Alex lumbers towards Mexico

June 29, 2010

Back when Alex was a tropical depression, I remarked that its large extent reminded me of a pacific storm that would be slow to come together, therefore, I was doubtful that it would quickly  become a tropical storm.  I was wrong on that point, of course, as the tropical depression became a tropical storm fairly quickly.  However, the road from there to a hurricane has been slow. To some exten it is due to Alex’ large size.  One really has to look at increasingly zoomed out shots to really appreciate it.

This is a recent shot of Alex, looking the best the storm’s looked yet, with most of the dry slot(s) that have been prevalent today now filled in.  Shots from a couple of hours earlier show the dry slots between rain bands.

Earlier in its mission this morning, the WC-130 Hurricane Hunter found flight level winds very near those of a hurricane.  However stronger winds were not found on subsequent passes.  Double wind maximums, where a peak wind is observed near the center with an equal or only slightly weaker wind is observed some distance away, were noted.  That is something typically seen in mature storms when an eyewall replacement cycle is about to occur. Whether in a  mature storm, or a developing one, double wind maxima is a sign that the storm isn’t going to be intensifying much, if at all, in the near term.  And that was observed later in the morning and this afternoon.

After the plane had made its last pass, the offices responsible for providing intensity estimates based on satellite imagery both gave an estimate that would make Alex a hurricane.  If Alex were away from land and no planes were flying into it, Alex would be a considered a hurricane now on the basis of those estimates.  Having near contemporaneous observations from the plane, however, caused the NHC to keep the intensity at 60 knots/70 mph.  The 4 PM discussion states that they expect the inbound mission, which arrives at the storm during the next hour, to find Alex to be a hurricane.

As has been the case for most of Alex’ life, the intensity forecast brings Alex up to 80 knots/90 mph (though this advisory’s forecast doesn’t do so explicitly as landfall falls between two forecast points).  As before, this is supported by SHIPS guidance. Also, the dynamic GFDL and HWRF models in their recent runs have shown Alex intensifying a fair amount in the hours right before landfall.  The TCHP chart shows that Alex is entering a local maxima of heat potential.  This may be part of the reason for the improved appearance of the storm’s core.  Values do not drop off markedly until Alex would be at the coast making landfall.  Shear is analyzed to be negligible.  The large structure and perhaps dry air ingestion appear to be  the only limitations on Alex potential to intensify before going ashore.

As for the track forecast, Alex has run into the edge of an expanding ridge of high pressure. That has caused an increasing westwards component of motion.  Most of the 12Z model guidance was unchanged in its predictions of landfall location, however, the GFS did shift to the south a bit.  That, combined with the apparent turn that was ocurring caused me to post that the NHC forecast would have to shift to the south and it did of course. The discussion notes that there is still room for adjustment to the south, if necessary.  Looking at the satellite loop, it’s easy to be tricked by short term motion, , especially when you don’t have recon fixes to support your perception of what the center is.  That said, it does appear Alex has continued to be more westward than forecast.  Because of that, the center of the forecast line needs to be shifted to the south accordingly, assuming that my perception is correct.

Due to how far south Alex appears to be headed to make landfall, wind damage from the core of the storm would not be a concern for Texans.  However, because of the far extent of rain bands, some severe winds and heavy rains are likely to be observed across the coastal areas of the state. National Weather Service Offices in Brownsville, Corpus Christi, and Houston are providing forecasts of the localized effects that Alex will have on those areas.  Residents are urged to use those as guidance.

As has been the case, I’ll post quick updates to my Friendfeed as warranted. I’ll probably have one more post here later this evening.


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