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HURRICANE GERT
08-03-2017, 11:39 AM (This post was last modified: 08-15-2017 03:44 PM by Her-icane.)
Post: #1
HURRICANE GERT
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Thu Aug 3 2017

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

1. A large area of showers and thunderstorms centered about 500 miles
southeast of the Cabo Verde Islands is associated with a vigorous
tropical wave. Environmental conditions are forecast to be conducive
for gradual development, and a tropical depression could form by
early next week over the eastern or central tropical Atlantic
Ocean. This system is forecast to move toward the west or
west-northwest at 10 to 15 mph for the next several days.
* Formation chance through 48 hours...low...30 percent.
* Formation chance through 5 days...medium...60 percent.

Forecaster Stewart

[Image: two_atl_0d0.png?031139]

[Image: two_atl_5d0.png]

JM


Quote: [Image: 99L-aug3.jpg] Impressive 99L off the Coast of Africa a Threat to Develop
Dr. Jeff Masters · August 3, 2017, 13:47


Above: MODIS true-color satellite image of 99L off the coast of Africa as seen on Thursday morning, August 3, 2017. Image credit: NASA.


A strong tropical wave that emerged from the coast of Africa on Wednesday was located about 500 miles southeast of the Cabo Verde Islands on Thursday morning, and was headed west to west-northwest at 10 – 15 mph. This system was designated 99L by the National Hurricane Center on Thursday morning, and has a greater potential to become a hurricane than the large majority of the Atlantic disturbances we’ve seen so far this year. However, 99L is expected to struggle with dry air next week, and could easily fizzle out.


99L was under moderate wind shear of 10 – 15 knots on Thursday morning, and had ocean temperatures adequate for development: 27.5 - 28°C (81 - 82°F). The dry air of the Saharan Air Layer was to the north of the wave, but not close enough to seriously impede development. Relative humidity at mid-levels of the atmosphere as analyzed by the 12Z Thursday run of the SHIPS model was high--about 75%. Satellite loops showed that 99L had impressive size, a good deal of spin, and low-level spiral bands already beginning to appear. An ASCAT passat 6:36 am EDT Thursday showed that 99L had a closed (but elongated) surface circulation centered near 8.5°N, 20°W, with top winds of about 25 mph in the heavy thunderstorms to the northeast of the center. This position is close enough to the equator that 99L will have trouble leveraging the Earth's spin to help get itself spinning over the next few days.



[Image: sal-aug3.jpg]
Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Thursday, August 3, 2017, showed a large tropical wave with plenty of moisture, 99L, off the coast of Africa. The dry air of SAL was well to the north, and not interfering significantly with development. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.



Forecast for 99L


The 0Z Thursday runs of our top three models for predicting tropical cyclone genesis--the European, UKMET and GFS models--all predicted development of 99L occurring early next week in the Central Atlantic. The UKMET model was the most aggressive with the storm, showing 99L impacting the Lesser Antilles Islands next Wednesday as a hurricane. The European model showed 99L developing by Sunday, but then predicted that dry air and increasing wind shear would kill the storm by Tuesday. Only 3 of the model’s 50 ensemble members showed 99L surviving long enough to become a threat to the U.S. mainland. Future struggles for 99L were also predicted by the 12Z Thursday run of the SHIPS model, which forecasted that the system would work its way far enough to the north early next week to encounter increasingly drier, more stable air associated with the Saharan Air Layer. Relative humidity at mid-levels of the atmosphere of 50% were predicted for Monday. At that time, the model also predicted that 99L would also be encountering higher wind shear near 20 knots, as well as SSTs of marginal warmth for tropical cyclones--near 26.5°C (80°F). However, the 0Z and 6Z Thursday runs of the GFS model predicted fewer struggles for 99L, showing that the storm would pass about 500 miles north of the Lesser Antilles Islands next Friday, August 11, and continue on to an eventual landfall as a hurricane on the U.S. East Coast the following week. All 20 of the 0Z GFS ensemble members showed 99L tracking near or north of the Lesser Antilles as a tropical cyclone next week, and about a third suggested the potential East Coast threat later on. Considering that 99L appears to be consolidating pretty far to the south, near 9°N, I think a more southerly track into the Lesser Antilles is more likely. In their 8 am Thursday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day odds of development of 30% and 60%, respectively. At this early stage of 99L's existence, there is high uncertainty on whether or not it will but develop, and where it will go. But keep in mind that we are now in the early portion of the dangerous peak part of the Atlantic hurricane season, and disturbances like today's 99L are the type of system that often become destructive hurricanes.


