Thursday, March 21, 2013

What is Driving this March Cold Spell?

At this point in the winter (now early Spring), we are all getting very annoyed with the cold. A few weeks ago when highs approached 70, the general populous assumed that this signified the end of the cold. The meteorological gods have had other ideas.

So what is driving this cold? The answer lies in the teleconnections derived from the pressure patterns in the higher latitudes. The North Atlantic Oscillation (NAO), Arctic Oscillation (AO) and the Pacific North American Index (PNA). I wrote extensively about each of these indices last year during our mild winter (2010-11) which you can read here. Even late in the season, these 3 teleconnections can still have a HUGE impact on the weather across the US.  As I write this, we are watching clusters of lake effect snow dropping several inches of "partly cloudy" across parts of the north coast

Back to the THREE indices...the more negative the NAO is, the stronger the northwest component which most times leads to a much stronger cold pattern. The AO is the NAO's close cousin. The more negative the AO, the better the propensity for colder periods and frequent clipper like systems. 

How strong is this NAO signature?  I looked back at Marches that had more than 2 straight days with NAOs below -1.5.  Why did I pick -1.5? Currently, the NAO is well below -1.5 and falling so -1.5 was a good starting point. Checking all of the data, I could only find 6 YEARS that match:  2011, 2001, 1980, 1977, 1962 & 1952.  I didn't use 2001 because those instances occurred in early march.  I plotted the NAO from March 10th through April 4th (to get some data overlap on both ends of the plot) to see if there was a pattern. Notice that in each year, the NAO bottomed out at the end of the month in EACH YEAR. Whether this is a statistical correlation or something more atmospheric, I'm not sure. More analysis would be needed to figure this out.

Where does this year rank in comparison to these years?  WAY BELOW THIS GRAPH. In fact, one model yesterday had the NAO dropping to -4.

Even if this model is a bit aggressive in its NAO forecast, we are still looking at temperatures staying WELL BELOW NORMAL for a while.

Is this cold spell some sort of record for this time of year? Surprisingly, not even close. Here is where this year's cold spell (starting after that warm Sunday a few weeks back) ranks ALL-TIME...How about 40th!

Tuesday, March 19, 2013

Do March Cold Stretches Lead to Milder Aprils?

Everyone is asking if this abnormally cold pattern will lead to a colder or warmer April?

Let's look at the Aprils of the past statistically after stretches of cold in mid to late March similar to this one.  The coldest stretches between March 15th and 24th were in these years. (So far, this stretch in 2013 (assuming that the extended forecast holds) is ranked 8th coldest.)

How were the temperatures in the following Aprils in these years? Here are the composites for each year. 4 of the 8 occurred before 1910.

APRIL 1906
APRIL 1960
APRIL 1872
APRIL 1885
APRIL 1896
APRIL 1956
APRIL 2006
APRIL 1967
The only years in this list that had COLDER than normal temperatures in NORTHERN OHIO were 1956 and 1885.  So statistically, the chances for an April with ABOVE NORMAL TEMPERATURES after a stretch of cold like what we are experiencing in this part of March are fairly good. Let's hope the actual meteorological pattern matches up.

Monday, March 18, 2013

Why is the Winter Pattern Lingering So Long?

This is the most asked question over the last week or so of northern Ohio residents. March of 2013 is the polar opposite of March of 2012 when we had a handful of 80 degree days en route to the warmest March in 140+ years of record keeping.  So far this March (through the 17th) temperatures rank 67th coldest---right in the middle of the 142 years of record keeping.

I only have a few minutes this morning due to the winter precipitation moving through the area along with my multiple weather segments so I'll keep my analysis brief:

The answer lies within the 3 major indices that govern our patterns. The AO (Arctic Oscillation), the NAO (North Atlantic Oscillation) and the PNA (Pacific North American Pattern). Last winter (2011-12) I wrote an extensive explanation on these three teleconnections. You can read it here.

Each index plays a different role at different times of year. In the summer, many of these indices are not as strong. But in March, they can play havoc with the development of spring warmups. The Sunday we had recently with temperatures at 70+ was the one bright spot in this colder than normal pattern.

Recently, these indices that measure the strength of arctic storm patterns (AO, NAO) are strongly negative which mean colder than normal. (graphic courtesy: Dr. Ryan Maue)

The PNA (pacific north american pattern) has been trending positive. All of these keep the NW flow aloft firmly entrenched keeping temperatures well below normal across the eastern US.

Not to worry. The "nickel-and-dime" cold punches WILL some point :)