Tropical Trouble This Weekend?

As Bill finally leaves North America, another disturbance is raising some interest to the north of the Caribbean.  Some model guidance is forecasting that this disturbance will develop into a strong tropical system and threaten the East coast.  Other guidance suggest that this system is a non-issue and will remain well away from the East coast if it generates at all.

The differences as I will show you in the following paragraphs will fall down to the time and orientation of upper level troughs over North America, and because of this, we are seeing several different outcomes in the model guidance.

First let’s look at the current situation over the Tropical Atlantic.  We have three key features.  The first is the disturbance to the northwest of the Leeward Islands.  This disturbance is becoming better organized by the hour with increasing convection and a developing low level circulation.  The outflow and inflow upper level structure is rather disorganized through due to the next feature to the north.

This feature is a weakening upper/mid level low found around 700 to 600 MB.  This disturbance is weakening and losing influence over the tropical disturbance.  How fast this upper level feature is no longer a factor will determine how fast the tropical disturbance will develop.  Currently, the upper level low is producing strong enough shear to disrupt the development of the upper level anticyclonic features of the tropical disturbance.  The third feature is the deep trough over the Southeastern United States and over the coastal waters.  This trough is punt the upper low eventually to the northeast and away from the tropical disturbance over the next 48 hours.  Then we have some interesting conditions unfolding!

I am using the NAM model guidance as a representation of the average model guidance for the 500 MB pattern.  I am leaning away from the current 12 Z GFS as the guidance is an outlier on the handling of the development of the tropical disturbance and upper level pattern as a whole.

So here we are at 48 hours and we have some interesting features to consider.  The most interesting is that the trough over the East is lifting towards the Canadian Maritimes.  This means that higher pressure can become established over the Southeast and the western Atlantic water around the Bahamas.  This also happens to be the same area that the tropical disturbance will be located.  Now, the Atlantic Ocean waters are rather warm around here, which is completely normal.  The Sea Surface temperatures off the Florida coast range from 85 to 90 degrees, which is like high octane fuel for any tropical system.  With light upper level winds and warm sea surface temperatures, conditions will be significantly favorable for development, possibly rapid development as some model guidance suggests.

By early Thursday morning, the trough over the Northeast is neutral in orientation, which is important because this means mid and upper level winds are orientated from south to north rather than southwest to northeast.  This is a significant observation as this will allow the tropical disturbance to continue to move west or northwest rather than north.  The other feature to key on is a weak trough diving into the Mississippi Valley.  This trough is key going towards the weekend.  The orientation of this trough will determine if this disturbance continues to the northwest or turns out into the Atlantic.  If this trough is tilted negative, then the disturbance will be drawn towards the coast on a southeasterly upper level wind.  If the trough is in a positive tilt, then the disturbance exits into the Atlantic without much impact.

As you can see, the NAM at this point and many model guidance in this family of model guidance has this trough over the Southeast tilted negative.  The ECMWF and CCM both do this and brings a well developed and moisture loaded tropical system into the Mid Atlantic and forecast area by Saturday night.

The trough is associated with a cold front that will focus the rain along the coast and could lead to a very wet and stormy weekend rather than the dry and cool weekend earlier expected.  The other issue is that the convergence and confluence over New England will support surface high pressure, much like a winter storm scenario.  In a way, we are looking at a negative NAO type pattern, which in turn will produce blocking over the Northeast and northern Mid Atlantic.  This upper level pattern originally was going to support surface high pressure off the New England coast, which would usher in a cool marine air mass into the forecast area.  Instead, with this tropical influence, we may see a situation set up where the high pressure over the New England coastal waters will enhance the pressure gradient and produce stronger than normal winds with this tropical system and cause a slower exit.  There is a lot to break down before we get to this point, but the threat is clearly there for a strong tropical system to impact the northern Mid Atlantic this weekend.

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