Friday, August 22, 2014

Upper 90°s This Weekend?!

By Jay Grymes & Steve Caparotta

WAFB First Alert Quickcast:
- getting even hotter over the weekend
- only a handful see cooling showers on Saturday & Sunday
- 96L still not much better organized
So today makes it four consecutive days at 95° for Metro Airport (BTR), and all indications are there is more of this heat to come.  In fact, ‘hotter’ is the forecast, with upper 90°s expected through the weekend and possibly on Monday as well.
That would mean seven consecutive days at 95° or above for BTR.  You bet that’s hot: we haven’t seen a run that long in two years (June 2012).  But it’s not rare either: this would be the 9th time in the past ten years with highs of 95° or more over a period of seven or more days.  In fact, it happened three times in the summer of 2011, including a 23-day stretch at 95° or above in August 2011.
The upper-level ridge (sometimes called a “heat ridge” in the summer) will remain in place through the weekend, adding to the daily heat-up by inhibiting the development of rain clouds and producing “sinking” air from above.
For some, you may be thinking, isn’t it cooler up above the surface?  Yes -- that’s why tall mountains can maintain snow cover at their tops even through the summer months.  So why does sinking air from above add to the daytime heating at the surface if it is cool enough to allow snow on mountain tops?  True, it is cooler where it starts, up at those elevations.  But as the air parcels from above start to sink, they are moving from a levels of lower pressure (the air pressure is lower is you go higher and higher up in the atmosphere) to levels of higher pressure.  For the atmosphere, the highest pressure is always at the Earth’s surface.
In other words, when air descends, it is moving into levels of increasing pressure: it is being compressed (squeezed) by the air around it.  As you may remember from your school days, as pressure increases, so does temperature.  In effect, the cool air from above warms considerably as it descends from the increasing pressure.  Meteorologists call this “adiabatic warming” and it is impressive: descending air will warm more than 5°F for every 1000 feet, or nearly 30°F for one mile of descent!  (Now that doesn’t mean that we are getting warmed by 30° from the sinking air!  But the descending air can easily add a few degrees to the already ‘hot’ surface temperatures.)
For the local forecast, we won’t say “no rain” for the weekend but we’re going with rain chances at 20% or less for both Saturday and Sunday -- in fact, it’s looking very spotty for Sunday.  What’s more, Sunday shapes up to be the hottest day of our current run with highs for just about everyone in the upper 90°s - - some neighborhoods may get awfully close to 100°!  Thankfully, by late Sunday into Monday, the overhead upper-level ridge begins to relax a tad, allowing for rain chances back to around 30% or so for Monday afternoon, although Monday's highs will still hit the mid to upper 90°s around the viewing area.  Into Tuesday and mid-week, the ridge retreats enough to get us back to something a little more typical  for August: scattered afternoon rains with highs in the lower 90°s.
As for morning lows, plan for more mid to upper 70°s likely for the next several days.
And now for Invest 96L … The National Hurricane Center fixes the main surface low-pressure zone north/northeast of Hispaniola and on a path towards the southern Bahamas.  96L has shown a bit more convection today, but nothing that clearly says that it is demonstrating a definitive center.  Indeed, as of this afternoon, there appear to be at least two or three places -- including one patch still in the Caribbean -- that could become the system’s core in the next 24-48 hours.  Remember, until we have a clear-cut center, forecasting the “where to” remains tricky.
For the time being, the consensus of model track forecasts keeps 96L (or possibly T.D. #4 or even T.S. Cristobal) in the western Atlantic, with the system being tugged northward by a developing trough near the U.S. East Coast in the next couple of days.  So, while we are not be ready to say that a visit to the Gulf is completely "off the table,” current signs are mainly good news for Louisiana and Gulf interests.

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