ATL: BERTHA - Post-Tropical - Discussion

[cycloneye]

This mornings discussion by Dr Jeff Masters.

An area of disturbed weather located near 10°N, 39°W at 8 am EDT Tuesday, about 1600 miles east of the Lesser Antilles Islands, (93L), has the potential to develop into a tropical depression by Wednesday, but is struggling with high wind shear today. Visible satellite loops on Tuesday morning showed improved organization to 93L with more spin and some low-level spiral bands beginning to form. A 7:30 am EDT July 29 pass from the ASCAT satellite showed plenty of west winds on the south side of 93L's center of circulation, so the storm is close to having a well-defined closed surface circulation. However, infrared satellite images showed that the system's heavy thunderstorm activity had diminished somewhat since Monday, and the storm is now fighting high wind shear of about 20 knots. Water vapor satellite loops and the Saharan Air Layer analysis showed that 93L had more dry air to contend with than on Monday, with some tendrils of the dry Saharan Air Layer to the north encroaching into the circulation. Ocean temperatures have cooled since Monday, and are now marginal for development, about 27°C.
Forecast for 93L
Given the high levels of wind shear affecting 93L today, it is more likely that NHC will classify it as a tropical depression on Wednesday than today. The 12 UTC Tuesday forecast from the SHIPS model predicted that the current high wind shear affecting 93L will relax to the moderate level, 5 - 15 knots, Wednesday afternoon through Friday, aiding development. Sea Surface Temperatures (SSTs) will slowly warm as the system approaches the Caribbean, reaching 28°C by Friday and Saturday. Two of our three reliable models for predicting tropical cyclone genesis, the GFS and UKMET models, predicted in their 00Z Tuesday runs that the disturbance would develop into a tropical depression by Thursday. 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 Tuesday Tropical Weather Outlook, NHC put the 2-day and 5-day odds of development at 70% and 80%, respectively.

All of the models predict that the disturbance will continue west or west-northwest at 10 - 15 mph for the next four days. The UKMET and the European models offer the fastest solution, predicting that the disturbance will arrive in the northeast Lesser Antilles Islands on Friday evening, move over Puerto Rico on Saturday evening, and approach the Southeast Bahamas and Turks and Caicos Islands on Sunday evening. The GFS is slower and more to the northeast, predicting a Saturday morning arrival in the northern Lesser Antilles, with passage a few hundred miles northeast of Puerto Rico occurring on Sunday morning. Dry air to the north of 93L will likely interfere with development throughout the week, and we will have to see if the moderate levels of wind shear forecast to occur 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 Tuesday morning runs of our two most reliable models for predicting intensity, the LGEM and DSHIPS models, forecasted that once 93L becomes a tropical depression, it will intensify into a hurricane within 72 hours. However, the dynamical GFDL and HWRF models, which made good intensity forecasts for Hurricane Arthur, were much less bullish. The Tuesday morning runs of these models predicted that 93L would never reach hurricane strength. I give a 10% chance that 93L will be a hurricane on Saturday when it makes its closest approach to the Lesser Antilles Islands. If 93L hits the Northern Lesser Antilles as a very wet Tropical Storm Bertha (not a hurricane), the storm could be more boon than bane for the islands. The Northeast Caribbean suffered its driest June in recorded history last month, according to NOAA, and many of the islands have significant drought problems.

The GFS and European models have come into better agreement on the long-range fate of 93L. The great majority of the 20 members of the 00Z and 06Z Tuesday runs of the European and GFS ensemble models (which run at low resolution 20 times with slightly different initial conditions to show a range of possible outcomes) showed 93L taking more of a northwesterly track early next week, passing to the north of Hispaniola and not making an extended track through the Caribbean Sea. This raises the odds that the strong trough of low pressure over the Eastern U.S. will be able to recurve 93L out to sea without the storm hitting the mainland U.S. coast.

Jeff Masters

http://www.wunderground.com/blog/JeffMa ... rynum=2740

[Gustywind]

I don't like this title even if we're a bit far away from what could happen during the next couple of days islanders... Keep an eye on that.

