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07-04-2017, 07:02 PM (This post was last modified: 07-05-2017 10:08 PM by ROLLTIDE.)
Post: #1
TD#4
JM


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[Image: goes-e-1830Z-7.4.17-835px.jpg] Slowly but Surely, 94L Organizing in Tropical Atlantic
Bob Henson · July 4, 2017, 19:21


Above: Enhanced infrared satellite image of extensive showers and thunderstorms surrounding the system dubbed Invest 94L (symbol "I" in center of map) in the central tropical Atlantic at 1830Z (2:30 pm EDT) Tuesday, July 4, 2017. Image credit: CIMMS/SSEC/University of Wisconsin–Madison.


While Americans celebrated a holiday on Tuesday, the tropical wave known as Invest 94L stayed on task in the central tropical Atlantic, where models indicated it could become a tropical storm as soon as Wednesday. As of 2 pm EDT Tuesday, 94L was located near 10°N, 34°W, where it had moved little since Monday.

Top sustained winds associated with 94L on Tuesday afternoon were 35 mph, just below the tropical storm threshold. However, the system remained embedded in the Intertropical Convergence Zone, the west-to-east band of heavy thunderstorms that predominates just north of the equator this time of year. The ITCZ was lending 94L plenty of broad cyclonic motion extending out more than 400 miles, as revealed in surface winds detected by the ASCAT scatterometer, but the presence of the ITCZ was also making it difficult for a low-level core to establish itself. Northeasterly wind shear of around 20 knots also impeded development late Monday into early Tuesday. Showers and thunderstorms were extensive but not well organized on Tuesday afternoon.

Working in 94L’s favor is its moist environment. The system has successfully avoided entraining large amounts of the dry air lying just to its north within the Saharan Air Layer (SAL). The environment immediately around 94L should remain fairly moist (with midlevel relative humidities of 70-80%) for the next couple of days. Sea surface temperatures along the path of 94L will remain adequate for tropical development—around 27-28°C (81-82°F), or about 0.5°C above average.

[Image: modis-94L-7.4.17.jpg]Figure 1. Invest 94L in the Central Atlantic as seen on Tuesday morning, July 4, 2017, by the MODIS instrument on NASA's Terra satellite. Image credit: NASA.


The outlook for 94L
Conditions should become more favorable for 94L over the next day or two. Wind shear was already dropping by Tuesday afternoon, and the shear was predicted by the 18Z Tuesday run of the SHIPS model to drop below 10 knots by Wednesday night. The shear should remain low for several more days as the system begins moving toward the west-northwest, which will pull it away from the ITCZ and help it establish a more distinct center. Model guidance increasingly agrees that 94L will likely become a tropical storm. All 70 members of the 0Z Tuesday GFS and European ensembles bring 94L to tropical depression strength by late Wednesday, and all GFS members—plus about 80% of ECMWF members—produce a tropical storm by late week. The next name on the Atlantic list is Don.

While the odds of a tropical storm this week are quite high, the chance of a hurricane appears low. Less than 10% of GFS and European ensemble members bring 94L to hurricane strength. Dry air seems to be the main impediment: as 94L angles more toward the west-northwest, it will likely ingest more of the dry air associated with the SAL. If the system can wall itself off from the arid environment and maintain a moist inner core, it will have a better chance at more sustained strengthening.

[Image: gfs-36hr-12Z-7.4.17.jpeg]Figure 2. A pocket of moisture (cool colors) surrounds the center of 94L in this prediction of total precipitable water (the amount of moisture in a column above the surface, in millimeters) and surface pressure (solid lines, in millibars). The forecast, for 8:00 pm EDT Wednesday, July 5, was generated by the 12Z Tuesday run of the GFS model. Image credit: tropicaltidbits.com.

