Other Factors: PNA Pattern

A negative PNA pattern favors a trough along the West coast of the United States and ridging along the

East coast of the United States. Thus, when the PNA is negative, the pattern is not extremely conducive for East coast snowstorms. Can snowstorms still occur along the

East Coast? Yes, but the storm track still has to be south and east of Philadelphia. Best option here for snow is a negative PNA pattern with a split flow jet

stream and other conducive factors in place like the 50-50 low, negative NAO, and position of surface high

pressure.

Pacific Decadal Oscillation Link

January 21–26, 1987January 21–26, 1987

0

0 .2

0 .4

0 .6

0 .8

1

1 .2

1 .4

1 5 9 1 3 1 7 2 1 2 5 2 9 3 3 3 7 4 1 4 5 4 9 5 3 5 7 6 1

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Other Factors: PNA Pattern

Other Factors: Positive PNA Pattern

A positive PNA pattern favors a trough along the East coast of the United States and ridging along the West

coast of the United States. Thus, when the PNA is positive, the pattern is conducive for East coast

snowstorms. However, many other factors must be in place as well. This is the case with every single one of

these points. Just one will not make a snowstorm. Having all of them in place does not mean a

snowstorm will happen. However, the more you have, the better chance that exists for a heavy snow

producing storm over the Delaware Valley.

Click here for current PNA index values

Other Factors: Positive PNA Pattern

Other Factors: PNA Pattern

Major snowstorms have occurred in the Delaware Valley during times of a negative PNA. However, it is more of a rare occurrence when compared to times of

a positive PNA.

If a snowstorm does develop along the East coast during a time of negative PNA, there has to be some

evidence of ridging to the south and/or west to promote the digging of a trough near the East Coast. This

ridging may be displaced just east of the West coast (Rockies/Plains) or sometimes as far east as the Deep

South, thus the PNA remains in a negative state.

Click here for current PNA index values

Other Factors: PNA Pattern

Also, just as is the case with the NAO, look for which direction the index is trending. A negative PNA

trending sharply neutral or positive is a good sign during times of a potential developing snowstorm.

Additionally, the state of the PNA (positive or negative) when evaluating snowstorm potential becomes less of a mitigating factor during late February, and especially

into March and April due to shortening wavelengths (courtesy of Donald Sutherland).

Click here for current PNA index values

Other Factors: PNA Pattern (Euro forecast for March 1, 2006)

When a negative PNA can be ideal…huge vortex in SE Canada, huge blocking ridge (high pressure) over Greenland connected with ridge in north central Canada, closed 500 low in eastern Gulf of Alaska ejecting energy into California, and energy undercuts the block in California (courtesy of DT).

Too big and too strong to be a 50-50 low, it is a gargantuan polar vortex (courtesy of DT)

PNA Index (Blizzard of 2006)• PNA was strong

positive during the storm.

• A strong positive PNA promotes ridging over the West coast and a trough over the East Coast. With this setup a shortwave (low) is better apt to develop into a storm and then ride up the Eastern seaboard.

Time of storm

PNA Index (February 14-17, 1958)PNA Index (February 14-17, 1958)

-1

-0.5

0

0.5

1

1.5

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58

Amplifying and Positive PNA Index

PNA Index (March 18-21, 1958)PNA Index (March 18-21, 1958)

-1

-0.5

0

0.5

1

1.5

2

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58

PNA Index (December 10, 1960)PNA Index (December 10, 1960)

-1.5

-1

-0.5

0

0.5

1

1.5

2

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61

PNA Index (January 18, 1961)PNA Index (January 18, 1961)

-1

-0.5

0

0.5

1

1.5

2

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61

PNA Index (December 23-25, 1966)PNA Index (December 23-25, 1966)

-2

-1.5

-1

-0.5

0

0.5

1

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61

PNA Index (February 5-7, 1967)PNA Index (February 5-7, 1967)

-2

-1.5

-1

-0.5

0

0.5

1

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58

February 5, 1978February 5, 1978

-0.5

0

0.5

1

1.5

2

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58

February 18, 1979February 18, 1979

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58

Series1

February 10, 1983February 10, 1983

-0.5

0

0.5

1

1.5

2

2.5

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58

Series1

February 24 and February 28, 2005February 24 and February 28, 2005

- 1

- 0 .5

0

0 .5

1

1 .5

1 6 1 1 1 6 2 1 2 6 3 1 3 6 4 1 4 6 5 1 5 6

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January 22, 2005January 22, 2005

- 2- 1 .5

- 1- 0 .5

00 .5

11 .5

2

1 6 1 1 1 6 2 1 2 6 3 1 3 6 4 1 4 6 5 1 5 6 6 1

December 5 – 7, 2003December 5 – 7, 2003

- 2

- 1 .5

- 1

- 0 .5

0

0 .5

1

1 .5

1 5 9 1 3 1 7 2 1 2 5 2 9 3 3 3 7 4 1 4 5 4 9 5 3 5 7 6 1

February 15-18, 2003February 15-18, 2003

- 3

- 2

- 1

0

1

2

1 5 9 1 3 1 7 2 1 2 5 2 9 3 3 3 7 4 1 4 5 4 9 5 3 5 7

February 5 and February 22, February 5 and February 22, 20012001

- 1

- 0 .5

0

0 .5

1

1 .5

1 5 9 1 3 1 7 2 1 2 5 2 9 3 3 3 7 4 1 4 5 4 9 5 3 5 7

January 20, 2001January 20, 2001

- 1

- 0 .5

0

0 .5

1

1 .5

2

1 6 1 1 1 6 2 1 2 6 3 1 3 6 4 1 4 6 5 1 5 6 6 1

December 30, 2000December 30, 2000

- 0 .5

0

0 .5

1

1 .5

2

1 6 1 1 1 6 2 1 2 6 3 1 3 6 4 1 4 6 5 1 5 6 6 1

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