Another area to watch: the Northwest Caribbean and Bay of Campeche
The European model ensemble from 0Z Thursday suggested that a tropical depression could form over the northwest Caribbean early next week, perhaps moving west-northwestward across the Yucatan Peninsula and into the Bay of Campeche. About 40% of the Euro ensemble members showed this feature. However, the GFS ensembles showed a plume of dry air sweeping through the region, and none of the 20 ensemble members showed tropical development in the northwest Caribbean. The operational UKMET model from 0Z Thursday also showed no development. Should a tropical system make it into the Bay of Campeche, where storms can intensify quickly, it would merit watching as a potential landfall threat to the northeast coast of Mexico or south Texas, as depicted by several Euro ensemble members.

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08-03-2017, 05:26 PM (This post was last modified: 08-15-2017 03:47 PM by Her-icane.)
Post: #2
RE: INVEST 99L
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Thu Aug 3 2017

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

1. A vigorous tropical wave accompanied by a broad low pressure system
is producing a large area of cloudiness and showers more than 300
miles south-southeast of the Cabo Verde Islands. Environmental
conditions are forecast to be conducive for gradual development, and
a tropical depression is likely to form by early next week over the
eastern or central tropical Atlantic Ocean. This system is forecast
to move toward the west or west-northwest at 10 to 15 mph for the
next several days.
* Formation chance through 48 hours...low...30 percent.
* Formation chance through 5 days...high...70 percent.

2. A strong tropical wave located over the southeastern Caribbean Sea
is producing a large area of cloudiness and thunderstorms, along
with tropical-storm-force wind gusts in squalls. Environmental
conditions are expected to become a little more conducive for
development by Sunday over the northwestern Caribbean Sea and by
early next week over the Bay of Campeche while the disturbance
moves westward or west-northwestward at 15 to 20 mph. This system
could produce brief heavy rainfall and gusty winds over Aruba,
Bonaire, and Curacao tonight and Friday. For additional information
on this system, see High Seas Forecasts issued by the National
Weather Service.
* Formation chance through 48 hours...low...10 percent.
* Formation chance through 5 days...low...20 percent.

High Seas Forecasts issued by the National Weather Service can be
found under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and
on the Web at http://www.opc.ncep.noaa.gov/shtml/NFDHSFAT1.shtml.

Forecaster Stewart

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08-03-2017, 06:43 PM
Post: #3
RE: INVEST 99L
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 PM EDT Thu Aug 3 2017

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

1. A large and complex area of disturbed weather, associated with a
broad low pressure system, extends from about 400 miles south of
the Cabo Verde Islands to about 600 miles southwest of the Cabo
Verde Islands. Environmental conditions are forecast to be
conducive for gradual consolidation and development, and a tropical
depression is likely to form by early next week over the eastern or
central tropical Atlantic Ocean. This system is forecast to move
toward the west or west-northwest at 10 to 15 mph for the
next several days.
* Formation chance through 48 hours...medium...40 percent.
* Formation chance through 5 days...high...80 percent.

2. A strong tropical wave located over the eastern Caribbean Sea is
producing a large area of cloudiness and thunderstorms.
Environmental conditions are expected to become a little more
conducive for development by Sunday over the western Caribbean Sea
and by early next week over the Bay of Campeche while the
disturbance moves westward or west-northwestward at 15 to 20 mph.
This system could produce brief heavy rainfall and gusty winds over
Aruba, Bonaire, and Curacao tonight and Friday. For additional
information on this system, see High Seas Forecasts issued by the
National Weather Service.
* Formation chance through 48 hours...low...20 percent.
* Formation chance through 5 days...medium...40 percent.

