Scientific Forecaster Discussion
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Area forecast discussion...updated
National Weather Service Gray ME
119 PM EST Tue Dec 12 2017
low pressure over New England will strengthen and lift north and
east of the region tonight and on Wednesday...with heavy snow and
rain through this evening giving way to mountain snow showers and
strong westerly winds on Wednesday. The coldest air of the season
will move into the region behind this system for the remainder of
the week...with a few chances for additional show showers.
Near term /until 6 PM this evening/...
115pm update: added coastal Rockingham/York/Cumberland to the
advisory as correlation coefficient off of gyx 88d matches with
observations showing warm air aloft moving ahead of coastal
front at the surface...resulting in a period of sleet and
freezing rain. No other significant changes at this time.
945am update: no changes to the headlines with this morning
update. Initial around of snow has produced 1-3 inches across
the region. Biggest changes are to match most recent local WRF
and GFS lamp data which indicates a much slower transition to
above freezing temperatures...with nearly all inland likely
remaining below freezing through the event. Primary low
continues to lift through New York with surface pressure falls
not really yet pin pointing the location of secondary
development. Coastal front is well evident on gyx 88d as a fine
line that is now beginning to lift north and west. Already
seeing precipitation filling in to the west in response to
strengthening mid and upper level forcing. Thus...snow will
redevelop with coastal changeover to rain during the mid
afternoon...with some mix likely north of this as warm air aloft
briefly lifts north before precipitation ends. Have to watch
interior York/Cumberland for warning level amounts...but amounts
thus far have been less than previous forecasts...so while
rates will shortly increase...inherited snow totals still
7am update... snow has started across most of southern Maine and
New Hampshire. Visibilities are decreasing as snow increases to
the southwest. The cold air damming has continued to strengthen
with even all buoys continuing to report northwest flow, this may keep
the precipitation as snow even longer along the coast.
***widespread snowfall expected ****
Low pressure will move through the region today bringing heavy
snow, mixing with and changing to rain along the coast.
Currently: at 3am a low pressure center was located over
Georgian Bay, with a secondary low forming over Pittsburg PA.
Pressure falls have been increasing to the southeast over the
past few hours with the PA low expected to take over as the main
center by daybreak. Ahead of this low a broad region of light
precipitation extended into New Hampshire.
Across northern New England a strong region of cold air damming
is in place, with a 1020mb high extending from New Brunswick
south across coastal Maine to the New Hampshire/Massachusetts
line. Within this region temperatures have continued to cool
overnight despite advancing cloud cover from the approaching
Morning: snow will continue to move into the region this
morning, overspreading the entire County Warning Area by mid-morning. The low
levels remain quite dry with dewpoints in the single digits
north and near 20 south. Thus the initial precipitation may take
a while to reach the surface. This has been reflected in
upstream observations over New York state. Snowfall will
increase this morning as the low intensifies and the low levels
saturate. With the cold air damming expect ptype to remain as
snow through the morning for all but extreme southern New
Hampshire where a bit of mixing is possible just before noon.
Expect a quick 2-3" of snow as the initial round of
precipitation moves in.
Afternoon: by afternoon we see two things occur. First the
heavier qpf makes its way into Maine and encounters the strong
coastal front. Expect this to result in a maxima in
precipitation rates just inland of the front. Snowfall rates of
1"/hr are possible in this area, extending across the
foothills. Meanwhile the low center begins to track across
southern New Hampshire and up Penobscot Bay. As it does it will
erode some of the same surface cold air that's helping to
enhance snow totals. This will result in a very tight gradient
in snowfall between the haves and the have nots. The area with
the most uncertainty remains Kennebec and Waldo counties.
Snow: 6-10 inches of snow across a broad brush of the mountains
and northern portion of the area. The sticking points remain
the coastal zones where rain can mix in. With the warm air
coming in all at once as the low passes overhead, expect a
minimal amount of sleet or freezing rain to occur during the
Headlines: a few changes to the existing headlines this
morning. The winter weather advisories have been expanded to
eastern Hillsborough and interior Rockingham counties. This
reflects the current colder temperatures there which should
allow them to pick up near advisory level snowfall. There may
also be a brief burst of sleet or freezing rain as the
precipitation type changes midday which will result in slick
roads and warrants an advisory expansion.
Winter weather advisories have been converted to winter storm
warnings for Sullivan, Merrimack, and Belknap counties in New Hampshire.
