If you follow social media or even some news outlets you may have encountered postings or have heard about computer forecast models striking the east coast with Irma or even perhaps a hit here in southern New England. In my opinion, posting these graphics so far out is very irresponsible. While of course a hit along the east coast is possible or even a hit here in southern New England is possible, we're talking about something which is 7+ days out. While we can forecast in this timeframe with rather decent confidence at times, when it comes to tropical system the forecast confidence decreases drastically and there is pretty much no confidence at all. In my mind, the goal for a forecaster, whether that forecaster be a degreed meteorologist or a hobbyist is to inform the public in a responsible manner, not scare the public. The later is something that is easily done nowadays as social media has blossomed and this is because so much information gets posted and when this is done irresponsibility the wrong idea gets out.
While we are still several-plus days away from any potential landfall along the east coast there are numerous factors which will come into play which will determine; 1) If Irma makes landfall along the east coast and 2) where Irma would make landfall. Within this post we will look at all of those factors.
Over the past few days there has been rather strong agreement between the computer forecast ensembles have been in rather strong agreement in bringing Hurricane Irma very close to the east coast of the United States. It is likely this reason alone which has led many to be discussing the possibility of a hit along the east coast. Below are ensembles from both the GFS and European computer forecast models showing the cluster of potential tracks. The GFS ensemble mean is from this morning's run of the GFS while the European ensemble mean is from last night's run (as of this writing this morning's run was not yet out):
While both computer forecast models show a great deal of members showing a landfall somewhere in the southeastern United States, the European model has several members which actually take Irma and curve it out to sea. While a landfall somewhere along the east coast is possible, a re-curve out to sea is just as possible. The million dollar question is; what determines whether Irma makes landfall or whether Irma re-curves out to sea? This is where we explore everything that will play a factor in what the ultimate outcome will be.
Believe it or not, one of the key pieces to all of this is thousands, and thousands of miles away well out in the western Pacific Ocean in the form of Typhoon Sanvu. Extensive research has shown that re-curving typhoons out in the western Pacific play a substantial role in the evolution of the synoptic weather pattern across the eastern central/eastern Pacific ocean and across North America. More times that not, tropical systems are quite large in nature. Within these systems a great deal of condensation occurs and the process of condensation releases what is called latent heat. When you have these large systems with a great deal of condensation, you're going to get a great amount of latent heat release. In fact, so much latent heat can be released that it actually shapes the weather pattern across the areas mentioned above. This can be illustrated by looking at this mornings run of the GFS computer forecast model. Looking at the jet stream at 250mb (almost 40,000' off the ground)) notice the extensive ridging (massive northward build in the jet stream) which develops across Alaska, western Canada, and the western United States over the next 5-days:
The significance of this ridging developing is it typically correlates with a trough (dip in the jet stream) either across the central and/or eastern United States. Looking back at the image, notice the troughing develop and amplify (strengthen) between the central and eastern United States. The significance of this feature is, when dealing with land-falling tropical systems across the east coast, and especially southern New England, this feature is nearly a prominent feature in many land-falls. However, there is also more to this. In order to increase the likelihood of a land-fall with this sort of scenario, you typically want the trough to amplify and become what is called negatively tilted as the tropical system is beginning to make the turn up the coast. When this all occurs, the negative-tilt of the trough 'captures' the system and tugs it right into the east coast. A great example of such a scenario would be with what occurred with Hurricane Sandy.
The next feature to focus on is across the northern Atlantic Ocean. Again, using this morning's run of the GFS computer forecast model we will focus on the large area of high pressure which becomes established over a large portion of the north Atlantic:
The importance of this feature is it could act as a block and actually prevent the tropical system from re-curving out to sea. In this scenario, the only possible track would likely be into the United States. Like with the troughing scenario, however, there is also more to this. If the extent of this high pressure cell shifts eastward, this now opens up an 'escape' route for the system to make a re-curve out to sea. How this high pressure center evolves over the next several days will be extremely critical in what Irma does as far as track is concerned.
The latest forecast track of Irma shows Irma potentially making striking Puerto Rico, the Dominican Republic, and Haiti regions:
Not only would this yield substantial destruction across portions of these areas but the topography, especially of Haiti would drastically alter the strength and organization of the system. When tropical systems go over land, they weaken significantly and rapidly as they need the ingestion of warm and moist air in order to keep going. Also, Haiti is home to some rather large mountains. If a tropical system goes over a mountain, if the mountain(s) are large enough, they can vastly disrupt the circulation of the tropical system which causes it to weaken substantially.
Some other factors to consider with regards to Irma are the degree of wind shear out ahead of Irma as well as available atmospheric moisture. Tropical systems tend to strengthen when they reside in areas with weak wind shear and high amounts of available moisture content. Assessing current wind shear across the Atlantic Ocean and just off the east coast of the United States we have low wind shear present around Irma and across the majority of the projected path of Irma. However, across the eastern United States and just off the east coast we have rather strong wind shear:
The forecasts are for this region of stronger shear to remain in place for at least the next several days. If this holds true, if Irma does take a turn towards the east coast or comes up the east coast, Irma could weaken quite a bit because of this.
Current water vapor imagery does show some drier air out ahead of Irma. This may not be all that significant, however, if Irma does ingest some of this drier air this could act as a weakening factor for Irma:
At this juncture its still extremely too early to say for certain what the likelihood is for a landfall across the eastern United States or if we have to worry about a landfall here in southern New England. At this time, however, my best guess would be either a landfall off the southeast coast (perhaps SC/NC/N FL) or either a sharp re-curve out to sea once Irma nears the east coast. However, the uncertainties are still rather high and any possibility remains on the table at this point. All the latest data will continue to be reviewed and over the coming days we'll have a much better idea as to what solutions are more likely and which aren't.
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