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Tropical weather analysis - August 19, 2014

By: KoritheMan , 7:06 AM GMT on August 19, 2014

Notice: All forecasts presented here are based upon my own knowledge of atmospheric dynamics. They are created using my knowledge of the various computer models, satellite interpretation, and other tools and parameters. These forecasts, while striving to be accurate, are not intended to supersede predictions by the National Hurricane Center. Always follow NWS protocol and forecasts.


Tropical Storm Karina has regained some strength over the last 24 hours. As of the 0300Z NHC advisory, the following information was available on the storm:

Wind: 65 mph, with higher gusts
Location: 15.9°N 133.6°W
Movement: WSW at 8 mph
Pressure: 996 mb

Satellite data show significantly warmer cloud tops, although they have recently become cold again in a loose band to the west of the low-level center. The 6z satellite estimates are certainly not impressive, and they suggest that Karina could be a little weaker at this time.

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

Karina's renewed struggles appear to be twofold: UW-CIMSS shows about 15 kt averaged 850-200 mb southeasterly shear over the cyclone due to a distant mid- to upper-level ridge. In addition, water vapor imagery and CIMSS TPW data show pockets of dry air to the west and east of the system, which appears to be becoming entrained into the southern portion of the circulation based on arc clouds observed on the south side of the center in satellite photography. This dry air has clearly not been handled well by the models, with most of their weakening due to more to shear than thermodynamics. Since the shear, while still evident, has decreased since yesterday, I will indicate a little bit of intensification while Karina is still over warm water, but I am below the NHC prediction and keep Karina just below hurricane strength. In a few days, the tropical storm will encounter cooler waters and a renewed surge of easterly to northeasterly shear due to outflow from the large circulation of Tropical Storm Lowell, located several hundred miles east of Karina. It is a little difficult to predict exactly how much weakening will occur as Karina interacts with Lowell and begins to decay, with the ECMWF generally suggesting less shear than the GFS at longer ranges. My forecast shows Karina winding down to a 30-kt depression at the end of the forecast period, in reasonable agreement with the GFS model, which shows Karina steadily weakening as it gets rather close to the circulation of Lowell.

Karina appeared to be moving a little north of the NHC forecast track earlier, but perhaps that was an illusion created when the system still had convection. Recent imagery indicates that the cyclone pretty much remains on track, and appears to be moving more westward now. Water vapor imagery shows a broad mid-level ridge stabilizing to the north of Karina, which should induce a westward motion for the next 24 hours, followed by deceleration and a rather sharp turn to the northeast as the circulation becomes more involved with the larger vortex associated with Lowell. All of the model guidance agrees with this scenario, which is surprising considering how aclimatological it is. There are still some differences in forward speed, with the ECMWF slower than the GFS. My forecast trends toward a blend of both.

Intensity forecast

Initial 08/19 0300Z 15.9°N 133.6°W 55 kt 65 mph
12 hour 08/19 1200Z 16.0°N 134.5°W 55 kt 65 mph
24 hour 08/20 0000Z 16.0°N 135.1°W 60 kt 70 mph
36 hour 08/20 1200Z 16.4°N 135.4°W 60 kt 70 mph
48 hour 08/21 0000Z 17.1°N 134.9°W 55 kt 65 mph
72 hour 08/22 0000Z 17.8°N 134.4°W 45 kt 50 mph
96 hour 08/23 0000Z 18.6°N 133.8°W 35 kt 40 mph
120 hour 08/24 0000Z 19.3°N 133.4°W 30 kt 35 mph

Track forecast

Figure 2. My forecast track for Karina.


Large Tropical Storm Lowell continues to spin the eastern Pacific. As of the latest NHC advisory, the following information was posted on the storm:

Wind: 40 mph, with higher gusts
Location: 17.0°N 118.6°W
Movement: WNW at 7 mph
Pressure: 1000 mb

Satellite data shows that Lowell remains a very large tropical cyclone with a large radius of maximum winds and little in the way an organized inner core.

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

Lowell is producing some rather deep convection -- colder than -80C -- in a large band south of the center. It is a little difficult identifying the center of circulation since the circulation is so large and elongated, but I assume it is well to the north of the -80C convection, although it doesn't appear to be fully exposed, either. Assuming that Lowell's circulation is a little closer to the deep convection, or even if it's not... if that convection can persist and wrap cyclonically into the center, some intensification could occur while Lowell remains over warm water and in an environment of moderate east-northeasterly wind shear. The shear appears to be more concentrated at the middle-troposphere rather than the upper-troposphere as evident by the cyclone's rather impressive outflow pattern on satellite imagery. It appears unlikely that Lowell will become a hurricane due to the large and sprawling nature of the cyclone vortex, although one can never quite rule that out with complete certainty. My forecast shows Lowell peaking at 55 kt in 48 hours, higher than the majority of guidance, and 5 kt higher than the 0300Z NHC intensity prediction scheme. Beyond that time, Lowell will encounter waters that cool only gradually first, but then do so quite rapidly later on. This should lead to weakening even though the shear is forecast to remain generally light for the next several days. Nevertheless, large circulations like Lowell frequently take a long time to spin down, and I suspect that we will still be tracking a remnant low over a week from now.

As I mentioned above, locating the broad center of Lowell is not easy, even with passive microwave imagery to assist me. It still appears to be moving northwestward roughly in line with the latest NHC forecast track, at least if we track the entire cloud system center and not jump on any random convective bursts within the gyre. Water vapor imagery shows a broad and distant mid-level ridge to the northeast of Lowell, while a cold low and trailing mid- to upper-level trough dig southward along California and into the eastern Pacific. The global models suggest that the unseasonably deep trough associated with the upper low will erode the western periphery of the subtropical ridge, allowing Lowell to continue moving northwestward to north-northwestward for the next 48 hours, which is consistent with current trends. Subsequently, the guidance shows Lowell's track bending a little to the left as mid-level ridging redevelops to the north of the storm as the trough lifts out. The TVCA model consensus has shifted left at 0z once again, and my forecast track closely mimics that guidance. I would not be surprised to see additional westward shifts by the model consensus, the NHC, and myself over the next couple of days.

Intensity forecast

Initial 08/19 0300Z 17.0°N 118.6°W 35 kt 40 mph
12 hour 08/19 1200Z 17.6°N 119.3°W 40 kt 45 mph
24 hour 08/20 0000Z 18.3°N 120.0°W 45 kt 50 mph
36 hour 08/21 1200Z 19.2°N 120.6°W 50 kt 60 mph
48 hour 08/22 0000Z 20.1°N 121.1°W 55 kt 65 mph
72 hour 08/23 0000Z 22.0°N 123.1°W 50 kt 60 mph
96 hour 08/24 0000Z 23.7°N 125.1°W 40 kt 45 mph
120 hour 08/25 0000Z 25.1°N 127.7°W 30 kt 35 mph: post-tropical/remnant low

Track forecast

Figure 4. My forecast track for Lowell.

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

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2. WunderAlertBot (Admin)
6:57 AM GMT on August 21, 2014
KoritheMan has created a new entry.
1. SolspotForecaster
12:36 AM GMT on August 20, 2014
Great tropical breakdown on TS Lowell! One think you might think about adding is the swell-output that these tropical systems will produce.

In the meantime if anyone is interested I have a full surf/swell rundown of what Lowell will be likely be producing for Southern California later this week. You can find the full post here...

http://solspot.com/content/solspot/adam-wright/tr opical-alert-ts-lowell-grinds-his-way-toward-south ern-california

Between the slow initial movement track (as well as his duration inside Socal's swell window) we should be looking at a pretty significant tropical S swell arriving later this week.
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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