The SAL is the Saharan Air Layer, a usually dry part of the atmosphere that comes off North Africa and inhibits storm development.
From Wunderblog:
99L a Potential Threat to Hispaniola, Bahamas, U.S. East Coast
The main Atlantic tropical weather threat to populated areas continues to be Invest 99L, a large tropical wave with an increasing amount of heavy thunderstorm activity that was located about 800 miles east of the northern Lesser Antilles Islands late Monday morning. This disturbance was moving west to west-northwest at 15 - 20 mph, and will bring heavy rains and gusty winds to these islands beginning on Tuesday evening. Conditions for development will steadily improve in the coming days, and the storm could be trouble for the Bahama Islands late this week--and is a threat to make landfall along the U.S. East Coast early next week. Satellite loops on Monday morning showed that 99L had finally managed to fire up a respectable amount of heavy thunderstorm activity near its core, in defiance of the dry air from the Saharan Air Layer (SAL) that had been interfering with development over the past few days. Water vapor satellite imagery showed that there continued to be a fair amount of dry air around the storm, though the amount of dry air had decreased since Sunday. Other conditions were generally favorable for development, with wind shear a light 5 - 10 knots and sea surface temperatures (SSTs) near 27.5°C (82°F), which was close to average.
Figure 1. Latest satellite image of 99L.
Track forecast: 99L a potential threat to Hispaniola, the Bahamas, and the U.S.
A strong ridge of high pressure will keep 99L headed north of due west over the next few days, and the storm should pass through the northern Lesser Antilles Tuesday night through Wednesday, track close to Puerto Rico on Wednesday night, and affect Hispaniola and the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, due to a potential weakness in the ridge of high pressure steering the storm caused by a trough of low pressure passing to the north of 99L. The storm should slow its forward motion to 5 - 10 mph, in response to this trough, and may turn to the north near the central Bahamas. The track of 99L may also be affected by the remnants of Tropical Storm Fiona, which could be a few hundred miles to the north or northeast. At this time, it appears that 90L near the Cabo Verde Islands off the coast of Africa, which is expected to become Tropical Storm or Hurricane Gaston late this week, will be too far from 99L next week to exert a steering influence on it. The steering situation is too complex next week to say how great a threat the storm may pose to the U.S., but 99L is a legitimate threat to make landfall along the East Coast.
Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Sunday, August 21, 2016 (top) and Monday, August 22, 2016 (bottom). The amount of dry air from the SAL interfering with 99L has decreased since Sunday. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.
Intensity forecast for 99L: commentary by Jeff MastersHere is the ensemble of tracking models:
Heavy rains from 99L will be capable of causing flash flooding problems in the Lesser Antilles, Virgin Islands, and Puerto Rico, but wind damage should not be an issue, since 99L will likely be, at worst, a moderate-strength tropical storm with 55 mph winds once it leaves the islands. The 8 am EDT Monday run of the SHIPS model showed moderately favorable conditions for development through Thursday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 27.5°C (82°F) to 29°C (84°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air of the SAL, potential interaction with the land areas of Puerto Rico and Hispaniola, and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). None of the Sunday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, GFS and UKMET models--showed development of 99L into a tropical depression or tropical storm over the next four days, though the UKMET model predicted it could be a tropical depression in the Bahamas in five days. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 50%, respectively. I think these odds are too low, and should be 30% and 60%, respectively. The Hurricane Hunters are scheduled to investigate 99L on Tuesday afternoon....MORE
