-latest first typhoon on record for the basin.

Eastern Pacific: Two more named storms, but activity likely to slow down soon

The Eastern Pacific tropical cyclone outbreak continues, with three simultaneous named storms now active in the basin. Tropical Storm Greg (left) is likely going to slowly weaken this week and eventually degenerate into a remnant low as it moves over cooler waters and into a drier air mass. Tropical Storm Irwin (center) is likely to strengthen a little this week, perhaps to a category 1 hurricane, before possibly weakening due to interaction with Hilary. Tropical Storm Hilary (right) is likely to undergo a period of rapid intensification and become a category 3 or 4 hurricane.

As of 8:00 p.m. PDT, Tropical Storm Greg was centered near 14.4°N 131.4°W and was moving westward at about 14 mph. Maximum sustained winds were 45 knots (50 mph) with an estimated minimum pressure of 1001 mb. Greg is expected to slowly weaken as it enters a drier air mass with relative humidity values below 50 percent, as well as decreasing sea surface temperatures. Sea surface temperatures are expected to drop below 26°C (78.8°F) by Wednesday, and the official National Hurricane Center (NHC) forecast calls for Greg to degenerate into a remnant low on Thursday. Greg is no threat to land.

As of 8:00 p.m. PDT, Tropical Storm Hilary was centered near 13.2°N 102.9°W and was moving west-northwestward at about 9 mph. Maximum sustained winds were 60 knots (70 mph) with an estimated minimum pressure of 997 mb. Hilary has begun to quickly strengthen today and is likely to become a hurricane tomorrow. Hilary is situated in an environment with sea surface temperatures near 29°C (84.2°F), relative humidity values over 70 percent, and vertical wind shear less than 5 knots. This favors rapid intensification, and the official NHC forecast predicts Hilary to become a major (Category 3) hurricane on Tuesday. Hilary could strengthen a little more than this if the intensification is more rapid than expected. By Wednesday, wind shear is expected to begin to increase due to nearby Tropical Storm Irwin. By Day 5 or so, Hilary could begin to interact with Irwin, although the exact details of this scenario are uncertain as of this time.

As of 8:00 p.m. PDT, Tropical Storm Irwin was centered near 14.9°N 117.3°W and was moving westward at about 8 mph. Maximum sustained winds were 45 knots (50 mph) with an estimated minimum pressure of 1001 mb. Irwin is expected to be over warm waters and light shear for the next 48 hours or so, and the official NHC forecast brings Irwin to hurricane strength at this time. Upper-level outflow from Hilary, which will likely be a strong hurricane at that time, will likely increase the shear around Irwin and cause a weakening trend to begin by the middle of this week. It will be interesting to watch the progression of Hilary and Irwin this week.

After Hilary and Irwin, the East Pacific might finally begin to slow down a little bit, and this is may be a sign that the Atlantic could come to life soon. When the East Pacific is active, it typically “robs” the upward motion that allows tropical cyclones to develop. The NHC does not list any other areas of interest in their Tropical Weather Outlook for the East Pacific, and no new tropical cyclones are expected during the next five days.

Noru becomes the first typhoon of 2017

Tropical Storm Noru quickly strengthened on Sunday to become the first typhoon of the slow 2017 Pacific typhoon season. As of 00:00 UTC Monday, Noru had 10-minute maximum sustained winds of 70 knots (80 mph) and 1-minute maximum sustained winds of 90 knots (105 mph). The estimated minimum pressure from the Joint Typhoon Warning Center (JTWC) was 956 mb. Noru is likely to strengthen a bit more before it begins to interact with Tropical Storm Kulap. It is notable that Noru is the second-latest first typhoon on record for the western Pacific basin.

I will be back tomorrow for an update on these systems. I will go into more detail with the West Pacific systems tomorrow since I discussed the East Pacific systems today.