1. Advanced SynopticM. D. Eastin Winter Weather Climatology and Forecasting

2. Advanced SynopticM. D. Eastin Review of precipitation types, hazards, and impacts Forecast Challenges Snowfall Climatology General Forecasting Lake-Effect Events Freezing Rain Climatology Physical Processes 4-5 December 2002 Carolina Ice Storm Forecasting Winter Weather Climatology and Forecasting

3. Advanced SynopticM. D. Eastin Winter Weather – Precipitation Types Snow: Occurs when air temperatures remain below freezing through the atmospheric depth Aggregates of ice crystals that grow in size via collisions as they fall Type of ice crystals are a function of the air temperature and supersaturation at time of development

4. Advanced SynopticM. D. Eastin Winter Weather – Precipitation Types Sleet (or Ice Pellets): Develops when falling snow encounters a “shallow” layer of warm air deep enough for the snow to completely melt and become rain The raindrops then passes through a “deep” layer of freezing temperatures, deep enough for the raindrops to freeze before reaching the ground Do not confuse sleet with hail, they form by completely different processes. How shallow and deep? Depends on many factors as we shall discuss soon…

5. Advanced SynopticM. D. Eastin Winter Weather – Precipitation Types Freezing Rain: Develops when falling snow encounters a “deep” layer of warm air, deep enough for the snow to completely melt into raindrops The rain then passes through a “shallow” layer of cold air just above the surface and the drops cool to temperatures below freezing The drops do not freezing before reaching the ground. Rather, they become supercooled Upon striking the frozen ground, the drops instantly freeze, forming a layer of ice (hence, freezing rain) How shallow and deep? Depends on many factors as we shall discuss soon…

6. Advanced SynopticM. D. Eastin Winter Weather – Hazards Blizzards: Definition: All of the following criteria must be satisfied Blowing snow Gale force winds (>34 knots) for at least 3 hours Air temperature less than 0.0ºC (< 32ºF) Visibility less than 1/4 mile There is no total snowfall criteria, however most blizzards have considerable snowfall totals Heavy snowfall limits travel, and effectively paralyzes large regions for several days Ice Storms: No official definition Occur when significant freezing rain accumulates Can cause extremely hazardous road conditions, down large trees and power lines, and immobilize large cities for several days or weeks

7. Advanced SynopticM. D. Eastin Winter Weather – Societal Impacts Damage: Ice storms are the most impactful with utility, residential, and commercial infrastructure (major concern in Southern U.S.) Blizzard are less impactful but strong winds and excessive rooftop snows can impact structures (major concern in Northern U.S.) Deaths: 165 deaths per year (1968-1992) (hurricanes = 25 / year) (tornadoes = 75 / year) Economic Impacts: Both storm types Retail losses > $10 billion per day Flight delays > $3 billion per year Snow removal > $2 billion per year

8. Advanced SynopticM. D. Eastin Forecasting Challenges Challenges: Precipitation type can be very difficult Model forecast uncertainty (even 12-24 hour forecast errors can be large) Zones of heavy snow, sleet, and freezing rain can be very narrow Zone locations can change very quickly as the system moves and evolves A location forecast error of only 50 miles can produce big socio-economic impacts if emergency managers prepared fro the wrong precipitation type

9. Advanced SynopticM. D. Eastin Forecasting Challenges Challenges: The amount of frozen precipitation can be critical → for freezing rain events the difference between a short-lived glaze and major power outages can hinge on this parameter Small variations in atmospheric and surface parameters can dictate changes in precipitation type Numerical models poorly represent many thermodynamic processes which dictate precipitation type Shallow cold / warm air advection Latent heat release / absorption Surface fluxes of heat / moisture Evolution of ground temperature Precipitation intensity Cloud radiation interactions

10. Advanced SynopticM. D. Eastin U.S. Snowfall Climatology Where does snowfall occur? Annual mean snowfall (30 year average) Charlotte region 3-6 inches What are these maxima?

11. Advanced SynopticM. D. Eastin 1000-500 mb Thickness: Used to identify the rain-snow line Small thickness = cold air Large thickness = warm air  Common threshold is 5400 m (or “540” on a thickness chart) for most areas of the country  Threshold varies with station elevation Forecasting Snow 5400 m

12. Advanced SynopticM. D. Eastin Other Factors: Numerical models: Determine which model gives the most accurate storm track Check which model provides an accurate depiction of the early mesoscale structure Know the model’s low-level temperature biases Storm / Environment Characteristics: Moisture transport → More moisture= More snowfall → Less moisture= Less snowfall Moisture Source (impacts snow-water ratio) Size/Area of Precipitation Region (large systems = more snow) Motion of System (slow moving systems = more snow) Check the vertical soundings through the region for the structure of any warm and cold layers Forecasting Snow

13. Advanced SynopticM. D. Eastin Lake Effect Snow Basics: Localized heavy snowfalls along the lee-coasts of large lakes (e.g. the Great Lakes) Occur during the fall and early winter when mean lake temperatures exceed mean land temperatures Mean annual snowfalls can exceed 200 inches in narrow zones Notable snowfalls: Greater than 10 inches per hour Single Day: 68” Adams NY (Jan 9, 1976) Storm Total: 102” Oswego NY (Dec 27-31, 66) Monthly Total: 149” Hooker NY (Jan 77) Annual Total: 467” Hooker NY (76-77) Snowfall Totals for November 10-13, 1996

