Remnant Low GASTON

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[dkmac]

Rob Lightbown:

Tropical Weather Discussion

Issued: Monday, September 6, 2010 820 am EDT/720 am CDT
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Ex-Tropical Depression Gaston:

A remnant low pressure system that was once Gaston is located about 400 miles east of the northern Leeward Islands. Currently, the reason why this hasn’t regenerated into a tropical cyclone is likely because of strong easterly winds aloft and dry air surrounding this system. It appears that the dry air is slowly being worked out of this system as is normal with systems as they track west of 55 West Longitude. In addition, satellite loops indicate that the convection has increased some over the past 24 hours and to get this to regenerate into a tropical cyclone is to have persistent and consistent organized deep convection for at least 12 to 24 hours. Now, will this occur??
Analysis this morning indicates that the upper level environment is more favorable for development the further west you go and at the same time, the atmosphere is also more moist. There is a huge discrepancy between the track intensity guidance from NHC and the global model guidance. The intensity guidance from NHC which includes the SHIPS and LGEM model forecast major strengthening in the next few days to a hurricane by Wednesday afternoon and a Category 4 or 5 hurricane in just 5 days from now. The reason why the NHC intensity guidance may be so gung-ho on intensifying this system is likely due to a forecast by these models of less than 10 knots of shear starting tonight and continuing through the end of the week.
The global models on the other hand forecast little or no development. The reason why that I can figure out is because of the model guidance’s forecast upper air environment this week. The GFS model as an example forecasts that Ex-Gaston remains on the southern edge of a ridge of high pressure and never gets under this high pressure system. At the same time, the GFS model forecasts that the upper level low pressure system will track westward to near western Cuba on Wednesday and then track to near the Yucatan Channel by early Thursday. So, even though the GFS model is forecasting little or no development, it is forecasting a somewhat favorable environment later this week in the central Caribbean.
So, I am scratching my head to say the least on why the huge discrepancy in the model guidance. There may be a few reasons including that the global models may not be resolving the small size of Ex-Gaston’s circulation correctly?; because the environment ahead of this system should become more and more favorable in the coming days. There is one recent case where the model guidance did not pick up on a storm and that is Category 5 Hurricane Felix from 2007. Could this be the case with Ex-Gaston?? It’s possible.
So, my take is that this system will remain status-quo today through tonight as it is still in an environment of somewhat dry air and moderate easterly wind shear. The wind shear is forecast to lessen starting tomorrow and the environment as a whole will become much more favorable as it gets into the eastern Caribbean later Tuesday and especially on Wednesday and I do believe that we will see this system intensify as it tracks westward across the Caribbean during the middle and later parts of this week.
Nevertheless, this system will bring heavy rainfall, strong gusty winds and isolated tornadoes to the northern Leeward Islands starting late today and continuing through tonight and then spread westward into the Virgin Islands and Puerto Rico starting Tuesday and continuing through Wednesday, even if this system does not regenerate into a tropical cyclone in the next day or two. All interests in this entire area should monitor the progress of this system very closely. Further west, all interests in Jamaica and the Cayman Islands should also closely monitor this system as it may affect you by late Friday or Saturday.

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[ConnecticutWX]

Its been out there for 2-4 days under a high risk! I wonder when it will develop... Im thinking either today (9/6) or tomorrow (9/7)

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12z Early Cycle NHC model tracks
Gaston
Statistical/Simple Models (CLIPER,BAMs,LBAR,other Statistical Models)






Dynamic Models (More sophisticated models)

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[dkmac]

StormW at WU:

Good morning!

REMNANT LOW GASTON:

Remnant low Gaston continues to move toward the west, along the southern periphery of the subtropical ridge.

Currently, Gaston is still having a big fight with dry air, mainly to his east. Wind shear doesn't seem to be too much of the problem anymore, as wind shear has been on the decrease, and is forecast to become negligible in the next few days.



My current thinking with this system...we will still see the waxing and waning of convection for at least another 24 hours. Based on analysis of forecast 700mb humidity and moisture levels, Gaston will be seeing improving conditions once close to the Caribbean.

