End of October storm?

We are now entering the third week of October and the transition from summer to autumn is already well underway as the leaves have gone through the color changing process and trees are beginning to lose their leaves.  While the weather during this third week of the month will be quite tranquil with temperatures running above-average for the most part unsettled weather looms on the doorstep.

In the southern Caribbean, Tropical Storm Sandy has developed south of Jamaica is is expected to slowly track northward over the next several days.  

(National Hurricane Center graphic of Tropical Storm Sandy and it's expected movement through Saturday)

Medium/long range computer models all indicate that Sandy will continue slowly tracking to the north through the remainder of the week and next weekend into the western Atlantic just east of the United States.  While some computer guidance does suggest slight strengthening of Sandy over the course of this period it doesn't appear all that likely she will reach hurricane strength as atmospheric conditions don't appear to be very favorable.  Eventually she will be working into an area of enhanced wind shear and some drier air streaming off the United States and both will eventually inhibit her from going through rapid/intense strengthening.  

While Sandy is slowly making her journey northward some interesting developments will be ongoing across the United States, the central/northern Atlantic Ocean, as well as in and around the Aleutians near Alaska and how these developments unfold across the aforementioned areas will impact what happens with Sandy and where she heads once she gets into the Atlantic.  First, we will take a look at the United States and how the computer models evolve the developing upper air pattern over the United States.

As we move towards the weekend computer models show a developing trough associated with the Polar Jet Stream working into the northwestern portion of the United States sliding an Arctic cold front eastward.  There is agreement within the computer models that as this trough works through the country the trough will begin to amplify and strengthen causing it to dig further south.  In response to the digging trough across the central United States a ridge would be developing across the eastern United States extending northward into eastern Canada.  How this trough develops and how strong (or how far south it digs) will play a major role in what happens with Sandy as it moves into the western Atlantic.  We'll get more into why a bit further down.

Now onto the central/northern Atlantic and the potential developments there.  Computer models are showing the continued weakening area of high pressure located over the Azores as the area of low pressure that is situated between Greenland and Iceland.  When this occurs this leads to what is known as a "blocking pattern".  This is termed blocking b/c the weaker area of low pressure located between Greenland/Iceland leads to ridging in the northern Atlantic, this can prevent storm systems from escaping out to sea and keep them closer to the east coast.  The strength of this northern Atlantic ridging will also play a major role in what happens with Sandy.  

When looking across the northern Pacific Ocean up around the Gulf of Alaska computer models are also struggling with the handling and placement of the Aleutian Ridge.  This will have major impacts upstream as the strength/placement of the Aleutian ridge will vastly impact the strength of the troughing across the central United States which would affect the strength of the riding across the eastern United States and western Atlantic. Now to tie these three paragraphs together!

Below is an ensemble mean showing the 500mb height anomalies across North America Saturday evening.  The image is displaying a fairly deep trough across the central United States with a massive ridge along the east coast and western Atlantic:

  

Below is an ensemble mean showing the 500mb height anomalies across the entire northern Hemisphere Saturday evening...for this graphic we will just focus on the highlighted area showing the ridging in and around the Aleutian Islands near Alaska: 

Below is an ensemble mean showing the 500mb height anomalies across the entire northern Hemisphere Saturday evening...for this graphic we will pay attention to the highlighted area across the northern Atlantic near eastern Canada and south of Greenland:

In the end the strength/placement of the Aleutian ridge will play a major role in how the upper air pattern across the United States develops.  The stronger the Aleutian ridge and the more the center of the ridge is placed over the Aleutians the stronger the ridging just west of the western United States will be and the deeper the trough across the central United States will become which will result in a much stronger ridge across the east coast and western Atlantic.  The weaker the Aleutian ridge and if it becomes further displaced from being centered over the Aleutian Islands this would flatted the ridging west of the United States and prevent the trough along the central United States from deepening too far south leading to a more flat ridge along the east coast of the United States and western Atlantic which would affect the strength of the blocking in the northern Atlantic.  The strength of the northern Atlantic blocking will be highly crucial as well as the stronger the blocking the less chance there is of Sandy escaping and moving into the Atlantic and a much higher chance of Sandy staying closer to the east coast and working northward.  

Over the past few days several medium and long-range forecast models have been showing that the riding in and around the Aleutians may be strong enough allowing for deeper troughing in the central United States and stronger ridging across the eastern United States and western Atlantic creating stronger blocking allowing for Sandy to work very close to the east coast.  By early next week, some of the models have Sandy actually interacting and phasing "combining" with the trough and the associated Polar Jet.  in this scenario we would see a rapidly intensifying storm system.  

What should we make of all this...

The first thing to keep in mind here is this is all 7+ days out there and the accuracy of computer models in that range is very, very low.  In this range, often times computer models will tend to dig those troughs too deep as they get to crazy with the strength of factors like blocking and ridging (or troughing) around the Aleutians.  Computer models also LOVE to go phase happy with systems in the Atlantic and troughs along the East coast.  So until we get closer (inside of 4-days) we shouldn't take much out of these extreme solutions.

With this said the pattern does appear that it will become at least favorable for a period of unsettled weather, somewhere in the Monday-Wednesday timeframe with at the least potential for some periods of rain across the Northeast.  As we move further through the week and closer to next week details will become much more clear as to how the pattern will evolve and what potential impacts we may see.