Hurricane Central
Tropical Storm Threat: Leeward Islands Could Feel Impacts This Weekend

By Jon Erdman Published: Jul 29, 2014, 10:32 AM EDT weather.com

http://www.weather.com/news/weather-hur ... t-20140729

Key Takeaways

- A tropical depression may form at any time in the central Atlantic Ocean.

- Potential threat to Lesser Antilles, including the U.S. Virgin Islands, as well as Puerto Rico this weekend.

- Too soon to call for future potential U.S. threat.

After an almost one-month Atlantic hurricane season hiatus, a tropical threat is emerging for the Lesser Antilles this weekend.
A tropical disturbance is roughly halfway between Africa and the Lesser Antilles, or about 1600 miles east of the southern Windward Islands.
The disturbance has a pronounced spin above the surface, but has yet to produce sufficiently persistent convection (translation: thunderstorms) and a pronounced-enough westerly surface wind to merit classification as a tropical cyclone.

Once those two events occur, the National Hurricane Center will likely begin issuing advisories on what would be Tropical Depression 3.

Forecast: Lesser Antilles This Weekend

First, the disturbance has to overcome some dry air and light to moderate wind shear over the next day or so.

Assuming a tropical depression does form, the system is expected to track toward the Lesser Antilles late Friday into the weekend, possibly strengthening into a tropical storm. Its name would be Bertha.
As the system nears the islands, an area of low pressure aloft (called a tropical upper tropospheric trough or TUTT) may begin to turn the system toward the northwest. If the system is stronger at that time, the TUTT would pull the system farther north. Conversely, a weaker, less vertically-developed system would not be pulled as far north by the TUTT.

Furthermore, this TUTT may produce wind shear (change in wind direction and/or speed with height) that is hostile for tropical cyclone intensification or development as the system nears the islands.

(MORE: Expert Analysis)

So, for now, interests in the Lesser Antilles, U.S. Virgin Islands and Puerto Rico should monitor the forecast closely for possible impacts this weekend.

(FORECASTS: Guadeloupe | St. Thomas | San Juan)

It is still far too soon to forecast any future potential impacts farther downstream, including in the Bahamas, the U.S. or Bermuda.

Incidentally, the average date by which the Atlantic hurricane season's second named storm will have occurred is Aug. 1, so this potential Bertha is right on time, climatologically.

Check back with us at The Weather Channel and weather.com for the latest on this potential tropical threat.

[Frank2]

Per the loop on Page 11, the swirl is near 8.9N and 39W - still in the ITCZ...

[Hyperstorm]

It looks like the system is struggling today with some easterly shear that is causing it to ingest some dry air into its already better defined circulation. This was actually predicted by the GFS, except that it happened a day early. Regardless, it looks like conditions are not really forecast to be as favorable for development until the day on Thursday. I don't see the system being upgraded today, and if the GFS forecast of dry air intrusion continuing tomorrow holds true, not even tomorrow. If we follow the GFS logic, we should still fully expect this system to strengthen into a tropical cyclone by Thursday. We'll keep watching!

[northjaxpro]

The TUTT north of Puerto Rico has been slow to move out and has been a persistent feature out there for about a week now. This will be a very key player going into the latter part of this week. This feature could maintain shear on whatever becomes of 93L by the time the system reaches the NE Caribbean area towards the end of the week.

As for today, 93L does have a tight circulation, but dry air from the SAL is infiltrating the periphrey of the circulation more today as compared to yesterday and no concentrated convection around the COC, and this will probably keep NHC from declaring the system a TD for probably at least the next 24 hours.

Leaning more on the fact that conditions I feel will stay only marginably conducive for further development as the week progresses. Still, you have the shear zone across the Caribbean and the TUTT north of Puerto Rico all awaiting 93L in a few days. 93L may get to be a TS toward the end of the week, but the conditions I feel will not enable the cyclone to fully develop into a strong system unless shear and sdry air really drops off later in the week.

