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Tropical weather analysis - August 4, 2014

By: KoritheMan , 9:56 AM GMT on August 04, 2014


Bertha is intensifying, and is just under hurricane strength as of the 0900Z NHC advisory:

Wind: 70 mph, with higher gusts
Location: 26.1°N 73.6°W
Movement: N at 16 mph
Pressure: 999 mb

While the satellite presentation is not as impressive as some 60 kt tropical storms I've seen, a NOAA aircraft has recently found surface data complimenting an upgrade to 60 kt at synoptic time. Rapid strengthening does not appear imminent even though the shear is lessening, as the central convection has been pulsating a bit, and the central pressure is still a little high.

Figure 1. Latest infrared satellite image of Tropical Storm Bertha. Image credit: NOAA's Satellite Services Division (SSD).

As previously mentioned, environmental wind shear appears to be decreasing over Bertha, with evidence that the upper flow is transitioning more to a southwesterly dominance, consistent with the approach of an amplified mid- to upper-level trough over the eastern United States. Most of the guidance still makes Bertha a hurricane, and my forecast will call for the cyclone to achieve that designation in about 24 hours, although it could occur a little sooner, as the GFS/ECMWF develop a small 200 mb ridge over the storm during the next 12-24 hours as it begins to enter the westerlies. By about 48 hours, southwesterly to westerly shear is forecast to increase significantly over Bertha; combined with rapidly cooling waters north of the Gulf Stream and interaction with the aforementioned trough, the end result should be extratropical transition over the north Atlantic as Bertha accelerates northeastward. It is possible that Bertha could become extratropical a little sooner than I have indicated below. It's interesting to note that both the GFS and ECMWF wind shear fields show a cold low currently over the central Atlantic moving westward and catching up with Bertha during the next couple of days. The northeasterly flow ahead of the low could combine with the southwesterly flow ahead of the western Atlantic trough and increase upper divergence over the cyclone. For that reason, and also because the models do not show Bertha getting appreciably detached from the trough for awhile, the post-tropical decay process will likely be less than what would normally be expected. It is still possible Bertha could get stronger than shown below before beginning extratropical transition.

Bertha has made the anticipated turn to the north as the cyclone rounds the western extension of the subtropical ridge. While some of the guidance still shows some binary interaction with a weak disturbance over the Bahamas that could theoretically induce a more westward component somewhat closer to the mid-Atlantic coast, there is no sign of organization with that particular system; in fact, the convection has weakened considerably, with much of the activity now located over the Gulf of Mexico. Bertha is forecast to accelerate northeastward over the next 24 hours, with a gentle eastward turn seen at the end of the forecast period. My forecast track is close to the National Hurricane Center prediction, and the model consensus TVCN. It is also left of my previous forecast at days four and five.

Intensity forecast

Initial 08/04 0600Z 25.4°N 73.5°W 60 kt 70 mph
12 hour 08/04 1800Z 27.4°N 74.3°W 60 kt 70 mph
24 hour 08/05 0600Z 30.3°N 73.3°W 65 kt 75 mph
36 hour 08/05 1800Z 34.3°N 70.8°W 70 kt 80 mph
48 hour 08/06 0600Z 38.8°N 68.6°W 70 kt 80 mph
72 hour 08/07 0600Z 41.2°N 60.0°W 65 kt 75 mph
96 hour 08/08 0600Z 46.3°N 48.8°W 55 kt 65 mph: extratropical
120 hour 08/09 0600Z 49.5°N 37.6°W 45 kt 50 mph: extratropical

Track forecast

Figure 2. My forecast track for Bertha. Note that the forecast track (and thus the initial position) was made using the 6z data, but the blog itself took me longer to finish than that. I apologize for the discrepancies.


