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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0843Z Nov 12, 2018)
 
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Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
343 AM EST Mon Nov 12 2018

Valid 12Z Mon Nov 12 2018 - 12Z Thu Nov 15 2018

...Great Lakes...
Days 1-3...

Continued periods of lake effect snow are likely, especially on
d2/d3 as W/NW winds develop behind a strong low pressure system
moving up the Atlantic Coast. As the surface low pulls away
Tuesday, a shortwave will move eastward across the Great Lakes in
its wake. This will cause column winds to become W/NW through a
deep portion of the column, producing favorable unidirectional
shear and cold advection across the lakes. Periods of heavy lake
effect snow are likely, especially southeast of Lake Superior,
Erie, and Ontario. Downwind of Lake Ontario may experience the
heaviest snow as instability becomes maximized and a connected
fetch occurs from Lake Huron. WPC probabilities are high for more
than 4 inches of snow east of Lake Ontario, with lighter amounts
likely near the other lakes. 


...Southern Plains into the Ohio Valley...
Day 1...

A swath of snow is likely to extend from near the Red River Valley
of TX/OK, northeast towards the southeast Great Lakes. This area
of snow is associated with lift beneath the diffluent right
entrance region of a polar jet streak, as well as mid-level
frontogenesis. Snow accumulations will generally be light, but WPC
probabilities do exceed 30 percent for 4 inches from far northeast
OK into southern MO. Locations northeast from here should see
lesser snow amounts as the subtropical jet phases with the polar
jet deflecting the best synoptic ascent to the southeast.
Available moisture is also less into the Ohio Valley, so total
accumulations there should only be 1-2 inches.


...Interior Mid-Atlantic and Northeast...
Days 1-2...

A positively tilted longwave trough will gradually shift eastward
into the Ohio Valley Tuesday. However, a reinforcing trough over
the Canadian Prairies cuts off the mid-level closed low over the
southern Plains while a Nor'Easter develops with the newly focused
northern stream trough. The associated surface low closes near the
Mid-Atlantic coast early Tuesday morning and is north of Maine by
Wednesday morning as a deep low.  Significant warm and moist
advection will occur ahead of this trough, with a strengthening
baroclinic gradient producing the potential for snow well north of
the surface low track beginning Monday night in the Ohio Valley.
Rapid cyclogenesis northeast from the Mid-Atlantic Tuesday would
broaden the precipitation shield to the eastern Great Lakes with
areas roughly along the northern Appalachians and west under
threat for snow.  Significant QPF should fall as snow on the west
side of the Appalachians where thermal profiles are cold enough
for snow. A sharp rain/snow line is expected with mainly southerly
flow east of the Appalachians through much of the QPF ahead of the
low. There has been a subtle shift eastward in the model consensus
this morning prompting a slight shift eastward of the heaviest
snow amounts. However, probabilities for 4 inches of snow are
still only high for the terrain of the Adirondacks, as well as the
mountains of Vermont, New Hampshire, and especially Maine where a
slight risk for 8 inches of snow exists.


...Pacific Northwest...
Day 3...

A shortwave trough swings off a Gulf of Alaska low and pushes
across British Columbia Wednesday before dropping towards Montana
late on day 3. The near zonal flow over northern WA will bring
Pacific moisture and a pronounced baroclinic zone over the
northern Cascades and Rockies of northern ID/MT by Wednesday
afternoon. Snow levels around 7500ft can be expected on the north
side of this zone with heavy snow for the north Cascades, pushing
into the northern Rockies near Glacier National Park Wednesday
night.


...Southern Appalachians...
Day 3...

Anomalously deep cutoff upper low will drift across the
Mississippi Valley on Wednesday before ejecting slowly to the
northeast while filling. A surface wave will develop along the
Southeast coast with deep moist advection lifting into the
Southern Appalachians Wednesday night. Cold high pressure to the
north will only slowly retreat, and as isentropic lift
intensifies, precipitation will overspread the region. Despite a
warm nose rapidly lifting to the north, surface temperatures
within the wedge of high pressure will remain below freezing, at
least during precipitation onset, and a period of freezing rain is
becoming more likely Wednesday night. There is good model
agreement in freezing rain, but placement and intensity vary
widely. There is enough consensus that WPC probabilities have been
raised, and low probabilities now exist for 0.25 inches of
accretion across the terrain of NC, VA, and WV.


Weiss