I expect the remnant to continue moving west for the next 24 hours, with a motion just north of due west after.

Based on current position from satellite loop imagery, and apparent steering flow from satellite loop imagery, it is unknown at this particular moment whether the center of Gaston will continue directly into the Caribbean, or enter just at the northern most Leewards, and cross over Puerto Rico, or just skirt the coast of Puerto Rico (south). Again, 60W is my benchmark, and will tell the tale of either just skirting the Greater Antilles (north possibly), or entering the Caribbean. I will say this however, for Gaston to survive and take advantage of the forecast conducive environment, both moisture and upper level support wise, he has to make it into the Caribbean, and drift southward of the Islands.



If he should be drawn more toward the WNW, then moisture conditions will not be as favorable, however the forecast upper level anticyclone will should be established to his favor.



Based on these analyzes, the remnant low should begin a better regeneration process in about 24 hours or so. Yes...it is taking a little longer than I first thought, as models on the various parameters had shown conditions to improve about 12-18 hours prior. I will continue to monitor this system as it continues west.

Elsewhere, a tropical wave lies just to SSE of the Cape Verde Islands, and will be monitored for development over the next 96 hours.



The African wave train remains active, and any waves that exit into the Atlantic WILL have to be monitored, as the shear forecast map, for the 4th day in a row, indicates a series of upper level anticyclones to develop as waves enter the Atlantic.

Elsewhere, Tropical Storm formation is not expected through Tuesday.

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2pm

Atlantic Graphical TWO Area 1 closeup

Return to Atlantic Graphical TWO 1. CLOUDINESS AND SHOWERS REMAIN DISORGANIZED IN ASSOCIATION WITH THE
REMNANT LOW OF GASTON...LOCATED ABOUT 300 MILES EAST OF THE
NORTHERN LEEWARD ISLANDS. ENVIRONMENTAL CONDITIONS ARE EXPECTED TO
BECOME MORE CONDUCIVE FOR DEVELOPMENT OVER THE NEXT COUPLE OF DAYS
AS THE SYSTEM MOVES WESTWARD AT 15 TO 20 MPH. THERE IS A HIGH
CHANCE...70 PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE
AGAIN DURING THE NEXT 48 HOURS. HEAVY SHOWERS AND WIND GUSTS TO
TROPICAL STORM FORCE IN SQUALLS SHOULD BEGIN TO AFFECT PORTIONS OF
THE LEEWARD ISLANDS LATER TODAY AND TONIGHT...AND INTERESTS IN
THESE ISLANDS SHOULD CONTINUE TO MONITOR THE PROGRESS OF THIS
SYSTEM.

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[richardginn]

Could it wind up hitting to much land to form???

[Batt2fd]

Dr. Jeff Masters Bolg

Gaston continues to suffer from dry air
An Air Force Hurricane Hunter aircraft spent the afternoon in Gaston's remains, and found a weak 1012 mb center of low pressure with only a limited region of westerly winds on the south side of the center of circulation. Top surface winds uncontaminated by heavy rain seen by their SFMR instrument were in the 30 - 35 mph range. The airplane found plenty of dry air in the storm's environment, and there are not enough heavy thunderstorms in ex-Gaston's circulation for it to qualify as a tropical depression. The remains of Gaston are now approaching the northern Lesser Antilles, and residents can expect a few heavy rain showers and wind gusts up to 40 mph beginning early this evening and continuing into the night. Latest radar out of Martinique doesn't show much in the way of heavy rain, and satellite imagery confirms that the thunderstorm activity associated with Gaston's remains is quite sparse. A large amount of dry air surrounds Gaston's remains on all sides, as seen on water vapor satellite loops.


Figure 2. Afternoon satellite image of the remains of Gaston, approaching the Lesser Antilles Islands.

