Transcript Document

Winter Weather
Climatology and Forecasting
Advanced Synoptic
M. D. Eastin
Winter Weather
Climatology 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)
Advanced Synoptic
M. 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 Synoptic
M. 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 Synoptic
M. 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 Synoptic
M. 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 Synoptic
M. 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 1949-1999
Advanced Synoptic
M. D. Eastin
North American Snowfall Climatology
Where does snowfall occur?
Annual mean snowfall
(1949-1999)
Note: 1000 mm is roughly
equivalent to 40 inches
Advanced Synoptic
M. D. Eastin
North American Snowfall Climatology
Where does snowfall occur?
Maximum annual snowfall
(1949-1999)
Note: 1000 mm is roughly
equivalent to 40 inches
Advanced Synoptic
M. D. Eastin
North American Snowfall Climatology
Where does snowfall occur?
Minimum annual snowfall
(1949-1999)
Note: 1000 mm is roughly
equivalent to 40 inches
Advanced Synoptic
M. D. Eastin
North American Snowfall Climatology
Where does snowfall occur regionally?
Mean annual snowfall (1949-1999)
Pacific Northwest
Northeast
What are these
maxima?
What are these
maxima?
Note: 1000 mm is roughly
equivalent to 40 inches
Advanced Synoptic
Charlotte region
~200 mm = 8 in
M. D. Eastin
North American Snowfall Climatology
Where does snowfall occur during El Nino and La Nina?
More snowfall during El Nino
Advanced Synoptic
More snowfall during La Nina
M. 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 (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 Rain
in a Mid-Latitude Cyclone
Advanced Synoptic
Freezing Rain Events (1982-1990)
M. 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 Synoptic
M. D. Eastin
Freezing Rain Event: 4-5 December 2002
Advanced Synoptic
M. D. Eastin
Freezing Rain Event: 4-5 December 2002
Advanced Synoptic
M. 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 Synoptic
M. 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 Synoptic
M. D. Eastin
Freezing Rain Event: 4-5 December 2002
Advanced Synoptic
M. D. Eastin
Freezing Rain Event: 4-5 December 2002
Advanced Synoptic
M. D. Eastin
Freezing Rain Event: 4-5 December 2002
Advanced Synoptic
M. D. Eastin
Freezing Rain Event: 4-5 December 2002
Advanced Synoptic
M. 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, but basic concept is
universal
Nomogram for the Carolina Piedmont →
(overlay of 4-5 December 2002 event)
Advanced Synoptic
M. 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
Snowfall Totals for November 10-13, 1996
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)
Advanced Synoptic
M. 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 Synoptic
M. 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 Synoptic
M. D. Eastin
Lake Effect Snow
Forecast Factors:
• Instability
• Fetch ←
• Upstream Moisture
• Synoptic-scale Forcing
• Topography
• Snow/Ice Cover on Lake
• Upstream lakes
Favorable Fetches for Lake Effect Snow
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
Advanced Synoptic
M. 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
Advanced Synoptic
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
M. 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 Synoptic
M. 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)
Note: Colored regions freeze over during winter
White regions do not freeze over
• Lakes Ontario and Michigan seasons
are year round
Advanced Synoptic
M. 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 Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
Advanced Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
Advanced Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
Typical Synoptic Situation:
H
• 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
L
• 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?
CAD
Advanced Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
00Z
Advanced Synoptic
10 April 2005
M. D. Eastin
Snowstorm: 10-11 April 2005
Advanced Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
12Z
Advanced Synoptic
10 April 2005
M. D. Eastin
Snowstorm: 10-11 April 2005
Advanced Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
00Z
Advanced Synoptic
11 April 2005
M. D. Eastin
Snowstorm: 10-11 April 2005
Advanced Synoptic
M. D. Eastin
Snowstorm: 10-11 April 2005
Advanced Synoptic
M. D. Eastin
Forecasting Snow
1000-500 mb Thickness:
Valid for High Elevations
• 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
Valid for Low Elevations
5400 m
Advanced Synoptic
M. D. Eastin
Forecasting Snow
10-11 April 2005 Colorado Snowstorm:
Present Weather
12Z - 11 April 2005
1000-500mb Thickness
12Z - 11 April 2005
5400 m
snow
T < 0ºC
850mb Temperature
12Z - 11 April 2005
Advanced Synoptic
M. D. Eastin
Forecasting Snow
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
Advanced Synoptic
M. D. Eastin
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
Advanced Synoptic
M. D. Eastin
Winter Weather
Climatology and Forecasting
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
Advanced Synoptic
M. 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.
Advanced Synoptic
M. D. Eastin