Major Hurricane Iselle continues moving westward across the eastern Pacific. As of the 0900Z NHC position, the following information was posted on the hurricane:

Wind: 125 mph, with higher gusts
Location: 16.0°N 135.6°W
Movement: W at 9 mph
Pressure: 958 mb
Category: 3 (Saffir-Simpson Hurricane Wind Scale)

The cloud pattern remains impressive since the release of the special advisory upgrading the winds to 110 kt at 6z. The eye is large and cloud free, and there is deep convection in the eyewall spiraling around the eye. Dvorak numbers are between 6.0 and 5.5 from CIMSS ADT and SAB, suggesting that the National Hurricane Center's currently assigned 110 kt intensity remains a good presumption. Upper-level outflow is well-defined in all quadrants, signifying little to no environmental wind shear.

Figure 3. Latest infrared satellite image of Hurricane Iselle. Image credit: NOAA's Satellite Services Division (SSD).

Iselle's intensity forecast is complex. The hurricane has been steadily assimilating annular characteristics for the last 24 hours, with little in the way of convective banding radiating outward from the center at this point. In addition, there is no evidence of any large-scale vertical shear affecting the hurricane, nor is there a lot of eastward-moving cirrus (westerly shear) downstream from its current position on water vapor imagery. As long as the shear remains as low as it is, the marginal thermodynamics or inner core fluctuations will be the primary contributors to the cyclone's intensity. Objective areal sea surface temperature analyses indicate that the SHIPS SST integration may be about a half a degree Celsius too cold, suggesting that Iselle may have more thermal energy to work with, at least in the short-term. A very slow and uncertain weakening is forecast from this point through the end of the forecast period. I say uncertain because annular hurricanes have a notorious tendency to remain stronger than advertised by the models or conventional meteorological knowledge; in fact, I am mostly advertising the initial weakening due to the likelihood of hard-to-time eyewall replacement cycles, and not because of any abrupt change in atmospheric parameters surrounding the hurricane. Later in the period, westerly shear is forecast to increase a little in association with a digging trough, so the cyclone is forecast to begin quickly weakening at those times as the shear helps inject dry air into the circulation as the system approaches Hawaii.

Iselle continues moving pretty much due west this morning. This general motion is likely to continue for at least the next 36 hours. After about that time, most of the guidance forecasts a west-northwestward motion to begin as a broad mid- to upper-level trough digs southward along 150W and erodes the subtropical ridge over Hawaii. While the models are in good agreement on this trough eventually lifting out, a second, much stronger impulse is forecast to dive southward from Alaska and reinforce the cyclonic flow in that area, which appears to be why the models continue moving the storm toward Hawaii instead of moving it westward to the south of the archipelago. My forecast track is similar to the latest ECMWF prediction, and takes Iselle over the Big Island Friday morning. Since the threat to Hawaii is still about four days away, additional northward or southward adjustments are entirely possible over the next few days. Residents there should continue to be very attentive to this situation.

Intensity forecast

Initial 08/04 0600Z 16.0°N 135.2°W 110 kt 125 mph
12 hour 08/04 1800Z 16.0°N 136.6°W 110 kt 125 mph
24 hour 08/05 0600Z 16.0°N 138.1°W 105 kt 120 mph
36 hour 08/05 1800Z 16.1°N 140.2°W 100 kt 115 mph
48 hour 08/06 0600Z 16.7°N 143.5°W 90 kt 105 mph
72 hour 08/07 0600Z 17.8°N 148.8°W 80 kt 90 mph
96 hour 08/08 0600Z 19.6°N 154.9°W 55 kt 65 mph: near Big Island
120 hour 08/09 0600Z 21.7°N 159.7°W 40 kt 45 mph

Track forecast

Figure 4. My forecast track for Iselle. Like with Bertha, my forecast track was made using the 6z coordinates, so there will naturally be some discrepancies with the synoptic data from 9Z


Tropical Storm Julio recently formed over the Pacific several hundred miles east of Iselle. As of the 0300Z NHC advisory, the following information was posted on Julio:

Wind: 40 mph, with higher gusts
Location: 13.4°N 118.4°W
Movement: W at 13 mph
Pressure: 1004 mb

The cloud pattern is still not terribly well-organized, but recent microwave and satellite data suggest that the low-level center could be a little more involved with the convection than it has been. A 0545Z ASCAT pass showed a slew of believable 30 kt vectors within the deep convection west of the center, which was the basis for an upgrade from the National Hurricane Center at 9z. There are currently little in the way of banding features, and upper-tropospheric outflow is limited to the western semicircle due to persistent easterly shear associated with a distant mid- to upper-level ridge.

