Going Negative - How Much for Philly?

The oft-repeated conventional wisdom concerning EC snow storms...especially in the mid-Atlantic region...holds there/s a requirement for the 'Big 3' teleconnection indices (PNA...NAO...and AO) to be on the 'right' side of zero. Most common is the idea unless and until the NAO goes negative...good snows just...don/t...won/t...ain/t gonna happen.

There may actually be some physical reasoning involved in reaching this conclusion about the 'Big 3' b/c 1) -NAO events are marked by arctic air masses flooding the eastern CONUS. The resultant baroclinic zones are strong and ripe for the development of ageostrophic...thermally-direct...vertical motion fields and cyclogenesis; 2) +PNA results from 'high' amplitude - shortened wavelength westerlies across the CONUS where Miller A-type cyclogenesis (.pdf) occurs preferentially over the northern waters of the GOM or over the Gulf Stream off the SE coast; and 3) -AO is observed when the polar vortex is 'weak,' which allows arctic air masses to drain into lower latitudes where it can excite strong winter storms.

But how important is it to have the 'correct' index sign to get a decent EC snow storm? Is the CW correct?

In an expanded follow-up to last season/s 'Going Negative' series...analyses of historical daily snowfall data and the 'Big 3' teleconnection indices from 1950 to 2007 for M-A stations seeks to answer that question. The CW is tested for PHL by evaluating joint probabilities its station snowfall relative to NWS Eastern Region/s 12- and 24-hour snowfall warning thresholds and the 'Big 3' teleconnection indices.

First up: Philly!

PNA...NAO...AO

Any amount: +PNA...+NAO...-AO
1 - < 4”: +PNA...+NAO...-AO
4 - < 6”: +PNA...+NAO...AO of either sign
+6”: +PNA...-NAO...-AO supports CW
6 - < 9”: PNA of either sign...-NAO...-AO
9 - < 12”: +PNA...NAO of either sign...AO of either sign
+12”: +PNA...+NAO...AO of either sign

CW strongly influenced by 6 - < 9” snowfalls. All combinations of NAO and AO with +PNA have equal chances for 9 - < 12+ events. +PNA and +NAO preferred for storms producing more than 12” in 24 hours.

PNA…NAO

Any amount: +PNA...+NAO
1 - < 4”: +PNA..+NAO
4 - < 6”: +PNA...+NAO
+6”: +PNA...-NAO supports CW
6 - < 9”: PNA or either sign...-NAO
9 - < 12”: +PNA...NAO of either sign
+12”: +PNA...+NAO

CW strongly influenced by +PNA for +9” snowfalls and -NAO for 6 - < 9” snowfalls. +PNA and NAO of either sign have equal chances for 9 - < 12 events. +PNA and +NAO preferred for storms producing more than 12” in 24 hours.
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PNA…AO

Any amount: +PNA...-AO
1 - < 4”: +PNA...-AO
4 - < 6”: +PNA...AO of either sign
+6”: +PNA...-AO supports CW
6 - < 9”: PNA or either sign...-AO
9 - < 12”: +PNA...AO of either sign
+12”: +PNA...AO of either sign

CW strongly influenced by +PNA for +9” snowfalls and -AO for 6 - < 9” snowfalls. +PNA and AO of either sign have equal chances for +9”.

NAO…AO

Any amount: +NAO...-AO
1 - < 4”: +NAO...-AO
4 - < 6”: +NAO...AO of either sign
+6”: -NAO...-AO supports CW
6 - < 9”: -NAO...-AO supports CW
9 - < 12”: NAO of either sign...AO of either sign
+12”: +NAO...AO of either sign

Bottom line:
12HR warning snows (+4", < 6”): +PNA...+NAO...AO of either sign
24HR warning snows (+6"): supports CW (+PNA...-NAO...-AO)
PHL/s best 24 hour snows (+12"): +PNA...+NAO...and AO of either sign

Ignoring NAO...
12HR warning snows (+4", < 6”): +PNA...AO or either sign
24HR warning snows (+6"): supports CW (+PNA...-AO)
PHL/s best snows (+12"): +PNA...AO of either sign

Ignoring AO...
12HR warning snows (+4", < 6”): +PNA...+NAO
24HR warning snows (+6"): supports CW (+PNA...-NAO)
PHL/s best snows (+12"): +PNA...+NAO

Ignoring PNA.
12HR warning snows (+4", < 6”): +NAO...-AO
24HR warning snows (+6"): supports CW (-NAO...-AO)
PHL/s best snows (+12"): +NAO...-AO

Can the CW for PHL stand the light of data?

Yes...it can for the general case of 24-hour snowfalls of 6" or more; however...decent snows of at least 4" and less than 6" (12HR ERH Warning threshold) occur with a preference for +NAO and no preference for sign of AO...as do historic events of 12" or more. +PNA plays a greater role than a -NAO or -AO for PHL/s snowfall in any amount.

The biggest surprise was found in the 9 - < 12" category where equal probabilities exist for PNA of either sign.