14. Advanced SynopticM. D. Eastin Lake Effect Snow Physical Processes: Result from cold air flowing over warm (ice-free) lakes Air acquires heat and moisture via surface fluxes and is destabilized Capping inversion limits cloud formation over the lake Frictional convergence and upslope flow along lee-coast provides needed lift Rising air saturates, develops localized clouds and heavy snowfall

15. Advanced SynopticM. D. Eastin Lake Effect Snow Forecast Factors: Instability Fetch Upstream Moisture Synoptic-scale Forcing Topography Snow/Ice Cover on Lake Upstream Lakes Instability: Degree of Instability: At least a 13ºC difference between the lake temperature and the 850-mb (~1.5 km) temperature for significant lake effect snowfall Depth of Instability: Mixed layer depth should be greater than 100-mb (~1.0 km) Capping inversion: A moderate capping inversion should be present

16. Advanced SynopticM. D. Eastin Lake Effect Snow Forecast Factors: Instability Fetch Upstream Moisture Synoptic-scale Forcing Topography Snow/Ice Cover on Lake Upstream lakes Fetch: Distance air travels over water Longer fetch, more moisture, more snowfall Determine from 850mb wind direction Small differences can significantly change the fetch (e.g. Lake Erie) 250º wind → 225 mile fetch 230º wind → 80 mile fetch Favorable Fetches for Lake Effect Snow

17. Advanced SynopticM. D. Eastin Lake Effect Snow Forecast Factors: Instability Fetch Upstream Moisture Synoptic-scale Forcing Topography Snow/Ice Cover on Lake Upstream Lakes Upstream Moisture: Impacts precipitation potential Initially low RH air will arrive at lee coast with less moisture → more difficult to get clouds and heavy snowfall Initially high RH air will arrive at lee coast near saturation (due to moisture fluxes), allowing for easy cloud formation and heavy snowfall production Synoptic–Scale Forcing: Cyclonic vorticity advection (PVA) aloft may enhance precipitation by lifting the capping inversion Cold air advection (CAA) may enhance the lake effect snowfall by increasing the instability

18. Advanced SynopticM. D. Eastin Lake Effect Snow Forecast Factors: Instability Fetch Upstream Moisture Synoptic-scale Forcing Topography Snow/Ice Cover on Lake Upstream Lakes Topography: Provides increased lift that promotes greater cloud formation and local snowfall Lake-effects snowfall increases when rapid elevation rises are along lee coast (e.g. Tug Hill Plateau, NY/PA) Annual snowfall increases ~10-12 inches for each 100 ft increase in elevation

19. Advanced SynopticM. D. Eastin Lake Effect Snow Forecast Factors: Instability Fetch Upstream Moisture Synoptic-scale Forcing Topography Snow/Ice Cover on Lake Upstream Lakes Snow / Ice Cover on Lake: Prevents needed moisture fluxes Diminishes or ends lake-effect season Lake Erie season often ends in late January (lake freezes over) Lakes Ontario and Michigan seasons are year round Note: Colored regions freeze over during winter White regions do not freeze over

20. Advanced SynopticM. D. Eastin Lake Effect Snow Forecast Factors: Instability Fetch Upstream Moisture Synoptic-scale Forcing Topography Snow/Ice Cover on Lake Upstream Lakes Upstream Lakes: Impacts the total moisture flux along the fetch Frozen upstream lakes limit total moisture arriving at lee coast on downstream lakes Less moisture, less snowfall Numerical simulations of total snowfall

21. Advanced SynopticM. D. Eastin Lake-Enhanced Snow Important Difference: Lake-enhanced snow applies to those cases where snow would have fallen without the influence of the lake, but the lake enhanced some aspect of the precipitation process beyond what the synoptic-scale system would have produced alone (e.g., enhanced convergence, moistening of the lower atmosphere, etc.) Notice the moisture plume extending from Lake Michigan down toward western Carolina where orographic snowfall was occurring

22. Advanced SynopticM. D. Eastin U.S. Freezing Rain Climatology Where does freezing rain occur? Robbins and Cortina (1996) compiled freezing rain reports for 9 years (1982-1990) Identified Four Regions:Pacific NW (rain falling through cold valley air) Central US (associated with CO leeside lows) New England (associated with nor’easters) Mid-Atlantic (associated with cold-air damming)

23. Advanced SynopticM. D. Eastin Freezing Rain Important Physical Processes: Elevated deep layer of saturation supports precipitation formation which will warm an elevated layer via latent heat release Elevated layer of veering winds (or warm air advection) further supports an elevated melting layer Shallow, but strong and continuous, near-surface layer of cold / dry air advection supports the prolonged cooling (evaporation) and freezing of any melted drops Cold ground temperatures (<5°C in the uppermost 10 cm) minimizes the upward heat flux and limits any warming of the near surface air GSO 1800 UTC on 4 December 2002

