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(T. F. “Storm” Walsh)
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Good evening everyone!
This post is the fifth Hurricane Tutorial, and will touch on the steering of hurricanes and track guidance models.
As we have seen for the most part, during the hurricane season, Tropical Waves, Tropical Depressions, Tropical Storms and Hurricanes, move in a general east to west direction. One of the main causes for this, is the well known subtropical ridge named the Azores / Bermuda High. The ridge takes its name from the fact that during the late fall through winter, this high is centered near, to almost due south of the Azores islands in the far eastern Atlantic. During the Northern Hemisphere summer and early fall, this high has the tendency to shift westward, and can become centered close to Bermuda. The predominate flow around this cell is clockwise, hence an east to west flow on the southern periphery. The barometric pressure associated with the high during the summer runs on average or “the norm” of 1024 mb (30.24 in.). Click the following graphic for the NOAA / NWS definition:
AZORES / BERMUDA HIGH
This is what “basically” guides our tropical cyclones. However, it’s a little more complex than this. A tropical system will be affected by not only the ridge, but also troughs moving off the east coast of the U.S. or across the Atlantic, as well as any “breaks” or weakness in the ridge. If there is a “break” in the ridge, a storm would have a tendency to head toward that break, and be drawn on a more northward path, or could slow or stall, until ridging builds back in. Think of it as in electricity, or a water pipe in plumbing. Electricity will take the path of least resistance. In the case of plumbing, water will take the path of least resistance, as in a break in the piping. The following graphic shows a break in the ridge. I have placed a red circle where the break in the ridge is. IF there were a storm moving west at approximately 25.0N;50.0W right now, it would have a tendency to head toward the break. If you click the graphic, it will take you to the West Atlantic Steering Layers page. I will explain in the next few paragraphs, how this page works. This is one of the tools I use to determine the motion of a Tropical Storm or Hurricane:
STEERING LAYER PAGE
Now, you’ll notice different millibar levels at the top of the page (i.e. 700-850 mb, 500-850 mb, etc.). This is because, as a storm becomes stronger, the higher up in the atmosphere it’s effects are felt. For example a CAT 3 storm would be steered by the 300-850 mb steering layer. This is one of the reasons we say a weak system as in a tropical storm or depression will generally move further west, vice being re-curved right away, as they do not extend that far into the atmosphere, and are less likely to feel the effects of a trough right away, if at all. The higher in the atmosphere a storm builds, the more it will feel the effects of a trough, or weakness in the ridge. Steering also depends on the location of the system, strength of the sub-tropical ridge, orientation of the ridge, and strength of any approaching troughs (affectionately referred to as “trof” by us meteorological types). Here is a good article from PSU Meteorology:
I’ve seen people mention over the past hurricane seasons about how strong the sub-tropical ridge was at a certain time, and it would drive a hurricane right into the U.S. This might be the case in the absence of any approaching trof heading toward the storm. However, in my experience, most of the time, there is usually the presence of a trof sitting along, or approaching the U.S. east coast. If this is the scenario, with a strong subtropical ridge, the chances are, the storm in question will most likely re-curve, as the trof to the west of the ridge will be strong as well, in reaction to the stronger high. Speaking of trofs, a “longwave”, deep trof will almost “always” allow for re-curvature, unless in a rare occurrence the hurricane is powerful enough to “bust” the trof, and go right through it. Shortwave trofs on the other hand have the tendency to cause a storm to slow, or even completely stall, depending on how strong it is, and the exact location (distance) from the storm. This is generally when we see steering currents weaken, and the storm stalls. Then, two things happen, either another ridge builds, and the system takes on a west to NW track again, or a stronger trof comes in and breaks the ridge, and the re-curvature process takes place (generally speaking). On another note regarding the subtropical ridge, there is a term used and it may sound funny and it’s called “pumping the ridge”. This phenomenon can occur with a very strong major hurricane (strong CAT 4 to CAT 5 hurricane). This occurs when the northern outer edge of the storm is strong enough, that it reinforces the southern or SW portion of the ridge. This allows the storm to travel on a slightly more WNW course briefly, vice a NW or northward component if the storm was moving in such a direction at the time. Put simply, the hurricane will have a slightly more westward component until it weakens, or something changes in the steering level. I don’t want to get long winded on this, so I will leave it at this point. You can always pose questions to me via email, or during hurricane season. The following graphic is linked to a page by the NWS “Jetstream” educational page and a tutorial on longwave and shortwave troughs:
LONGWAVE AND SHORTWAVE TROUGHS
I’m sure everyone by now, has come across the infamous “Spaghetti Model” posts that various news channels bring up, or folks in our various weather groups on social media. I especially like this graphic of the guidance models, to which folks will pick out one or two models and say “it’s gonna hit there” or another will pick out some other models and “predict” where the storm is heading. The thing is, about half to three quarters of what you see in the following graphic, is not necessary, and I kind of wish people would refrain from posting like “50” different model projections (being facetious).
SPAGHETTI MODEL PLOT
What a mess, right? I’m going to try to simplify it though. There are 3 different types of modeling used in forecasting track and intensity. In this case we are speaking of forecast track. There are DYNAMIC models, CONSENSUS models and STATISTICAL models. During my attendance of the 2010 National Hurricane Conference, I had the opportunity to “pick” the brains of some of the NHC forecasters regarding track models. It was conveyed that STATISTICAL models are not used for track guidance. We use the DYNAMIC models, along with ENSEMBLES. There are some preferences among various forecasters as to which model is the best. As far as dynamic global, the ECMWF over the past few years for me, has been one of the best as far as nailing track. I did learned a pointer back in 2010 from the NHC to watch the consensus models TVCN, TVCA, and TVCE. THESE have been fairly accurate over the past hurricane seasons in my opinion since 2010, and the NHC “OFFICIAL” track is usually derived from these, along with input sometimes from other models, usually the FSSE (Florida State Super Ensemble) model. In some instances, other global models like the GFS will provide an accurate track, and every once in a while, the CMC (Canadian Meteorological Center) model will have an accurate track forecast occasionally. However, when I’m making a track guidance forecast, I prefer to use the consensus models mentioned, and the ECMWF (European Center for Medium range Weather Forecasting) model, along with its ensemble model ECMWF EPS.
I have noticed that folks are anxious to post track guidance maps as soon as they are updated. The only problem is, most of the models are either statistical or less accurate forecast track guidance models. I recommend going with the DYNAMIC and / or the CONSENSUS models listed, and of course the NHC official track. This is a little hard to see, but I have arrows pointing to the area on the map so you can pick out the consensus models. You’ll note the TVCA, TVCE, and NHC OFCL tracks.
HILDA EASTERN PACIFIC TRACK GUIDANCE (CLICK GRAPHIC FOR LARGER PICTURE)
The following link is to an updated display of the modeling used for track and intensity guidance from the NHC.
NHC TRACK AND INTENSITY MODELS
You may direct any questions you have by contacting me personally, ANYTIME, at: email@example.com
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
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