Atlantic's First Invest of the 2015, 90L, Organizing Over the Bahamas

The first Atlantic Ocean "Invest" of 2015 has arrived, as the National Hurricane Center (NHC) designated the area of disturbed weather over the Northwest Bahamas as Invest 90L on Wednesday morning. Note that there is no formal definition of what qualifies as an "Invest"; declaring an "Invest" is merely done so that a set of forecasting aids like computer model track forecasts can be generated for the disturbance. NHC gives an "Invest" a tracking number 90-99, followed by a single letter corresponding to the ocean basin--"L" for the Atlantic, or "E" for the Eastern Pacific. Other warning agencies assign "Invests" for the other ocean basins--"W" for the Western Pacific, "A" for the Arabian Sea, etc. When the numbering reaches 99, the next disturbance gets the recycled name "90". The appearance of 90L on May 6 this year marks the third earliest arrival of the year's first "Invest" over the past ten years:

2015: May 6
2014: June 4
2013: May 18
2012: February 5
2011: March 10
2010: May 24
2009: May 18
2008: May 31
2007: May 8
2006: June 10


Figure 1. Latest satellite image of Invest 90L.

Satellite loops show heavy thunderstorms between the Southeast coast of Florida and the Northwest Bahamas in association with 90L increased on Wednesday morning, but there was no evidence of an organized surface circulation trying to form. Long-range radar out of Melbourne, Florida showed no low-level spiral bands trying to form, and the activity was not well-organized. Wind shear was a moderate to high 15 - 25 knots. Water vapor satellite loops show a large area of dry air to the west of 90L over Florida and the Gulf of Mexico, and this dry air is retarding development, thanks to strong upper-level winds out of the west driving the dry air into the core of 90L. Ocean temperatures were near 26°C (79°F), which is about 1.7°C (3°F) above average for this time of year, and just at the limit of where a tropical storm can form. The Hurricane Hunter mission scheduled for Wednesday afternoon was cancelled, and has be re-scheduled for Thursday afternoon, if necessary.


Figure 2. Wind forecast for Saturday, May 9, 2015 at 11 am EDT made by the 00Z Wednesday run of the European model. The model is predicting a subtropical depression to be off the coast of the Southeast U.S.

Forecast for 90L
The 8 am EDT run of the SHIPS model predicted that wind shear over 90L would fall to the moderate range, 10 - 20 knots, on Thursday and Friday, which should allow 90L to approach subtropical depression status by Friday at the latest. Phase space diagrams from Florida State University from Wednesday morning's 06Z run of the GFS model support the idea that this system could be a subtropical or tropical system by Friday. Ocean temperatures fall to about 25°C (77°F) in the waters off of the North Carolina coast, so the farther north the storm wanders, the tougher time it will have developing tropical characteristics--though if the storm manages to find a sweet spot over the core of the warm Gulf Stream current, it has better odds of development. Steering currents are weak over the waters off the Southeast U.S. coast, so expect a slow and erratic motion for 90L. The Wednesday morning 00Z runs of our two top models for predicting tropical cyclones tracks, the European and GFS models, both showed the system making landfall this weekend, with the GFS model predicting landfall in South Carolina on Saturday, and the European model taking the storm ashore in North Carolina on Sunday. Beginning on Friday, coastal regions of both of these states can expect heavy rains and high surf causing rip currents and coastal erosion. Note that the west side of 90L will be weaker and drier, due to the dry air to the west of the storm, and the heaviest rains and stongest winds of 90L will be on the east side of the storm, over North Carolina. In their 8 am EDT Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 60%, respectively.

Wunderblogger Steve Gregory has a more detailed look at the meteorology of 90L in his Wednesday afternoon post.


Figure 3. Latest projected track of Typhoon Noul from the Joint Typhoon Warning Center (JTWC); a time of 9:00 p.m. JST Wednesday is 1200 GMT or 8 am EDT.

Typhoon Noul posing an increased threat to northern Philippines
After dumping more than 10 inches of rain at Yap International Airport, Category 1 Typhoon Noul is on its way toward the northern Philippines. Noel is gradually intensifying, with sustained winds of 90 mph reported at 8 am EDT (1200 GMT) Wednesday. Satellite loops show that the storm’s center is obscured by central dense overcast, but microwave data obtained by satellite shows that an open eye is already present. Noel is well structured and passing over warm waters, with only weak to moderate wind shear, so continued strengthening into a Category 4 storm is expected. Noel will encounter a trough as it approaches the Philippines, which may force the storm to recurve before making landfall there. However, model guidance has been trending westward, albeit with some uncertainty, and the official Joint Typhoon Warning Center track (see Figure 3) now brings Noul ashore along the northeast coast of Luzon, the largest and northernmost island of the Philippines. Noel is expected to peak in intensity just a few hours before approaching Luzon, with sustained winds of 105 knots (120 mph) projected by the JTWC near landfall. The northeast part of Luzon is mountainous, which could increase the risk for very heavy rains as Noul moves onshore or nearby, although a grazing landfall would put most of Luzon on the weaker western side of the circulation. Noul will be referred to as Dodong in the Philippines’ naming system.

Another tropical system, Invest 93W, is organizing to the east, and is likely to develop late this wee. It is too soon to know what its chances are of affecting the Philippines or Japan next week.

An exceptionally busy early portion of typhoon season
Noul's formation date of May 3 marks the second earliest appearance on record for the Northwest Pacific's sixth named storm of the year, according to statistics of the Japan Meteorological Agency's database from 1951 - 2015 maintained by Digital Typhoon. The average is 1.8 storms before May 8. The record is held by 1971, when the sixth named storm of the year (Babe) formed on May 3, six hours earlier than Noul's formation time. Noul will be the second tropical cyclone to affect the Philippines so far in 2015. The first was Tropical Storm Maysak, which hit the Philippines exceptionally early in the season--during Easter weekend, April 4 - 5. Fortunately, Maysak was weakening rapidly as it made landfall, and no deaths or significant damage were reported (though four people were injured after huge waves generated by Maysak hit them while they were taking selfies along the shoreline of Dipaculao town in Aurora province on April 4.)

Jeff Masters and Bob Henson