Posted this on both threads since it talks about Synoptic and lake effect..
SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
There will be two distinct phases for accumulating snowfall. The
first will primarily affect far SW NY Thursday night, which roughly
includes higher elevations of Chautauqua County, Western Cattaraugus
County, and Southwestern Erie County. This area will get clipped by
the heavier synoptic snow, potentially up to 12", on the cold side
of a developing surface low moving from PA into the North Country by
Friday morning. With the above in mind, issued a Winter Storm
Warning for SW NY, knowing that this will need to be converted to a
LES Warning after the synoptic snow ends. This should also match our
neighbors across the NYS line.
The 2nd phase will be lake effect that will develop during Friday,
starting out as a relatively weak band initially over far Western NY
during the morning. This should intensify however as the band
develops a full lake fetch as the coldest air aloft (easily seen by
following forecast 500mb temps) moves toward the region from the
WSW. Expect a full blown LES band over the Buffalo and possibly
northtowns by Friday eve, before the band begins to move S a little
again Friday night and settles over the more traditional areas just
South of Buffalo and over the Southtowns, where it should slowly
weaken through Saturday. There should only be 1-2" from the
synoptic snow, then a long enough break before the LES band begins.
Therefore, went just with a LES Warning for Northern Erie and
(Western portions of) Genesee and Wyoming counties.
Adding up both the synoptic snow and LES snow, some areas could see
1-2` of snow.
Briefly covering the pattern responsible...a well-formed longwave
trough now covers much of the Central US. This has spawned a
surface low with associated blizzard conditions over the Upper
MidWest. But this feature allows a strong jet on the south side of
the trough to spin up a secondary low over the Midwest and into PA
by Thursday evening, while abundant moisture streams north from the
Gulf of Mexico. Cold air over the midwest should then move east,
changing any ongoing rain over to snow, at least for WNY. Highest
snow totals will be closest to the PA state line/over the Chautauqua
Ridge where there may be a little lake enhancement by Friday
morning. Far less is expected further inland. Meanwhile, areas from
near KROC east to the Eastern Lake Ontario may still wake up to
grass instead of a White Christmas. But this will change, at least
for lake snow-prone areas as colder air aloft rushes into the region
later Friday.
As noted above, a full blow LES band will be underway by Friday
evening, and a similar lake response is expected ~Friday night ENE
of Lake Ontario. This is fairly far out the the forecast though, so
left a Watch in place. By Saturday, the LES bands should be
ongoing. One concern will be another Canadian-sourced shortwave
trough that will race ESE during the day. This may force the lake
bands to back a little, particularly east of Lake Ontario, before
being snapped back to the ESE later in the day or Saturday eve. With
uncertainty and details still a little fuzzy, the Watch seems
reasonable for now.