Hurricane Norbert Brushing Baja; 90L Emerges From the Coast of Africa

Hurricane Norbert took advantage of unusually warm 29.5°C (85°F) Sea Surface Temperatures (SSTs) and intensified into the Eastern Pacific's tenth hurricane of the year on Wednesday evening. The Eastern Pacific has seen an unusually active hurricane season, with 14 named storms, 10 hurricanes, and 6 intense hurricanes so far. An average Eastern Pacific hurricane season sees 15 named storms, 8 hurricanes, and 3 intense hurricanes during the entire year, with about 2/3 of that activity occurring by September 9. Satellite loops on Thursday morning showed that Category 1 Norbert had no eye but some very intense eyewall thunderstorms with cold cloud tops, and Mexican radar showed an outer band of Norbert bringing heavy rains to the tip of the Baja Peninsula and adjacent areas of Mainland Mexico. Norbert should be able to take advantage of warm SSTs, a moist atmosphere, and moderate wind shear over the next two days to maintain Category 1 hurricane strength, but the models all show the core of the hurricane remaining just offshore as it moves northwest parallel to the Baja Peninsula. Norbert is a small storm, and it's hurricane-force winds are only expected to reach out about 25 miles from the center when it makes its closest pass by the tip of the Baja Peninsula on Thursday night and Friday morning. Hurricane force winds will likely stay offshore, but Baja can expect tropical storm-force winds from Norbert. In their 2 am PDT Thursday WInd Probability Advisory, NHC gave Cabo San Lucas on the southern tip of the Baja Peninsula a 63% chance of experiencing tropical storm-force winds of 39+ mph, and a 0% chance of experiencing hurricane-force winds of 74+ mph. Heavy rains of 3 - 5" causing flash flooding will be the primary threat from Norbert to Baja.


Figure 1. MODIS true-color image of Tropical Storm Norbert at approximately 5 pm EDT September 3, 2014. At the time, Norbert had top winds of 70 mph. Image credit: NASA.

New tropical wave 90L emerges from the African coast
A well-organized tropical wave (designated 90L by NHC on Thursday morning) has emerged from the coast of Africa, and is moving to the west at about 15 mph. Satellite images show the wave has plenty of spin, but little heavy thunderstorm activity. Wind shear is moderate, 10 - 20 knots, and the ocean temperatures are warm, near 28°C (82°F.) The core of the wave will pass a few hundred miles south of the Cape Verde Islands on Friday. Two of our three reliable computer models for predicting tropical storm formation show development by Monday, and in their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odd of development odds of 10% and 40%, respectively. The Thursday morning long-range runs of the GFS and European ensemble models favored the storm taking a more west-northwesterly track into the open ocean next week, with a low threat to the Lesser Antilles Islands indicated. The Atlantic is expected to be dominated by dry, sinking air next week, due to the phase of the Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, so I expect that 90L will struggle to develop. Indeed, the 2 am EDT Thursday run of the SHIPS model showed 90L encountering increasingly dry air over the next five days in the face of moderate wind shear--conditions hostile for development.


Figure 2. MODIS true-color image of Invest 90L off the coast of Africa, at approximately 8 am EDT September 4, 2014. Image credit: NASA.

Jeff Masters