Sunday, March 2, 2014

Coming in the Back Door

This winter has been a wild one for temperatures in South Carolina.  There have been big swings from hot to cold and back again.  Now that meteorological spring has begun (March 1st) the pattern continues.

A dramatic change to our weather is about to take place.  Cold arctic air is already in place across much of the central U.S.  Normally a cold front moving in from the west would bring the cold air to our state.  However, occasionally it comes in through the “back door”.

High pressure builds east and the cold air slides down the eastern seaboard faster than it can come in from the west.  It becomes trapped between the ocean to the east and the Appalachians to the west.  The cold front that surges south is known as a “back door” cold front.

Typically a wedge of cold air stays in place until something comes along to move the cold air out, which is a very difficult thing to do.  The result can be days of overcast skies and chilly northeast winds.

This time the back door cold front will be accompanied by a dramatic temperature drop.  When the front passes the temperatures will likely drop 20-25° F in an hour or two.  The front will be preceded by a band of rain which will begin the temperature drop.  Winds will shift from a westerly direction to a northerly direction as the front passes.  The winds will increase and there will be a wind chill to worry about in the late afternoon.

The RPM model forecast for 1:30 p.m. EST on Monday, March 3, 2014.  This is from the 21z run of the model using the 12 km grid.  Click on the image for a larger version.  Image Credit: WLTX-TV\WSI.

The Regional Precision Model (RPM), that we use, captures the dramatic change in temperature.  In the panels below it starts at 1:30 p.m. and ends at 3 p.m.  Notice the sharp drop in temperature in Columbia, SC.  It is a change of 21° F in 90 minutes.  Furthermore, by 7 p.m. the model projects a temperature of 32° F with a wind chill temperature of 21° F.  High temperatures will likely occur in the late morning or early afternoon in the mid to upper 60s.

The RPM temperature forecast for Monday afternoon March 3. 2014.  You can tell where the front is from the contour color of temperatures.  Note the precipitous drop in temperatures as the front passes.  Click on the image for a larger version.  Image Credit: WLTX-TV\WSI.

Don’t get too “hung up” on the time.  The model could be off by an hour either way.  The important thing to take from this is the large temperature drop.  It is highly advisable to take a warm coat when you leave in the morning for work or school, because it will be dramatically colder when you come home.

Temperatures will likely be near or below freezing by early evening.  There could be some light freezing rain or freezing drizzle as the clouds will remain overnight Monday.  It will not be enough to cause any travel problems and there will be little accumulation.

This weather event is more of a temperature event than a precipitation one.  So there will be no snow or ice storm for this part of the country.  However, the change of temperature can be a problem.  The high temperature in Columbia, SC on Sunday was 76° F, but the temperature just 24 hours later will be in the mid-30s.  Fortunately the warm spell was brief and plants have not begun to flower.  The pollen season has begun, but the growing season has not.

It is not often that we see back door cold fronts with temperature gradients of this strength.  When we do they tend to be in late winter or early spring.  This latest one may have the strongest temperature gradient in recent memory.  In addition, temperatures will drop into the mid to upper 20s Tuesday morning with wind chill temperatures in the upper teens.

This may not be winter’s last gasp.  I have been advising everyone to delay planting this year.  Don’t be in a hurry.  The weather pattern we have been seeing since late October suggests the possibility of a late freeze.  This could happen in the first half of April.  Until then we are likely to continue to see wild swings in temperature.