Northwest Flow Snow (NWFS) Aspects of Sandy

1. Northwest Flow Snow (NWFS) Aspects of Sandy Part I: A General Overview of NWFS in the Southern Appalachians NWA 38th Annual Meeting, Charleston SC 15 October 2013

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. NWFS Aspects of SandyPart 1: A General Overview of NWFS in the Southern Appalachians Outline • The NWFS collaboration group • Definition of NWFS events in the Southern Appalachians • Summary of aspects studied and current knowledge • Operational application

4. Study Area & Collaborators Other Acknowledgments: Chip Konrad (UNC) Brian Etherton (Charlotte, RENCI) Sandra Yuter (NCSU) Gary Lackman (NCSU) Greg Dobson (UNC-A, RENCI) Blair Holloway (GSP, CHS) Jim Hudgins (RNK) Kevin McGrath (RLX) Jeff Hovis (RLX) Gary Votaw (JKL) Ed Ray (JKL) Steve Nelson (FFC) Jonathan Blaes (RAH) Steve Zubrick Steve Keighton Baker Perry David Hotz Doug Miller Larry LeePat Moore

5. Northwest Flow Snow (NWFS)as defined for the Southern Appalachians: • 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 with moist layer cold enough for efficient ice crystal growth • Localized heavy snowfall rates and significant accumulations can result in highly variable snowfall distributions

6. Impacts can be significant • Often come with strong cold advection and bitter wind chills • Blowing/drifting snow • Can last for 2-3 days • Increasing population in many higher mountain communities, including some interstates

7. Favored locations for NWFS in the Southern Appalachians

8. High percentage of snow in southern Appalachians occurs with low-level NW flow Wind direction at event maturation for all snow events (n=122) on Poga Mountain, NC, 2006-2012 (From Perry et al, 2013)

9. Shallow in nature compared to other snow events in region, and often subsidence at 700mb Special sounding at PogaMtn, courtesy of UNC-A.  Vertical MRR data courtesy NCSU, showing several types of snow events containing low-level NW flow. Bottom one is common pure NW flow with no significant synoptic features.

10. Many NWFS events associated with strong cold advection and high snow-to-liquid ratios Snow-to-Liquid ratios for NWFS Events at PogaMtn, NC from 2006-11 AvgSfc Temp (C): -4.9 Coldest: -12.4

11. Trajectory analysis sometimes shows a Great Lakes Connection…

12. …which can lead to heavier snowfall

13. High resolution NWP output • Has been helpful with terrain signals • Can simulate the lake-influenced banding, and associated diurnal signals • UNC-A study showed importance of upstream surface heat/moisture fluxes (even without Great Lakes influence)

14. Several synoptic map patterns may include or evolve into NWFS events • “M”: Miller A & B East Coast cyclones (biggest snows) • “SE”: Southeastward-moving clippers • “NE”: Northeastward-tracking lows that pass to the north and drag cold front through Appalachians (short-lived) • “CL”: Cut-off upper lows (rare, often late season) • “LC”: Surface low develops in lee of Appalachians (not depicted on map) • “U”: NW flow in low levels in absence of significant synoptic features (not depicted) From Perry et al (2013)

15. Improving forecasts spatially, temporally, and for max amounts • Grid “Edit areas” created from NWFS climatology • Incorporation of high res model output • Adjustments for varying snow-to-liquid ratios • Extended headlines when long-duration anticipated Observed event-total snow (Feb 27-28, 2008) Forecast event-total snow (Feb 27-28, 2008)

16. References • Holloway, B.S., 2007: The role of the Great Lakes in northwest flow snowfall events in the southern Appalachian mountains. M.S. Thesis, Dept of Marine, Earth, and AtmosSci, NCSU, 2004 pp. • Keighton, Lee, Holloway, Hotz, Zubrick, Hovis, Votaw, Perry, Lackmann, Yuter, Konrad, Miller, Etherton, 2009: A Collaborative approach to study northwest flow snow in the southern Appalachians. Bull Amer. Meteor. Soc., 90, 979-991. • Miller, D.K, 2012: Near-term effects of the lower atmosphere in simulated northwest flow snowfall forced over the southern Appalachians. Wea. Forecasting., 27, 1198-1216. • Perry, L.B., and C.E. Konrad, 2006: Relationships between northwest flow snowfall and topography in the southern Appalachians, USA. Climate Research, 32, 35-47. • Perry, L.B., C.E. Konrad, and T.W. Schmidlin, 2007: Antecedent upstream air trajectories associated with northwest flow snowfall in the southern Appalachians. Wea. Forecasting, 22, 334-352. • Perry, L.B., D.K. Miller, S.E. Yuter, L.G. Lee, and S.J. Keighton, 2008: Atmospheric influences on new snowfall density in the southern Appalachian mountains, USA. Proc 65th Eastern Snow Conf., Fairlee, VT, Eastern Snow Conf, 123-133. • Perry, L.B., C.E. Konrad, D. Hotz, and L.G. Lee, 2010: Synoptic classification of snowfall events in the Great Smoky Mountains, USA. Phys. Geogr., 31, 156-171. • Perry, L.B, S.J.Keighton, L.G. Lee, D.K Miller, S.E. Yuter, and C.E. Konrad, 2013: Proc 70th Eastern Snow Conf., Huntsville, Ontaria, Canada, Eastern Snow Conf, DRAFT. • Yuter, S.E., and L.B. Perry, 2007: Storm structures and precipitation characteristics of snow events in the southern Appalachian mountains. Abstracts, 12thConf on Mesoscale Processes, Aug 2007, Waterville Valley, NH.

17. NWFS 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?