Forecast for Gaston
There is little change to the forecast for Gaston's remains. Dry air will continue to be the major impediment to development, and the system is unlikely to become a tropical depression today. However, wind shear, which is currently a moderate 10 knots, is forecast by the latest SHIPS model run to fall very low, 0 - 5 knots, tonight through Wednesday. With almost no wind shear affecting it, Gaston will a better chance of keeping the moisture from its heavy thunderstorms near its core on Tuesday. This will insulate the storm from the dry air surrounding it. The atmosphere is also moister in the eastern Caribbean, further increasing the chances of development. I believe it is probable Gaston will become a tropical depression again on Tuesday. NHC is currently giving Gaston a 70% chance of developing into a tropical depression by Wednesday, which is a reasonable forecast.

Heavy rain showers and gusty winds from the storm will affect Puerto Rico Tuesday morning through Wednesday afternoon. This activity will spread to the Dominican Republic Tuesday afternoon through Thursday morning. Assuming dry air and an encounter with Hispaniola's high mountains do not destroy Gaston, heavy rain from the storm should move over Haiti, eastern Cuba, and Jamaica Wednesday night and Thursday morning. The models don't give much support for Gaston surviving past Wednesday. The ECMWF, GFS, NOGAPS, Canadian, and HWRF models all dissipate Gaston. However, two models--the GFDL and UKMET--predict that Gaston will survive the dry air and an encounter with Hispaniola, and pass far enough south of the island to find a favorable environment in the Central Caribbean for development on Wednesday. Wind shear will be low, water temperatures will be hot, and the atmosphere will be plenty moist. Gaston could intensify into a hurricane in the Western Caribbean by the end of the week, as predicted by the latest run of the GFDL model.

[SevereWxEnthusiast]

Could it wind up hitting to much land to form???

Yes, that is a possibility - we have to keep watching it. Worst case is if it gets into the Caribbean waters south of Puerto Rico and Cuba, then shoots the gap (between Cuba and the Yucatan) and turns in to the Gulf and tracks over a warm current eddy. That would make for a very dangerous system.

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Too much dry air for now

is product is updated at approximately 2 AM, 8 AM, 2 PM, and 8 PM EDT from June 1 to November 30. Special outlooks may be issued as conditions warrant.
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GIS data: .shp ZCZC MIATWOAT ALL
TTAA00 KNHC DDHHMM
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
800 PM EDT MON SEP 6 2010

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

THE NATIONAL HURRICANE CENTER IS ISSUING ADVISORIES ON TROPICAL
STORM HERMINE...LOCATED ABOUT 80 MILES SOUTH-SOUTHEAST OF
BROWNSVILLE TEXAS.

1. MULTIPLE AIRCRAFT RECONNAISSANCE REPORTS INDICATE THAT THE SURFACE
CIRCULATION ASSOCIATED WITH THE REMNANTS OF GASTON REMAINS POORLY
DEFINED...AND THAT THE SURROUNDING ENVIRONMENT IS ONLY MARGINALLY
CONDUCIVE FOR DEVELOPMENT DURING THE NEXT COUPLE OF DAYS.
THEREFORE...THERE IS A MEDIUM CHANCE...50 PERCENT...OF THIS SYSTEM
BECOMING A TROPICAL CYCLONE AGAIN DURING THE NEXT 48 HOURS.
REGARDLESS OF DEVELOPMENT...THIS SYSTEM IS CAPABLE OF PRODUCING
HEAVY SHOWERS AND WIND GUSTS TO TROPICAL STORM FORCE IN SQUALLS AS
IT MOVES OVER THE LEEWARD ISLANDS TONIGHT AND TUESDAY.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

PUBLIC ADVISORIES ON HERMINE ARE ISSUED UNDER WMO HEADER WTNT35 KNHC
AND UNDER AWIPS HEADER MIATCPAT5. FORECAST/ADVISORIES ON HERMINE
ARE ISSUED UNDER WMO HEADER WTNT25 KNHC AND UNDER AWIPS HEADER
MIATCMAT5.

$$
FORECASTER ROBERTS/BRENNAN
NNNN

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