Tuesday, April 17, 2012

Freak Show?

Has this "freakish" weather pattern stumped meteorologists?  Is the weather pattern even freakish?  It's warm, yes, and much of the last year plus has been warmer than average and arguably the warmest stretch of time on modern climate record in Philadelphia.  It's also been dry...we've beat that horse into the ground a few times over the past couple of weeks.

Is this something that is stumping folks of the weather persuasion?

Not really.

The only thing that may be "freakish" in all of this is perhaps the headline writer's inability to see that only one meteorologist commented in the linked article and that a horticulturalist was the one that was stumped.   I'm not indicting Tony Wood, who is a very good writer and the article is relatively informative.  However, the headline is not only poorly written...it's also pretty misleading.

High pressure has "controlled" the pattern over the Eastern US for most of the winter.  Yeah, we have had occasionally forays of cold and occasional rain events through the Eastern US...but  since November we have had a lack of cold air in the East and many times a high pressure ridge off of the Carolinas.  The map below shows the 500 mb anomaly average since November -- that's the mid level in the atmosphere where many times cold/warm patterns can be driven.  You can notice the oranges and reds over us -- that yields higher pressure over the East Coast, with the result being a warmer than average winter and a warmer Spring so far. The core of cold had been over Alaska during much of this timeframe.

The result of this high pressure ridge has been fair weather....this, in concert with a strong Pacific jet blasting the Northwest US and West Coast of Canada with moisture and storm systems, has resulted in benign weather over the East and has resulted in a rather dry pattern overall for us along the East Coast as storm systems that do organize in the Midwest and Ohio Valley lift northeast around the ridge of high pressure.  Many times, we ultimately get a glancing cool front at the end of the storm that has little or no precipitation.

Storm tracks (red and black lines) and precipitation since mid January.  Core of storm tracks have been west  or northwest of the Delaware Valley thanks to dry pattern, while many storms hit the Northwest or Western Canada and weaken once inland.

The pattern has been eerily similar to that of ten years ago, at least as far as our region is concerned.  High pressure dominated over the US East Coast for much of the winter, resulting in one of the warmest and least snowy winters on record.   Ten years ago today it was 95 degrees outside -- a record high for the date and for the month of April.   It was the byproduct of a dry, warm winter that resulted in a dry, warm Spring that continued into a dry, hot summer.  In fact, some of our warmest twelve month running temperature averages on record occurred during parts of 2002.   The only substantive difference between 2002 and 2012 was a wetter and cooler March in 2002, which resulted in a relatively cooler (but still mild) March 2002.

Given that rainfall in four of the last eleven Aprils has totaled less than 2.50" (2001, 2002, 2003, and 2008) in a month where the average rainfall is 3.57", it's not unusual to have a dry April or a warm April.  And we've been here before...see ten years ago.

Like then, we could use some rain.  This weekend offers us that real possibility of it.