For tonight precipitation is expected to expand in coverage as a S-N
oriented LLJ develops near the deepening surface low. This jet will
bring copious amounts of Atlantic moisture over eastern Mid-Atlantic
and New England regions, and convergence ahead of the jet will bring
widespread rain showers across our eastern zones. Dynamic cooling
through the evening hours will push the entire thermal profiles to
at or below freezing resulting in a change over to snow...likely
across the higher terrain (mainly Lewis County). We will have the
highest PoPs from the Finger Lakes eastward, while chance PoPs will
remain in place for areas west of the Genesee Valley. Overall snow
totals tonight will range 1 to 3 inches on the higher terrain east
of Lake Ontario.
During the day Friday the deep Atlantic moisture over New England
and southern Quebec will fold westward back over our region as the
second upper level low drops by our region just to the west. This
abundant moisture in the lower levels, combined with deepening cold
air and increasing low level lapse rates will result in lake
enhanced, and upslope snow showers through the day across WNY, and
then shifting eastward to areas east of Lake Ontario. This second
wave of snow will have greater accumulations than the snow
originating with the initial coastal low and shortwave. Also this
second wave of snow will have higher snow to liquid ratios...and
allow the increasing winds to create blowing snow.
Temperatures aloft at 850 hPa will drop to around -7 to -10C...not
quite cold enough for pure lake effect snow...but cool enough that
we should still see some enhancement. The moist, northwest flow will
also generate orographic lift snow Friday and into Friday night.
While accumulations will be minor through the first half of
Friday...the deepening cold air and increasing northwest winds will
begin to increase the snow accumulation Friday afternoon across the
higher terrain...and then everywhere by sunset Friday night. This
period of late Friday to Friday night is when the bulk of the snow
for this event will fall.
Winds... Winds tonight will be light across the region, with an
inverted surface trough extending across our eastern zones from the
passing coastal low. Once the surface low passes by our latitude, a
northwest wind will rapidly increase Friday midday through Friday
night. Gusts will generally be 30 to 40 mph, with occasional gusts
to 45 mph along the southern shoreline of Lake Ontario.
Temperatures...Lows tonight will remain just above freezing for much
of our CWA. The airmass will warm a little Friday morning before the
coastal low passes by to our east, and a much colder airmass invades
the region from the northwest. Temperatures will likely begin to
fall through the afternoon hours, and continue to fall Friday night
with overnight lows in the upper teens across the higher terrain and
lower 20s near the Lakes. The combination of winds and temperatures
will make wind chill values drop into the single digits across the
region Friday night.