|Above: Invest 94L over the northwest Bahamas, as seen at 10:00 am EDT Monday, July 22, 2019. Image credit: NOAA/RAMMB.|
A trough of low pressure (94L) located over the northwest Bahama Islands was headed west-northwest to northwest at 15 mph, and had dumped rains between 1.0 and 1.5” on New Providence and Eleuthera islands in The Bahamas as of 11 am EDT Monday. The disturbance is likely to bring 1 – 2” of rain to portions of southern and central Florida beginning late Monday afternoon, and could develop into a tropical depression by Wednesday. The Hurricane Hunters are on call to investigate 94L on Tuesday.
Favorable parameters for development of 94L into a tropical depression included sea surface temperatures (SSTs) near 30°C (86°F) and moderate wind shear near 10 knots. Unfavorable for development was the dry atmosphere 94L was embedded in, with a mid-level relative humidity near 55%. Satellite images on Monday showed that the wave had plenty of spin and a modest amount of heavy thunderstorms that had grown in areal coverage and organization since Sunday. There were signs of a surface circulation beginning to develop on satellite imagery late Monday morning, with surface winds out of the west evident over Andros Island in The Bahamas.
[#NerdTweet] Decreasing westerly shear over #94L is allowing convective updrafts to refire repetitively within the wave envelope, likely gradually moistening the column and allowing just enough sustained convection to strengthen the circulation. Small systems can be very sneaky. pic.twitter.com/o2dO1zav2s— Levi Cowan (@TropicalTidbits) July 22, 2019
Forecast for 94L
The 12Z Monday run of the SHIPS model predicted that SSTs would remain warm, near 30°C (86°F), and wind shear would be light to moderate, 5 – 10 knots, though Wednesday. During this time, the atmosphere was predicted to stay dry, with a mid-level relative humidity of around 55%. These conditions may allow 94L to develop into a tropical depression. None of our top three models for predicting tropical cyclone development--the GFS, European, and UKMET models--predicted development of 94L into a tropical depression with their 0Z Monday runs. The disturbance also had very little support for development from the 0Z Monday GFS and European model ensemble forecasts.
94L will turn more to the northwest on Tuesday and Wednesday, and is likely to get absorbed by Thursday by a cold front expected to push off the U.S. East Coast. Strong upper level winds associated with the front will bring high wind shear in excess of 20 knots beginning on Wednesday night, likely putting an end to further development of 94L. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 94L 2-day and 5-day odds of development of 30%. Given the developing surface circulation seen late Monday morning on satellite imagery, as well as the increase in 94L's organization and heavy thunderstorm activity, I put these odds higher, at 40%. The next name on the list of Atlantic storms is Chantal.
Update: In its 2 pm Monday tropical weather outlook, NHC raised the odds of 94L development to 60%.