93L will get farther westward if it plays out to be a weaker, sheared system. I can see it traversing at least as far as 75 degrees longitude. I won't speculate on long range track too much until the NHC designates this as a TD and then go from there. Too many questions about if the trough will pull it out to sea, or if the Bermuda ridge will build back in to block 93L to escape out to sea. I will wait until about Thursday to assess this matter.

This post is NOT AN OFFICIAL FORECAST and should not be used as such. It is just the opinion of the poster and may or may not be backed by sound meteorological data. It is NOT endorsed by any professional institution including storm2k.org. For Official Information please refer to the NHC and NWS products.

[northjaxpro]

Thanks cycloneye for adding the disclaimer. I forgot.

[wxman57]

I'm thinking a WNW track across the NE Caribbean as a weak, disorganized TS. Perhaps right over you in PR, Luis. Continue WNW to near the eastern Bahamas, possibly as a weakening system. May recurve around 75W.

[northjaxpro]

I'm thinking a WNW track across the NE Caribbean as a weak, disorganized TS. Perhaps right over you in PR, Luis. Continue WNW to near the eastern Bahamas, possibly as a weakening system. May recurve around 75W.

This is my line of thinking too wxman, which I touched on in my post just above.

[cycloneye]

I'm thinking a WNW track across the NE Caribbean as a weak, disorganized TS. Perhaps right over you in PR, Luis. Continue WNW to near the eastern Bahamas, possibly as a weakening system. May recurve around 75W.

You know what? Bring it on as the gov announced rationing of water would begin next week depending on what this does. We are going thru a moderate drought that other islands are going thru.

[AEWspotter]

Despite the 20+kt easterly wind shear, the LLC is tightening up. The latest ASCAT pass (below) shows that the LLC is still a bit elongated from SW to NE, but there is a 30kt wind vector in there I think, for what it's worth. Something else worth noting... both dynamical and statistical models agree that the wind shear will abate to below 10kts in 48 hours or so. Both the GFDL and HWRF models are not very bullish on this system, keeping it a tropical storm near the northern extent of the Lesser Antilles. Any reasonable increase in convection near the LLC would convince me to give it a number. Let's see what NHC does...

For the record, the GFS weakens the TUTT and meanders it to the WSW. To me, it looks like it might be lifting out with the trough off of the eastern US. See the Wide-View of the Atlantic below

ASCAT: http://manati.star.nesdis.noaa.gov/asca ... Bas113.png
Models: http://einstein.atmos.colostate.edu/~mc ... 291200.PNG
Atlantic Wide-View Vis: http://www.ssd.noaa.gov/goes/east/tatl/flash-vis.html

[tropicwatch]

Despite the 20+kt easterly wind shear, the LLC is tightening up. The latest ASCAT pass (below) shows that the LLC is still a bit elongated from SW to NE, but there is a 30kt wind vector in there I think, for what it's worth. Something else worth noting... both dynamical and statistical models agree that the wind shear will abate to below 10kts in 48 hours or so. Both the GFDL and HWRF models are not very bullish on this system, keeping it a tropical storm near the northern extent of the Lesser Antilles. Any reasonable increase in convection near the LLC would convince me to give it a number. Let's see what NHC does...

ASCAT: http://manati.star.nesdis.noaa.gov/asca ... Bas113.png
Models: http://einstein.atmos.colostate.edu/~mc ... 291200.PNG
Atlantic Wide-View Vis: http://www.ssd.noaa.gov/goes/east/tatl/flash-vis.html

It is almost out of the higher wind shear.

[TropicalAnalystwx13]

My forecast has this becoming a tropical depression by tomorrow night (It should already be designated, but I'll move on...) and a tropical storm on the day Thursday as wind shear lessens significantly. I could see this being a 50-60 mph tropical storm as it approaches the northern Leeward Islands over the weekend. After that time, the trough over the East Coast should be able to turn this more poleward, and I see a recurvature west of Bermuda as an intensifying hurricane.

[Alyono]

just stop! There is NOTHING that has yet to prove a well defined LLC. It's ebcoming better defined, but now the convection is dropping off. Notice all the mets are saying this has yet to form? That should tell you something

[TheStormExpert]

Do I smell a repeat of TS Dorian from last season coming on? Anyways that Kelvin Wave explains why the Tropical Atlantic looked so impressive yesterday (for once).

