Another de-amplifying shortwave lifts NE into New Eng as it
encounters strong downstream high latitude blocking. Parent low will
weaken across the eastern Lakes with secondary low becoming dominant
storm as it tracks just south of the New Eng coast. This track will
help to lock in low level cold air, especially away from the coast,
but for this event we will have a pretty strong SW mid level jet
which will bring a warm nose aloft into New Eng. The challenge for
this forecast is how far N this warm nose gets Fri night, how much
of a front end thump is expected Fri night before the warming aloft
and also how much snow falls during Sat over interior northern MA
assocd with mid level low. A lot of moving parts which makes for a
very challenging snowfall forecast. The GFS is on the colder side of
the guidance envelope and we think it is too cold as it typically
has a cold bias aloft as it tends to mix out inversions. The other
consideration will be snow liquid ratios as marginal boundary layer
temps will keep SLR below 10:1, especially outside of the higher
terrain.
Timing, ptype and snowfall...
Snow should overspread the region from SW to NE between 8 pm and
midnight Fri evening. We are not expecting any impacts to the
evening commute. Mainly snow to start across all SNE as temps aloft
are plenty cold enough to support snow, then will have to watch warm
nose aloft lift NE into SNE assocd with a 70-80 kt 700 mb jet.
Current thinking is this will lift further N than what GFS is
indicating and likely get into northern MA. Confidence is not as
high as we would like at this time range as a slight shift north or
south will have a big impact on potential snow accum. We expect a
mix and possible changeover to sleet after midnight in the interior,
with a changeover to sleet then rain near the coast as increasing
easterly winds overwhelm the boundary layer. Soundings show a fairly
deep cold layer in the low levels supportive of more sleet than
freezing rain where changeover occurs. The best chance for all snow
will be across far northern MA, along and north of route 2 corridor.
We have some decent snowgrowth on the front end so a brief period of
moderate to heavy snow is likely Fri night with snowfall rates up to
1"/hr, especially away from the coast. The challenge on the coast is
SLR and potential melting as temps will be near or slightly above
freezing, but nighttime and potential brief heavier rates should
help to make up for this and bring minor accum here before the
changeover to rain.