Hurricane Joaquin discussion

The prolonged stretch of dry and tranquil weather here across southern New England is about to come to an abrupt end as we enter the first week of October.  As we move towards the later half of the week and towards the weekend, our attention is strictly focused on what is now Hurricane Joaquin which is located just northeast of the Bahamas.  Several pieces of computer forecast guidance, including ensemble members all indicate that Hurricane Joaquin will make landfall somewhere along the east coast, with perhaps anywhere from VA to NJ receiving the highest likelihood of a direct hit.

The latest (2:00 PM EDT on September 30th, 2015) advisory from the National Hurricane Center (NHC) has Hurricane Joaquin as a category 1 hurricane with maximum sustained winds of 85 mph near the center and with hurricane force winds extending outward up to 35 miles from the center with tropical storm force winds extending outward up to 125 miles from the center.  The estimated central pressure is 968mb.  The NHC is also forecasting further strengthening of Joaquin over the next couple of days and aren't ruling out the possibility of Joaquin becoming a major hurricane (sustained winds of at least 115 mph).  Below is the latest forecasted track:



Notice the large shape of the cone in the projected track.  This cone is referred to as the "cone of uncertainty".  The larger the spread within the cone, the lower the confidence in the exact track of the storm.  The reasoning behind such a large spread is due to the fact that the atmosphere will be undergoing some incredibly complex changes over the next several days and how the pattern exactly evolves is going to vastly influence what happens with Joaquin's track.  We will explore this further below and get into the reasoning as to why the pattern will become so complex and why our computer forecast models are/will continue having a very difficult time.

One reason for the large spread in track is purely due to the large spread with computer forecast model ensembles.  Below is a graph composing of various computer forecast models and their mean projected track.  This chart is called a spaghetti chart because...well all the different tracks and such resemble a piece of cooked spaghetti.
 

Notice you have a cluster of models bringing Hurricane Joaquin into VA/NC , then another small cluster up around northern VA/MD and then a few tracks into portions of southern New England and even a couple straight out to sea.  From this we can gather a couple pieces of information:

1)  There is much spread within the models which means there is a very low confidence at this time as to exactly where the storm will track and where the storm will make landfall.  When these plots and lines are much closer together, this is when confidence is typically much higher.

2)  It does appear there is a stronger cluster of the data out across the mid-Atlantic coast (NC/VA/MA) area.  What we can decipher from this is that perhaps the likelihood of a landfall anywhere in this area may be more likely.  However, given the spread as mentioned above, we can't say this with the highest of confidence.

When it comes to the forecasting aspect, as mentioned a bit earlier, the atmospheric pattern configuration is going to be quite complex with many variables all tying into how the system evolves and where it tracks.  Depending on how each feature (which we will discuss below) evolves, that will impact what happens with the system.  We will take a look at some of the important features below which are labeled on the map:
 

 The above graphic is from one of our computer forecast models, the GFS and this is from the 12z run (8:00 AM EDT) from this morning projected out to 48-hours.  So we're looking at 8:00 AM EDT Friday morning.  This particular graphic was just chosen to provide a visual of the key features at play.

1) Trough - This trough will be a major focal point over these next several days.  Computer forecast models really amplify the trough as it moves into the southeast and digs it very far south.  The trough is also expected to become negatively tilted.  What computer forecast models do is as Hurricane Joaquin begins to turn more to the north and west, the trough will eventually capture Joaquin and bring the system right towards the East coast.  So the question here becomes, if/where this capture takes place.  If it takes places further south and earlier, the chances vastly increase for a landfall perhaps across VA/MA area.  If it's later and slower with the capture, then the chances for a landfall further north increase.

#2, #3, and #4 - I grouped all these together because they are will seemingly work in conjunction with one another...well at least with respect to what happens with Joaquin.  The strong high pressure to our north, and ridging off to our east will ultimately do its very best to keep Joaquin closer to the coast.  That cut-off low (#4) will also work to prevent the ridging (#3) from sliding east as well which would allow the opportunity for the Hurricane to slide east.

So with all this...what can we perhaps expect here in southern New England?  At this juncture it appears that the highest likelihood for the most significant impacts will be somewhere along the mid-Atlantic coast where substantial impacts may be possible.  Up here across southern New England we may get spared, however, we could still deal with the potential for a quite a bit of rain, some wind and maybe some flooding.

Confidence should continue to build over the next few days as hopefully our computer forecast models develop a better consensus.  

Cold Front to Bring Relief From Heat/Humidity and Much Needed Rain

The high heat and humidity is set to come to an end as a cold front slowly progresses towards New England late Wednesday and on Thursday.  As the cold front traverses New England, the front is expected to stall across the region.  As this occurs, weak waves of low pressure are expected to develop and rid up along the cold front.  The presence of the stalled front overhead along with areas of weak low pressure riding up along the front will set the stage for numerous periods of showers and thunderstorms and some of the rains will be quite heavy.  Once the cold front passes the region, we will see much more seasonable temperatures and humidity levels filter in.

Moving through the remainder of Wednesday, as of 4:00 PM EDT, the cold front was slowly pushing through western and central New York state where a broken line of showers and thunderstorms have developed, including a few strong to severe thunderstorms.  As this front continues to slowly slide south and east, the risk for showers and perhaps some thunderstorms will increase as we work through the evening and into the overnight hours.  With very weak shear aloft, poor mid-level lapse rates, and instability expected to wane with loss of dayitme heating, the risk for strong to severe thunderstorms will be very low.  The main threat would be for torrential downpours.  However, gusty winds can't be ruled out.

On Thursday, clouds will be present region wide, holding back temperatures more closer to 80F rather than 90F, however, the humidity will still be pretty high with dewpoints right around 70F.  While it will be cooler, it will still be quite humid.  Mid-level lapse rates will be continued weak with weak shear in place as well.  While thunderstorms will be possible given the presence of weak instability thanks to very moist low-level airmass, the above factors indicate the threat for strong to severe t'storms will be very low.

For widespread heavy rainfall, it would be beneficial to see a much stronger low-level jet forecasted, however, that is not the case here.  Computer forecast models though are indicating PWAT values as high as 2'' throughout the day on Thursday, a very strong indicator of how moist the airmass will be.  With multiple waves of low pressure developing, presence of weak instability, and moist airmass, we will see periods of showers and thunderstorms throughout the day tomorrow.

As far as rainfall totals go, computer forecast guidance is suggesting anywhere from 1''-2'' of rainfall would be possible.  Looking at the SREF forecast model below, (a compilation of numerous pieces of numerical guidance to generate an average) it seems like a good bet most folks can expect anywhere from 0.50'' to 0.75'' of rain.  However, there will be some locations who potentially exceed 1.50'' to 2'' of rainfall tomorrow.