Northwest Flow Snow (NWFS) Aspects of Sandy

1. Northwest Flow Snow (NWFS) Aspects of Sandy • Douglas K. Miller –UNC Asheville, Asheville, NC •  Steve Keighton – NOAA/NWS Blacksburg, VA • Steven M. Zubrick – NOAA/NWS Sterling, VA Part II: Unique Synoptic Setting for the Production of Northwest Flow Snow

2. NWFS Aspects of Sandy4-part series • Part I: A General Overview of NWFS in the Southern Appalachians • Part II: A Unique Synoptic Setting for the Production of Northwest Flow Snow • Part III: Moisture Anomalies and Trajectory Analysis • Part IV: Radar and Satellite Observations

3. Outline • Northwest flow snow review • Synoptic classification scheme • Synoptic evolution of Sandy • Impacts of Sandy in the southern Appalachian Mountains • Contrast with “classic” NWFS case study • Research questions

5. Northwest Flow Snow Review (Part I) • NW low level winds (upslope component on western slopes) • Typically post-frontal, with absence of deep moisture and synoptic scale upward motion (in fact, large scale subsidence more common) • Shallow moist and unstable layer below deep stable layer • Cold air advection and cold enough temperatures in moist layer for good ice crystal growth • Localized heavy snowfall rates and significant accumulations can result in highly variable snowfall distributions

6. Synoptic Classification From Perry et al. (2013)

7. Synoptic Classification

8. NWFS Aspects of Sandy4-part Series Fig. 2 of Perry et al. (2013)

9. Synoptic evolution of Sandy http://www.nasa.gov/mission_pages/hurricanes/archives/2012/h2012_Sandy.html

10. HPC Sea level pressure, 0000 UTC 29 Oct – 0000 UTC 1 Nov 2012

11. SPC 700 hPa analyses, 0000 UTC 29 Oct – 0000 UTC 1 Nov 2012 700 hPa level moisture? Mean 700-500 RH (%)

12. WRF-NMM 500 hPa level, 0000 UTC 29 Oct – 0000 UTC 1 Nov 2012

13. SPC 300 hPa analyses, 0000 UTC 29 Oct – 0000 UTC 1 Nov 2012 300 hPa jet structure?

14. Sandy storm track [Fig. 1 of Galarneau, Davis, and Shapiro (2013)] dt = 12-h

15. A hypothesis (Galarneau, Davis, and Shapiro 2013) • Diabatic heating on northwest flank of Sandy [~0000 UTC 28 October] and resulting upper-level divergent outflow; anti-cyclonic vorticity advection downstream of 500 hPa subtropical trough, impeding eastward progression of the overall 500 hPa trough • Allowed Hurricane Sandy to continue tracking northward rather than out to sea A prolonged period of northwesterly flow over the southern Appalachian Mountains

16. Impacts of Sandy in the southern Appalachian Mountains • Snow accumulation • Winds • Drifting snow • Mountain waves

17. Total snow accumulation associated with Sandy NWFS Aspects of Sandy4-part Series courtesy: Daniel Martin at Appalachian State University

18. Wind gusts (mph) NWFS Aspects of Sandy4-part Series courtesy: Daniel Martin at Appalachian State University

19. Sunrise on Mount LeConte, taken on Halloween Morning 2012. Snow drifts of 5 to 6 feet were reported at the lodge. Picture courtesy of Allyson Verden. slide courtesy of Sam Roberts and David Hotz

20. Fig. 1 of Perry et al. (2013) NWFS Aspects of Sandy4-part Series

21. RAP Skew-T analysis at Poga Mountain, 1200 UTC 30 Oct 2012 100 200 300 500 700 850 1000

22. Vertical cross section orientation 608 km, 129.75o

23. RAP θes [K] and ω [+ shaded, x 10-3hPa s-1], 1200 UTC 30 Oct 2012 Mountain wave slide, vertical cross section @ maturation KY SC Poga Mountain

24. Contrast with “classic” NWFS case study 26 – 28 February 2008 • 44-h duration • Surface low center located far from southern Appalachian Mountains

25. HPC Sea level pressure, 1200 UTC 27 February 2008 A comparison 0.54 inches

26. SPC 700 hPa analyses, 1200 UTC 27 February 2008 700 hPa level moisture? Mean 700-500 RH (%)

27. WRF-NMM 500 hPa level, 1200 UTC 27 February 2008

28. SPC 300 hPa analyses, 1200 UTC 27 February 2008 300 hPa jet structure?

29. Contrast with “classic” NWFS case study Poga Mountain observation comparisons • 0000 UTC 29 October – 1700 UTC 31 October 2012 (Sandy) • 2300 UTC 26 February – 1900 UTC 28 February 2008

30. Micro Rain Radar - Sandy dBZ 65-h duration 3 km Time (UTC)  Doppler Velocity 1000 UTC 30 October 2012

31. Micro Rain Radar – 27 Feb 2008 dBZ 44-h duration 3 km Time (UTC)  Doppler Velocity 1100 UTC 30 February 2008

32. Contrast with “classic” NWFS case study

33. Sandy A comparison 0.54 inches Precipitable water (in.) lowest 400 mb – 1200 UTC 30 Oct 2012

34. 27 February 2008 A comparison 0.22 inches Precipitable water (in.) lowest 400 mb – 1200 UTC 27 Feb 2008

35. Study Conclusions Uniqueness of synoptic setting for northwest flow snow • Transitioning tropical system merging with a potent negatively-tilted mid-latitude synoptic-scale trough • Northwestward track of surface system resulting in westerly/ northwesterly flow of long duration (65-h) • Proximity of surface system to southern Appalachians leading to unusual snow and wind; high societal impact X Synoptic class:

36. Research Questions What is the source of the high density snow of the 29-31 October 2012 event? • Unusual air parcel trajectories? • Unique cloud microphysical processes?

37. Northwest Flow Snow Aspects of Sandy • Part I: A General Overview of NWFS in the Southern Appalachians • Part II: A Unique Synoptic Setting for the Production of Northwest Flow Snow • Part III: Moisture Anomalies and Trajectory Analysis • Part IV: Radar and Satellite Observations Questions? • Galarneau, T., C. Davis, and M. Shapiro, 2013: Intensification of Hurricane Sandy (2012) through Extratropical Warm Core Seclusion. Mon. Wea. Rev. doi:10.1175/MWR-D-13-00181.1, in press. • Perry, L.B., S. J. Keighton, L. G. Lee, D. K. Miller, S. E. Yuter, C. E. Konrad, 2013: Synoptic Influences on Snowfall Event Characteristics in the Southern Appalachian Mountains. Proceedings of the 70th Eastern Snow Conference, in press.