Tropical Storm Danny Weakening; TD Kilo Less of a Threat to Hawaii

Tropical Storm warnings are flying for the northern Lesser Antilles islands of Antigua, Barbuda, Montserrat, St. Kitts, Nevis, and Anguilla, as a weakening Tropical Storm Danny heads west at 15 mph. Danny passed very close to Buoy 41300 on Sunday afternoon, which measured top winds of 42 mph at 11 am EDT. High wind shear and dry air continue to take their toll on Danny, which had weakened to top winds of 50 mph as of 2:00 pm EDT Sunday. Danny remains a very small tropical storm, with tropical storm-force winds that extend out about 60 miles to the northeast of the center and 20 miles to the southwest. Satellite loops on Sunday afternoon showed that Danny's low-level circulation had become exposed to view, with only a small clump of heavy thunderstorms clustered on the storm's north and east sides.


Figure 1. A visible image of Tropical Storm Danny at 2 pm EDT on Sunday, August 23, showed that the low-level center had become exposed to view, with all of Danny's heavy thunderstorms clustered on the north and east sides of the center.

Forecast for Danny
Given Danny's small size and the continued hostile wind shear and dry air affecting the storm, further weakening is likely. Our two top models for predicting hurricane tracks, the GFS and European models, both dissipate Danny by Tuesday. Interaction with high terrain of Puerto Rico and Hispaniola could well finish Danny off, and I doubt the storm will be around on Wednesday. Rainfall from Danny could end up being more boon than bane for the islands, which are suffering one of their worst droughts in decades.


Figure 2. MODIS image of 98L from NASA's Terra satellite taken at approximately 10 am EDT Sunday, August 23, 2015. Image credit: NASA.

98L in Eastern Atlantic well-organized
In the Eastern Atlantic about 800 miles west of the Cape Verde Islands, Invest 98L appears poised to become the next tropical depression of the 2015 Atlantic hurricane season as it steams westward at a rapid 20 mph. Satellite loops on Sunday afternoon showed that 98L had a well-developed spin and some low level spiral bands beginning to form, but heavy thunderstorm activity was limited, due to the dry air. The Saharan Air Layer (SAL) analysis offered by the University of Wisconsin shows plenty of dry air to the north and west of 98L, and this dry air will potentially impede development of 98L throughout the week. Sea surface temperatures (SSTs) are favorable for development, near 27°C. The 8 am EDT Sunday run of the SHIPS model diagnosed moderate wind shear of 10 - 15 knots over 98L, and predicted the shear would remain in the moderate range through at least Wednesday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 2-day and 5-day odds of development of 70% and 80%, respectively, to 98L. Steering currents for 98L are very similar to what Danny experienced, and the 8 am EDT (12Z) Sunday run of the GFS model showed 98L taking a track into the northern Lesser Antilles Islands on Wednesday, passing near Puerto Rico on Thursday, then over the Dominican Republic by Friday. The European model was about 12 hours slower with 98L, bringing the system into the Southeast Bahamas by Saturday. Wind shear will rise at 98L approaches the islands, which should force the system to weaken

New tropical wave coming off of Africa a marginal threat to develop
A strong tropical wave emerged from the coast of Africa on Sunday, and may well be classified as Invest 99L by NHC by Monday. This wave is headed west at 15 - 20 mph, and will encounter similar conditions as Danny and 98L did--warm ocean waters, moderate wind shear, but plenty of dry air from the Saharan Air Layer (SAL) to contend with. These conditions favor some modest development, and in their 2 pm EDT Sunday Tropical Weather Outlook, NHC gave the system 2-day and 5-day odds of development of 0% and 20%, respectively.

97L north of Bermuda no threat
In the Northwest Atlantic, Invest 97L, a cluster of storms associated with a weakening upper low and frontal zone a few hundred miles north of Bermuda, is accelerating to the north and is about to be absorbed by a non-tropical low pressure system. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 2-day and 5-day odds of development of 0% to 97L.

Threat to Hawaii from Kilo diminishes
Residents of Hawaii should monitor the progress of Tropical Depression Kilo, which is expected to mill around in the waters well west of the islands the next few days. Briefly a tropical storm, Kilo was downgraded on Friday night as its large area of westward-moving showers and thunderstorms (convection) outran the low-level circulation. Kilo will still likely organize into a hurricane, but the latest forecasts from the GFS and European model keep the storm well to the west of Hawaii for at least the next five days.

Hawaii should keep an eye on three tropical disturbances to its east, in the waters of the Eastern Pacific to the southwest of Mexico. The easternmost of these disturbances (which was given 5-day development odds of 70% by NHC in their 2 pm Sunday Tropical Weather Outlook) could pose a long-range threat to Hawaii next week, according to the 12Z Sunday run of the GFS model, but it is too early to be confident of this.

Elsewhere in the Pacific, there is plenty of activity, with Category 3 Typhoon Goni pounding Japan's southern islands. The highest reported gust from the typhoon was 71.0 m/s (159 mph) at Ishigakijima. Category 1 Typhoon Atsani was heading northeast out to sea, and Tropical Storm Loke was headed northwards towards Midway Island. I don't have time for a full update, as I have to run to catch a plane to Atlanta for Monday's 6 pm EDT premiere on The Weather Channel of the new Weather Underground TV show!

I'll have a new update on Monday.

Jeff Masters