Looking at WV it looks now like it's moist envelope is shrinking as I type. It'll be lucky enough to weak minimal TS status at this point.

[TheStormExpert]

just stop! There is NOTHING that has yet to prove a well defined LLC. It's ebcoming better defined, but now the convection is dropping off. Notice all the mets are saying this has yet to form? That should tell you something

Many spoke too soon yesterday about how impressive it looked.

[cycloneye]

TCFA issued earlier this morning.

WTNT21 KNGU 290701
SUBJ/TROPICAL CYCLONE FORMATION ALERT//
RMKS/1. FORMATION OF A SIGNIFICANT TROPICAL CYCLONE IS POSSIBLE WITHIN
100 NM EITHER SIDE OF A LINE FROM 9.9N 37.3W TO 11.4N 43.8W
WITHIN THE NEXT 24 HOURS. CURRENT MODEL GUIDANCE IS SUGGESTING
INTENSIFICATION OF A TROPICAL DISTURBANCE IN THE CENTRAL ATLANTIC,
APPROXIMATELY 1000NM WEST-SOUTHWEST OF THE CAPE VERDE ISLANDS.
AVAILABLE DATA DOES NOT JUSTIFY ISSUANCE OF NUMBERED TROPICAL
CYCLONE WARNINGS AT THIS TIME. WINDS IN THE AREA ARE ESTIMATED TO
BE 25 TO 30 KNOTS. METSAT IMAGERY AT 290545Z INDICATES THAT A
CIRCULATION CENTER IS LOCATED NEAR 9.9N 37.3W. THE SYSTEM IS MOVING
WEST-NORTHWESTWARD AT 15 KNOTS.
2. REMARKS:SATELLITE IMAGERY INDICATES THAT THE DISTURBANCE HAS BECOME BETTER
DEFINED OVER THE LAST 12 HOURS AND SCATTEROMETRY PASSES INDICATE
CLOSED CIRCULATION WITH WIND SPEEDS OF 25 TO 30KTS. THE UPPER LEVEL
ENVIRONMENT IS CONDUCIVE FOR INTENSIFICATION IN THE NEXT 24
HOURS. SEA SURFACE TEMPERATURES OF 80 TO 84 DEGREES WILL FURTHER
AID IN THE DEVELOPMENT OF THIS TROPICAL FEATURE.
3. THIS ALERT WILL BE REISSUED, UPGRADED TO WARNING OR CANCELLED
BY 300700Z.//

[TheStormExpert]

What ever happened to the MJO, is it forecasted to appear next week still or will it remain nonexistent like it has all season so far?

[cycloneye]

My forecast has this becoming a tropical depression by tomorrow night (It should already be designated, but I'll move on...) and a tropical storm on the day Thursday as wind shear lessens significantly. I could see this being a 50-60 mph tropical storm as it approaches the northern Leeward Islands over the weekend. After that time, the trough over the East Coast should be able to turn this more poleward, and I see a recurvature west of Bermuda as an intensifying hurricane.

Any new Kelvin Wave coming to help it?

[wxman57]

I'm thinking a WNW track across the NE Caribbean as a weak, disorganized TS. Perhaps right over you in PR, Luis. Continue WNW to near the eastern Bahamas, possibly as a weakening system. May recurve around 75W.

You know what? Bring it on as the gov announced rationing of water would begin next week depending on what this does. We are going thru a moderate drought that other islands are going thru.

I've heard it's been rather dry there. Not many tropical waves this year. This could be more of a beneficial system than a threat.

[Ntxw]

What ever happened to the MJO, is it forecasted to appear next week still or will it remain nonexistent like it has all season so far?

The MJO has been and remains in large part over the Pacific (WPAC). What aided 93L was a CCKW (convectively coupled Kelvin wave) they move quicker than the MJO when not coupled with the MJO. CCKW's can enhance tropical activity on shorter time scales and help intensify developed systems or seed them. Once they pass in large part the environment of the basin determines what happens next without their assistance.