1. 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

2. January 21–26, 1987

3. Other Factors: PNA Pattern

4. 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

5. Other Factors: Positive PNA Pattern

6. 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

7. 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

8. 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)

9. 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

10. PNA Index (February 14-17, 1958) Amplifying and Positive PNA Index

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

12. PNA Index (December 10, 1960)

13. PNA Index (January 18, 1961)

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

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

16. February 5, 1978

17. February 18, 1979

18. February 10, 1983

19. February 24 and February 28, 2005

20. January 22, 2005

21. December 5 – 7, 2003

22. February 15-18, 2003

23. February 5 and February 22, 2001

24. January 20, 2001

25. December 30, 2000