Going Negative - How Much for DC?

Where the CW about the -NAO/s 'critical' importance to mid-Atlantic snowfall comes from is hard to tell…since more and more it appears to be something of a pleasing urban legend bordering on pure myth.

Just last week…the CW was held out again as gospel for getting snow in the DC area in this blog entry from The Washington Post/s “Capitol Weather Gang.”

“Some other ingredients, but not all, that make up the classic snow pattern for the region include the North Atlantic Oscillation (NAO), the 50-50 low, and the Polar Vortex.

While you don't need every ingredient in place to get snow here, the more you have in place the better. Let's start with the NAO. The NAO is traditionally measured by the difference in pressure between Iceland and the Azores. When this index is negative, it is often indicated by a block of high pressure over Greenland. When this block is strong and well-placed, it tends to lock the cold air in place over the East Coast, which is critical for snow in our region.”

Never mind how it parrots uncritically the CW about the ‘critical’ need for -NAO while ignoring how important the ‘positive PNA’ long wave flow regime is to producing more than nuisance snows for DC or how the ingredients described are all part of the same phenomenon or missing the mark by attributing the source of snow-enabling cold air along the EC to the Newfoundland LOW.

So...how well does the CW hold up for DC...located in the heart of the mid-Atlantic and close enough to the coast to see more than its share of cold rain in winter? Is a -NAO critical for good snows in the nation/s capital?

PNA…NAO…AO

CW for ‘any amount’ strongly influenced large number of snowfalls (292 days) less than 5” and for historic amounts of 12” or more. The probability for +NAO is higher once daily snowfall reaches 5 - 12” range.

PNA…NAO

CW for ‘any amount’ strongly influenced large number of snowfalls (292 days) less than 5” and for historic amounts of 12” or more. The probability for +NAO is higher once daily snowfall reaches 5 - 12” range.

PNA…AO

CW for ‘any amount’ strongly influenced large number of snowfalls (292 days) less than 5” and above 7” (27 days). The probability for PNA of either sign and -AO is highest when daily snowfall occurs in the 5 - < 7” range.

NAO…AO

+NAO and -AO are more likely for daily snowfalls from 5 - less than 12”.

Bottom line

'Big 3’

12HR warning snows (5" - < 7”) indicated by PNA of either sign...+NAO...-AO
24HR warning snows (+7") indicated by +PNA...+NAO...-AO
DCA/s best 24 hour snows (+12") indicated by +PNA...-NAO...-AO supports CW

The CW is supported for the ‘Big 3’ teleconnection indices when DCA/s daily snowfall is 12” or more.

Ignoring NAO…

12HR warning snows (5" - < 7”) indicated by +PNA...-AO supports CW
24HR warning snows (+7") indicated by +PNA…-AO supports CW
DCA/s best snows (+12") indicated by +PNA...-AO supports CW

The CW for +PNA and -AO is supported for both warning thresholds…all divisions of daily snowfall…and for historic daily snowfalls. The CW is not hard and fast for DCA/s 12HR warning criteria as the 5 - < 7” amount with +PNA and -AO occurs with an equal probability as -PNA and -AO.

Ignoring AO…

12HR warning snows (5" - < 7”) indicated by PNA or either sign...+NAO
24HR warning snows (+7") indicated by +PNA…+NAO
DCA/s best snows (+12") indicated by +PNA...-NAO supports CW

CW for +PNA and -NAO is unsupported for 12- and 24HR NWS warning criteria when daily snowfall is 5 - < 12”.

CW for +PNA and -NAO is supported for daily snowfalls < 5” and historic (+12”) daily snowfalls.

Ignoring PNA…

12HR warning snows (5" - < 7”) indicated by -NAO...-AO supports CW
24HR warning snows (+7") indicated by +NAO…-AO
DCA/s best snows (+12") indicated by -NAO...-AO supports CW

CW for -NAO and -AO is supported for 12HR warning criteria and historic (+12”) daily snowfalls.

CW for -NAO and -AO is unsupported for 24HR warning criteria and for 7 - < 12” daily snowfalls.

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The NAO CW is valid for nuisance snows and rare events that produce historic amounts. It cannot; however...be applied without regard to the sign of PNA for daily amounts that trigger warnings. +PNA and / or -AO are far more reliable indicators of favorable conditions for snow in DC.