Scientific Forecaster Discussion
fxus61 kgyx 171153
Area forecast discussion
National Weather Service Gray ME
653 am EST sun Nov 17 2019
high pressure will remain over the forecast area today allowing
for a sunny day with light winds. Low pressure off the mid
Atlantic coast will move north on Sunday night and
Monday...spreading moisture into New England likely resulting in
a wintry mix of precipitation types. Unsettled weather will
continue through the midweek with temperatures near seasonal
Near term /through tonight/...
645 am update...
near term forecast is on track with no substantial changes made
at this time, except to tweak sky coverage a bit according to
satellite trends. A deck of 3k ft clouds has formed over
southeast Massachusetts and is expected to expand northward into
southeast New Hampshire/SW ME this morning.
by dawn today, high pressure will be centered over northern
Maine, providing another quiet and cool day with northerly to
northeasterly flow as the high pressure center crests across the
north. After a frigid morning in the single digits below zero
across the north (into the balmy teens over the southern coastal
areas), temperatures rebound into the upper 20s and 30s today.
Temperatures could make a run for 40 across southern zones
however increasing onshore flow is expected to increase cloud
cover there, so stuck closer to a general consensus for
Attention then turns to a surface low pressure system associated
with a cut off low aloft strengthening off the Carolina coast.
As high pressure and upper ridging moves off to the NE, the low
moves up the coast. Aloft, warm advection starts in the mid-
levels Sunday with h850 temperatures climbing above freezing
Sunday evening and overnight. Northeast flow at the surface
brings surface temperatures back below freezing overnight
except for the New Hampshire seacoast and extreme southern ME where onshore
flow brings more mild temperatures near freezing into the mid-
30s. There is a chance for drizzle and freezing drizzle tonight,
but would have to erode quite a bit of dry air to occur. The
best chances for boundary layer saturation come near the coast
where temperatures wouldn't support ice accumulation. There is a
non- zero chance of a glaze forming from freezing drizzle over
southern New Hampshire and northwest ME but without confidence to issue headlines
at this time.
Short term /Monday through Monday night/...
a complex but likely impactful forecast to start the work week
awaits as a low pressure system quickly moves northward offshore
of the East Coast. A sharply amplified trough over the plains
travels east, tilting negatively as it does so, kicking the cut
off low aloft out over the Gulf Stream allowing the low pressure
to deepen considerably Monday, to around 990mb by the time it
reaches just beyond the 40/70 benchmark. Although the low
center stays offshore, taking aim at Nova Scotia, mixed
precipitation and potentially significant ice accumulation is
becoming more of a concern for New England as a well-forced
shield of stratiform precipitation crosses south to north across
the area Monday with cool temperatures upon onset.
As is typically the case, surface temperatures are the primarily
challenge with this forecast. Starting Monday morning, most
areas start below freezing with widespread 20s or cooler outside
of the immediate coastal areas and where onshore flow (see near
term discussion) allowed temperatures to rise. Aside from
possible drizzle in the morning, model guidance suggests
precipitation onset across the coast during the second half of
the morning. A simple top-down approach with the very warm mid-
levels and cool surface results in freezing rain upon onset. Sooner
arrival would result in more freezing rain taking better
advantage of cool surface temperatures on the cooler side of the
diurnal trend. Alternatively, a later arrival results in less
freezing rain, particularly across the coastal plain of Maine.
As the low approaches the Gulf of Maine Monday, warm advection
helps the diurnal trend, bringing temperatures above freezing
along a SW/NE oriented line pushing inland from the coast
changing mixed precipitation over to rain. Current timing brings
the 95 and 93 (south of the whites) corridors above freezing by
around noon and the upper Connecticut valley late in the
afternoon. The thermal ridge aloft peaks over New England during
this time with h850 temperatures ranging +2 to +6c making for a
bit of a messy precipitation forecast over the mountains with
rain, freezing rain, and sleet all possible depending on the
depth of the low level cold layer especially over the
international border region of Maine.