There’s been little change in the longer-range scenario for 94L outlined in Monday’s post. A strong ridge of high pressure should steer the system mostly west-northwest at 5 - 15 mph for the next five days, which would bring the storm near or north of the Northern Lesser Antilles Islands this weekend. We’ll need to watch this path for potential impacts to the islands, especially if the track falls toward the southern end of the possibilities in the ECMWF ensemble runs.

The main question for next week is whether we’ll see a weak system continuing west-northwest and perhaps dissipating along the way (a solution favored by many members of the 0Z Monday European ensemble) or a potentially stronger system angling more to the northwest, toward the Bahamas and perhaps further north (as depicted by a number of GFS ensemble members). A broad weakness in the flow across eastern North America will sharpen over the weekend, then flatten early next week before potentially sharpening again. It’s too soon to know how the timing of these features might shape the path of whatever 94L has become by that point.

Happy Fourth to all of our U.S. readers!
Dr. Jeff Masters contributed to this post.


Unquote




ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 PM EDT Tue Jul 4 2017

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

1. Satellite images indicate that the cloud pattern associated with the
broad area of low pressure centered about 800 miles west-southwest
of the Cabo Verde Islands has changed little in organization since
yesterday. Environmental conditions are still favorable for a
tropical cyclone to form within the next day or two while the low
moves westward or west-northwestward at 10 to 15 mph across the
tropical Atlantic. After that time, the system is expected to
encounter a dryer and more stable air mass, which should be less
favorable for development.
* Formation chance through 48 hours...high...70 percent.
* Formation chance through 5 days...high...80 percent.

Forecaster Beven


ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM




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07-05-2017, 08:36 AM (This post was last modified: 07-05-2017 10:06 PM by ROLLTIDE.)
Post: #2
TD#4
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Wed Jul 5 2017

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

1. Satellite images indicate that the circulation associated with the
area of low pressure centered about 800 miles west-southwest of
the Cabo Verde Islands has become better defined, but the associated
thunderstorm activity is not well organized. This system has the
potential to become a tropical depression today or Thursday before
conditions become unfavorable for tropical cyclone formation
. The
low has been moving little, but it should begin a west-northwest
track at 10 to 15 mph today.

* Formation chance through 48 hours...high...70 percent.
* Formation chance through 5 days...high...70 percent.

Forecaster Avila
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07-05-2017, 08:03 PM (This post was last modified: 07-05-2017 08:15 PM by Her-icane.)
Post: #3
RE: 94L
JM


Quote: [Image: jul5-94L.jpg] 94L in Central Atlantic Develops Closed Surface Circulation
Dr. Jeff Masters · July 5, 2017, 15:30



Above: Invest 94L in the Central Atlantic as seen at 11:45 am EDT July 5, 2017. The storm had a meager but increasing amount of heavy thunderstorm activity on the west side of a fairly well-defined surface circulation. Image credit: NOAA

Tropical wave 94L in the Central Atlantic has developed a fairly well-defined low-level circulation and is likely to become a tropical depression by Thursday. At 8 am EDT Wednesday, 94L was located about 800 miles west-southwest of the Cabo Verde Islands, near 11°N, 36°W. After remaining nearly stationary for about two days, 94L finally began a long-anticipated west to west-northwesterly motion at about 10 mph late Wednesday morning.

Satellite images on Wednesday morning showed that 94L had a moderately well-defined surface circulation, but heavy thunderstorm activity was meager and limited to the west side of the circulation. An 8:17 am EDT Wednesday pass from ASCAT showed the surface circulation as well, but saw top winds of only 20 – 25 mph. However, the satellite did not sample the west side of the circulation, where all of the heavy thunderstorms are located. Development was likely being retarded by dry air, thanks to a presence of the Saharan Air Layer (SAL) just to the north. Wind shear was moderate, 10 - 15 knots, and sea surface temperatures (SSTs) were warm enough for development, near 27°C (81°F)--about 1°F above the seasonal norm.



