Advanced SynopticM. D. Eastin Winter Weather Climatology and Forecasting

Advanced SynopticM. D. Eastin Review of winter precipitation types and hazards Climatology Snowfall Freezing Rain Forecasting Challenges Freezing Rain 4-5 December 2002 Carolina Ice Storm Forecasting Lake-Effect Snow Physical Processes Forecast Factors Mountain Snow Storms April 2005 Colorado Snow Storm Forecasting (all snow storms) Winter Weather Climatology and Forecasting

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

Advanced SynopticM. D. Eastin Winter Weather Precipitation Types Sleet: 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.

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 thin layer of ice (hence, freezing rain)

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

Advanced SynopticM. D. Eastin North American Snowfall Climatology Where does snowfall occur? Kluver (2007) Used quality controlled data from NWS cooperative stations and Canadian surface observations Computed mean snowfall amounts on a 1 x1 grid using interpolated daily snowfall data from

Advanced SynopticM. D. Eastin North American Snowfall Climatology Where does snowfall occur? Annual mean snowfall ( ) Note: 1000 mm is roughly equivalent to 40 inches

Advanced SynopticM. D. Eastin North American Snowfall Climatology Where does snowfall occur? Maximum annual snowfall ( ) Note: 1000 mm is roughly equivalent to 40 inches

Advanced SynopticM. D. Eastin North American Snowfall Climatology Where does snowfall occur? Minimum annual snowfall ( ) Note: 1000 mm is roughly equivalent to 40 inches

Advanced SynopticM. D. Eastin North American Snowfall Climatology Where does snowfall occur regionally? Mean annual snowfall ( ) Note: 1000 mm is roughly equivalent to 40 inches Pacific NorthwestNortheast Charlotte region ~200 mm = 8 in What are these maxima? What are these maxima?

Advanced SynopticM. D. Eastin North American Snowfall Climatology Where does snowfall occur during El Nino and La Nina? More snowfall during El NinoMore snowfall during La Nina

Advanced SynopticM. D. Eastin U.S. Freezing Rain Climatology Where does freezing rain occur? Robbins and Cortina (1996) Compiled reports of freezing rain during a 9 year period ( ) 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) Typical Locations of Freezing Rain in a Mid-Latitude Cyclone Freezing Rain Events ( )

Advanced SynopticM. D. Eastin Forecasting Winter Weather Challenges: Precipitation type can be very difficult Model forecast uncertainty (even 1-2 day 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

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

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

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

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

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

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

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

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

Advanced SynopticM. D. Eastin Forecasting Freezing Rain / Sleet Partial Thickness Nomograms: Use model forecast soundings at a location Determine the mb thickness Determine the mb thickness Consult the nomogram Thickness values on nomograms are location dependent, but basic concept is universal Nomogram for the Carolina Piedmont → (overlay of 4-5 December 2002 event)

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

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

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: there needs to be at least a 13ºC difference between the lake temperature and the 850 mb (or 1.5 km) temperature for significant lake effect snowfall Depth of Instability: Mixed layer depth must be greater than 100 mb (or 1.0 km) Capping inversion: At least a moderate capping inversion must be present

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

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

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

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

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

Advanced SynopticM. D. Eastin Snowstorm: April 2005

Advanced SynopticM. D. Eastin Snowstorm: April 2005

Advanced SynopticM. D. Eastin Snowstorm: April 2005 Typical Synoptic Situation: Weismuller and Howard (1988, unpublished) constructed a climatology of heavy snowfall events in Colorado ( ) An intense “Four Corners” low develops in association with a plunging west coast trough and upper-level PVA forcing An arctic air mass (anticyclone), located north of the heavy snowfall area, produces pre-existing inter-mountain (valley) cold pools or cold air damming (CAD) events As the low moves east, it taps the Gulf of Mexico air mass and begins to generate strong moist upslope flows over any CAD How does the April 2005 snow storm compare to this typical situation? L H CAD

Advanced SynopticM. D. Eastin Snowstorm: April Z 10 April 2005

Advanced SynopticM. D. Eastin Snowstorm: April 2005

Advanced SynopticM. D. Eastin Snowstorm: April Z 10 April 2005

Advanced SynopticM. D. Eastin Snowstorm: April 2005

Advanced SynopticM. D. Eastin Snowstorm: April Z 11 April 2005

Advanced SynopticM. D. Eastin Snowstorm: April 2005

Advanced SynopticM. D. Eastin Snowstorm: April 2005

Advanced SynopticM. D. Eastin 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 Valid for Low Elevations Valid for High Elevations

Advanced SynopticM. D. Eastin April 2005 Colorado Snowstorm: Forecasting Snow Present Weather 12Z - 11 April mb Thickness 12Z - 11 April m snow T < 0ºC 850mb Temperature 12Z - 11 April 2005

Advanced SynopticM. D. Eastin Partial Thicknesses: mb thickness mb thickness Used to discriminate between precipitation types near (< 100 km) the rain-snow line Helps resolve thin warm layers not easily identified by the mb thickness Forecasting Snow

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

Advanced SynopticM. D. Eastin Summary: Precipitation types (snow, freezing rain, sleet) Winter weather hazards (blizzards and ice storms) Climatologies (snowfall and freezing rain) Freezing rain Synoptic / Physical processes Forecast factors Lake Effect Snow Synoptic / Physical processes Forecast factors Mountain Snowstorms Synoptic / Physical processes Forecast factors Winter Weather Climatology and Forecasting

Advanced SynopticM. D. Eastin References Cortina, J. V., and C. C. Robbins, 1996: A climatology of freezing rain in the contiguous United States. NOAA Tech Report Kluver, 2007: Characteristics and trends of North American snowfall from a comprehensive gridded dataset. M.S. Thesis, University of Delaware.