Best forcing and highest quantitative precipitation forecast also exists during the Monday
afternoon/evening period, with heavier precipitation possibly
mixing warmer temperatures from aloft to the surface, further
complicating the p-type/impact forecast, particularly north of
the western ME mountains. Overall, a low confidence ice
accumulation forecast with conservative amounts up to 0.10" upon
onset over the 95 corridor during the morning (before warming
and likely melting), and also over the north where cool
temperatures remain when best forcing arrives.
Monday night the low crosses the Gulf of Maine with a northerly
to northwesterly wind shift and cold advection taking over. As
temperatures aloft come down, precipitation switches to snow
with around 2-4" possible over the north with favorable
upsloping winds enhancing amounts over the mountains.
Long term /Tuesday through Saturday/...
the long term period will start out pretty active as one
northward-moving short wave arrives right on the heels of a
departing one Tuesday morning. This likely means another burst
of steady precip (albeit brief). The issue continues to be
temperatures at the surface and aloft that will be pretty marginal
throughout and will likely continue to result in a variety of
precip types through midday Tuesday, especially inland. On the
coastal plain, boundary layer temperatures should be warm enough
Tuesday morning to allow for mostly rain with temps in the mid
30s. However, it is possible that a good portion of the forecast
area could flip to all snow for a couple of hours if ascent and
associated cooling is strong enough. The most likely scenario
Tuesday morning is rain along the coast and across most southern
zones, with a mix inland changing to all snow in the mountains
with a couple of inches possible.
Thereafter, on Wednesday and Wednesday night another short wave
trough looks to move through, but the members of the 00z
guidance suite isn't handling the amplitude and track very
consistently. The most likely scenario is for a period of light
snow showers will will continue to watch out for trends that may
allow for a period of heavier precipitation.
The next in the long train of short wave troughs arrives around
Friday. Run to run consistency with this one continues to be
poor, especially with amplitude. Therefore, low confidence
forecast on ptype, amounts, and winds. However, consensus at
this time would yield not much more than a strong cold frontal
passage on Friday.
Aviation /12z Sunday through Thursday/...
VFR prevails today with light northerly flow turning
occasionally variable under high pressure. Clouds build in
coverage and depth, from S to N, tonight with ceilings lowering
to MVFR along the coast and southern New Hampshire by 12z Monday. Freezing
rain is likely upon onset before switching to rain during the
afternoon. Periods of IFR ceilings and some IFR visibilities
are likely to develop Monday with heavier areas of
precipitation. Winds will also increase out of the north and
northeast Sunday night with about 15g25kt by the afternoon. As
the low exits to the northeast later Monday, winds back to the
north and eventually northwest Monday night with an array of
precipitation types possible over the north (rain south). IFR
conditions expected for this period.
Long term...Tuesday through Thursday...
IFR conditions improve to MVFR then VFR midday Tuesday.
Lingering shsn and MVFR ceilings for hie and perhaps leb into
short term...today through Monday night...
high pressure crosses today with generally light offshore flow.
Tonight, northeasterly winds strengthen in response to the
approaching coastal low, to gale force for all of Monday. Have
upgraded the gale watch to a Gale Warning as a result. Winds
decrease below gales Monday night as the low pressure pulls
away to the northeast.
Long term...Tuesday through Thursday...
winds and seas will diminish Tuesday and Tuesday night as low
pressure moves away. However, winds and seas look to increase
again later Wednesday and Wednesday night as offshore low
pressure tightens the pressure gradient on the coastal waters.
prolonged NE flow to gale force, along with building seas ahead
of the coastal low may result in minor splashover and erosion at
high tide on Monday, mainly from Portland south. This is
supported by snap- ex, etss, and estofs models which show storm
surge of around 1.5 ft.
Marine...Gale Warning from 3 am to 10 PM EST Monday for anz150>154.