High Seas Forecasts issued by the National Weather Service can be
found under AWIPS header NFDHSFAT1, WMO header FZNT01 KWBC, and
on the Web at http://www.opc.ncep.noaa.gov/shtml/NFDHSFAT1.shtml.

Forecaster Beven

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08-04-2017, 07:17 AM (This post was last modified: 08-04-2017 07:22 AM by Her-icane.)
Post: #4
RE: INVEST 99L
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 AM EDT Fri Aug 4 2017

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

1. A broad low pressure system, associated with a tropical wave, is
producing a large area of disorganized showers and thunderstorms
several hundred miles south and southwest of the Cabo Verde
Islands. Environmental conditions are conducive for this system to
consolidate and develop during the next few days, and a tropical
depression is likely to form by early next week while moving
west-northwestward at about 15 mph across the tropical Atlantic
Ocean.
* Formation chance through 48 hours...medium...50 percent.
* Formation chance through 5 days...high...[color=#ff0000]80 percent.

2. A large area of cloudiness and thunderstorms located over the
central and eastern Caribbean Sea is associated with a tropical
wave. This disturbance is expected to move west-northwestward at
about 15 mph across the western Caribbean Sea and Bay of Campeche
through the middle of next week, where environmental conditions
appear conducive for development.
* Formation chance through 48 hours...low...20 percent.
* Formation chance through 5 days...medium...60 percent.

Forecaster Berg
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08-04-2017, 05:03 PM
Post: #5
RE: INVEST 99L
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Fri Aug 4 2017

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

1. A broad and elongated area of low pressure is producing disorganized
shower and thunderstorm activity several hundred miles south and
southwest of the Cabo Verde Islands. Environmental conditions
appear conducive for this system to consolidate and develop during
the next few days, and a tropical depression is likely to form by
the early or middle part of next week while moving
west-northwestward at 15 to 20 mph across the tropical Atlantic
Ocean.
* Formation chance through 48 hours...medium...50 percent.
* Formation chance through 5 days...high...80 percent.

2. A tropical wave is producing disorganized showers and thunderstorms
over the central and eastern Caribbean Sea. This disturbance is
expected to move west-northwestward at about 15 mph across the
western Caribbean Sea and Bay of Campeche through the middle of next
week, where environmental conditions appear conducive for
development.
* Formation chance through 48 hours...low...20 percent.
* Formation chance through 5 days...medium...60 percent.

Forecaster Berg
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08-04-2017, 06:08 PM
Post: #6
RE: INVEST 99L
JM


Quote: [Image: nhc-5day-15Z-8.4.17-835px.jpg] Two

Tropical Systems to Watch: 99L in Eastern Atlantic, 90L in Caribbean

Bob Henson · August 4, 2017, 17:41


Above: The five-day tropical weather outlook from the NHC/NOAA National Hurricane Center, issued at 2:00 pm EDT Friday, August 4, 2017, showed two areas of potential development in the Atlantic tropics. Image credit: NHC.


It’s quite possible we will be tracking two named storms in the Atlantic basin by early next week. A tropical wave dubbed Invest 99L is spinning its way through the eastern tropical Atlantic, while another sizable wave, Invest 90L, is moving through the Caribbean. Computer models are giving both systems a reasonable chance of developing as they head west-northwest over the next few days, and both need to be watched for potential U.S. impacts.



99L taking its time organizing


As of 8:00 am EDT Friday, 99L consisted of two weak surface lows, one around 9°N, 24°W, and the other near 10°N, 30°W. The lead wave near 30°W exhibited a good deal of spin on Thursday, but the presence of dual lows is now complicating the picture. Satellite imagery on Friday morning showed an elongated zone of disorganized showers and thunderstorms (convection) between the two centers. Because 99L is at such a low latitude, it’s more difficult for Earth’s rotation to impart a spin to the system, and the currently fractured state of 99L implies it will take time for the system to organize.


Conditions will be quite favorable for nurturing 99L over the next few days, assuming that it can consolidate around a single low-level center. Wind shear is quite low—only around 5 to 10 knots—and the 12Z Friday SHIPS model output indicates that shear will remain below 10 knots through at least Monday. 99L will be passing over sufficiently warm sea-surface temperatures of 27-28°C (82°-84°F), which is about 0.5-1.0°C above average for this time of year. The warm surface layer is not exceptionally deep, though—another reason why it may take 99L awhile to organize, assuming it does. Mid-levels of the atmosphere are quite moist around 99L (70-75% relative humidity), although 99L may encounter some dry air from the Saharan Air Layer (SAL) as it heads west-northwest.



