I made this comment on facebook/twitter yesterday morning, which lead to many questions:
"91L is something that needs to be watched closely. A legitimate threat for Puerto Rico, Hispaniola, and the Bahamas. From there the SE US/Florida. The Japan/Asian trough/ridge teleconnection holds several clues into the future of what should be Emily."
So instead of me trying to answer everyone on facebook/twitter, I figured I would just write a blog post about the Asian trough/ridge teleconnection and how it relates to 91L's future. I'm going to try to keep this as short and informative as possible.
First, let's look at 3 model output charts from the JMA (12z July 29th run), valid 24, 72, and 144 hours out. You can clearly see Typhoon Muifa, which is bombing out right now! Well on the way to becoming a super typhoon. Here is the forecasted path:
So what do you see? Remember, this is like putting a puzzle together. First you can see that Typhoon Muifa is moving (tropical cyclones propagate, but anyways) in a northerly direction for the most part. This is caused by interaction with a trough, which you can clearly see on the JMA charts. It's not like this trough is very strong/deep, hence the slow movement to the north. If the trough was strong and dug in deep, it would just pick up Typhoon Muifa, pulling it out to sea.
This is not the case though, as Typhoon Muifa will interact with a weak trough for the next several days, before it lifts out and a ridge builds in over the top. Again, make sure you refer to the JMA charts for guidance. This will eventually "push" Typhoon Muifa towards China. Keep all of this in mind as I'll eventually show you how it relates to 91L and the US.
When using the Asian trough/ridge teleconnection you have to keep a few things in mind. First, Tokoyo, Japan correlates to Cape Hatteras, NC (Pretty close to the same latitude). SE China correlates to the Gulf of Mexico. This trough/ridge pattern takes 6-10 days to materialize. Basically, if a trough enters Japan, it should take 6-10 days to get to east coast of the US. The time of year will give you a better handle on whether it's a 6 day telconnection, or a 10 day teleconnection. This is based on wavelengths and is where experience is HUGE! The atmosphere doesn't have a calendar, so each year is unique. As we get later into the summer months, the wavelengths shorten in general. Right now, I would personally say it's around a 7 day correlation, based off experience. I really don't have any guidelines here, as it is really based off feel and experience.
I know some people may laugh at this type of forecasting, but trust me it works! Think of it this way... Say I give you a rope, and told you to stand across a room. If I yanked on the that rope, you will feel the yank (wave) in a certain time period. The harder I yank the rope, the faster/stronger you will feel it. So, the stronger the trough/ridge (this is especially influenced by typhoons like Muifa, as it will amplify troughs/ridges), the faster/stronger the US should "feel" it. Again, this is where experience of watching trough/ridge patterns over and over again comes into play. It's honestly just pure experience. This teleconnection is NOT going to pick up on smaller feature like a subtle shortwave or a cut off low. It works best when major amplification is involved! Also, I can't tell you specific details on what will happen 6-10 days later, but I can at least tell you if parts of the US "should" be under the influence of a trough/ridge. I try to use this all year round with long range forecasting, but I especially like using this teleconnection with the tropics.
With that all in mind, how does this relate to 91L? Ok, I feel confident that 91L will become Emily soon enough, gaining latitude towards Puerto Rico. I think it will be a solid hurricane at this point as well (maybe a major hurricane?). There is a danger this could hit Hispanolia and get torn to pieces. That's very possible! However, let's assume it just misses Hispanolia to the north, and heads towards the Bahamas. This will be a slow process, as Hurricane Emily should be moving slowly at this point. Now comes the million dollar question, does it hit the US, or does it head out to sea?
I'm not going to lie, as this is very difficult to predict. Typhoon Muifa will amplify this trough, so there is a solid chance it could just pick up Emily and take her out to sea, even though it would still be a close call for the US. The other option, is for the trough to lift out, and a ridge to build in over the top, just like with Typhoon Muifa. This is going to be close, as it is all about timing and "guessing" the correct movement of the trough/ridge pattern through amplification. Actually, guessing is probably the wrong word, as I have many clues on my side, but nailing this down to a 24 hour timeframe is very difficult.
My instincts says the trough will just barely get Hurricane Emily, "pulling" her out to sea. It's going to be so darn close though! Honestly, it is very difficult to predict! The trough could easily miss, as a ridge builds in over the top, "pushing" Emily towards the SE US. I'll need another day or two to really have a good feel for it. I'll post my official track forecast Monday. At least though, you now have an understanding of the Asian trough/ridge teleconnection and how it works. Hopefully, this will help enough so you understand what I'm referring to in future posts about the tropics. There are several more details involved, but I think this is a good start. I hope everyone has a great weekend!
*Sunday update 3:22am: Couldn't sleep, so I decided to look at some data. Things are beginning to hint that the trough may not completely grab what will be Emily. If so, this could be a threat for the Carolinas. I still want to watch it for another day or so before I post my track forecast. Like I said above, it's going to be really close! My instincts still lean towards Emily barely going out. I will say, Joe Bastardi made an excellent point about trough splitting which I should have thought about! Trough splits are something we tend to see a decent amount in the overall pattern we are currently in, so that could be a wild card. Let's see how 91L develops Sunday, and I'll be back on here Monday...