A tropical wave in the Central Atlantic Ocean about 550 miles southwest of the Cape Verde Islands is gradually becoming better organized and has a medium chance of becoming a tropical cyclone during the next 48 hours as it moves generally westward at 10 to 15mph.
Elsewhere in the North Atlantic, the Caribbean Sea and the Gulf of Mexico, Tropical Cyclone formation is not expected during the next 48 hours.
Tropical Atlantic Wide Infrared Satellite Image:
USA National Weather Service Forecast
6:00 am EST on July 28, 2014
For the North Atlantic...Caribbean Sea and the Gulf of Mexico:
A tropical wave located about 550 miles southwest of the Cape
Verde Islands is producing a large area of cloudiness and
thunderstorms. Shower activity has increased and become a little
better organized during the past several hours, and environmental
conditions are expected to be conducive for additional development
of this disturbance over the next several days while it moves
generally westward at 10 to 15 mph.
* Formation chance through 48 hours...medium...near 30 percent.
* Formation chance through 5 days...high...70 percent.
48 Hour Forecast – Favorable Environmental Conditions For Tropical Development
Infrared Satellite in Belize City
Tropics Quiet Across the Atlantic
7/28/2014 9:00:50 AM
The Atlantic Tropical Basin remains relatively quiet with no organized weather features.
There are a couple of tropical waves moving across the Atlantic Basin as is usual for this time of year. The first wave is moving inland across Nicaragua and Honduras and will help fuel heavy showers and thunderstorms across Central America. A second tropical wave located near 40 west longitude will continue to track westward today across the open Atlantic and towards the Lesser Antilles. A deep layer of dust from North Africa will inhibit development, and conditions do not appear conducive for the wave to develop into a tropical cyclone over the next 48 hours. However, conditions will become somewhat more conducive for organization of this feature into a tropical system over the central Atlantic as we head into the middle of this week.
120 Hour Forecast – Favorable Environmental Conditions For Tropical Development
Confidence Is Growing That Tropical Storm Bertha Will Likely Develop By Wednesday Or Thursday As Invest 93L Tracks Westward Towards The Lesser Antilles; Everyone Across The Central & Northern Lesser Antilles, Virgin Islands & Puerto Rico Should Closely Monitor Invest 93L As Tropical Storm Conditions Are Possible By Saturday
Rob Lightbown of Crown Weather Services
Monday, July 28, 2014 6:07 am
Invest 93L Located 550 Miles Southwest Of The Cape Verde Islands In The Eastern Atlantic: A tropical wave that is located near 10.3 North Latitude and 31.2 West Longitude or about 550 miles to the southwest of the Cape Verde Islands in the eastern Atlantic has been designated Invest 93L by the National Hurricane Center. The reason for this is because satellite imagery in the last 12 hours or so has shown that thunderstorm activity around this system has increased and has also become a little better organized. In addition, it appears that a pouch of moisture that has developed around Invest 93L which will likely protect it from any dry air. Looking at things this morning, it appears that this system will likely develop first into a tropical depression and then a tropical storm by about Wednesday or Thursday. In addition, it still looks possible that the central and northern Lesser Antilles will be impacted by tropical storm conditions on Saturday followed by the Virgin Islands and Puerto Rico being impacted by tropical storm conditions on Sunday.
Looking at current conditions around Invest 93L reveals that wind shear values are around 10 knots and water vapor satellite loops shows that there is a pouch of moist air that has developed around Invest 93L. In addition, it appears that the amount of dry air has decreased over the last 24 hours across the central and eastern Atlantic. This tells me that the environmental conditions continue to become more favorable for development across the central and eastern Atlantic and this trend is expected to continue throughout this week.
There is now overwhelming model support for the development of Tropical Storm Bertha this week. The GFS, Canadian, UKMET, FIM and NAVGEM models all forecast that Invest 93L will develop into first a tropical depression and then a tropical storm as early as Wednesday. This model suite then forecasts that this system will impact the central and northern Lesser Antilles on Saturday afternoon and Saturday night with tropical storm conditions and then the Virgin Islands and Puerto Rico with tropical storm conditions on Sunday.
Once Invest 93L/Bertha tracks past the northern Lesser Antilles, Virgin Islands and Puerto Rico, there is a growing trend for a stronger Atlantic ridge of high pressure system to push westward and a weaker trough of low pressure over the eastern United States. If these trends continue, then this system could pose a threat to the Bahamas and the US East Coast around the middle to later part of next week (between August 6th and August 8th). Right now, only the Canadian model guidance forecasts a track that would bring it close to the Bahamas and the US Southeast coast next Wednesday (August 6th). This is something that is very far out in time, but it is something that will need to be watched closely.
Here are my thoughts: The fact that the environmental conditions are trending towards a favorable environment between the Lesser Antilles and the coast of Africa and that the thunderstorm activity is becoming better organized with Invest 93L tells me that the global models, especially the GFS model, appears to be correct in forecasting tropical development from Invest 93L.