[Image: viz-99L-1445Z-8.4.17.jpg]
Figure 1. Visible satellite image of Invest 99L as of 1445Z (10:45 am EDT) Friday, August 4, 2017. Image credit: NOAA/NESDIS.
Outlook for 99L


Models are in agreement that 99L will continue on a west-northwest track into early next week, heading near or just north of the Leeward Islands by Tuesday or Wednesday. Beyond that point, upper-level high pressure is projected to remain firmly in place across the North Atlantic, which implies that 99L could pose a threat to the U.S. East Coast in about 8 to 12 days. Among our three top global models for hurricane prediction, the GFS has been the most gung-ho in developing 99L, and the operational runs of the GFS from 00Z and 06Z Friday both bring a significant hurricane near the East Coast late next weekend. Our top models are far from unanimous on this idea, however. The operational runs of the European and UKMET models from 0Z Friday both keep 99L disorganized, perhaps becoming a depression or weak tropical storm at best over the next few days.


Of the 50 members of the Euro ensemble run from 0Z Friday, all 50 produce at least a tropical depression from 99L. However, most of those tracks bring a weak depression into the northeast Caribbean—a notoriously hostile environment for embryonic storms because of divergent low-level winds and sinking air. Only 2 of the 50 members depict a U.S. East Coast threat. The 20 GFS ensemble members from 0Z Friday are in far better agreement on a U.S. threat, with about 75% of the tracks recurving into or near the East Coast (though most are only at tropical-storm strength by that point). We can infer that the main question at hand is: will 99L organize beyond depression strength at all? If it does, there are ample signs that it could become a hurricane to watch, especially given its location and the time of year. (The worst Cape Verde hurricanes tend to develop from August into September.)


Our best intensity model, the HWRF, steadily develops 99L into a significant hurricane by Monday, but we should be cautious about intensity projections from HWRF (or any model, for that matter) until a distinct low-level center has formed. In its 2:00 pm EDT tropical weather outlook on Friday, the NOAA/NWS National Hurricane Center (NHC) gave 50% odds that at least a tropical depression would emerge from 99L by Sunday, and 80% odds by Wednesday.



[Image: gefs-168hr-sfc-6Z-8.4.17.jpeg]
Figure 2. Surface pressure centers analyzed in one-week forecasts issued by the 20 ensemble members of the GFS from the 2:00 am EDT Friday run, valid at 2:00 am EDT Friday, August 22, 2017. Each two-digit center should be preceded by a “10” or “9” for the predicted surface pressure in millibars. Most ensemble members produce at least a tropical depression for both Invest 99L (right) and Invest 90L (left, as discussed below). Image credit: tropicaltidbits.com


[Image: ir-99L-1700Z-8.4.17.jpg]
Figure 3. GOES-16 infrared satellite image of Invest 90L as of 1700Z (1:00 pm EDT) Friday, August 4, 2017. GOES-16 data are preliminary and non-operational. Image credit: NASA/MSFC Earth Science Branch.
90L aiming for Yucatan coast, and perhaps the Bay of Campeche

A more immediate risk is 90L, which may become a significant tropical storm or hurricane in the Gulf of Mexico early next week. The tropical wave associated with 90L extended north-south across the Caribbean Sea at around 68°W on Friday morning, with a weak surface low at its southwest end in far northern Colombia. The strongest convection was displaced well north of this center, in the central Caribbean. It is possible a new center might form closer to the convection if the showers and thunderstorms persist over the next day or two.


