Tropical weather analysis - August 17, 2012

By: KoritheMan , 3:41 AM GMT on August 18, 2012

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Gordon

Tropical Storm Gordon continues racing eastward toward the Azores. As of the latest NHC advisory, the following was posted on the storm:

Wind: 70 mph, with higher gusts
Location: 34.2°N 42.1°W
Movement: E at 18 mph
Pressure: 990 mb

Gordon is not quite as ragged on satellite imagery as it was 12 hours ago. There is more deep convection encompassing the center, and recent microwave data suggested a better-defined inner core structure, though the mid-level circulation remains displaced slightly to the northeast, probably due to continued westerly shear.



Figure 1. Latest infrared satellite image of Tropical Storm Gordon. Image credit: NOAA

Gordon still has about another day over waters of 26C. Since the strongest of the shear appears to be avoiding Gordon for the moment, some intensification is possible in the short-term. Gordon is forecast to become a hurricane before reaching cooler waters and encountering a sharp increase in westerly shear. Looking at the cyclone phase diagrams on FSU, the global models are in general agreement that Gordon will lose tropical characteristics shortly after passing through the Azores during the wee hours of Monday morning. No reintensification is expected once the transition is complete, as Gordon will be well to the south of a strong baroclinic low over the north Atlantic. Instead, a gradual decay is forecast while Gordon moves southeastward under the nose of the subtropical ridge.

There isn't much to say track wise. Gordon is well-embedded within the westerlies, and is moving toward the east. A turn to the east-northeast is forecast over the next 24 hours, followed by a gradual bend to the east-southeast with a decrease in forward speed subsequent to crossing the Azores. Model guidance remains in good agreement with this scenario. It is possible that Gordon could eventually bring some locally heavy showers and gusty winds to portions of Portugal and Spain late next week.

Although Gordon is forecast to be weakening by the time it approaches the Azores, some rather strong winds are possible in the archipelago, especially in areas of rugged terrain. Because of the forecast track, the government of Portugal has issued a tropical storm watch for portions of the island chain. Tropical storm warnings will likely be required for a portion of these islands on Saturday, since the average lead time for such warnings is 36 hours.

5-day intensity forecast

INITIAL 08/18 0300Z 55 KT 65 MPH
12 hour 08/18 1200Z 60 KT 70 MPH
24 hour 08/19 0000Z 65 KT 75 MPH
36 hour 08/19 1200Z 60 KT 70 MPH
48 hour 08/20 0000Z 55 KT 65 MPH
72 hour 08/21 0000Z 45 KT 50 MPH...EXTRATROPICAL
96 hour 08/22 0000Z 35 KT 40 MPH...EXTRATROPICAL
120 hour 08/23 0000Z 25 KT 30 MPH...EXTRATROPICAL

5-day track forecast



Figure 2. My 5-day forecast track for Gordon.

Watches and warnings


A TROPICAL STORM WATCH IS IN EFFECT FOR...
* THE CENTRAL AND EASTERN AZORES

A TROPICAL STORM WATCH MEANS THAT TROPICAL STORM CONDITIONS ARE
POSSIBLE WITHIN THE WATCH AREA...GENERALLY WITHIN 48 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Helene

Tropical Storm Helene formed over the southwestern Gulf of Mexico today from the remnants of Tropical Depression Seven. As of the latest NHC advisory, the following was provided:

Wind: 40 mph, with higher gusts
Location: 21.0°N 96.4°W
Movement: N at 2 mph
Pressure: 1006 mb

Helene is looking rather sickly this evening, to say the least. There is virtually no deep convection anywhere within the center. Instead, the only convection is confined to shallow bands over southern Mexico. Satellite images suggest that these bands are diminishing, which could be an indication that Helene will soon begin to reestablish some convection over the center.



Figure 3. Latest infrared satellite image of Tropical Storm Helene. Image credit: NOAA

The center of Helene is difficult to locate, but reconnaissance observations and surface data suggest that it remains offshore. Earlier microwave data acquired just after 0000 UTC also indicated this. Sometimes satellite and radar data can offer a different perspective, but alas, nothing beats state of the art real-time observations.

