Sunday, May 5, 2013

A Cool Start To May 2013

Just as we were getting used to hotter springs, Mother Nature pulls a fast one.  It has been unusual to say the least for South Carolina weather since April 26th.  Cloudy, cool conditions have been the rule with plenty of rain.  The griping has begun as many are tied of conditions we normally attribute to Seattle.

My favorite tweet during the past week has been “Breaking news: This just in – Cleveland wants its weather back.”  I’m sure those in Cleveland might think this an improvement over what they normally get in early May.

So what has been happening?  Why has it been so cloudy and cool for so long?  When will this go away so that we can get on with summer?

The first half of spring was influenced by a weather pattern associated with a negative phase of the Arctic Oscillation.  This was described in the previous post.  However, that has not driving the weather pattern recently.  Instead the jet stream has retreated to the north leaving upper-level low pressure systems isolated from the main flow.  This has resulted in a blocking pattern where weather systems slow to a crawl and can persist for days.

The weather pattern at 500 mb (~18,000 ft) on Saturday, May 4, 2013 at 12z UT.  The jet stream had retreated farther to the north leaving isolated pockets of low pressure over the mid-latitudes.  Click on the image for a larger version.  Image Credit: WLTX-TV.

Several low pressure systems aloft had become cut-off from the main flow as the picture above shows.  South Carolina was affected last week by the low off the coast and a high pressure ridge centered over the Northeast.  This brought in cool air at the surface with moist air aloft resulting in persistent cloud cover.  The pattern shifted eastward ever so slightly each day until the low pressure system to the west affected the state over the weekend.  It remained cloudy and cool, but this time brought abundant rainfall to the Upstate.

A high-resolution composite of the polar orbiting satellites for Saturday, May 5, 2013.  The cloud cover over the Mississippi Valley is associated with the low pressure in the chart above.  Clear skies over Louisiana and Mississippi are the result of dry air rotating around the base of the system, known as the dry slot.  Click on the image for a larger view.  Image Credit: NASA.

Notice the dry slot in the picture over the lower Mississippi Valley where skies have cleared.  This will move into South Carolina Sunday night providing a break in the action.  However, the cold pool of air associated with the upper-level low with result in more clouds Tuesday with periods of rain as the low passes overhead.  Some of this may last into Wednesday.

The low pressure system should be far enough away for the pattern to finally break on Thursday.  If the timing holds, this will represent persistence in cool, cloudy weather spanning a full nine days follow by two more days after a break.  The good news is that the sun will return and temperatures will climb into the mid-80s as we would expect at this time of year.  This will also be good news for SEC baseball which has had terrible weather for the “boys of summer”.

However, this will not be the last cool period for May.  The Climate Prediction Center is forecasting cooler than normal and wetter than normal conditions for the 11th through the 15th of the month.  So don’t expect to see 90-degree heat very soon.

The 6 to 10 day outlook produced Sunday, May 5, 2013.  It covers the period of Saturday, May 11, to Wednesday, May 15.  Click on the image for a larger view.  Image Credit: NOAA\CPC.

Climate Change

It is not that rare to see blocking patterns of this nature in May.  The jet stream is retreating north and sometimes the planetary waves develop into cut-off lows resembling the type of structure we have seen for the past couple of weeks.  However, research is showing that blocking patterns are becoming more frequent.  Scientists are studying the effects on atmospheric flow patterns as a result of global warming and changes in the Arctic.

This year we have been the recipients of a persistent wet pattern since January.   It has wiped out the drought for much of the Southeast.  The current weather pattern does not tell us what to expect this summer.  It is likely to be hotter than normal, but we probably will not be setting record all-time highs during the day.  However, it will be more humid which may affect low temperature records, i.e. warmest low temperatures.