Tropical weather analysis - July 2, 2013

By: KoritheMan , 4:32 AM GMT on July 02, 2013

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Dalila

Tropical Storm Dalila is just under hurricane strength. As of the most recent NHC advisory, the following information was made on the storm:

Wind: 70 mph, with higher gusts
Location: 18.2°N 106.9°W
Moving: WNW at 6 mph
Pressure: 992 mb

The satellite presentation consists of a small but well-defined central dense overcast with cold convection. Most of the convective banding that was seen earlier has loosened and morphed into the CDO, although a formative curved band is beginning to redevelop in the western semicircle of the cyclone circulation. Recent microwave data suggests a narrow warm spot surrounded by a fragmented eyewall.



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

Water vapor imagery still suggests some southeasterly shear over Dalila, which is contributing to the somewhat asymmetric appearance of the convection in that quadrant. The global models suggest that the shear will soon change to southerly, but will remain in at least the 15 to 20 kt range throughout much of the forecast period; this is expected to mitigate any significant strengthening despite of warm sea surface temperatures for the next three or so days. In addition, water vapor imagery and analysis of the SHIPS model file suggests that Dalila will soon encounter a much drier airmass, which should also help to exacerbate the weakening process. Late in the period, Dalila is forecast to encounter the 26C isotherm, although if the forecast track continues to shift to the south, the cyclone will remain over warmer water and would likely weaken more slowly. In the meantime, there is still a little more time for intensification, and my forecast shows Dalila peaking at 70 kt over the next day or so, and remains at the upper end of the guidance envelope. An alternate scenario is that Dalila does not strengthen anymore, as a microwave pass just after 3z shows that the inner core and developing eye has collapsed. It is possible the shear is getting to it faster than I thought.

Dalila remains south of a strong mid-level ridge over the western United States. With the help of a couple passive microwave images, it appears that the tropical cyclone has been gradually turning to the left as it fully comes underneath the influence of the ridge. A definite westward motion should soon ignite, but there remains a considerable dichotomy between the models, with the GFS continuing to take Dalila slowly westward throughout the forecast period, but even that model shows something of a southwestward bend beyond Wednesday afternoon. The ECMWF continues to abruptly turn Dalila southwest in about 24 hours, while the GFDL erratically takes the storm over southern Baja. What this suggests is that there will likely be little overall movement of the tropical cyclone, particularly in the near-term. Later in the period, some acceleration is shown relative to earlier forecast times, but is held slower than the National Hurricane Center forecast just in case some erratic motion occurs later in the period. I am unsure why the discrepancy, but I presume it is related to the depth of the cyclone vortex, with the ECMWF showing a weaker cyclone. For now, the stronger solution is preferred, but if the southward movement turns out to be reality, additional southward adjustments will become necessary in later forecast packages.

Because Dalila is turning away from Mexico, all coastal watches and warnings have been discontinued. However, locally heavy rain and high surf will continue to impact a portion of the southwest coast for the next day or so.


5-day intensity forecast

INITIAL 07/02 0300Z 60 KT 70 MPH
12 hour 07/02 1200Z 65 KT 75 MPH
24 hour 07/03 0000Z 70 KT 80 MPH
36 hour 07/03 1200Z 65 KT 75 MPH
48 hour 07/04 0000Z 60 KT 70 MPH
72 hour 07/05 0000Z 50 KT 60 MPH
96 hour 07/06 0000Z 40 KT 45 MPH
120 hour 07/07 0000Z 30 KT 35 MPH

5-day track forecast



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



Invest 97E

An area of disturbed weather located a couple hundred miles south of the extreme southeast coast of Mexico is association with a tropical wave. The system is currently experiencing easterly shear, but these winds are forecast to slowly relax over the next 48 hours, which should allow for steady development beyond that time. Most of the makes this system into a tropical cyclone in a few days as it moves west to west-northwest at 10 to 15 mph.

Probability of development in 48 hours: 10%



Elsewhere

The GFS has been hinting at the possibility of a trough split occurring over the western Atlantic late this week into early next week, in association with a large baroclinic zone east of Bermuda; the southern portion of this feature breaks off due to ridging and begins to move westward toward the Bahamas. However, this solution has been very inconsistent. The CMC made this system into a hurricane that made landfall in southeast Louisiana during the 0z run on Sunday, but this solution seems highly improbable, and the model has since dropped it. Interestingly, that model also wants to bring a tropical cyclone into the Florida panhandle in about 3 or 4 days; this system comes from the northern portion of the tropical wave over Hispaniola as it moves west-northwestward under the ridge into the Gulf. None of the other models show this, and this is probably just the CMC spewing crap again.

All things considered, the Atlantic remains quiet for now, with the upward MJO pulse likely in the process of leaving our area of the world. It is still my expectation that activity will pick up in the next couple weeks as the Bermuda high weakens and allows for more convergence in the MDR, but this is speculative for now.

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2. WunderAlertBot (Admin)
7:03 AM GMT on July 03, 2013
KoritheMan has created a new entry.
1. Astrometeor
4:41 AM GMT on July 02, 2013
Thank you very much sir!
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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.