Atmospheric conditions are seemingly favorable for strengthening, with the only obvious impediment being the limited amount of time over water that Helene has. After moving inland, Helene should quickly dissipate over the rugged terrain of Mexico, but since it is not forecast to penetrate too far inland, I have held the winds a little higher through the next two days than the National Hurricane Center, as the close proximity to the Gulf of Mexico could allow for convection to linger over the open water. That convection could help to maintain some strong winds even after the system moves inland. Regardless, Helene is forecast to dissipate in three days or less. Although Helene possesses a rather vigorous circulation as denoted by surface data, the current lack of convection and limited time over water argues against it becoming a hurricane. However, the funneling effect of Mexico's mountains, especially in this location, could help to locally enhance winds in any significant convective bursts. Thus, it would not be surprising to see Helene become a little stronger than forecast before it moves inland.

Water vapor and 0z upper air data suggest that Helene is south of a low- to mid-level ridge over the northern Gulf of Mexico, while a trough is amplifying over the east coast. The global models gradually weaken this feature as a well-defined trough begins to manifest over the central and southern United States. Although Helene is forecast to be inland by the time the trough begins to exist, the sheer scope of the trough as depicted in the models suggests that the cyclone will not get very far inland before it begins to move more poleward. Indeed, the global models actually take Helene back over the Gulf of Mexico in about three to four days. The CMC suggests an eventual threat to the northern Gulf Coast, while the GFS suggests a slow northward track into Corpus Christi. The ECMWF model quickly pulls Helene back into Mexico under a building ridge, which doesn't seem entirely realistic given the trough. While I doubt Helene moves as far northward as the CMC suggests, given the models and the strength of the trough, it seems a bit unlikely that Helene will continue moving toward the west after landfall. It appears that Helene will slow significantly in the event it moves back over water.

5-day intensity forecast

INITIAL 08/18 0300Z 35 KT 40 MPH
12 hour 08/18 1200Z 40 KT 45 MPH
24 hour 08/19 0000Z 45 KT 50 MPH...ON THE COAST
36 hour 08/19 1200Z 30 KT 35 MPH...INLAND
48 hour 08/20 0000Z 30 KT 35 MPH...INLAND
72 hour 08/21 0000Z...DISSIPATED

5-day track forecast



Figure 4. My 5-day forecast track for Helene.

Watches and warnings

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* THE COAST OF MEXICO FROM BARRA DE NAUTLA TO LA CRUZ

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN THE
NEXT 24 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA ...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.



Invest 94L

A tropical wave in the far eastern Atlantic about 300 miles southwest of the southern Cape Verde Islands ("94L") is producing limited shower activity, mainly to the west of the wave axis due to easterly shear. This is not uncommon for westward-propagating tropical waves in this part of the Atlantic.



Figure 5. Latest infrared satellite image of Invest 94L. Image credit: RAMMB

Conditions should become more favorable over the next 24 hours, and the GFS suggests that an anticyclone will accompany 94L for the next five days. Indeed, the 18z run of this model turns it into a powerful hurricane in the eastern Caribbean before moving it northwest. Although the thematic pattern this year has so far closely mirrored the last three years, with a mean trough over the eastern United States capable of inducing recurvature, I am not completely convinced that 94L will recurve. The GFS has in general trended farther west over the last few cycles, and the Euro has been rather adamant about a threat to the Lesser Antilles. While both models still recurve the system, there are indications within the GFS ensembles as well as the synoptic pattern portrayed in those models that recurvature may not initiate until at least 70W, possibly as far west as 80W. This is close enough that the east coast of the United States could be a viable target in the extreme long-range. However, such long-range predictions are prone to some rather large errors, and one should not make their plans based off them.

Regardless, it appears that this system could eventually become a hurricane, and quite possibly a major hurricane, as suggested by the GFS and the purely statistical guidance. It should be noted that the 850 mb vorticity fields in the global models indicate that a weaker system would tend to move more west. So if 94L remains weak throughout the next several days, we could be looking at another long-range threat to the Caribbean. As usual, there is some dry air in the central Atlantic, which could be an inhibitor for strengthening in a few days.

Probability of development in 48 hours: 20%

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2. VirginIslandsVisitor
7:51 AM GMT on August 18, 2012
Thanks for your thoughts, Kori.


Member Since: July 30, 2011 Posts: 2 Comments: 624
1. TropicalAnalystwx13
3:51 AM GMT on August 18, 2012
Cool.
Member Since: July 6, 2010 Posts: 113 Comments: 32046

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.