Over its last several runs, the operational European model has consistently developed a tropical cyclone in the far northwest Caribbean by Sunday or Monday, taken it quickly across the Yucatan Peninsula, and then strengthened it rapidly in the Bay of Campeche, with a potential hurricane landfall in northeast Mexico or southern Texas by late next week. About 75% of the Euro ensemble members show at least a depression in the northwest Caribbean by Monday, and almost 90% project a tropical cyclone moving through the Bay of Campeche later next week. Most of these runs are weaker at landfall than the Euro operational run, though. The operational GFS has been unimpressed with 90L for several runs, but there is increasing support within the GFS ensemble. Nearly all GFS ensemble members from 0Z Friday have a tropical cyclone in the western Gulf of Mexico, with some tracks angling toward the northwest Gulf Coast.


Conditions will be quite supportive for 90L by the time it reaches the northwest Caribbean. SHIPS model output from 12Z Friday indicates that sea-surface temperatures will be very warm (30°C or 86°F), with light to moderate wind shear and an extremely moist environment (mid-level relative humidity above 80%). The Bay of Campeche is a favorable spot for rapid intensification, because of its typically warm sea surface temperatures and a concave topography that helps generate atmospheric spin, so if 90L does make it across the Yucatan as a tropical cyclone, it will bear very close watching. In its 2:00 pm EDT outlook on Friday, NHC gave 20% odds that at least a tropical depression would emerge from 90L by Sunday, and 60% odds by Wednesday.

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08-04-2017, 07:34 PM
Post: #7
RE: INVEST 99L
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08-05-2017, 10:09 PM (This post was last modified: 08-05-2017 10:25 PM by Her-icane.)
Post: #8
RE: INVEST 99L
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
800 PM EDT Sat Aug 5 2017

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

1. Showers and thunderstorms associated with a strong tropical wave
located over the central Caribbean Sea are showing signs of
organization. Additional development of this system is possible
while it moves west-northwestward at 10 to 15 mph, and a tropical
depression or tropical storm could form over the northwestern
Caribbean Sea before it reaches the Yucatan peninsula late Monday or
Tuesday. Even if formation does not occur before the system
reaches the Yucatan peninsula, a tropical depression or tropical
storm could form over the Bay of Campeche during the middle part of
next week. Development would likely not occur if the system moves
inland over Central America and southeastern Mexico and does not
re-emerge over water. An Air Force Reserve reconnaissance aircraft
is scheduled to investigate this system tomorrow afternoon, if
necessary. Interests along the coasts of Honduras, Belize, and the
Yucatan peninsula of Mexico should monitor the progress of this
disturbance.
* Formation chance through 48 hours...medium...50 percent.
* Formation chance through 5 days...high...70 percent.

2. Shower activity associated with an elongated area of low pressure
located about 1000 miles west-southwest of the Cabo Verde Islands
has become a little better organized during the past 24 hours.
Some additional slow development is possible during the next two to
three days before the system encounters less favorable
environmental conditions during the middle part of next week
. This
system is expected to move generally west-northwestward across the
tropical Atlantic Ocean at about 15 mph.
* Formation chance through 48 hours...low...30 percent.
* Formation chance through 5 days...medium...50 percent.

Forecaster Beven

JM


Quote: [Image: ir-99L-1530Z-9.5.17-835px.jpg] 90L Slowly Organizing in Caribbean; 99L Remains Diffuse in Central Atlantic
Bob Henson · August 5, 2017, 16:07

Above: Infrared GOES-16 satellite image of the tropical wave known as Invest 90L, located over the central Caribbean at 1530Z (11:30 am EDT) Saturday, August 5, 2017. Image credit: NASA/MSFC Earth Science Branch.


Heavy rains will be heading toward Nicaragua and Honduras from a strong tropical wave in the central Caribbean. Dubbed Invest 90L, the system could become a Gulf of Mexico tropical storm early next week if it doesn’t run aground over Central America. Meanwhile, the wave known as Invest 99L continues slogging through the tropical Atlantic, still disorganized but maintaining the potential to become a named storm next week as it nears the Lesser Antilles. The next two names on the Atlantic list are Franklin and Gert.


Showers and thunderstorms (convection) blossomed around Invest 90L on Friday night, and satellite imagery showed an increasing amount of spin near the center of the convection, located midway between Jamaica and Colombia. The wave may continue to slowly organize on Saturday as it moves west to west-northwest at about 10 mph. Conditions will be very favorable for 90L to develop further on Sunday into Monday in the northwest Caribbean. The 12Z Saturday run of the SHIPS statistical model predicts that 90L will encounter sea surface temperatures (SSTs) near 30°C (86°F), deep oceanic heat content, light wind shear (below 10 knots), and a very moist environment (mid-level relative humidity around 80%).


