Hurricane Matthew: A Threat to Southern New England or Not?

Hurricane Matthew is a VERY dangerous storm wrecking havoc on Haiti and is poised to do the same for much of Cuba as well as the Bahamas.  Turning the attention to us here in southern New England, there has been some talk that Hurricane Matthew will make his way up the east coast and perhaps make a direct landfall somewhere in southern New England.  While there are some computer forecast model guidance which shows this type of scenario the prospects of this likelihood is more unlikely than not.  When forecasting 5-10+ days out one of the key things to recognize and understand is the projected 500mb pattern and how the pattern at this level is set to evolve.  While forecasting in this range can be quite difficult, the 500mb level can give a forecast great clues about what may happen or what some of the more likelihood scenarios are.  Why the 500mb level?  This level is called the "steering" level as the main jet stream at this level typically drives our weather and patterns.  As far as Matthew is concerned, one computer forecast model, the American-run Global Forecast System (GFS) has shown a track into southern New England (some runs have shown a direct hit and some other runs have shown an initial landfall down around the Carolina's before making a second landfall here), there is more to forecasting than just projected sea-level pressure charts.  Despite the track shown on the GFS, the projected pattern on the GFS actually suggests that this sort of track into southern New England is indeed not likely.  We will highlight why below.

We will use the 6z run of the GFS computer forecast model to take a look at the 500mb pattern and how it is projected to shape up over the next several days.  The following images show the projected 500mb heights and normalized height anomalies.  Height anomalies are EXCELLENT to look at because they give you a GREAT clue about well how anomalous a pattern may be.   Not only can you see your ridge and troughs but you can see how "strong" your troughs and rides are.  Below is the initial 500mb height anomalies for 2:00 AM EDT Tuesday morning.  We see a deep trough digging into the western third of the United States, some ridging into eastern Canada, a cut-off low over New England, and Hurricane Matthew right near Haiti/Cuba:

Looking at an animation of how the pattern is projected to evolve over the next 5-days there are several key features to focus on.

1) How the trough in the west evolves as it pushes east across the country
2) Riding that develops off the east coast in the western Atlantic
3) Hurricane Matthew himself

1) West coast trough - Notice how over time that trough goes from digging very deep to sort of lifting and becoming weaker as it pushes through the Plains and towards the Great Lakes region.  Initially, the height anomalies associated with the trough range from -3 standard deviations to -5 standard deviations below average which indicates a trough that is digging very deep.  However, as the days progress these anomalies weaken and this indicates a trough which is much weaker.  What is the significance of this?  Anytime you have a storm system working up the east coast you want a very deep digging trough to be working into the southeastern United States and becoming negatively tilted (we'll touch upon this aspect a bit later) in order for the system to remain close to the coast.  When this occurs the trough captures the system and keeps it locked on the east coast.  If you have a weaker trough which isn't digging or negatively tilted the system doesn't really have any means (although there are some other ways) to hang on the coast and it is more than likely to be kicked out to sea by either the jet stream or the trough.

2) Western Atlantic ridging - Notice how some ridging (indicated by the above-average height anomalies in the red-ish shadings) becomes established just off the east coast in the western Atlantic.  This feature is actually something that will help turn Hurricane Matthew towards the east coast and initially prevent the system from going out to sea.  This actually spells some very bad news for anywhere from FL up to the Virginia's.  This area needs to pay attention to Matthew extremely closely.

3) Hurricane Matthew himself - You can actually see how the establishing ridging helps to steer Matthew towards the southeast coast and then you can see how the weakening trough sort of kicks Matthew out to sea before striking southern New England.

Now what about this "negative" tilt of the trough?  Looking at 6z Saturday morning (2:00 AM EDT) at the 500mb height/vorticity map we see the orientation of this trough as pretty neutral or if anything very slightly negative.  Notice how the height contours (isohytes) dig pretty much nearly north to south..this is what is referred to as a neutral tilted of the trough.  If these dips were oriented more NW to SE this would indicate a more negative tilt of the trough.  Initially as the trough digs through the western United States and into the Plains the trough is negatively tilted, however, as it begins to amplify (or weaken) it becomes more neutral tilted.  When a trough reaches a negative tilt that indicates the system as at maturity and will eventually begin to weaken.  In this case the negative tilt happens very far west:

What about some of the other computer forecast models?  The latest European model is actually pretty similar with the 500mb pattern evolution though there are timing differences with how things go down.  We see a trough which really isn't digging deep into the south and we see a positive tilt to the trough:

  

So what are some of the possible scenarios we can expect up here in southern New England?  While we can't rule out a track into southern New England that scenario is extremely, extremely unlikely due to the reasons stated above.  The more likelihood scenario right now is that Hurricane Matthew takes aim on the southeastern United States and could make landfall anywhere from the east coast of FL up to SC.  From here is where things could diverge.  As that trough works into the east it is possible it captures moisture from Matthew and interacts it with the cold front associated trough and provides much of southern New England with beneficial rainfall.  This scenario would all come down to the timing of everything such as when the trough approaches New England and where Matthew is located...these details can be ironed out a bit later in the week.  It is also possible that after Matthew makes landfall or comes close to the United States coast could just get pushed out to sea all together.  Either way, here in southern New England we should see no significant impacts outside of perhaps some heavy rains (maybe some pockets of flooding depending on how much).

Also, as Matthew works northward, if it were to make a run towards southern New England the likelihood of Matthew weakening substantially would be pretty high.  The forward speed may not be as quick as with a typical tropical system thanks to the building ridge to the east and a slower system moving into slightly cooler waters would weaken it and any interactions with the front/trough would de-characterize its tropical characteristics.

Matthew will continue to garner bit news and attention (and rightfully so due to impacts oh Haiti, Cuba, the Bahamas, and perhaps threat to SE US coast, however, up here in southern New England we should be spared from any significant damage or impacts from a hurricane.