Lets outline some timing of the onset and changeover of the pcpn.
The snow will start across the Southern Tier early this evening and
overspread the remainder of the western counties by midnight. The
leading edge of the snow will quickly work across the Eastern lake
Ontario region after midnight...while the transition to mixed pcpn
will start to take place over the southern Tier.
On Thursday...the remaining moderately heavy wet snow will continue
to change to a mix of sleet and freezing rain before completely
changing to rain over the western counties by midday. Its worth
noting that the pcpn over the western counties Thursday afternoon
will be more spotty due to the weaker forcing and mid level drying.
Otherwise...the change from snow to a mix will take about six hours
from the onset. Areas east of Lake Ontario may not complete the
change over to just rain...rather experiencing a longer duration of
snow sleet and or freezing rain.
SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
Another surface wave will develop across the mid Atlantic States
Thursday night then move northeast along the eastern seaboard into
Friday. This low will develop another round of precipitation across
the area, especially the second half of the night. Developing
northerly flow will pull down colder air changing the precipitation
back to snow, although there are still are model differences on when
this occurs. The 00z GFS remains the warm outlier, so we favored the
colder NAM/Canadian solutions and went for a quicker change over
with snow continuing into Friday. The general expectation is a 1-3
inch accumulation Thursday night with most of this falling after
midnight with the higher amounts focused across the higher terrain.
The colder solutions showing 850 mb temperatures lowering to near
-14c during Friday, favoring a period of northwest flow lake
enhanced snow, as synoptic moisture lingers across the area at least
into early Friday night. It seems plausible that there may be a
window where moderate to possible heavy accumulations could occur
Friday into Friday night. The best chance to see locally higher
accumulations would be from Rochester to the southern Tug Hill,
perhaps as far south as the Bristol Hills and over the higher
terrain east of Lake Erie. Still some uncertainty to how all this
will play out, but there is enough model support to issue a winter
storm watch for the entire area for the potential of greater than 6
inches of snow during this time frame, with the exception of the far
northwest part of the area where there should be a minimum in
snowfall around the Buffalo metro area.