Severe Neg. NAO, Little March Snow for DCA: What Happened?

[donsutherland1]

The March NAO average is now in: -2.757. As a result, March 2005 ranked third lowest for the month of March. Yet, while abundant snows fell in such cities as New York (6.9") and Boston (14.5"; Tied 16th highest for March), in Washington, DC only tears fell and hearts ached.

With the warm breezes of the rising spring whispering of summer days to come, now is a good time to look back. Perhaps the kind embrace of spring warmth might ensure that pain from looking back might just be a little less than otherwise.

What went wrong? The PNA was too high for the most part!

March Averages:
NAO: -2.757
PNA: +0.230

Experience with Late-Season "Kocin-Uccellini" (KU) Storms:
These memorable snowstorms often provide a harvest of snowfall that makes March extraordinary.

Since 1950, there have been 6 KU snowstorms that commenced on March 1 or afterward:

March 2-5, 1960
March 12-14, 1993
March 18-21, 1958
March 18-19, 1956
Mar 31/Apr. 1, 1997
Apr. 6-7, 1982

In 5/6 (83%) of cases, these storms occurred in March or April when the PNA averaged below -0.500. Moreover, in all 3 cases when the NAO averaged negative, the PNA also averaged -0.500 or below.

March: NAO -1.000 or Below:

The same kind of relationship holds up with regard to total monthly snowfall. In general, when the NAO has averaged -1.000 or below and the PNA has been positive, March has produced very little snowfall for Washington, DC.

Since 1950, 4 seasons have met such criteria. 3/4 (75%) saw less than an inch of snow for DCA. The most was 2.2". The average snowfall for such seasons was: 0.9".

In seasons where the NAO averaged -1.000 or below in March and the PNA was positive, 6/10 (60%) received 3" or more snow and 3/10 (30%) received 6" or more. The average snowfall for such seasons was 4.9".

For all seasons (1950-2005), 17/56 (30%) received 3" or more snow in March, 8/56 (14%) received 6" or more snow in March and 33/56 (59%) received less than 1". The average snowfall for March came to 2.1".

Conclusion:

In sum, a strongly negative NAO-positive PNA combination is more than 25% more likely than the climatological probability to produce less than 1" of snow. Moreover, such snowfall comes to just over 40% of the average figure for March.

In contrast, a strongly negative NAO-negative PNA combination is twice as likely as the climatological probability to produce 3" or more snow and 6" or more snow. The monthly average for such situations is approximately 230% of the average figure for March.