Sunday, October 21, 2012

Coastal Storm Followed By Cold Weather?

The South Carolina State Fair was lucky and enjoyed great weather for its entire run which ended today.  Dry weather is expected to continue over the Midlands most of the coming week and it will be delightful fall weather.  However, the weather pattern will begin changing this week and the Midlands, as well as much of the East coast, will be challenged by the changing conditions next weekend.

All of the computer models, used for medium-range weather forecasts, are converging on a solution that changes the weather for much of North America.  The change is already underway.  It is happening in the middle troposphere (500 mb) over western Canada where a double-barrel low pressure system is pushing the polar jet stream south from Alaska to California.

The 500 mb chart for 00z, 22 Oct 2012.  Winds generally blow parallel to the contours lines.  Click on the image for a larger view.  Image Credit: WSI.

This will bring colder air south into the western U.S.  The low pressure system will slowly swing east and become an elongated trough of low pressure.  It will extend through the Plains states on Saturday intensifying a surface low pressure system and moving it north toward the Arctic.  At the same time a tropical system will be headed north from the Caribbean Sea into the Bahamas.

The is the 500 mb chart valid for 12z Sunday (8 a.m. EDT), 28 Oct 2012.  Notice the broad trough of low pressure over the central U.S. and the disturbance off the Carolina coast.  The pattern will change rapidly after this time period.  Image Credit: WSI.

The National Hurricane Center (NHC) is keeping tabs on a system south of Jamaica.  As of Sunday evening NHC forecasters say there is a 70% chance of the system becoming a tropical cyclone over the next two days.

The satellite picture of the Caribbean Sea Sunday evening at 8 p.m. EDT.  The I in the center of the picture represents the area being watched by the National Hurricane Center.  Image Credit: CIMSS.

It is unclear when the system will develop and how long it will remain tropical.  However, all of the models pick up on this system and move it north.  It would move through the Bahamas next weekend and would be along the Mid-Atlantic coast by early next week.  What caught my attention was the intensity of the system moving north and the intrusion of the cold air from the west early next week which could make this a monster storm.

The scenario I am describing uses the European computer model (ECMWF) which has regularly been the superior model at longer ranges.  The American (GFS) and Canadian (GEM) are similar, but do vary in details.  At this time frame the details are usually transitory and may disappear in later model runs.

According to the ECMWF there should be some development of the low in the Caribbean either Tuesday or Wednesday.  The weather map across North America will look something like the map below.

The surface forecast map for Tuesday, Oct. 23, 2012.  Notice the system in the Caribbean Sea and the cold front in the Rockies.  The computer model used here is the ECMWF.  Click on the image for a larger view.  Image Credit: WLTX-TV.

The tropical system will move north across Cuba late in the week and into the Bahamas by Saturday.  Initially this will be a slow northward movement.  However, as the system gets farther north the speed will increase.  At the same time a cold air mass will begin invading the central U.S.

Late the in the week the picture changes dramatically for the eastern third of the country.  The pictures below show the day-by-day changes beginning on Friday and ending on Monday.

Daily forecast of the surface map for Friday, Oct. 26, through Monday, Oct. 29, 2012.  The system coming out of the Caribbean could be a powerful storm system either along the East coast or offshore by Monday.  Windy conditions are expected for the Carolinas Sunday and Monday.  The computer model used here is the ECMWF.  Click on any of the maps for a larger view.  Image credit: WLTX-TV.

The tropical cyclone could become a hurricane off the southeastern U.S. before moving northward and becoming a powerful mid-latitude storm system.  This transformation will likely happen late in the weekend into early next week as cold air rushes into the system from the west.  It will be interesting to watch, but the effects could be devastating.  The long easterly fetch along the coast could lead to significant storm surge and beach erosion.  Seas could be dangerous all along the East coast.

In South Carolina there could be high surf, coastal flooding, and beach erosion over the weekend.  There may even be some rain for the eastern part of the state, but this will depend on the ultimate track of the system.  A cold front will pass through the state on Sunday bringing windy conditions both Sunday and Monday.  The coldest air of the fall season will be pushing into the Southeast behind the front.  There could be a frost or freezing temperatures by the middle of next week once the winds subside.

What I have presented here is a broad overview of events that will take place over the next ten days.  Obviously, there are many details yet to be determined and I recognize that this scenario may be wrong.  However, whenever all of the models are pointing in the same direction, it is best to take heed.

I will update this information when needed.  Stay tuned!