[Image: jul5-sal.jpeg]
Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Wednesday, July 5, 2017, showed a large area of dry Saharan air in the tropical Atlantic north of about 12°N. Invest 94L was just to the south of the SAL. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Favorable conditions for development through Thursday, then turning unfavorable
Conditions through Thursday are marginally favorable for development, as diagnosed by the 12Z Wednesday run of the SHIPS model: wind shear should mostly be moderate, mid-level relative humidity will be moderately moist (65 - 75%), and SSTs will be marginally warm enough for development--in the 26 - 27°C (79 - 81°F) range. The Madden-Julian Oscillation (MJO), a periodic pulse of thunderstorm activity that circles equatorial regions of the globe every few weeks, is in a phase that will help development of Atlantic tropical cyclones this week. 94L has separated itself from the large band of heavy thunderstorms called the Intertropical Convergence Zone (ITCZ), so 94L will not have to compete with thunderstorm clusters of the ITCZ for moisture any longer.

These conditions should allow 94L to develop into a tropical depression on Wednesday or Thursday. In their 8 am EDT Wednesday Tropical Weather Outlook, the National Hurricane Center (NHC) gave 94L 2-day and 5-day odds of tropical cyclone development of 70%. About 70% of the 50 members of the 0Z Wednesday European ensemble forecast and 100% of the 20 members of the 0Z Wednesday GFS ensemble forecast predicted development of 94L into Tropical Storm Don this week. The models showed a west-northwest track for the storm this week, bringing it about 200 - 500 miles north of the Lesser Antilles Islands by Sunday. On this track, the storm will move into a large area of dry, stable air associated with the Saharan Air Layer (SAL), which will interfere with development. On Friday, wind shear is expected to rise to a high 20 knots, and increase further to 30 knots over the weekend. These are very hostile conditions for a tropical storm to survive, and most of the models and their ensembles show 94L weakening or dissipating this weekend. At this point, 94L is not looking like a long-range threat to affect any land areas as a tropical cyclone.





ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 PM EDT Wed Jul 5 2017

For the North Atlantic...Caribbean Sea and the Gulf of Mexico:

1. Satellite images indicate that shower activity has become more
concentrated in association with the area of low pressure centered
about 900 miles west of the Cabo Verde Islands. Some additional
development is possible tonight and Thursday, and a tropical
depression could form before conditions become less conducive on
Friday. The low is expected to move west-northwestward at 10 to 15
mph for the next several days across the open Atlantic.

* Formation chance through 48 hours...medium...60 percent.
* Formation chance through 5 days...medium...60 percent.


Forecaster Beven

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07-05-2017, 10:06 PM
Post: #4
RE: 94L
TD#4

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07-06-2017, 10:42 PM (This post was last modified: 07-06-2017 10:46 PM by Her-icane.)
Post: #5
RE: TD#4
JM


Quote: [Image: td4-jul6.jpg] Tropical Depression 4 Struggling With Dry Air
Dr. Jeff Masters · July 6, 2017, 15:42




Tropical Depression Four formed in the Central Atlantic on Wednesday evening, but is struggling against dry air and is unlikely to survive long enough to threaten any land areas. At 11 am EDT Thursday, TD 4 was located about midway between the Cabo Verde Islands and the Lesser Antilles Islands, and was headed west-northwest at a speedy 21 mph.


Satellite images on Thursday morning showed that the circulation of TD 4 was weakening. The storm had a decreasing amount of thunderstorm activity, and the cloud tops of these thunderstorms were warming, showing that they did not extend as high in the atmosphere due to weaker updrafts supporting them. This weakening trend was likely due to dry air from the Saharan Air Layer (SAL), which had wrapped into the circulation center from the east. Wind shear was favorable for development, a light 5 - 10 knots, and sea surface temperatures (SSTs) were marginal for development, near 26.5°C (80°F)--about 1°F above the seasonal norm.


