Monday, October 22, 2012

Why The Model Disparity?

The future tropical system that's yet to develop but is causing model hell for next week is still spinning around in the western Caribbean.  It might develop into a tropical storm in the next day or so but still lacks central organization around low pressure.  The tropical energy from this storm system may play a role in a model hyped coastal bomb that various computer modeling has shown for the past couple of days.

It's still model hype at this point but I do want to share the differences in modeling to help explain what is causing the setup.

As we head into next weekend, whatever that tropical entity becomes is modeled to be in the Atlantic -- whether it's near Florida or or east of the Bahamas a fair distance is to be determined.  At the same time, there are two other features to note.  First, a trough that's pushing colder air into the Midwest and will bring us our chill next week.  Two, a storm modeled to develop near Newfoundland.

A couple of differences already stand out to you by Saturday morning.  One, the Euro depiction of this tropical entity is stronger, located farther west, compared to the weaker and shallower GFS.  The location part is due to the ridge to the tropical entity...a little bit stronger at the surface on the Euro, acting as a bit of a block on northward progression but because the Euro is farther west, the storm is near the Gulf Stream and is a bit stronger thanks to its warmer water.  Second, the GFS' depiction of the low in the center of the Atlantic is farther southeast than the Euro's.  The Euro's take on this low is to push it farther west and southwest than the GFS as time progresses through next weekend....this is important since the farther west this other low hangs back in comparison, the harder it will be for the tropical low to escape the East Coast.

Those factors end up leading us to what happens down the line...early next week.  The Euro, closer to the coast, stronger, and also dealing with the Central Atlantic storm diving farther west and southwest, pushes the tropical entity on a wobbly track parallel to the coast before turning it inland near the Delmarva as a very strong coastal bomb.  You can see the interaction and intensification taking place between a deepening trough over the Midwest and this tropical entity.

Important to note in this are two other factors -- the trough diving into the central parts of the US on the Euro is deeper and stronger than on the GFS...and the corresponding ridge downstream from the trough is stronger on the Euro (the ridge is stronger over Newfoundland).  Having the storm intensifying as it comes north feeds into both factors playing out and thus the result is the coastal bomb.   On the GFS side, the weaker low that's farther east scoots away and misses the interaction with the weaker trough.

Whatever ultimately develops in the Caribbean and moves north from there into the Atlantic will NOT be tropical by the time it reaches our latitude, if it even follows the extreme scenario the Euro is modeling.

If the storm bombs out and comes up the coast, it would traverse over cooler water.  Based on this alone, the system wouldn't be able to strengthen as water temperatures are around 70 and tropical systems would need 80 degree water to strengthen.  It needs to rely on something else for it to intensify   In order to intensify, it will be phasing in with mid-latitude energy from the deepening trough over the Midwest, resulting in your classic nor'easter with tropical moisture infused.   If you remember Ida's remnants from 2009 as they spun offshore, it was a phasing of tropical energy with a developing low off of the Carolinas.  This scenario is a different in that the tropical entity would intensify but as it reaches our latitude it will likely evolve into a more nontropical-looking and acting system.

There is a pretty vast difference between the Euro and the GFS at this point regarding this storm...and given that the tropical part of this puzzle hasn't even developed into a tropical storm yet this vast swath of difference is not surprising in the least.  The modeling bomb on the Euro and also the Canadian is also not surprising given the other players on the field and how the tropical entity reacts to them.  That said, if the tropical entity that may ultimately develop remains weak and diffuse, the odds of a big bomb will be a lot lower without a stronger, well-defined system that comes north out of the Caribbean.  Before we sound any alarm bells of hype, let's get the tropics to do their part and develop something notable first!