Our top three global models for hurricane prediction have largely stuck to their guns regarding 90L, with the European model the only one consistently calling for 90L to develop in its operational runs. There is ample reason to be cautious when only one of the three leading models foresees development, but given the organization now under way and the prime conditions lying ahead of 90L, at least some strengthening seems plausible. In its tropical weather discussion issued at 8 am EDT Saturday, the NOAA/NWS National Hurricane Center gave 90L a 30% chance of development into at least a tropical depression by Monday and a 50% chance by Thursday.


One of the biggest short-term questions is whether 90L will track mainly due west, which could take the wave along the Honduras coast or just inland from it on Sunday. In this case, 90L may never progress beyond tropical-wave status. However, if 90L maintains a distinct low-level center that stays well north of the coast, it could become a tropical depression or tropical storm by the time it reaches Belize and the Yucatan Peninsula of Mexico around Monday. About a third of the European model ensemble members from 0Z Saturday favor this northern trajectory and tropical-storm status. The northern scenario also leaves the door open for further development in the Bay of Campeche, a very favorable location for tropical cyclones because of its warm SSTs and concave topography. Again, roughly a third of the Euro ensemble members from Friday night call for a tropical storm in the Bay of Campeche or just to its north, approaching northeast Mexico or far southern Texas around the middle of next week. In contrast, only about a quarter of GFS ensemble members calls for development, with lower tropical-storm intensities and a track into the far southern Bay of Campeche.



[Image: viz-99L-1515Z-8.5.17.png]
Figure 1. Visible satellite image of Invest 99L at 1515Z (11:15 am EDT) Saturday, August 5, 2017. Image credit: NOAA/NESDIS.
99L approaches the Central Atlantic


Located several hundred miles southwest of the Cape Verde Islands, Invest 99L is having some trouble getting its act together. The wave remains quite elongated, and convection remained scattered and relatively weak on Friday night, failing to take advantage of the typical nocturnal bump-up in tropical thunderstorm development. In its 8 am EDT Saturday outlook, NHC identified a surface low near 10°N, 33°W, with other areas of convection to its west and east. We’ll have to see if this low remains the predominant center as the wave continues moving west at 10 mph.


While the European model has been dubious about 99L for days, the GFS and UKMET models continue to favor development. However, the Friday night UKMET run depicts only a weak system, and the GFS ensemble and operational runs have pulled back notably on the potential they had flagged earlier for a strong hurricane to approach the U.S. East Coast roughly a week from now. More than 80% of GFS ensemble members from Friday night called for 99L to become a tropical storm by Monday, when the wave is expected to be approaching the northern Lesser Antilles. The big difference this time is a more southerly track than previous runs, which would take 99L closer to the northeast Caribbean. Such a track would produce much more interference from land interaction with the Greater Antilles, and increasing wind shear from an upper-level trough could also be affecting 99L by early next week. A quarter of the GFS ensemble members from Friday night insisted that 99L could survive these factors to potentially affect the U.S. Gulf or Atlantic coast as a tropical storm or a hurricane more than a week from now. Meanwhile, less than 20% of the European ensemble members called for 99L to develop into a tropical depression or weak tropical storm over the next couple of days on approach to the Lesser Antilles. All of them snuffed out 99L as it enters the eastern Caribbean, a notoriously difficult location for tropical development.


In its Saturday morning discussion, the NOAA/NWS National Hurricane Center gave 90L a 40% chance of development into at least a tropical depression by Monday and a 70% chance by Thursday.