Figure 5. Latest infrared satellite image of Tropical Storm Julio. Image credit: NOAA's Satellite Services Division (SSD).

The models show the shear slowly decreasing during the next 24-36 hours, so a more rapid pace of strengthening is shown then. I expect Julio to become a hurricane in about 48 hours, although if the shear eases up sooner it could occur in as little as 36 hours. Subsequently, a period of rapid intensification is certainly possible given the very warm sea surface temperatures and the deep upper ridge the GFS and ECMWF predict will build over the top of the storm. However, it is best to err on the side of conservatism until this becomes more certain, and my forecast does not explicitly indicate the traditional rate of rapid intensification for now. It is, however, a fair bit higher than the current NHC prediction which only takes Julio up to 80 kt. It is close to a blend of the GFDL, which brings Julio up to 85 kt, and the SHIPS, which brings Julio just shy of major hurricane intensity. The situation we have here is a little analogous to the one that allowed Iselle to become a major hurricane, and it is a distinct scenario that her predecessor could follow in her wake. Perhaps unnervingly, the GFS/ECMWF show a decently favorable upper wind configuration even as Julio begins to approach Hawaii beyond day five; this is not typical, as Hawaii is normally protected by shear from the mid-oceanic trough. If Julio ends up following the track of the models and passing near or over Hawaii, it could by far be stronger of the two storms, likely still at hurricane intensity at the time of closest approach. Some weakening still seems likely at days four and five as Julio approaches a cold water wake left behind by Iselle.

Julio is embedded within the deep easterlies south of a strong low- to mid-level ridge. The models are in good agreement that Julio should move generally westward over the next couple of days to the south of this ridge, with a few of the models even suggesting a possible south-of-west motion for more than just a brief duration. When the cyclone gets west of 125W, a turn more toward the west-northwest is likely to commence as Iselle and a strong mid- to upper-level trough along 150W weaken the ridge in the vicinity of the Hawaiian Islands. There is some discrepancy in regards to the timing of the west-northwestward turn, with the ECMWF indicating a more gradual turn and keeping Julio just south of the islands, while the GFS shows a quicker turn into the trough, and consequently a track farther north and much closer to the islands. While there's currently no hard evidence to favor one scenario over the other, if Iselle doesn't weaken too significantly, it may be able to keep a deep enough weakness in the ridge to induce a motion closer to what the GFS suggests, My forecast track attempts to blend both models but is biased a little closer to the GFS solution, at least for now. I'll have to look into making possible adjustments to the forecast track in either direction over the next few days.

Once again, I want to emphasize that Julio could be a rare hurricane for Hawaii if the current forecast tracks in the models pan out, and interests there should monitor Julio very carefully over the next 7 days.

Intensity forecast

Initial 08/04 0900Z 13.4°N 118.4°W 35 kt 40 mph
12 hour 08/04 1800Z 13.3°N 120.0°W 40 kt 45 mph
24 hour 08/05 0600Z 13.4°N 122.3°W 45 kt 50 mph
36 hour 08/05 1800Z 13.7°N 125.3°W 55 kt 65 mph
48 hour 08/06 0600Z 14.1°N 128.4°W 70 kt 80 mph
72 hour 08/07 0600Z 14.6°N 133.6°W 90 kt 105 mph
96 hour 08/08 0600Z 15.3°N 139.9°W 85 kt 100 mph
120 hour 08/09 0600Z 16.2°N 147.2°W 80 kt 90 mph

Track forecast

Figure 6. My forecast track for Julio.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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2. WunderAlertBot (Admin)
6:25 AM GMT on August 06, 2014
KoritheMan has created a new entry.
1. Astrometeor
11:33 PM GMT on August 04, 2014
Iselle went all the way to 140 mph, just a tad above the NHC's original peak of 80 mph.

This is now 3/3 on major hurricanes in the EPAC where they underestimated the peak strength.
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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