Good write-up from GYX for tomorrow night/sunday
KEY MESSAGE 3 DESCRIPTION...
Low pressure moves through the Great Lakes Saturday night, as a
second low develops in the Gulf of Maine late Saturday night
and Sunday. The first low brings warm air in aloft, while the
coastal low helps brings colder air southward at the surface
Sunday morning.
Like we have seen several times so far this winter, cold air
damming will help to drive temperatures and precip types through
the event. With the NBM`s reliably poor performance through
these systems, the forecast heavily weighted the available high
res guidance, most notably from the Nam 3km. As the HRRR becomes
more available through the system, there is likely room for
continued refinement of temperatures into the day on Sunday.
The set up overall is not as ideal at CAD set up as we have
seen from other systems this winter. Saturday`s highs are mild
with temps in the low to mid 40s, and precip arrives too early
in the evening to allow for any meaningful radiational cooling.
This means we`ll have to rely on wet bulbing and cold air
advection for the colder temperatures.
It`s pretty uncommon to wet bulb below freezing into sleet in
freezing rain. For this reason, along much of the coastal plain
and southern New Hampshire it`s likely to be in the 32-34 degree
range with rain for much of Saturday night. Just north of this
area, temps are likely to be at or just below freezing to
produce a narrow stripe of sleet and freezing rain. Then north
of here through the mountains and foothills, mainly snow is
expected.
This is the anticipated outcome through Saturday night. Then by
daybreak Sunday, temperatures begin to fall as the coastal low
deepens and winds shift to northerly. Colder air is expected to
slosh southward at all levels of the atmosphere as the low
deepens. This transitions precip to mainly rain or snow Sunday
morning, with the rain/snow line moving southward. Precip likely
runs out before the cold air arrives through the Seacoast as
the dry slot moves in, but a period of snow likely ends the
event through much of coastal Maine.
Further north through the mountains, foothills, and across
central Maine, moisture likely continues to blossom through the
day as the coastal low deepens. Through these areas, there is an
increasing potential for advisory level snowfall through the
event.