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08-06-2017, 01:07 PM (This post was last modified: 08-06-2017 01:10 PM by Her-icane.)
Post: #9
RE: INVEST 99L
ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
200 PM EDT Sun Aug 6 2017

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

1. Satellite data indicate that a broad area of low pressure has
formed over the western Caribbean Sea about 150 miles east of the
eastern coast of Honduras. In addition, winds just below
tropical-storm-force are occurring to the northeast of the center.
The associated showers and thunderstorms are gradually becoming
better organized, and a tropical depression or tropical storm will
likely form before the low reaches the Yucatan Peninsula of Mexico
late Monday or early Tuesday. After the system crosses the Yucatan
Peninsula, it is expected to move across the Bay of Campeche by
midweek where additional development is expected. An Air Force
Reserve reconnaissance aircraft is scheduled to investigate this low
on Monday, if necessary. Interests along the coasts of Honduras,
Belize, and the Yucatan Peninsula should monitor the progress of
this system as heavy rains and strong winds are possible at those
locations.
* Formation chance through 48 hours...high...80 percent.
* Formation chance through 5 days...high...90 percent.

2. An area of low pressure located about midway between the Cabo Verde
Islands and the Lesser Antilles remains elongated, and the
associated showers and thunderstorms are not well organized. Some
slow development of this system is possible during the next several
days while it moves generally west-northwestward across the tropical
Atlantic at about 15 mph.
* Formation chance through 48 hours...low...20 percent.
* Formation chance through 5 days...medium...50 percent.

Forecaster Cangialosi

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08-06-2017, 09:24 PM
Post: #10
RE: INVEST 99L
Henson for JM


Quote: [Image: ptc7-track-21Z-8.6.17-835px.jpeg]
Tropical Storm Warnings Issued for PTC 7 in Northwest Caribbean
Bob Henson · August 6, 2017, 21:15


Above: Projected track of PTC 7 as of 5:00 pm EDT Sunday, August 6, 2017. Image credit: NOAA/NHC.

Tropical storm warning flags are flying for the Yucatan Peninsula, with a tropical storm watch for the Belize coast, as Potential Tropical Cyclone 7 could become a named storm before it sloshes ashore late Monday. Models agree the system will likely strengthen more dramatically in the Bay of Campeche before an expected second landfall later this week in northeast Mexico.


The NOAA/NWS National Hurricane Center (NHC) upgraded the wave known as Invest 90L to PTC 7 at 5:00 pm EDT Sunday. The new “potential tropical cyclone” designation, which was introduced this year, tracks systems that are not yet tropical storms but that could affect land as tropical storms within 48 hours.


ASCAT scatterometer data at midday Sunday showed that easterly winds of around 30-35 mph extended across a broad zone north of PTC 7. NHC placed the center of this broad low-level circulation about 90 miles east of the northeast tip of Honduras as of 5:00 pm EDT Sunday, moving west-northwest at about 15 mph. The system remained quite asymmetric, with westerly winds weak to nonexistent on its south side. Satellite imagery showed widespread but still poorly organized showers and thunderstorms (convection), with little spin evident. Westerly upper-level winds feeding into a large upper low north of Puerto Rico have imparted strong wind shear (15-25 knots) to the eastern part of PTC 7, with additional shear being produced by a small upper low north of Honduras.



[Image: ir-90L-2015Z-8.6.17.jpg]
Figure 1. Infrared GOES-16 image of PTC 7 as of 2015Z (4:15 pm EDT) Sunday, August 6, 2017, when the system was still designated as Invest 90L. Image credit: NASA/MSFC Earth Science Branch.




Outlook for PTC 7: Two landfalls are likely

Despite PTC 7’s less-than-textbook appearance on Sunday, there are strong signals that the wave will become a tropical storm before it moves west-northwest into the coast of Belize or Mexico’s Yucatan Peninsula late Monday. The wave is heading into a part of the far northwest Caribbean with unusually deep warm water. Sea surface temperatures, or SSTs, are close to 29°C (84°F), more than 1°C above average. Moderate wind shear of 10-15 knots is predicted to slacken below 10 knots by Monday, and the atmosphere around PTC 7 will remain very moist, with mid-level relative humidities around 70%. In its 2:00 pm EDT tropical weather outlook, NHC gave PTC 7 an 80% chance of becoming at least a tropical depression by Tuesday afternoon and a 90% chance through Friday.