This reflects slightly higher snow totals overall for these
areas, especially in the higher terrain.
Short term /6 PM this evening through 6 am Wednesday/...
as the low moves off into eastern Maine this evening cold air
will come rushing back in behind it as the precipitation cuts
off. For the northern mountains, this will mean another few
inches of snow on the backside as the precipitation wraps around
behind the cold front. At the coast precipitation will come to
an end with a chance for some light drizzle or freezing drizzle
to take over at the end.
Long term /Wednesday through Monday/...
upper low pressure is expected to cross the forecast area Wed.
This will bring several sensible wx impacts to the region. The
two higher confidence impacts will be much colder air and gusty
winds. As the cold front crosses the area behind the trof
axis...temps will likely hold steady or fall during the day Wed.
In addition...with the strong cold air advection winds will become gusty...and
could approach 30 to 35 kts into Wed evening. Gradient flow is
expected to continue overnight...so I used raw model 2 M temps
for lows in the well mixed environment. As readings fall into
the teens and single digits...wind chill values will also drop
to near zero...and as cold as ten below in the nrn zones.
The more uncertain part of the Wed forecast will be coverage and
intensity of snow showers. The upper low itself should provide
some support for snow showers as it swings overhead...so I have
at least slight chance to chance pop for part of the day Wed all
the way to the coast. Behind the trof axis plenty of low level
moisture will remain...but flow will become wly and downsloping
commences off the higher terrain. This may dry out some of the
areas downstream of the mtns. Upwind of the mtns however...very
cold temps will have the prime snow growth zone in the same
region of the atmosphere where orographic lift will be
maximized. That will support widespread snow showers thru the
day...possibly heavy at times...and efficient accumulations due
to high ratio snow. It is possible some of the higher terrain
will pick up an additional 3 to 5 inches or more beyond what
they receive today.
A trailing shortwave trof looks to pass S of the forecast area and
out to sea Thu. But the pattern remains active...and fast moving
waves thru the nrn stream will have the potential to phase with
the srn stream and bring precip to the area. There are two
windows Worth keeping an eye...one being Sat and another early
next week. At the moment the shortwave model guidance is keying on
for Sat is over the Pacific Ocean and some 24 to 36 hours from
moving over land observations. For now I see no reason to
deviate from a chance of snow near the coast.
Aviation /18z Tuesday through Sunday/...
LIFR/IFR in snow changing to rain mht/pwm/rkd with a period of
freezing rain/sleet at these locations during the transition.
Mix also expected con/Aug...with all snow hie/leb.
Precipitation ends this evening with slow improvement in
conditions overnight. VFR outside of the mountains on Wednesday
with westerly winds 20-30kts. Continuing shsn at hie on
Wednesday with IFR/MVFR restrictions.
Winds gradually diminish Wednesday night with VFR conditions
Long term...upper low pressure will cross the forecast area Wed
with areas of MVFR cigs. Widespread shsn expected...which will
bring local IFR conditions...most likely at leb and hie. In
addition wly wind gusts near 30 kts possible Wed and Wed
evening. Shsn linger near hie into Thu...with VFR conditions
expected elsewhere. Ocean low pressure will develop Sat...and at
this time looks like it will remain out to sea. There is a
chance this spreads some -sn back towards coastal
terminals...but confidence is low in that scenario.
short term... low pressure will cross the Gulf of Maine with
winds briefing increasing to gale force this afternoon as the
low moves through.
Long term...strong cold air advection Wed will allow a wly gale to develop. The
bays should be more of a marginal gale...but outside the bays
may gust to near 45 kts Wed evening. Winds and seas will
gradually diminish into Thu...but Small Craft Advisory conditions will likely
linger into Thu night. Low pressure looks to develop in the wrn
Atlantic to start the weekend...and may prolong Small Craft Advisory
ME...Winter Storm Warning until 1 am EST Wednesday for mez007>009-
Winter Weather Advisory until 1 am EST Wednesday for
Winter Weather Advisory until 6 PM EST this evening for mez023-
New Hampshire...Winter Storm Warning until 1 am EST Wednesday for nhz001>009.
Winter Weather Advisory until 1 am EST Wednesday for
Winter Weather Advisory until 6 PM EST this evening for nhz014.
Marine...Gale Warning until 1 am EST Wednesday for anz150>154.