I think that it is likely that Invest 93L will develop into first a tropical depression and then Tropical Storm Bertha sometime between Wednesday morning and Thursday morning. From there, it looks very possible that the central and northern Lesser Antilles will be impacted by tropical storm conditions, including winds of up to 50 to 70 mph, heavy rainfall and very rough seas, starting on Saturday afternoon and continuing through Saturday night. After that, the Virgin Islands and Puerto Rico may be impacted by tropical storm conditions during the day Sunday.
Everyone across the central and northern Lesser Antilles, Virgin Islands and Puerto Rico should keep very close tabs on Invest 93L as it looks very possible that this system will have an impact on your weather in terms of tropical storm conditions from Saturday into Sunday.
Looking way ahead to next week, it is still far too early to determine whether this system will pose a threat to the Bahamas or the US East Coast, but given the trends of a stronger high pressure system next week which could push this system further west towards the Bahamas and the US East Coast, it is something that will need to be watched for.
I will have frequent updates on Invest 93L/Bertha throughout this week.
Invest 93L Information:
Model Track Forecast:
93L in Eastern Atlantic Growing More Organized
2:43 PM GMT on July 28, 2014
An area of disturbed weather located near 10°N, 33°W at 8 am EDT Monday, about 500 miles southwest of the Cape Verde Islands, was designated Invest 93L by NHC early Monday morning. This disturbance is a more serious threat than Tropical Depression Two of last week, and has the potential to develop into a strong tropical storm before reaching the Lesser Antilles Islands on Friday or Saturday. Visible satellite loops on Monday morning showed that the disturbance had only a modest amount of spin, but infrared satellite images showed that the system's heavy thunderstorm activity had increased significantly since Sunday. Wind shear was light, 5 - 10 knots, which should aid development. Ocean temperatures were decent for development, about 27.5°C. Water vapor satellite loops showed that the atmosphere was reasonably moist in the area, though a large area of dry air lay a few hundred miles to the north.
Figure 1. Analysis of the Saharan Air Layer (SAL) made at 8 am EDT Monday July 28, 2014 using data from the Meteosat-9 satellite. Dry, dusty air was present from the coast of Africa westwards across the tropical Atlantic, but was located well to the north of tropical disturbance 93L. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.
Figure 2. MODIS true-color image of 93L from approximately 9:30 am EDT July 28, 2014, when the storm was about 500 miles southwest of the Cape Verde Islands. The disturbance was embedded in the Intertropical Convergence Zone (ITCZ), the band of heavy thunderstorms that circles the globe in the tropics. Image credit: NASA.
Forecast for 93L The 12 UTC Monday forecast from the SHIPS model predicted that wind shear will remain low to moderate, 5 - 15 knots, for the remainder of the week, aiding development. Sea Surface Temperatures (SSTs) will cool a bit to 27°C on Tuesday and Wednesday, which will tend to slow development. Two of our three reliable models for predicting tropical cyclone genesis, the GFS and UKMET models, predicted in their 00Z Monday runs that the disturbance would develop into a tropical depression by Thursday. Several of our less reliable models, the NAVGEM and Canadian models, also predicted development. The only reliable model that did not predict development was the European model, which historically has had the highest incidence of failing to predict development when development actually occurs. The fact that two out of three of the reliable genesis models predict development bolsters the odds that development will actually occur. In their 8 am EDT Monday Tropical Weather Outlook, NHC put the 5-day odds of development at 70%, up from their 40% forecast from Sunday.
All of the models predict that the disturbance will continue due west or west-northwest at 10 - 15 mph for the next five days. The UKMET is the fastest of the models, predicting that the disturbance will arrive in the Lesser Antilles Islands on Friday evening. The GFS is slower, predicting a Saturday morning arrival in the islands. Once the disturbance crosses west of about 55°W longitude on Thursday, ocean temperatures will warm to about 28°C, which should aid development. Dry air to the north will likely interfere with development late in the week, and we will have to see if the moderate levels of wind shear forecast to occur over the tropical Atlantic will be capable of driving this dry air into the core of the system, disrupting formation. The disturbance may also have trouble disentangling itself from the Intertropical Convergence Zone (ITCZ), the band of heavy thunderstorms that circles the globe in the tropics, which lies just to the south of the disturbance. Clusters of thunderstorms in the ITCZ may compete for moisture and energy, slowing development of the disturbance.
The long-range fate of 93L remains highly uncertain, and will depend upon exactly how far south the center ends up consolidating when the storm develops. Most of the 20 members of the 06Z Monday run of the GFS ensemble model (which runs the GFS model at low resolution 20 times with slightly different initial conditions to show a range of possible outcomes) showed the disturbance missing the U.S. East Coast and recurving out to sea next week; four of the members showed 93L hitting the Southeast U.S. coast. Most of the members of the 00Z Monday run of the European ensemble model showed 93L moving into the Eastern Caribbean on a more southerly trajectory without recurving.
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