Looking Ahead; End of April and Early May

It has turned out to be a fairly decent April in Southern New England.  Temperatures have averaged slightly above normal through the 22nd

NCEP CFSv2 Month to Date Temp Anomaly April 2014 (image Weatherbell)

The last seven days are below normal, thanks in large part to last Wednesday-Friday.  Even the warmth of the weekend and past few days couldn't overcome the negative anomaly

NCEP CFSv2 Last 7 days Temp Anomaly 4-15 4-22 (image Weatherbell)

There are several things to note from the two images.  First is strength of the cold anomaly in South Central Canada.  Second is the warmth in the last seven days in Alaska and the NW Territories of Canada. Third is the cool anomalies in Atlantic Canada and Greenland.   Why are these features important?

1. According to a 1950 report by Jerome Namias found here April and May tend to be the opposite it terms of weather; in fact the difference is second to only October and November.  He also concluded that the strength of the reversal can be tied to the strength of the Canadian cold pool.  This makes sense; if April is warm and May is cold then there must be cold air lurking to the North.  Therefore seeing the cold pool in Canada gives weight to a cool May.
2. The recent warmth in Alaska is a result of a moving block.  Rossby in 1939 found a general 3-8 week weather pattern shift that often occurs in late winter or early spring.  Seeing Alaska and NW Canada move from cool to warm (relative to average) signals a shift of this block in the Pacific.
3. According to Namias and Rossby the best place to watch for this reversal is in the Atlantic.  So far we have seen lower heights across Greenland in April.  The long range EURO Ensemble mean/control both forecast upper level heights to rise across Greenland over the next 1-2 weeks.  This will allow the jet stream to weaken and buckle into the United States

You may ask why would I put faith in a 1939/1950 report and that is fair to ask.  First of all the past 15-20 years have been similar to the 1930s-1950s period due to similar ocean conditions (Atlantic warm, Pacific warm to cool).  These guys were bad ass.  They did not have computer models to solve complicated formula's.  They had to figure out why thing happened through observation and analysis.  There is a ton of meteorology to be learned from reading accounts of meteorologists from the 1930-1960.  A good place to start is here, the Monthly Weather Review.  Much was learned during World War II.  

So what does this mean for Southern New England?  Here is the Climate Prediction Centers 8-14 day analogs.

CPC 8-14 Upper Air Analog Chart (4/23/14, valid 5/4/14)

The dates jump out.  Here is May 9, 1954 at 8 PM EST

Plymouth State Reanalysis May 10, 1954 0 UTC 500 mb height

Here is this mornings EURO, 500 mb heights valid 8 days from now, Thursday at 8 am

12z EURO 500 mb heights 5/1 8 am (image Weatherbell)

Its not a perfect match as the trough is hung back a bit further in the Great Lakes and the Ridge in the west is more over California.  I expect this to slowly continue to push east as early May progresses.  The case from 2005 looks more similar to the CPC outlook, and it looks more similar to the day 10 EURO forecast.

Plymouth State Reanalysis April 26, 2005 0 UTC 500 mb heights

Here is the EURO next Saturday (keep in mind EURO will change slightly over next few days)

12z EURO 500 mb heights 5/3 (image Weatherbell)

The 2005 event stretch from late April in early May which is what I expect this year  The 1954 event was a May event.  Precipitation will be above average

May 1954 Precipitation Anomalies (ERSL)

April-May 05

April-May 05 precipitation anomalies (ERSL)

Temperatures should be below average for the Eastern 1/2 of the country

May 1954 temp anomaly (ERSL)

April-May 05

April-May 05 Temp Anomaly (ERSL)

As you can see there will be regional differences compared to the two years.  I expect the temperatures to be closer to 1954 given the cold pool available in Canada.  Next week looks rainy.  This could be one of those mid spring stretches where in rains for 7-8 days off/on and the sun goes into the witness protection for awhile.

It also looks like an El Nino is coming this year.  Its not likely to be a Super El Nino given the overall cool state of the Pacific Ocean.  However a moderate-strong event like 02-03 or 09-10 is certainly possible.  Questions/comments/complaints can be left on facebook or tweeted to me @zgreenwx .  Thanks for reading.  Sorry to bring bad news and I hope I am wrong.

Zack Green