The 12Z Sunday run of the HWRF model, our best short-term intensity guidance, shows PTC 7 becoming a strong tropical storm before a landfall in the Quintana Roo district of Mexico on Monday evening. The 12Z runs of the GFS, European, and UKMET models agree on this general track, though without as much intensification. Given PTC 7’s current lack of organization, I would peg its most likely strength at landfall as a weak tropical storm. Heavy rains across the region should be the main threat.



The second chapter of PTC 7’s saga will unfold after it crosses the Yucatan Peninsula. The low-lying, water-surrounded terrain of the peninsula should dent PTC 7’s strength only slightly, especially since the system may still be loosely organized at that point. Global models have come into stronger agreement on a track that would keep PTC 7 moving west-northwest through the Bay of Campeche from Tuesday to Thursday, when it will likely reach the northeast coast of Mexico. The Bay of Campeche is a notoriously favorable area for tropical development, with very warm waters and a concave topography that helps focus storm spin. Models suggest that 99L will be angling leftward as it moves through the bay, which would impart further cyclonic spin and could also hasten intensification.


The official NHC outlook from 5 pm EDT Sunday brings PTC 7 into the coast of northeast Mexico as a strong tropical storm. If PTC 7 enters the bay with a low-level circulation, I would give better than 50-50 odds that it will reach northeast Mexico as a hurricane. The 12Z HWRF suggests that rapid strengthening is possible from Wednesday until a Thursday landfall as a strong Category 2 storm, and the Rapid Intensification index in the 18Z SHIPS model gives a 62% chance that PTC 7 will be a 95-knot hurricane (top-end Category 2) by late Wednesday. Although our three top global models show less strengthening, the HWRF and SHIPS scenario is a real possibility.



[Image: vis-99L-1945Z-8.6.17-637px.jpg]
Figure 2. Visible satellite image of Invest 99L as of 1945Z (2:45 pm EDT) Sunday, August 7, 2016. Image credit: NOAA/NESDIS.





Little change with 99L in the central Atlantic

The wait-and-see game continues with Invest 99L, a large tropical wave rolling through the central Atlantic midway between Africa and the Lesser Antilles. 99L has shown little inclination to organize over the last several days, and it remains quite elongated as it pushes west-northwest at about 15 mph.


The strong zone of high pressure in the central Atlantic pushing 99L westward is also leading to subsidence and drying, and this dry air will be a thorn in 99L’s side. The wave will be encountering a large extension of the Saharan Air Layer (see Figure 3 below) over the next couple of days. The 18Z SHIPS model shows mid-level relative humidities around 99L will drop from around 60% to around 55% from Monday onward. Moderate wind shear of 10-15 knots, increasing to 15-20 knots by Tuesday, may also impede PTC 7’s growth.


In its 2:00 pm EDT tropical weather outlook, NHC gave PTC 7 an 20% chance of becoming at least a tropical depression by Tuesday afternoon and a 50% chance through Friday.



[Image: sal-18Z-8.6.17.png]
Figure 3. A patch of the Saharan Air Layer (SAL) lies ahead of 99L west-northwest track over the next several days. Image credit: UW-CIMMS/NOAA Hurricane Research Division.


The leading global models have come into closer agreement on 99’s fate, which seems most likely to be unimpressive but with a small chance of U.S. impacts more than a week from now. Of the ensemble runs from 0Z Sunday, less than 20% of the 50 European model members led a tropical storm near or just north of the Leeward Islands by Wednesday. A small fraction (less than 10%) continued 99L on a northwestward track that had the system approaching the U.S. East Coast as a tropical storm or hurricane around 7 to 9 days from now. As in past days, the GFS model was more bullish on 99L, with about two-thirds of its 20 ensemble members producing a tropical storm and about 25% carrying the system toward Florida and/or the Gulf of Mexico as a tropical storm or hurricane by next weekend. The operational UKMET model from 0Z Sunday suggested the potential for an East Coast system, while the operational GFS and ECMWF runs did not develop 99L significantly through 7 days.


There is plenty of time to see if 99L manages to organize as it approaches the Leeward Islands. Based on the trend of global models, 99L or its remnants will ultimately head toward the United States. We may need to keep at least one eye on 99L even if it approaches the U.S. as a weak system, since rapid spin-ups are possible in the Bahamas and Gulf of Mexico this time of year.

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