[Image: sal-jul6.jpeg]
Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Thursday, July 6, 2017, showed a large area of dry Saharan air to the east of TD 4. This dry air was being entrained into the center of the storm. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.



Unfavorable conditions for development Friday and beyond
Dry air will be TD 4’s main impediment to development through Friday, as diagnosed by the 12Z Thursday run of the SHIPS model--relative humidity at mid-levels of the atmosphere will fall from 60% Thursday morning to 50% on Friday morning, which will make it difficult for the storm to develop. However, with wind shear expected to stay low through Friday morning, there is a brief opportunity for TD 4 to get its act together and become Tropical Storm Don. By Friday night, though, strong upper level winds out of the west will begin impacting the storm, bringing high wind shear of 15 – 25 knots through the weekend. The combination of dry air and high wind shear should be enough to destroy TD 4 by Monday. Only 4% of the 50 members of the 0Z Thursday European ensemble forecast--and none of the 20 members of the 0Z Thursday GFS ensemble forecast--predicted that TD 4 would still be alive on Monday evening. The models showed a west-northwest track for the storm, bringing TD 4’s remnants about 200 - 500 miles north of the Lesser Antilles Islands by Sunday. Given the dry air surrounding TD 4 and the meager amount of heavy thunderstorms associated with the storm, I’m not expecting any dangerous flooding rains in the Lesser Antilles Islands from TD 4.

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07-07-2017, 06:39 PM (This post was last modified: 07-07-2017 06:40 PM by Her-icane.)
Post: #6
RE: TD#4
JM


Quote: [Image: td4-dust-jul6.jpg] Tropical Depression 4 Weakening, May Dissipate Friday Night
Dr. Jeff Masters · July 7, 2017, 14:27



Above: Tropical Depression 4 in the Central Atlantic as seen by the new GOES-16 satellite on Thursday afternoon, July 6, 2017. A large area of orange-brown African dust was chasing TD 4 as it sped to the west, as seen in this excellent video from NOAA/CIRA/RAMMB.


Tropical Depression Four continues to struggle against dry air and is likely to dissipate Friday night or Saturday morning before passing just north of the northernmost Lesser Antilles Islands. Satellite images on Friday morning showed that the circulation of TD 4 was weak, and the storm had a decreasing amount of thunderstorm activity. The cloud tops of these thunderstorms were warming, showing that they did not extend as high in the atmosphere due to weaker updrafts supporting them. This weakening trend was likely due to dry air from the Saharan Air Layer (SAL), which had wrapped into the circulation center from the east. Wind shear was favorable for development, a light 5 - 10 knots, and sea surface temperatures (SSTs) were adequate for development, near 27.5°C (82°F)--about 1°F above the seasonal norm.


[Image: sal-jul7.jpg]
Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Friday, July 7, 2017, showed a large area of dry Saharan air to the east of TD 4. This dry air was being entrained into the center of the storm. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.



Unfavorable conditions for development
Dry air will be TD 4’s main impediment to development, as diagnosed by the 12Z Friday run of the SHIPS model--relative humidity at mid-levels of the atmosphere were at 50% Friday morning, and are expected to fall to 45% by Saturday. This dry air, in combination with increasingly strong upper level winds out of the west that will start creating moderate wind shear of 15 – 20 knots by Saturday morning, should cause TD 4 to dissipate Friday night or Saturday morning. The models predict a continued west-northwest motion at about 20 mph for TD 4 or its remnants over the weekend, and the system may pass close enough to the northern Lesser Antilles Islands on Sunday to bring a few rain showers and gusty winds. Given the dry air surrounding TD 4 and the meager amount of heavy thunderstorms associated with the storm, I’m not expecting any dangerous flooding rains in the Lesser Antilles Islands.


We'll have to keep an eye on the remnants of TD 4 when they enter the Bahama Islands by the middle of next week, where increased low level moisture, warm SSTs and low to moderate wind shear may allow some re-organization of the system to occur.
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