Download Lecture 543

Lecture 11 (11/18)
Winter Storms and
Lake Effect Snow
Winter Storms
• A winter storm is just a low pressure area (storm)
that moves across the U.S. with particular cold air
behind it.
• Important to forecast a couple days in advance so
public can prepare, close airports, close schools,
etc.
Winter Storms
• Snowfall almost always occurs in the cold sector
of a cyclone (NW of cold front and N of warm
front).
• Most common just to the North or Northwest of
the center of the cyclone.
• Exception: Upslope snow in Denver – when moist,
cold air blows in from Kansas, it must go uphill
(2000 ft)
• This is often enough to induce clouds and snow
Precipitation Types
• Freezing Rain
– Snow melts in warm layer
– The cold layer below is too shallow and water only
freezes on contact
• Sleet
– Snow melts in warm layer
– Refreezes in the lower cold layer
• Snow
– Begins as snow and layer is cold enough to remain
frozen
Blizzards (not the thing at DQ)
• Blizzard = A snowstorm with winds of at
least 35 mph, and visibility reduced to less
than 1/4 mile with snow and blowing snow
• A severe blizzard has 45 mph or greater
winds
• Blizzards can make “white out conditions”
where visibility is so bad you cannot tell the
ground from the sky (everything is white)
• Also make snow drifts
Snow Drifts
• Snow drifts happen when the wind blows
snow and deposits it into mounds
• Remember the snow fence example in the
book
• In extreme cases, snow drifts can get as
high as 40+ feet!
A Typical Snow Drift
Not a typical Snow Drift
Start shoveling lady!
Lake Effect Snow
• Lake Effect Snows (LES) most common in
late fall through early winter
• Cold arctic air (typically associated with
strong surface high pressure from Canada)
flows over the warm lakes
• Heat and moisture from the lake surface is
transferred into the boundary layer (the
layer where the air from the ground is
mixed into the air above it.)
LES process
• Surface heating beneath cold air above
destabilizes the lowest 1 to 2 km of the
atmosphere
• Initiates convection
• If the air resides over a lake long enough, it
picks up enough moisture to condense water
vapor and form clouds
• Condensation and freezing in the clouds
releases latent heat which adds to buoyancy
Moving towards shore
• The warm, moist, unstable air moves
onshore
• Rougher surface beneath and topography (if
there is any) decelerates the air and induces
convergence winds coming together)
• Enhanced lift at the shore
• Cumulus and stratocumulus clouds
precipitate
Results of LES
• Land breeze circulation occurs (circulation
from land to water at surface and water to
land aloft)
• All of this produces heavy snow squalls in
narrow regions
• Skies can be clear just a few miles away
• If these heavy snow resides in same area for
hours or even days you can get huge snow
accumulations
LES Picture
LES Climatology
Forecasting Lake Effect Snow
• Lake effect snow is seriously under-forecasted by
the models
• If the models predict lake-effect snow, it will most
likely happen
• Determine the wind direction: examine the 850mb
analysis or forecast.
• Add 13 deg F to the 850mb temp over the lake (to
calculate potential temp and determine stability of
air between lake and 850
Rule of Thumb
• If temp at 850 + 13° > temp of lake then
you won’t get LES situation (since air is
stable)
• Cyclonic curvature favors lake effect snow
of winds
• If the 850 mb height contours curve to the
left as you go downstream, strong lakeeffect snow is more likely
For Next Time
• Do your homework.
• Read Chapter 8 (Thunderstorms and
Tornadoes). I know, I’m really getting into
wishful thinking.