Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 PM EST Fri Dec 07 2018

Valid 00Z Sat Dec 08 2018 - 00Z Tue Dec 11 2018


Days 1 through 3..

...Southern Plains into the Lower Mississippi Valley...
A long wave trough crosses the Southern Plains and Lower MS Valley
during the first part of Day 1. Colder air oozing south from the
Central Plains will aid in the production of snow and ice across
portions of the area. The 00z NAM looked too cold in the lower
levels, so the thermal portion of the forecast was based primarily
on a blend of the 12z GFS/ECMWF. The QPF portion of the forecast
was based on the most recent WPC QPF.

Day 1..
Ahead of a long wave trough crossing the Southern Plains and Lower
MS Valley during Day 1, warming in the mid levels (mostly between
750-700 mb) will produce a pronounced warm nose during the peak
vertical motion, which is expected mainly before 08/12z. The warm
nose is expected to result in a precipitation phase problem over
portions of north TX, where low level cold air seeps south from
the Central Plains. Model soundings showed a sleet profile in
these areas, which is expected to cut back on snowfall amounts. A
stripe of 4+ inches of snow is possible near and south of Lubbock,
where the best vertical motion overcomes the mid level warming.

Further north across far northern portion of the TX Panhandle,
drier air in place (shown well in the 07/12z soundings, as well as
surface dew points) is expected to produce a sharp northern extent
of the precipitation shield and snowfall amounts. An axis of 2 to
4 inches of snow was stretched across southern OK into northern
AR, where model soundings showed sufficient moisture in the
dendritic growth zone for precipitation to fall mainly as snow.
Just south of the best vertical motion, the warm nose should limit
snowfall, but cold air in the lowest levels will produce less than
a tenth of an inch of ice accumulation across norther AR into far
western TN.

After the best vertical motion exits the Southern Plains and Lower
MS Valley, moisture in the column becomes too shallow to support
snowflake production. With low level cold air remaining wedged in
place, light freezing rain or freezing drizzle is possible from
northeast OK across northern AR into western TN. Additional ice
accumulations are expected to be light.

Day 2..
A closing mid level low dropping out of the Central Rockies
provides additional synoptic scale lift over the Southern Plains
into the Lower MS Valley during Day 2. Moisture left in the column
from the exiting system could be tapped for snowfall production,
especially over northern OK into northern AR and western TN. While
there is moisture in place, most of it lies below the dendritic
growth zone, so snowfall production will not be maximized.
Snowfall amounts of 2 inches or less are expected in these areas,
along with a coating of ice, mainly outside of the bets vertical
motion with the next mid level impulse.

...Southern Appalachians into the Mid Atlantic...
Deep moisture from the Gulf of Mexico becomes entrained into the
lift associated with short wave energy lifting out of the Southern
Plains into the southern Appalachians from late Day into early Day
3, with the greatest threat of winter weather occurring during Day
2. The 12z NAM appears to be too cold across the southern
Appalachians, while the 12z GFS was likely too fast and too warm
with the mid level system. With this in mind, the thermal portion
of the forecast was based on a multi model blend, with the
greatest contribution from the 12z ECMWF. The QPF portion of the
forecast was based on the most recent WPC QPF.

Day 1...
Moisture streaming eastward, well ahead of the mid level system
over the Southern Plains reaches the southern Appalachians late
during Day 1. Sprawling surface high pressure extending from the
Central Plains into the northern Mid Atlantic resulting in cold
air damming along the lee of the southern Appalachians as the
deeper moisture arrives. Over the higher terrain of far northwest
SC into far southwest NC, the column should be cold enough (at
least initially) to support snowflake production, and
accumulations of 2 to 4 inches were placed here. Further south
into northern GA, a warm nose in the southwest mid level flow
should cut off snowflake production, but cold air dammed in
against the terrain. This should result in an areas of 0.10 to
0.10 inches of ice before 09/00z.

Day 2...
In response to the lift associated with the approaching mid level
short wave, surface low pressure forms on a frontal boundary
stretched across the Gulf Coast. Frontogenetic banding with the
developing surface low is expected to drift northwest into the
southern Appalachians, with the best mid level lift centered
around 09/12z. During this time, model soundings showed deep
moisture and lift in the dendritic growth zone to support banded
snow across far northern NC into far southwest VA and far northern
SC. The lift and moisture could support snowfall rates exceeding
an inch an hour, especially over the higher terrain. There was a
multi model signal (less by the most recent ECMWF output) support
12 to 24 inches of snow across the mountains of western NC.

Further south, a warm nose (generally between 750-700 mb) ahead of
the short wave robs the column of its ability to support snowflake
production. However, with surface high pressure cresting over the
northern Mid Atlantic states, cold air damming remains in place.
This is expected to result in ice as far south as northern GA
(possibly across the northern suburbs of Atlanta). There was a
multi model signal for local
0.25+ inch ice accumulations over far southwest NC, where the cold
air remains in place at the surface the longest.

On the northern edge of the precipitation shield, dry air in the
column is expected to result in a sharp snowfall gradient over
portions of southeast KY and eastern TN into central VA. At this
point, the northern extent of the 4+ inch snowfall amounts should
lie across south central VA, with the gradient tapering to little
in the way of accumulation across central VA.

Day 3...
As the mid level system crossing the southern Appalachians during
Day 3, the precipitation is expected to end from west to east, as
the flow becomes downslope in many locations. By that time, with
the best vertical motion exiting the Mid Atlantic coast, snow
level are expected to rise as the precipitation rates decrease.
Because of this, local 2 to 4 inch snowfall amounts are expected
across the higher terrain of western NC in western VA, where the
column remains cold with the passage of the mid level trough.
Further east, the column becomes too warm to support snow, but
lingering low level cold air could result in a period of icing
following the exit of the best mid level vertical motion. Ice
accumulations are expected to remain below 0.10 inches.

...Pacific Northwest...
A series of short waves crossing the Pacific Northwest will bring
the potential for light to moderate snow (as well as some icing)
during Day 1 and 2. Deeper moisture ahead of a long wave trough
approaching the West Coast poses a greater threat for heavy
snowfall, especially over the northern Cascades of WA. For the
most part, there was good model agreement with the synoptic scale
features, so the thermal portion of the forecast was based on a
multi model blend. The QPF portion of the forecast was based on
the most recent WPC QPF.

Day 1 and 2...
Moisture ahead of short wave crossing the Pacific Northwest during
Days 1 and 2 should be sufficient to support local 2 to 4 inch
snowfall across the higher terrain of the Olympic Range and the
northern and central WA Cascades. In these areas, snow levels
hover between 6000 and 7000 feet in the broad warm air advection
pattern ahead of the mid level impulses.

Day 3...
The moisture profile ahead of the long wave trough approaching the
West Coast is more impressive than Days 1 and 2. There should be
enough lift and moisture to support higher QPF amounts over the
northern and central WA Cascades, where the upslope flow result in
local QPF maxima. The increased warm air advection pattern ahead
of the long wave trough is expected to keep snow levels between
5500 and 6500 feet over the higher terrain in WA. There was a
multi model signal for 6 to 10 inches of snowfall over the higher
terrain of the northern WA Cascades, and based on the amount of
lift and moisture available, those values appear to be reasonable.

Hayes