Thursday, September 8, 2011

Just When You Thought We Were Safe...

Just when you thought we were safe, the latest run of computer models has done quite a flip, with several pointing toward Nate making landfall along the northern Gulf Coast.

But before we go getting all worked up, let's consider some important points:

  • All but one of those tracks that you see pointing toward the northern Gulf Coast is either a variant of the same computer model or gets its starting conditions from that computer model -- the GFS (Global Forecast System). For instance, AVNO represents the operational or real world run of the GFS model. AEMN represents a consensus of possible tracks presented by that same GFS model.  AEMI is pretty similar to AEMN. And while the GFDL is somewhat separate, it gets its starting conditions (location of Nate, condition of the surrounding atmosphere) from the GFS.
  • This is a dramatic change for the GFS model, which for most of the last couple of days had Nate heading into Mexico. One of the things we look for in forecasting tropical systems is model consistency. A dramatic change such as this gives us serious reason to question its accuracy.
  • The real key to see if we have something to worry about will be to see if this model trend continues. The GFS will have another update out within the next couple of hours. Just as important, we'll look to see if the European (ECMWF) model follows its lead. If one or both points toward the northern Gulf Coast, then we'll become a bit more concerned.
  • The National Hurricane Center has not yet bought into this scenario. Check out the official forecast track from this morning.

The bottom line here is that I want you to realize that essentially all of those lines pointing in our general direction are really the result of a single computer model run overnight. It could easily have been a fluke. However, it's typically a fairly reliable model, so we'll wait to see if others follow suit.

If you caught my 5 p.m. or 9 p.m. weathercasts on Wednesday, I showed how 2 high pressure areas would have a big impact on Nate's track. One is located over the northern Caribbean, while the other is over northern Mexico. I showed how there was essentially an opening for Nate to move north in the next day or two, but that opening was expected to close over the weekend as the 2 highs essentially merged. If those highs for some reason don't 'bridge the gap', a northward motion would certainly be possible.

As for Nate right now, 'he' looks a tad better this morning when compared to yesterday evening. Satellite imagery shows t-storm activity trying to become a bit more concentrated around the center, but it's still mostly focused southwest of the center of circulation. This seems to be indicative of at least some modest northeasterly shear.

While the official forecast does call for Nate to become a hurricane, there is considerable uncertainty about the potential impact of some extremely dry air over the northern and western Gulf of Mexico. We've seen dry air time and time again take a toll on tropical systems this season. The water vapor image below shows that dry air in the orange shading.

So, folks, while this latest run of computer models this morning gives us reason to pause, don't go getting too worked up just yet. Forthcoming runs of the models will give us a better idea of whether there is something to be concerned about or whether the overnight run was just a fluke.

We've already gotten several questions about potential impacts from Nate on LSU and Southern home football games this weekend.  Even if Nate did decide to head this way, it looks as though the motion would be slow enough to prevent any serious problems on Saturday. We'll certainly keep you updated on that in the coming days.

Wednesday, September 7, 2011

Tropical Development Chances Increasing in Gulf

The area of disturbed weather that we have been discussing for the last couple of days in the southern Gulf of Mexico is showing signs of getting its act together this morning. The latest Tropical Weather Outlook from the National Hurricane Center is now giving the system a 60% chance of development over the next 48 hours.

A look at the latest infrared satellite imagery from this morning shows an increase in t-storm activity around the disturbance in the Bay of Campeche. 

You also get the notion of some spin (indicative of low pressure development) when you look at the visible satellite loop.

The Hurricane Hunters are scheduled to investigate the area in the southern Gulf this afternoon, so it should be interesting to see what, if anything, they find. Remember, on these missions in the early stages of development, the Hurricane Hunters are primarily looking for evidence of a closed low-level center of circulation -- something enough to classify a disturbance as a tropical depression or tropical storm.

The big question is where does this thing go from here? The computer models are essentially split into two camps: westward into Mexico or northward toward the northern Gulf Coast. The steering forecast is quite tricky in the coming days. Something to note is that the farther north something forms, the more likely it is to be drawn northward. A system developing farther south in the Bay of Campeche would be more likely to meander or move westward into Mexico. Here's a look at the latest computer model runs from Wednesday morning.

If you haven't already noticed, we've got the computer model runs for Katia and what is now Tropical Storm Maria posting to our website. I will likely replace the Katia models with those for the disturbance in the Gulf this afternoon.

So why do we have 2 different camps with the models? A lot of it has to do with the influences of an upper-level trough forecast to be over the center of the U.S. by the weekend. Some models believe this trough will be enough to draw the Gulf disturbance north, while others think the trough misses it and allows high pressure to build back over the Gulf by early next week and push this system westward. It's one of those things that is just too close to call at this point.

Be sure to check back with us on 9News at 5 and 9News at 6 for updates on the Hurricane Hunter mission this afternoon (assuming it doesn't get canceled). You can also get the latest updates from our WAFB Storm Team Facebook page or by following us on Twitter (@WAFBweather).

Tuesday, September 6, 2011

Another Tropical System in the Gulf?

Just as we say goodbye to Lee and begin to enjoy some beautiful weather in its wake, we take note that there could be another tropical system developing in the Gulf this week. If you caught any of our shows last night, we mentioned the area of disturbed weather in the southern Gulf along a stalled front. Chances seem to be increasing that something will form here later this week. The National Hurricane Center is now giving it a 20% chance of development in the next 48 hours.

