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

Advanced Synoptic M. D. Eastin

Winter WeatherClimatology and Forecasting


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• 10-11 April 2005 Colorado Snow Storm• Forecasting (all snow storms)



Winter Weather Precipitation TypesSnow:

• 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


Winter Weather Precipitation TypesSleet:

• 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.


Winter Weather Precipitation TypesFreezing 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)


Winter Weather HazardsBlizzards:

• 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


North American Snowfall ClimatologyWhere 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 1949-1999



Annual mean snowfall(1949-1999)

Note: 1000 mm is roughly equivalent to 40 inches



Maximum annual snowfall(1949-1999)




Minimum annual snowfall(1949-1999)



North American Snowfall ClimatologyWhere does snowfall occur regionally?

Mean annual snowfall (1949-1999)


Pacific Northwest Northeast

Charlotte region~200 mm = 8 in

What are thesemaxima?

What are thesemaxima?


North American Snowfall ClimatologyWhere does snowfall occur during El Nino and La Nina?

More snowfall during El Nino More snowfall during La Nina


U.S. Freezing Rain ClimatologyWhere does freezing rain occur?

• Robbins and Cortina (1996)• Compiled reports of freezing rain during a 9 year period (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)

Typical Locations of Freezing Rainin a Mid-Latitude Cyclone

Freezing Rain Events (1982-1990)


Forecasting Winter WeatherChallenges:

• 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


Freezing Rain Event: 4-5 December 2002


Freezing Rain Event: 4-5 December 2002Synoptic 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


Freezing Rain Event: 4-5 December 2002Synoptic 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


Freezing Rain Event: 4-5 December 2002


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, but basic concept is universal

Nomogram for the Carolina Piedmont → (overlay of 4-5 December 2002 event)


Lake Effect SnowBasics:

• 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


Lake Effect SnowPhysical 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


Lake Effect SnowForecast 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



• 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



• 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



• 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



• 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



• 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


Snowstorm: 10-11 April 2005


Snowstorm: 10-11 April 2005Typical Synoptic Situation:

• Weismuller and Howard (1988, unpublished) constructed a climatology of heavy snowfall events in Colorado (1948-1987) • 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 10-11 April 2005 snow storm compare to this typical situation?

L

H

CAD



00Z 10 April 2005



12Z 10 April 2005



00Z 11 April 2005


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

Valid for Low Elevations

Valid for High Elevations


10-11 April 2005 Colorado Snowstorm:

Forecasting Snow

Present Weather12Z - 11 April 2005

1000-500mb Thickness12Z - 11 April 2005

5400 m

snow

T < 0ºC

850mb Temperature12Z - 11 April 2005


Partial Thicknesses:

• 1000-850 mb thickness• 850-700 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 500-1000mb thickness

Forecasting Snow


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


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



ReferencesCortina, 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.

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

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