24. Advanced SynopticM. D. Eastin Freezing Rain Important Physical Processes: Events are self-limiting since the (re-)freezing of raindrops releases latent heat and warms the colder sub-freezing near-surface air Most freezing rain events are short-lived (last less than 6 h) Major ice storms are accompanied by either (1a) initially very cold air at the surface or (1b) strong low- level cold/dry air advection, as well as (2a) deep snow cover and/or (2b) sub-freezing soil temperatures Minor ice storms are associated with (1a) near-freezing cold air at at the surface and/or (1b) weak cold air advection as well as (2a) limited snow cover and/or (2b) above-freezing soil temperatures GSO 0600 UTC on 5 December 2002

25. Advanced SynopticM. D. Eastin Freezing Rain Important Physical Processes: In the Carolinas, many significant freezing rain events are associated cold-air damming (CAD) events The amount of freezing rain is often a function of how long the near-surface air and ground can remain below freezing

26. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002

27. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002

28. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002 Synoptic Situation: A cold front passed through the Carolinas on December 2-3 bringing a very cold air mass into the region An intense “cP” anticyclone was located northeast of the Carolinas → its weak easterly flow impinged along the eastern slopes of the Appalachians A strong capping inversion below the ridge line (mountain tops) prevented the cold air from being lifted over mountains The cold air began to “pile-up” along the eastern slopes, creating a “wedge” A cold-air damming (CAD) event developed

29. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002 Synoptic Situation: Meanwhile, a cyclone was developing along the Texas Gulf coast and began to move eastward (Type II cyclogenesis) This primary cyclone began advecting a deep layer of moist air northward from the Gulf of Mexico over the cold air wedge of cold air and a wintry mix of precipitation began to fall At the same a time a secondary coastal low developed along the primary low’s warm front (situated over the Carolina coast) The coastal low also advected a deep layer warmer moist air eastward over the cold air wedge, contributing to the precipitation formation and the switch from snow to freezing rain

30. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002

31. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002

32. Advanced SynopticM. D. Eastin Freezing Rain Event: 4-5 December 2002

33. Advanced SynopticM. D. Eastin Forecasting Freezing Rain / Sleet Partial-Thickness Nomograms: Use model forecast soundings at a location Determine the 1000-850mb thickness Determine the 850-700mb thickness Consult the nomogram Thickness values on nomograms are location dependent (one for Southeast U.S is shown to the right) but basic concept is universal

34. Advanced SynopticM. D. Eastin Forecasting Freezing Rain / Sleet Nomogram for the Carolina Piedmont → (overlay of 4-5 December 2002 event)

35. Advanced SynopticM. D. Eastin Summary: Precipitation types (snow, freezing rain, sleet) Winter weather hazards (blizzards and ice storms) Societal Impacts (damage, death, and economic) Snowfall Climatology General forecast factors Lake-effect events Freezing rain Climatology Physical processes Forecast factors Winter Weather Climatology and Forecasting

36. Advanced SynopticM. D. Eastin References Ballentine, R. J., A. J.Stamm, E. E. Chermack, G. P. Byrd and, D. Schleede, 1998: Mesoscale model simulation of the 4-5 January 1995 lake effect snowstorm. Wea. Forecasting, 13, 893-920. Bourgouin, P., 2000: A method to determine precipitation type. Wea. Forecasting, 15, 583-592. Cortina, J. V., and C. C. Robbins, 1996: A climatology of freezing rain in the contiguous United States. NOAA Tech Report Cortina, J. V., B. C. Bernstein, C. C. Robbins, and J. W. Strapp, 2004: An analysis of freezing rain, freezing drizzle, and ice pellets across the united states and Canada. Wea. Forecasting, 19, 377-390. Gedzelman, S. D. and E. Lewis, 1990: Warm snowstorms: A forecaster’s dilemma. Weatherwise, 43, 265-270. Hjelmfelt, M. R., 1990: Numerical study of the influence of environmental conditions on lake-effect snow storms over Lake Michigan. Mon. Wea. Rev., 118, 138-150. Kain, J.S. S. M. Goss, and M. E Baldwin, 2000: The melting effect as a factor in precipitation-type forecasting. Wea. Foreacasting, 15, 700-714. Keeter, K. K., and J. W. Cline, 1991: the objective use of observed and forecast thickness values to predict precipitation type in North Carolina. Wea. Forecasting, 6, 456-469. Lackmann, G. M., 2011: Winter Storms, Midlatitude Synoptic Meteorology - Dynamics, Analysis, and Forecasting, Amer. Meteor. Soc., Boston, 219-246. Lackmann, G. M., K. Keeter, L. G. Lee, and M.B. Elk, 2002: Model representation of freezing and melting precipitation: Implications for winter weather forecasting. Wea. Forecasting, 17, 1016-1033. Niziol, T. A., 1987: Operational forecasting of lake effect snow in western central New York. Wea. Forecasting, 2, 310-321. Niziol, T. A., W. R. Snyder, and J. S. Waldstreicher, 1995: Winter weather forecasting throughout the eastern United States. Part IV: Lake effect snow. Wea. Forecasting, 10, 61-77.