NHC Tropical Weather Outlook from 7 a.m. on Tuesday, Sept. 6, 2011.

It seems quite possible that the NHC will deem this area an 'invest' at some point today, which would initialize a run of computer models that are specifically designed to handle tropical systems. The computer guidance that we already have access to shows quite a split, much like we saw in Lee's early development stages. There are scenarios showing the low going anywhere from Mexico to the Florida panhandle.

 Infrared satellite image from 7:45 this morning showing t-storm activity gathering in the southern Gulf along a stalled front.

Much of that spread seems to be a reflection of differences in timing for possible development of the system. Simply put, a quicker developing system would likely be pulled to the north or northeast late this week, while a slower developing system might be left to meander in the southern Gulf or head west towards Mexico. It is WAY too early to speculate on which of these scenarios might pan out. We just wanted to remind you that we're still in the heart of hurricane season even though we're enjoying a nice taste of fall this week.

Later today, we'll add a post from Chief Meteorologist Jay Grymes discussing just how active this season has been so far and why we shouldn't let our guard down in the coming weeks.

Thursday, September 1, 2011

Nasty Looking Labor Day Weekend

The tropical disturbance in the Gulf -- 'Invest 93L' -- still isn't terribly well organized this morning. However, take note of just how large the system is, with shower and t-storm activity covering just about the entire eastern half of the Gulf.

The National Hurricane Center has increased chances of development to 70% as of its latest update at 7 a.m. this morning. However, regardless of development, the bigger concerns for this weekend appear to be heavy rainfall and coastal flooding.  Most of our guidance still points toward low pressure being centered off the Louisiana coast by tomorrow and then has it meandering through the weekend, and in some cases, even longer. We often refer to these composites of computer models as 'spaghetti plots' and I think you can see where the 'spaghetti' term comes from in this case!

The Hurricane Hunters are scheduled to investigate the mess in the Gulf this afternoon to see if Tropical Depression #13 may be forming. While it does look quite likely that we'll get Tropical Depression #13 and/or Tropical Storm Lee out of this over the next day or two, the primary concern for the next few days will be heavy rainfall because of the slow movement.  The National Weather Service has already posted a Flash Flood Watch for areas generally south of I-10. I hesitate to show you this next graphic, but since it is a possibility, it's worth a look. The Hydrometeorological Prediction Center (HPC; a branch of the NWS) is painting a scary picture in terms of potential rainfall over the next 5 days.  Note the bullseye along the coast topping 18"! Now, this is far from a certainty, but the potential for BIG rain totals is a serious concern.

Strong westerly winds continue in the upper levels of the atmosphere across the northern Gulf of Mexico this morning, resulting in some shear impacting 93L. While the shear may let up a bit in the next day or so, it doesn't appear as though it will totally go away over the weekend. That's why at this point we're thinking the intensity of the system may not be a huge issue, at least for the next few days. However, if it does manage to linger in the Gulf long enough, conditions could become more favorable and allow it to intensity by late in the weekend or early next week.

Even if the winds aren't terribly strong, a persistent flow out of the east and southeast would likely mean coastal flooding concerns this weekend. That is something we'll certainly have to watch.

The possibility of a looping/meandering storm certainly brings back memories of a couple others. We have to be careful about making comparisons since each system is unique, but 2001's Allison and 1985's Juan do come to mind. Most of you likely remember the historic flooding from Allison; Juan actually made 3 separate landfalls along the Louisiana coast as it completed a couple of loops.

Let's hope that our current system does NOT resemble these in the coming days, but there is a real potential for this disturbance to be pestering us well into next week.

A final note...lost in all of this mess is that fact that today marks the 3rd anniversary of Gustav's landfall in Louisiana (Sept. 1, 2008; Labor Day). As we went days -- and in some cases weeks -- without power  3 years ago, I felt our struggles were lost on the national media. Since the storm didn't devastate New Orleans, there was little attention paid to the rest of us. Today, I still think Gustav is a forgotten hurricane.  How many times did you hear Ike referenced during the Irene coverage last week? But did you hear anyone mention Gustav?

Gustav's peak winds easily surpassed that of what we saw with Andrew in 1992 and essentially matched those of Betsy in 1965.  Here's a quick comparison for Baton Rouge:

Storm          Peak Sustained Wind          Peak Wind Gust
Betsy                     58 mph                           92 mph
Andrew                 46 mph                           70 mph
Gustav                   61 mph                           91 mph

Here are a couple of radar snapshots of Gustav.  The first is the hurricane making landfall near Cocodrie around 10 a.m. Note how the heaviest storms are located in the western eyewall at this point. It looked like metro Baton Rouge would miss out on the worst. However, those same storms rotated completely around the eye and did hit Baton Rouge shortly after 1 p.m., producing the 91 mph wind gust.

Radar image of Hurricane Gustav at landfall near Cocodrie on Sept. 1, 2008 around 10 a.m.

Radar image of Hurricane Gustav taken at 1:10 p.m. on Sept. 1, 2008.  The eyewall was impacting metro Baton Rouge at this point, producing a wind gust to 91 mph at BTR Metro Airport.